Desktop CNC Drill


A desktop computer controlled drill for printed circuit boards

This report examines the design and development of a computer controlled printed circuit board drill. It covers the activity from the initial conception through market opportunity to detailed design and manufacturing. The technical areas investigated include software, electronics and mechanical design.

The interfacing techniques describe how to control stepper motors using a standard PC with a parallel port. The driver electronics are also examined. Software techniques are used to interpret the Hewlett Packard Graphics Language file format (HPGL) and it is shown how it can be utilised in CNC applications. The mechanical components for each axis are designed with cost and ease of manufacture being amongst the primary considerations.

The final design is critically examined and suggestions are made for further improvements.

 

Contents
  1. Aims and Objectives

    Objectives

    Aims

2. Introduction

Product Introduction


Design Intent Statement


What makes the Drill Different to Existing Products?


Overview of Printed Circuit Board Fabrication

3. Market Feasibility

Market Opportunity

Product Placement


Customer Requirements

Benchmarking of Existing Products

4. Technical Feasibility

Introduction to Technical Feasibility


Electronics

Software and Interfacing

Hardware

5. Product Design Specification


1.0;Product Description and Operation


2.0;Market


3.0;Performance

4.0;Environment


5.0;Maintenance


6.0;Target Product Cost


7.0;Competition


8.0 ;Quantity


9.0 ;Size


10.0;Weight


11.0;Aesthetics, Appearance and Finish


12.0;Materials


13.0;Product Life Span


14.0;Standards and Specifications


15.0;Ergonomics and Human Interface


16.0;Customer


17.0;Quality and Reliability


18.0;Testing


19.0;Safety


20.0;Installation


6. Conceptual Design


Conceptual Design of Subsystems


X-Y Drive System Concepts


Bearing Possibilities


PCB Clamp Mechanism


Drill Up/Down Mechanism

Concept Choice and Justification


Appearance


System Integration


Software User Interface and Human Factors

7. Detailed Design and Development

Design for Manufacture and Design for Assembly

8. Costing;65

9. Discussion

10. Conclusions


11. Bibliography


12. Appendix;72

Aims and Objectives

Objectives

 

Aims

Product Introduction

The product is a desktop computer controlled drill that drills holes in printed circuit boards or any other similar sheet material. The drill comes with software that interprets HPGL (Hewlett Packard Graphics Language) files that are generated in a CAD package and sent to the drill via the interfacing software.

Design Intent Statement

"To design a computer controlled printed circuit board drill that interfaces to the parallel port of any PC compatible and is simple to use, for a direct cost of less than £100"

What makes the Drill Different to Existing Products?

The drill's unique selling point is its low cost. Existing CNC solutions are very expensive, and appear to be treated by vendors as a specialist technology, whereas the fundamentals of CNC control are quite straightforward and easy to utilise.

The drill can achieve its low cost by:

Overview of Printed Circuit Board Fabrication

There are several types of substrates available for prototyping such as stripboard, veroboard TM, wire-wrap, etc. but all of them come under pressure when the hole and component count rises above a few dozen components. Layout becomes difficult, confusing, and the chances of connection error are high. That is when printed circuit board methods become beneficial. The low cost of computer aided design (CAD), and the proliferation of PCs makes layout design with computer an accessible option. The 'traditional' design process for producing a prototype printed circuit board is shown in figure 2.1

Fig 2.1 Typical PCB construction procedure

Market Opportunity

Using analysis from relevant internet newsgroups it was discovered that there was a need for accurate and automated PCB drills and other similar XY computer controlled tools. There were several people who designed their own machines. This was done using some of the following methods.

A variety of control techniques were used, including:

Existing CNC products for PCB prototyping are expensive. It is possible to buy a turnkey solution, or to build a system from scratch using linear slides and steppers with leadscrews However, both of these solutions give more accuracy than is required and are very expensive e.g. 6.35 diameter 300mm long leadscrew and nut costs £72.45 ex. VAT. (Source: HPC Drives Ltd., catalogue D7)

The cheapest tools for drilling are manually operated hand drills and dedicated power PCB drills. They are cheap, but they have the following disadvantages:

Disadvantages of Manual Drilling

Advantages of Manual Drilling

There is an opportunity in the market for a computer-controlled drill that is cost effective for smaller runs of PCBs. The cheapest non-dedicated CNC product found cost over £550, and the cheapest dedicated CNC PCB drill was the 'Quickcircuit' system costing from £8200 (see Fig 3.2, benchmarking table for more information on competitors, page 12)

The 'Desktop CNC Drill' is aimed at anyone with CAD facilities who wishes to produces short runs of PCBs, specifically those who wish to produce circuit boards with many holes, or several copies of the same board design. Such cases would involve tedious manual drilling, and a CNC solution would be ideal. At present there is no low cost integrated CNC drilling solution available to customers.

Advantages of CNC over Manual Drilling

Disadvantages of CNC Drilling

The primary function of the CNC drill is for PCB prototyping, but it will also be capable of drilling holes in sheet materials up to a thickness of 3mm, once they have a similar hardness and density to that of epoxy/glass fibre PCB.

Product Placement

There are already sophisticated CNC machines available for commercial PCB production, and high end PCB prototyping machines capable of milling tracks and drilling holes. However, the cost of such machines makes them prohibitive for smaller development operations, or those wishing to introduce themselves to CNC. The Desktop CNC drill is not intended to compete in the high end/production market, where there are many excellent but expensive products.

The drill is aimed at the following types of customers:

An important feature of the product is that the drill head can be removed and replaced with a tool of the users choice. This adaptability is a feature of the product that increases functionality and makes the product potentially a useful tool for other tasks including:

 

Customer Requirements

In order for the CNC drill to satisfy the customer, it must meet their requirements, by providing the features that they want in an accessible format. The drill should incorporate all the benefits of cheaper means of drilling, e.g. the flexibility of hand drilling, and also incorporate ideas used in high end machines, scaling them to a more economical solution.

The following is a list of customer requirements generated through market research into existing products, and through contact with people who produce PCBs:

Features of Desktop CNC Drill

Every part and feature of the drill should directly satisfy a customer requirement, meet the needs of the specification, manufacturing and assembly considerations, and be as cost effective as possible. The following is a table of customer requirements together with the feature of the drill that meets those requirements.



Customer Requirements

Feature of Drill

(for more details on features, see the PDS page 26)

Cost effective Low price, good performance
Takes up small amount of space Small footprint
Reasonably quick operation Faster than manual, 30 holes per min
Accurate drilling Accuracy and Repeatability +/-0.1mm
Easy to use Graphical software, on-line help and diagnostics
Easy to control hole positioning CAD file controls hole position
Allow for easy removal of piece part Quick hold/release mechanism
Minimum maintenance No maintenance
Allows for large PCBs Max PCB size of 150x150mm
Easy to clear away swarf and debris Swarf tray to catch debris
Flexible Removable drill head, with mounting plate for other tools, e.g. pen, engraver etc.

Table 3.1 - Customer requirements of a CNC PCB drill.

 

Benchmarking of Existing Products

There are quite a few drills that can be used for PCB work, and they can be categorised as follows:

Manual Drilling Products

CNC Drilling Products

As there is no commercially available product in direct competition, a sample of products with prices relatively close to the Desktop CNC Drill has been examined. This rules out the most expensive production level machines. The purpose of this benchmark is to compare features and performance against price. This will clarify what kind of features and performance is required in order to be competitive in the marketplace.



Manufacturer /Drill

Description

CNC /Manual

No load RPM

Torque g/cm

Bit Size /mm

Weight /kg

Vcc

I at max load /A

Cost /£ Ex.VAT
Reliant PCB/Mini Drill Manual 9000 100 <=2.9 0.160 12V DC 1.5 13.01
Zircon/Zircon2 PCB/Mini Drill Manual 11200 150 <=3.0 c.0.200 12V DC 2.25 20.82
Como Hobby Drill Manual 14500 530 <=3.0 c.0.200 6-18V DC 0.75 22.97
Minicraft/MB150 Hobby Drill Manual 16000 25W <=3.2 0.190 9-18V DC ? 22.97
Minicraft/Buffalo2 Hobby Drill Manual 12500 100W <=6.0 0.420 9-18V DC ? 36.58
Skil/Cordless Cordless Man Manual 1100 ? <=10 1.300 12V DC ? 51.05
Dremel/Multi DIY /Mini Drill Manual 37000 ? <=6.0 ? 240V AC ? 75.66
Black&Decker/BL400 DIY /Power Drill Manual 2500 400W <=10 ? 240V AC ? 19.06
Black&Decker/KD574CK DIY /Power Drill Manual 3000 680W <=13 ? 240V AC ? 61.91
T-Tech/Quick Circuit 5000 CNC PCB Prototyping Machine CNC 23000 ? 0.1 to unlim. 18.000 240V AC ? 8200.00
Chester Eagle Mill/Drill Mill/Drill Manual 3000 2HP <=32 300kg 240V AC ? 849.36

Table 3.2 - Performance benchmark of existing products

Introduction to Technical Feasibility

All the technology in the CNC drill has been used before. The challenge is in the bringing together hardware, software and electronics to form a design that meets the required performance, is cost-effective, competitive and useful to customers. There are existing products that are similar, but are aimed at the mass manufacture market, or the high end prototyping market.

Technical feasibility is discussed under the following headings:-

Electronics

Parallel Port Information

The PC printer port is an inexpensive method of implementing control of external systems. The standard has been in existence for many years and is well documented. A single port can have eight outputs, five inputs and four bidirectional lines. The parallel port typically has a 25 pin D type female connector, which contains the 3 sub ports; data, status and control. Most parallel ports are located at address hexadecimal 378, or denary 888. The following is a table of common port addresses found in most PCs.



Printer;

Data Port

Status Port

Control Port


LPT1		 888 (hex 378) 889 (hex 379) 890 (hex 37a)


LPT2		 632 (hex 278) 633 (hex 279) 634 (hex 27a)


LPT3		 956 (hex 3bc) 957 (hex 3bd) 958 (hex 3be)

Figure 4.1: Table of common printer port addresses

The software that controls the drill will default to data address 888 as it seems to be the most popular, but in order to accommodate differing machine configurations, the user can easily define their required port address.

There are now more modern specifications of parallel ports with enhanced specifications.

These add extra features such as allowing the data direction of bits to be switched, giving more flexibility. (Source: 'Interfacing to the IBM-PC Parallel Printer Port', Ian Harries 1997) For the purposes of compatibility the drill uses the original port specification, now referred to as the Standard Parallel Port (SPP).

The port voltage levels conform to the TTL (transistor to transistor logic) standard, where low is 0 to 0.8V and high is 2.4 to 5V. A data high is 1 and a data low is 0.

Motor Control Chips

There are a few custom stepper motor driver chips, with varying degrees of complexity and cost. After researching what was available, It was concluded that it would be cheaper and give more control if I drove each coil separately from the parallel port. Most of the cheaper chips had a current capacity of 500mA max, some up to 1A. An example of this is the SA10047 from Phillips as in fig 4.2 below.

Figure 4.2 - Stepper motor driver using Phillips SAA1042

Using separate drive lines makes the system more flexible and easier to adapt to a more general type of control if desired by the customer.

Drive Circuit

The following circuit has been tested in a test rig and has been found to be satisfactory in terms of function. Further tests will determine its reliability and performance.

Figure 4.3 - Control circuit for each motor coil

Circuit Considerations

7407 buffer and current driver

As the project involves controlling high voltages and currents, using low TTL voltages, there is the danger of damage to the PC I/O board should an fault or error occur. Therefore it is good practice to isolate the power electronics from the low power side. There are three isolation possibilities:

  1. Buffer/driver isolation

Optoisolated I/O interface to PC

This method provides a very high degree of over voltage protection. The cost of an optoisolator chip is relatively expensive when compared to a 7407 or a 7404 buffer.

Buffer/driver chip, e.g. 7407

This method employs a buffer to handle the difference in voltages between both sides of the electronics (5V TTL, and 12V for the motor). The 7407 is a cheap chip, providing 6 buffers and current drain of up to 30mA

Figure 4.4 - Pinouts for a 7407 Buffer driver

 

Power Supply

The power supply will be provided by the user, and needs to be 12V DC, tip positive, capable of supplying 2-3 Amps. Most bench power supplies can easily provide this. Some of the electronics require a stable regulated 5V supply (TTL) for interfacing with the PC, and this is provided internally using a 7805 regulator, with 1000m F smoothing capacitor. The motors and the drill need 12V and this can be a relatively coarse supply. Cirkit Distribution Ltd. offer a 12V 2A regulated PSU for £17.95

 

Driver Transistor

In order to provide the high currents required for the coils of the motors, some form of current driver is needed. A cheap, rugged and reliable method is to use a power transistor such as the TIP122 Darlington pair type. It can conduct up to 5Amps, and together with a heatsink will be a reliable and cheap choice.

TIP122 Specification



Material

NPN


Max I

5A


Hfe

5000 @ 2A


Vceo max

100V


Vcbo max

100V


Vebo max

5V


Ptot max

65mW

Current Limiting Resistor

By providing the motor with a higher supply voltage that its rated voltage, there is the benefit of increased rotor speed for a given torque, as shown figure 4.6 below.

Fig 4.6 - Effect of forcing resistors

This is quite a common technique in order to increase the step per second performance, as the current build up in the coil is much quicker. However, in order not burn out the coils, there needs to be a resistor in series with each coil (see chapter 7, page 59 for calculation of the resistor value)

 

Shunt Diodes

As the load driven by the transistors is a coil and inductive, there will be quite high reverse voltages when the coils are discharged. In order to protect the transistors from over voltage damage there is a diode place in parallel with each coil.

Drill On/Off Circuit and Drill Up/Down Circuit

These circuits are essentially the same as the circuit for each coil of the stepper motor, but there will be no current limiting resistors, as they are not needed.

Limit Switches and Drill Up/Down Sensor Circuit

Each axis of the drive system has a 2 limit switches in order to detect its home position, to initialise the step count and to protect against moving outside the traverse boundary.

The circuit used will be similar to the figure 4.7. When the 'X switch' is open, the voltage on 'Y output' is 5V, or TTL high. Conversely, when closed the output is 0V, or TTL low.

Figure 4.7 -Limit switches circuit

This circuit is the same for the switch that detects when the drill has left its home position

 

Software and Interfacing

Writing and Reading to/from the Parallel Port

The logic levels of each port can be controlled by sending a byte to the respective address. This is done in Qbasic by using the 'OUT' command. The format of this command is OUT [port address],[byte (decimal 0-255)] To switch all bits high on port 888, the command would be: OUT 888,255

The binary result of the above command on port 888 is to switch all its eight bits TTL logic high, i.e. 5V, so the logic level of the port would be 11111111. The equivalent command in C has the format outportb [(port address)], [byte (decimal 0-255)]. To read the logic levels of port 888, and print the result, the Qbasic command is: x = INP(888)

This would return a decimal value for variable x between 0 and 255, representing the binary status of each bit, e.g. an x value of 34 means that the bit pattern is 0010010. Similarly, the equivalent command in C has the format x=inportb[(port address)].

Bitwise ANDing

In order to read or write the logic levels of a particular bit, the port byte is read as above, and by ANDing the result with an appropriate number, the undesired bits can be masked out.

Example: To read logic level of bit 2

 

Binary;

Decimal


Byte Read		 11011010 218


AND		 00000010 2


Result		 00000010 2

Is result=2? Yes, so bit 2 logic is high (1)

 

Binary;

Decimal


Byte Read		 11011000 216


AND		 00000010 2


Result		 00000000 0

Is result=2? No, so bit 2 logic is low(0)

File Formats

There are many file formats available; HPGL, DXF, Gerber, Exellon, NC to name a few. The most important consideration was which file type would the customer be most able and likely to generate, and which would be particularly suitable for a drilling application. HPGL was chosen because it is the native language of the pen plotter, and its commands are analogous to a CNC drill. Any package that has a HPGL plotter driver can be used to generate a control file by choosing a 'plot to file' option usually found under the 'print' command.

Some of the basic HPGL commands are:

PU;-pen up


PD;-pen down


PA;-pen absolute

An example extract of a HPGL file is as follows:-

&#27;.(;&#27;.I81;;17:&#27;.N;19:IN;SC;PU;PU;SP7;LT;VS36;PU;PA199,199;PD;PA199,199;PU;PA279,279;PD;PA279,279;PU;PA398,398;PD;PA398,398;PU;PA0,0;SP;EC;PG1;EC1;OE;

Each command is separated by a semicolon, and the positional commands like "PA199,199" are in the format PA [x distance],[y distance]. The units are 'HPGL units', which have a real-world scale of 0.025mm, or » 1 thousandth of an inch. This is user definable in the control software, so the output can be scaled if required.

 

Hardware

The following pulley/timing belt drive layout is the basis for the mechanical calculations:

Figure 4.8 - Basic single axis drive layout

Stepper motors have been chosen over servo motors because of the following reasons:

Types of Stepper Motors

There are a few different types of stepper, namely:- Unipolar, bipolar, bifilar, permanent magnet, etc.. but the easiest interface and cheapest to use is the bipolar. See diagram for basic coil schematic:

Figure 4.9 - Schematic of stepper motor coils

Stepper motors can be driven either in half step or full step mode.

Full Step Mode

Full step mode energises each coil in turn and steps the rated step resolution of the motor, i.e. 200 steps per rev in this case. The sequence for full step drive is as follows:

 

a

b

c

d


1

1		 0 0 0


2		 0

1		 0 0


3		 0 0

1		 0


4		 0 0 0

1

To change the direction of rotation, the sequence is reversed from whichever row sequence was last output.

Half Step Output

This stepping mode can position the armature halfway between coil positions, giving twice the rotary positioning resolution. The sequence for half step mode is as follows:

 

a

b

c

d


1

1		 0 0 0


2

1

1		 0 0


3		 0

1		 0 0


4		 0

1

1		 0


5		 0 0

1		 0


6		 0 0

1

1


7		 0 0 0

1


8

1		 0 0

1

Again, to change the direction of rotation, the sequence is reversed from whatever row sequence was last output.

When changing rotor direction, the software needs to know the previous output sequence so it can start the reverse sequence from the correct row. The motor will jump steps if the sequences are not in the right order, thus step count positional accuracy will be lost.

 

Timing Belt

The linear position of the PCB is determined by a toothed timing belt. Using a toothed synchronous drive belt has the following advantages:

Source: 'Mechanical Engineers Pocket Book', Timings and May 1990

Given the scale of the project, a belt with a 9mm wide with a 3mm tooth pitch (SYNCHROBELTâ HTD stock sizes)is more than adequate in terms of load carrying ability. The belt is wider than necessary to allow a better connection to the PCB vice mechanism, this also increases the robustness of the design.

Pulleys

Pulleys come with a variety of tooth pitches and with single or double walls as an option. The following chart gives the amount of linear movement on the belt for a 1/400th of a revolution in the stepper, calculated using the following formula:

Total Linear Movement on belt=2p r/No of steps per Rev.

Figure 4.10 - Linear Belt movement per step

Given the required resolution of 0.1mm, the pitch diameter for the drive pulley should be approximately 12.7mm. This equates to giving a 1 pulse linear movement of 0.1mm.

To achieve the required linear speed of 25mm/sec, the required number of pulses per second is calculated by using the formulae:

Pulses per Second (PPS)=Required Speed/Distance moved by 1 pulse

in this case, PPS=25/0.1=250 pulses per second. It is likely that the motor will skip steps if this PPS is applied instantaneously, so it will need to be gradually accelerated from a standing start.

Note: See chapter 7, Design and Development for more detailed information on acceleration, velocities, torque and pulses per second.

 

Drill Mechanism

The diagram below shows a the DC motor controlled rotary to linear mechanism that will be used to depress and return the drill. The total depression of the drill is required to be at least 5mm, so allowing for a 10mm diameter disc, this will give 10mm depression.

Figure 4.11 - Drill up-down mechanism concept

Drill Feed Rate

The feed rate of the drill will be directly proportional to the speed of the DC motor driven disc, and will follow a sinusoidal model, because of the mechanism. The feed rate can be calculated using the following formula.

Drill Feed Rate Average =(RPS of disk/2)*diameter of disk

e.g. for a disk RPS of 0.5 and a disk diameter of 10mm, the average feed rate would be:

(0.5/2)*10=2.5mm sec

However, because of the mechanism, the maximum feed rate will occur at the 3 o'clock position, so

Drill Feed Rate Max =(RPS/2)*p r

given r=5mm and RPS=0.5

Drill Feed Rate Max = (0.5/2)*p 5=3.93mm/s (2d.p.)

Figure 4.12 - Feed rates for 3, 4, and 5mm disc sizes

Note: See chapter 7, Design and Development for more detailed information.

1.0;Product Description and Operation

1.1;The product is a desktop CNC drill, intended for printed circuit boards and drilling of other thin sheet materials. Drilling co-ordinate information is generated in a CAD package, and exported via HPGL (Hewlett Packard Graphics Language). The HPGL file is loaded into a parser application that controls the drill operation, and sends the appropriate data to the parallel port on a standard IBM-PC. The data is fed to an interface/driver PCB, and on to the x-y motors and the drill in the diagram below.

Figure 5.1- Top level operation of CNC PCB drill

The blank PCB material is held by a vice mechanism, and the drilling starts upon instruction from the user via the PC software.

2.0;Market

2.1;The target markets for the product are those of small scale PCB prototyping, sheet material prototyping, home workshop, hobbyist, small scale production of any drilled sheet material. Manual drilling of PCBs is an especially time consuming process. This makes automation of drilling very productive, and suited to computer control.

2.2;See the chapter 3, Market Feasibility for more information.


3.0;Performance

3.1;General

Max. material Size 150mm x 150mm
Max. material thickness 3mm
Edge border required 5mm on x axis edges only

3.2;Stepper Motor Drive System

No of motors 2
Motor type 4 phase Permanent Magnet Stepper
Steps per revolution 200
Phase Excitation V 5V spec. (12V driven with I limiting)
Current per phase 1A Max.
Resolution and Repeatability Both 0.1mm
Running Torque 320Nmm
Static Torque 460Nmm
Load inertia 135gcm2
Excitation 2 phase, software controlled half step
Speed X direction 25mm/sec max after 200ms
Speed Y direction 25mm/sec max after 200ms
Running temperature 80° C max. above ambient
Winding DC resistance 5W per winding
Pulse per second 220 steady state (half wave operation)

3.3;Drill Motor

No load speed 6000 rpm
Torque at stall 200Nmm
Supply Voltage 12V
Drill bit size 0.5 - 3mm

 

3.4;Drilling Performance

Max No of Holes per minute, in 2mm x 2mm rectangular array 30
Time for 4 x 1mm holes in corners of 10mm square 5 sec

3.5;Power Drive Electronics

Control Connector 25 way sub D type
Step Mode Half step mode, software controlled
Motors and drill 12V DC 3A max consumption
TTL components 5V DC 0.5A regulated and smoothed

 

3.6;Drive Software

Output port Parallel Port, defaults to &378 address (user definable by software)
Input File Format HPGL
Input File Length Limited by RAM
Channel Data for Motors 4 bits O/P per motor, 1 byte total
Channel Data for Drill 1 bit O/P for spindle, 1 bit I/P for drill depression and return switch
Channel Data for Limit Switches 4 bits for limit switches on x and y

Figure 5.2 - The core software interface model

 


3.7 Minimum Computer Requirements (IBM-PC compatible)

CPU 386 - 16Mhz
RAM 1MB min. Free
Hard Disk 1MB min. Free
I/O 25 pin D type Parallel port
Monitor VGA Mono or better
Operating System MD-DOS v5.0 or later

3.8;Misc. Features

3.8.1;The product shall have a removable drill head and a plate with holes suitable for mounting customer designed heads.

3.8.2;The product shall have a swarf tray, to allow easy removal of debris. see 5.1 and 5.2

3.8.3;The product shall have a menu driven user interface front end to software


4.0;Environment

      Operating Temperature Range

      0C to 60C Ambient

      Operating Humidity Range

      10%to 90% RH

4.2;The machines operation and performance should not be adversely affected by dust and dirt.

4.3;The machines operation and performance should not be adversely affected by being dropped from a height of 300mm onto a carpet tiled concrete surface. Test by being released by hand from the mentioned height.


5.0;Maintenance

    1. The product should require minimum maintenance except for:

5.2;The swarf tray shall capture all particles and not allow contamination of the drive mechanism or the material vice.


6.0;Target Product Cost

6.1;The product's suggested retail price is £225 ex VAT via mail order.

6.2;Total direct cost of product is to be less than £100


7.0;Competition

7.1;A company called Dolphin Systems sell a CAD/CAM/DNC mill, lathe, engraver for £595. This is aimed at model engineers, but could be used for PCB drilling.

7.2 ;CNC products with similar market positioning are EMCO lathes and milling machines, which retail at £2650 and £3500 respectively.

7.3;T-Tech produce a 'Quick Circuit System' costing £8200 ex VAT. It can mill tracks and drill holes. (no need for masking/etchant and so on) It also has an accessory for producing through-hole inserts on double sided boards.

7.4;See drill product benchmark in the Market Research chapter, page 12.


8.0 ;Quantity

8.1;The estimated volume of sales/production is 1000 per year, selling by mail order via specialist press within the UK and EU.


9.0 ;Size

9.1;The product's footprint shall fit inside a 400x275mm area.


10.0;Weight

10.1;The product's weight is to be 5kg max


11.0;Aesthetics, Appearance and Finish

11.1;The product shall have a painted finish to any sheet metal parts visible to the customer.

11.2;Harnessing/wiring shall not be visible wherever possible and shall be securely fastened to avoid catching on any moving parts.

11.3;Wiring shall be of a sufficient power rating and colour coded to show function.

11.4;Sheet metal parts shall not have any sharp edges or corners. All burrs to be removed on parts that customer can access.


12.0;Materials

12.1;Materials used in the product shall not become stained discoloured or react with human sweat, or petroleum based oils

12.2;The chassis shall be made primarily of painted sheet steel parts, given the relatively low volume of units.

12.3;The shall be no custom injection moulded parts due to the high cost of tooling and low volumes estimated for this product.


13.0;Product Life Span

13.1;The product should last for 4 years given 1 hour continuous operation per day, i.e. 1460 hours operation, before servicing.

13.1.1;The product shall be capable of 6 hours continuous operation allowing for drill bit and raw material replacement.

13.2;Standard replaceable parts will include motors, bearings, driver/interfacing PCB, drill motor and chuck assembly.

13.3;Consumable parts are drill bits and drilling substrate, e.g. blank PCB


14.0;Standards and Specifications

14.1;The product shall conform to the CE standard

14.2;The product shall confirm to EU standards on electromagnetic emission, i.e. Part 15 of FCC rules.


15.0;Ergonomics and Human Interface

15.1;The only human interaction with the machine shall be the insertion and removal of PCBs, the removal and replacement of drill bits, the removal of the swarf tray and the optional removal of the drilling head assembly.

15.2;There will also be a hardware interrupt push button that cuts power to the motors and halts operation in an emergency.


16.0;Customer

16.1;The following are likely users of this type of drill:

16.2;See Market Research chapter 3, page 11, for more information on the customer requirements.


17.0;Quality and Reliability

17.1;The drill shall be capable of 6 hours continuous operation at 23 degree C and 50% relative humidity, with no loss of accuracy or repeatability, while drilling holes in a 2mm rectangular array


18.0;Testing

18.1;The product shall be tested to ensure that the performance specification is adhered to. Testing schedule to be defined at a future date.


19.0;Safety

19.1;The product shall have the case, chassis, or exposed metal parts grounded.

    1. The PSU shall have a 1A slow blow fuse
    2. There shall be safety guards where necessary
    3. No sharp corners or edges on sheet metal parts

 


20.0;Installation

20.1 ;The product shall be packaged/delivered/sold in a labelled cardboard box, adequately packaged with the following items

20.2;The product shall come with software on 3.5" HD floppy disk, and be installed automatically by running a file called setup.bat from DOS

    1. The software shall have a self test routine, with software diagnosis of any problems, reporting to the user any corrective actions required.

Conceptual Design of Subsystems

The drill encompasses many areas of engineering design, summarised by the following chart:

The following are the conceptual design areas that have been considered in this chapter.

X-Y Drive System Concepts

An important consideration to take into account in the early design stage is what exactly will be moving, i.e. the drill or the PCB. In order to keep the stepper motor sizes down to a minimum, the payload should be kept to a minimum, e.g. keeping the drill stationary and using a flexible drive shaft, air motor, etc.

The choice of drive layout also effects the co-ordinate system used. A Cartesian XY system works out better in this case because of positioning errors get greater further away from the fulcrum using cylindrical or polar co-ordinates. For example, if the polar positional resolution is 0.5° this results in a linear resolution of 0.785mm at a distance 100mm from the fulcrum (Tan0.5*100=0.785). See figure 6.2 below.

Figure 6.2 Linear distance moved by 0.5° step

This does not take into account backlash and slop, so it is not really a feasible option even though it would get rid of more expensive linear bearings, as all joints could be rotary.

Using Cartesian co-ordinate methods, the layout of the X-Y drive system can be one of 3 possibilities, as shown below.

There are several drive possibilities that can be used to gain the required accuracy, namely

The best in terms of accuracy, resolution and load carrying ability is definitely the leadscrew, but its is by far the most expensive of all the options. Although expensive, an anti-backlash nut could be used to greatly enhance repeatability. A possible arrangement for single axis motion is shown below.

Figure 6.6 - Leadscrew and nut coupled direct to motor

Timing belt and gears

This arrangement gives good positional accuracy and repeatability together with low cost with 'off the shelf' components. The belt used would be 'non-stretch' polyurethane, with glass tensioning fibres. The pulleys teeth pitch match the belt, resulting in minimal backlash.

Figure 6.7 - Timing belt and gears

As it is the belt that moves, it needs to be attached to the mounting plate, or what ever needs positioning. This is achieved with a stock belt clip as shown below.

Figure 6.8 Belt clip

Wire cable and pulleys

This option is the cheapest of the three discussed, but is also the least rugged, and does not give very good accuracy or repeatability without additional components. This type of arrangement is seen in many plotters, printers and scanners where the mass to be moved is low.

Figure 6.9 - Wire rope and pulleys

Figure 6.10 - Wire rope and pulleys, top down view

Bearing Possibilities

The carriage needs to run smoothly with minimal play to meet the performance specification. One possibility is a circular shaft with self lubricating bearings e.g. Oiliteâ or plastic. This type of bearing on a circular shaft would give good performance but bearing life is a consideration, and needs to be addressed in testing. Plastic bearings using nylon 66 or Nylatronâ GS would be cheaper, but have less life expectancy.

Figure 6.11 Linear bearing possibilities

Dovetail/'Vee' with linear ball bearing

An excellent and accurate method, but heavy and expensive. This method is standard in many lathes and mills.

Precision Linear Slide

This is a very good solution in terms of accuracy, but it is expensive. HPC Drives Ltd. sell it for £30 per 200mm, excluding the sliding carriage.

Figure 6.12 - Precision linear slide

PCB Clamp Mechanism

In order for the user to place and remove a PCB quickly, a quick release method is desirable. The PCB fixing mechanism needs to hold the PCB rigid and give support against the force of drilling. Depending on the deflection of the PCB under the force of the drill, it may be necessary to employ some form of disposable backing, which would also improve hole quality. This will be addressed in the technical feasibility and testing.

Sliding V-edge vice

The diagram below shows a concept using a sliding quick release lever mechanism.

Figure 6.13 - Sliding V-edge vice

Spring loaded clips

This method could be used as a quick and cheap method for retaining the PCB. It can be duplicated for each corner of the PCB.

Figure 6.14 - Spring loaded clips

Drill Up/Down Mechanism

If the max thickness to be drilled is 3mm, and allowing 1mm tip clearance and 1mm through overlap, then the total distance to be moved by the up/down mechanism is at least 5mm. See figure 6.15 below.

Figure 6.15 - Drilling clearance

There are several ways to achieve the type of up/down motion required by a drill. Some of the mechanism concepts considered are shown on the next page.

 

Mechanism Concepts

 Figure 6.16 - Lead screw, with reversible DC motor

 Figure 6.17 Solenoid with hinged or linear bearing

Concept Choice and Justification

The final concept chosen was a split axis timing belt driven system. The reason was the balance between accuracy and cost. The wire and pulley option could be feasible but it would take testing to confirm that option, and within the scope of the project this time was not available. Timing belts are used frequently for medium loads, and where good accuracy is required, so their ability to perform is well documented.

Rather than performing a traditional matrix analysis to choose a final concept for each mechanical, software, and electronic design area, the performance criteria have been used as an ongoing 'sanity check' for each design option. This has reduced time spend on developing concepts that have little chance of performing to the required level, both in terms of performance and cost. Due to the nature of the project, with its many design areas, more time could be spent formulating a common sense decision. In any case, there can be a high level of confidence that the concept choices will perform to specification.

Appearance

Considerations:

System Integration

Considerations:

Software User Interface and Human Factors

There are many programming language possibilities including:

Some of the considerations to take into account when choosing a language include:

Prototyping has been undertaken in Qbasic and final application development will be in C. The reason for this is because of Basic's easy to use code, and ability to get results quickly, although perhaps not efficiently. Basic allows for rapid development modifications and changes while not having to worry about code syntax as much as C. The developer's existing knowledge of Quickbasic will allow rapid development to get the hardware up and running, while not being frustrated by lack of programming knowledge. The intention is to get the program fully functional and running in Basic before conversion to C.

 

Interface Style

Required: Pull down menu, text/graphic, hybrid mouse & keyboard driven

Considerations:

Initial interface development in Quickbasic:

Figure 6.19 - Menu driven interface

Figure 6.20 - Initial Menu Command Structure

Introduction

This chapter examines the technical aspects of the detailed design of the 'Desktop CNC Drill'. The areas covered are DFM, DFA, detained design, mechanical calculations, electronics calculation, and software design.

Design for Manufacture and Design for Assembly

During the design process much consideration was given to Design for Manufacture and Design for Assembly. As the product will have relatively low production numbers (1000 per annum), the parts did not need to be designed with automated assembly in mind. The majority of the parts are made from 'Zintecä ', which is a trade name for a zinc coated sheet steel. Most of the parts will be made by CNC laser cutting the blanks from sheet and the bending operations performed with a press using jigs where applicable.

The bearing rods are made from silver steel which has improved finish, wear and corrosion resistance to mild steel while not being as expensive as stainless steel.

The pulley and link pins are made from turned mild steel stock and it is intended that they are produced on a CNC lathe.

DFA Considerations

DFM Considerations

Manufacturing Quantity

If assembly time for one unit is 1 hour, and assuming a 7 hour day and a 5 day week, then the number of units produced per week should be 35.

If there are 48 production weeks in the year, then there is a theoretical yearly capacity of 1680.

To make the target 1000 units per year as required by the specification would require about 29 weeks production.

 

Design Description and Justification

Each drive assembly is identical apart from the length of the bearing rods and the length of the drive belt. This was done to improve assembly and cost effectiveness. The axes are separate to reduce payload on the motors as described in chapter 6, Conceptual Design.

Figure 7.1- General arrangement (covers removed)

Figure 7.2 - General arrangement alternate view (covers removed)

Figure 7.3 - Free running pulley pin and bracket

Figure 7.4 - Bearing rod bracket

 

PCB Vice Mechanism

The PCB vice is capable of holding flat sheet material up to 150x150x3mm. It can be adjusted to clamp PCBs by releasing the wingnut on the clamp stop at the end of each slide rod and moving the jaw until it is firmly up against the edge of the PCB. The butterfly nuts are then tightened.

Figure 7.5 - PCB vice mechanism

Figure 7.6 - X axis layout, with PCB vice (covers removed)

 

 

Mechanical Calculations and Development

Deflection of Guide Rails

The aim of this calculation was to chose a diameter of guide rail that would not deflect too much under the forces of drilling, and support the PCB adequately.

Given the maximum span of 315mm the deflection can be calculated using the formula shown below The best choice diameter for the rails can then be chosen.

The above graph is based on a normal load of 9.81N or the equivalent of 1kg. mild steel was assumed as the material, Modulus=210GN/m2 . The following formulae are used:

;;;;

An appropriate rail diameter of 8mm was chosen from the chart,(relevant row shown)



Rail Diameter (D)

Normal Load at Center (W)

Distance to Supports (l)

Second Moment of Area

Max Bending Moment (Nm)

Max Deflection (mm)

0.0080

9.81

0.315

2.01062E-10

0.772538

0.15

Summary: A guide rail diameter of 8mm was chosen

See Appendix D, Spreadsheets and calculations for more detail.

Deflection of Printed Circuit Board Under Drilling Forces

A check was made to ensure that the PCB has enough support to withstand drilling without too much deflection, which could decrease accuracy and lead to drill bit breakage.

The following formulae were used:

;;;

The modulus of glass filled epoxy resin is 20kN/mm2 The normal load was assumed to be 9.81N


Summary: It is recommended that when using 150mm long PCB, the minimum width should be 50mm which gives 1.035mm deflection.

The maximum recommended width is 150mm.

See Appendix D, Spreadsheets and calculations for more detail.

 

 

Linear Movement of Belt for One Motor Step

The aim of this calculation was the need to determine the drive pulley radius so that it would give a 0.1mm linear movement of the belt, which is the required linear resolution. The motor is capable of 400 steps per rev in half step drive mode.

Fig 7.7 - Drive arrangement

Formula used:

Summary: The required drive pulley radius is 6.36mm

See Appendix D, Spreadsheets and calculations for more detail.

 

Drill Feed Rate

The drill feed mechanism is based on the design below.

;;

Figure 7.8 - Drill feed mechanism: concept to detailed design

Owing to the type of mechanism, the feed rate will follow a sinusoidal model. The maximum feed rate will be when the top link pin is in the horizontal position.

It is important that the feed rate is below 1.8mm/sec (0.36 mm/rev at 300 RPM) to avoid stalling of the drill when drilling 3mm holes.

Summary: The drill mechanism feed motor should have a speed of 13 RPM with a 5mm disc to give a feed rate of 0.13mm/rev at 300RPM drill bit speed.

See Appendix D, Spreadsheets and calculations for more detail.

 

Drill Torque and Power Requirements

These calculations determine the torque required by the drill motor, given a drilling speed of 300RPM, and a max bit diameter of 3mm. The formula used was:

Where:

kd= drilling factor, approximated 0.06 for PCBs (adapted from cutting speeds for thermoplastics, (Tables, Data and Formulae for Engineers and Mathematicians, Greer and Hancox, 1993, p70)

F= feed rate, mm/rev;;D= drill diameter mm

Summary: Recommended feed rate for thermosetting plastic (3mm diameter hole) is 0.13mm/rev, requiring 94Nmm torque for this situation. Recommend drill torque specification is 200Nmm before stall to give FOS of about 2.

Calculation of Series Resistor

In order to improve the current rise times in the coil and improve the torque/speed characteristic a resistor is place in series with the motor coil. The resistor value is calculated using Ohm's law, V=IR

;;;

Figure 7.9 - Effect of higher supply voltages series resistor and drive schematic

The motor ratings are 5V at 1A and 5W coil resistance.

The supply voltage is 12V, so allowing 1.2V drop across TIP122 (12-1.2=10.8V)

10.8V-5V=5.8V;( must be drop across the series resistor.)

If current is 1A, then resistance is

5.8/1=5.8W

The power of the resistor required can be calculated using the formula P=VI

P=5.8*1=5.8Watts

See Appendix D, Spreadsheets and calculations for more detail.

In summary, the series resistance needs to be 5.8W and have minimum 5.8W power dissipation. The closest commercially available values are 5R6 (10%), 6R2(5%) and 6R8(10%). Choose 6R2.

 

Calculation of Current Rise Time in Coil

The current rise in the motor coil takes the form of exponential growth, and is calculated using the following formula:

The calculation was based on the following data:

Final current:;;;1A

Supply Voltage:;;12V

Coil Inductance:;;9mH

Combined Resistance:;10.8W


From calculation, over 95% (0.95A) of the current has build up in the coil after 2.5ms

See Appendix D, Spreadsheets and calculations for more detail.


 

Software Development

The primary purpose of the software is to interpret the HPGL file the user specifies, and to output it to the parallel port in a form which the interfacing electronics can understand. The screenshot below illustrates the layout of the front end. The interface is menu based and lists each command under the relavent menu heading.

Figire 7.10 - Front end of control software

The program is a DOS program which will run under DOSv5, Windows 3.11, Windows 95 and Windows NT. The advantage of it being a DOS program is that it can be run in a variety of operating systems. If booted up under pure DOS (rather than a DOS window) the program will not suffer from timeslice interrupt in Windows 3.11, which can slow down the program operation, especially when other programs are running concurrently.

;;;

Fig 7.11 Edit menu commands;;Fig 1.12 Test menu commands

Figure 7.13 - Status report window

Figure 7.14 - Flow of program operation

 

 

List of Software Features

See Appendix E for sample code listing.

 

Prototyping and Testing

During the course of development, the two main areas of prototyping and test were the software and the interface electronics. A prototype circuit board was constructed and successfully interfaced to the software and demonstrated the control of a single stepper motor. The x values in a HPGL file were read from file, interpreted in terms of steps required and the direction with the correct pattern and sequence of pulses successfully output to the parallel port. This test demonstrated how PC control of stepper motors and the conversion of HPGL to real-word output was eminently possible.

Other Software Tests to Ensure Reliability

Electronic and Mechanical Tests



Part No

Name

Material

Bought?

Qty

Unit cost

Total

M--001

10mm toothed belt x Polyurethane

ü

1

2.20

2.20

M--002

10mm toothed belt y Polyurethane

ü

1

2.00

2.00

M--003

12mmpulley ABS

ü

4

0.50

2.00

M--004

8mm bearing Nylon 66

12

0.30

3.60

M--005

Angle column 1 Zintec

1

0.40

0.40

M--006

Angle column 2 Zintec

1

0.40

0.40

M--007

Bearing rod x Silver steel

2

0.30

0.60

M--008

Bearing rod y Silver steel

2

0.30

0.60

M--009

Belt clamping strip Zintec

2

0.40

0.80

M--010

Clamp rod stopper Aluminium

2

0.60

1.20

M--011

Drill motor + chuck .

ü

1

3.00

3.00

M--012

Drill fixing bracket Zintec

1

0.70

0.70

M--013

Drill link pin MS

1

0.70

0.70

M--014

Drill link pin small MS

1

0.70

0.70

M--015

Drill mechanism flange MS

1

2.50

2.50

M--016

Drill mechanism link Zintec

1

0.70

0.70

M--017

Drill mechanism motor .

ü

1

2.50

2.50

M--018

Drill mount plate Zintec

1

1.20

1.20

M--019

Drill slider plate Zintec

1

0.60

0.60

M--020

Drill slider rod Silver steel

2

0.30

0.60

M--021

Pulley bracket free run Zintec

2

0.50

1.00

M--022

Microswitch .

ü

4

0.70

2.80

M--023

Stepper motor cover Zintec

2

1.20

2.40

M--024

PCB clamp profile left hand Zintec

1

2.00

2.00

M--025

PCB clamp profile right hand Zintec

1

2.00

2.00

M--026

PCB clamp rod MS

2

0.30

0.60

M--027

Pulley cover Zintec

2

1.80

3.60

M--028

Pulley pin -free running MS

2

0.70

1.40

M--029

Rod bracket Zintec

4

0.90

3.60

M--030

Stepper bracket Zintec

2

1.10

2.20

M--031

Stepper motor .

ü

2

10.00

20.00

M--032

X base support Zintec

1

1.40

1.40

M--033

Y base support Zintec

1

1.30

1.30

Sub totals

65 71.30


Misc. Items

M--034

PCB + electronics .

1

7.00

7.00

M--035

25 pin cable .

1

0.70

0.70

M--036

Software disc + manual .

1

1.00

1.00

M--037

Fixings + cable harnessing .

1

2.00

2.00

M--038

Finishing and packing

1

5.00

5.00

Sub total 15.70

Total Material Cost

Total Labour Cost

Total Direct Cost

87.00
20.00
107.00

The product cost is based on estimates from various trade catalogues, and quotes from sheet metal fabricators on parts with similar characteristics. The unit part cost is based on a part batch of 1000.

The cost of each part is assumes the manufacturer has all the machines necessary to make the part, and includes all costs up to the point where the parts are in front of the operators in bins. The labour cost is based on two assembly operators being paid £10 per hour, assembling the same machine, and assuming that the product takes one hour to fully assemble and test.

 



Assy No.

Main Assemblies
MA-01 CNC drives main
MA-02 Y axis assembly
MA-03 X axis assembly
MA-04 Drill mechanism assembly

 

Overall the product satisfies its brief and the PDS, but there is plenty of room for improvement and optimisation given more time. It was found that some solutions were not optimal, but due to the amount of work already done it was decided not to change because of time restraints.

Because of the number of parts and the possibility for assembly and tolerances to affect the reliable functioning of the product a working prototype should ideally be built. This would allow problem areas to be examined more realistically and would show other areas in the design that could be improved. One such area that is difficult to predict using computer aided design in the overall robustness and stiffness of the assembly. The stiffness can be theoretically worked out, but it is very difficult to predict every nuance of the final hand assembled product. Another area that is difficult to predict with out building a prototype is that of resonance and vibration, which could potentially be a problem area.

 

Items that could be improved in retrospect

 

Further work

 

 

Conclusions

Books

'Practical Electronics Calculations and Formulae', F.A. Wilson, Babani 1995

'Further Practical Electronics Calculations and Formulae', F.A. Wilson, Babani 1995

'Practical Electronics Handbook', Ian Sinclair, Newnes 1995

'Mechanical Engineers Pocket Book', Timings and May 1990

'Tables, Data and Formulae for Engineers and Mathematicians', Greer and Hancox, 1993

Catalogues

HPC Drives Catalogue 1997 (D7)

HPC Gears Catalogue 1997 (C15)

Quick Circuit catalogue 1997

Electromail CD-ROM catalogue 1997-98

'Maplin Electronics Catalogue', March-August 1997, Maplin MPS

 

Web Sites

'Jones on Stepper Motors', "Jones on Current Limiting", Douglas W Jones

Electronics on the Web Magazine

Technology Resource Centre (Technology Education in Northern Ireland)

'Motorise Your Telescope' - Mel Bartels Page

'Secrets of simple CNC'

Arrick Robotics Homepage

'HPGL tutorial', Paul Bourke

'Parallel Port FAQ/Tutorial', Zhahi Stewart,

'Interfacing to the IBM-PC Parallel Printer Port', Ian Harries 1997

Magazines

Richard Bartlett, 'Introduction to CNC for practical Engineers'. Model Engineers Workshop, issues Mar-Sept 1997,

Ray Stuart, 'A computer controlled X-Y- table', Model Engineers Workshop - Jan/Feb 1995

Ray Stuart, 'Stepper Motors', Model Engineers Workshop - Nov/Dec 1994

 

Miscellaneous

Stepper Motor Driver IC SAA 1027, J5566, RS Data Library

L6219 Stepper Motor Driver data sheet, SGS Thompson Microelectronics

Appendices Contents:

 APPENDICES - not included in html - -contact the author for details

A.;Circuit Diagrams

 

B. Assembly Hierarchy

 

C. ;Part and Assembly Drawings

 

D. Spreadsheets and Calculations

 


E. Sample of control program code ;;;'CNC Drill Interface Program', v1.0

 

Assembly Hierarchy

The following is an I-DEAS generated indented hierarchy of the whole assembly showing sub-assemblies and the instances of each part.

"CNC drill drives main:;1,assemblies"

IN "Hi76-xbase support_8"

Assembly: CNC drill drives main-assemblies

:....Assembly: y axis main

: :....Assembly: y drive assy

: : :....rod bracket

: : :....bearing rod y

: : :....rod bracket

: : :....bearing rod y

: :....Assembly: y stepper main

: : :....12mmpulley

: : :....10mm toothed belt y

: : :....stepper motor1

: : :....pulley pin-free

: : :....12mmpulley

: : :....free run pulley bracket

: : :....stepper bracket new

: :....Assembly: drill slider mech

: : :....Assembly: drill mechanism

: : : :....drill mount plate

: : : :....drill

: : : :....drill fixing bracket

: : : :....8mm bearing (2.5mm thick)

: : : :....8mm bearing (2.5mm thick)

: : : :....8mm bearing (2.5mm thick)

: : : :....8mm bearing (2.5mm thick)

: : : :....drill link pin

: : : :....drill mech link

: : :....drill slider plate1

: : :....drill slider rod

: : :....drill slider rod

: : :....drill mech motor

: : :....8mm bearing (2.5mm thick)

: : :....8mm bearing (2.5mm thick)

: : :....8mm bearing (2.5mm thick)

: : :....8mm bearing (2.5mm thick)

: : :....drill mech flange

: : :....drill link pin small

: :....belt clamping strip

: :....microswitch 1

: :....microswitch 1

: :....yaxis support

: :....motor cover

: :....pulley cover

:....Assembly: x axis main

:....Assembly: x drive assy

: :....bearing rod

: :....rod bracket

: :....bearing rod

: :....rod bracket

:....Assembly: pcb clamp assy

: :....pcb clamp profile rh

: :....pcb clamp rod

: :....pcb clamp rod

: :....pcb clamp profile lh

: :....belt clamping strip

: :....clamp rod stopper

: :....clamp rod stopper

: :....8mm bearing (2.5mm thick)

: :....8mm bearing (2.5mm thick)

: :....8mm bearing (2.5mm thick)

: :....8mm bearing (2.5mm thick)

:....Assembly: x stepper main

: :....Assembly: x stepper assy

: : :....12mmpulley

: : :....stepper motor1

: : :....10mm toothed belt x

: : :....stepper bracket new

: :....Assembly: x free pulley

: :....12mmpulley

: :....free run pulley bracket

: :....pulley pin -free

:....PCB 150 square (Suppressed)

:....microswitch 1

:....microswitch 1

:....xbase support

:....pulley cover

:....motor cover

:....angle column 2

:....angle column 1