Building Inexpensive CNC Machines

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ing Inexpensive CNC Machines http://www.fullnet.com/u/tomg/goo

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Curve Tracer Kit page

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Building Inexpensive CNCMachines

for Computer-Controlled Drilling and

Milling

(This page is under "ongoing construction"...)

(C) Copyright 2002 by Thomas P. Gootee

Introduction

When I started thinking about CNC machines, I just wanted acomputer-controlled machine that would be able to automatically drill allof the holes in the printed circuit boards that I made, for my CurveTracer kits (see the link, above). But, the "good" commercially-availablemachines were priced higher than the amount that I could justifyspending. So, I started thinking about what it would take to build one,


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myself. And, I didn't want to build one that would cost almost as muchas a commercial machine: Otherwise it might have been smarter to justBUY one, to begin with! So, I decided to add "low cost" as one of thedesign goals. I also expected to end up with one or more additionalproducts to sell, as a result of this project, to (help) justify the time that I

spent on it.

This page was started on 6/6/02. It should be updated, occasionally, as Imake progress on the machines I'm building.

Machine Number 1:

Starting a project like this one, while having so little knowledge of whatis needed to complete it, makes progress very slow, at first. I did a lot ofsearching and reading of the Usenet newsgroups, and many websites,through http://groups.google.com. (I do have an electrical engineeringdegree, but had never worked with or studied stepper motors, and knewalmost nothing at all about milling/drilling machines, nor CNCmachines.)

I learned about stepper motors, and their driver circuits, and the softwareused to run and control them. And I saw some of the ways that othershad designed and built their own CNC machines. I also began tounderstand some of the limitations imposed by different types ofmachine designs. I started to get my design goals better-defined, and alsosaw where the machine I wanted to build would fit into the continuum ofsizes and types of CNC drill/mill machines, which, by the way, hehe, is

very near "the bottom": All I needed was a small machine, able to drillboards of up to 6"x4" in size, with a worst-case hole-placement accuracyof about 0.01-inch or so, which would be about 1/3 to 1/4 of a hole'sdiameter (using 0.035" or 0.04" holes). And I didn't need to do a veryhigh volume of drilling. I figured I'd be happy if it could drill at least2,000 holes per day (although, hopefully, not taking all day to do that!).


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I started to feel like I could probably design and build a machine. But,having both my "low cost" design-constraint, and the fear of "messingup" on something I'd never done before, I wanted to START with adesign that would be VERY quick and easy to build, and EXTREMELYinexpensive, that could serve as a "testbed" for learning more about

stepper motors, their driver circuits, and the software used to control themachines driven by them. After that, I hoped I would bemuch-better-positioned to build something that was closer to being a"real" CNC mill/drill machine.

I had considered several possible designs, while doing the initialresearch:

I considered trying to use an old flatbed X-Y plotter, either to move asmall drill-head that was at the end of a Dremel-Tool-type flex-shaftdrive cable, or to carry the circuit board while the drill was mountedabove, on a separate third axis. Old plotters are quite inexpensive (under$50, on www.ebay.com). And I actually already owned some.

I also thought about just building the x, y, and z axes from scratch, using

threaded rods (leadscrews) connected to stepper motors to move them.And I am considering using a purchased "compound slide" milling tableas the x-y positioner portion of a machine, which would be modified byreplacing the leadscrews' manual cranks/handles with stepper motors.One possibly-big advantage of that approach, which would also begained if building the x-y table "from scratch", using leadscrews, is thatthe leadscrews provide a larger gear ratio, which in turn gives betterlinear resolution, per step. Even the small, cheap milling tables usuallymove each slide-table by 0.1-inch per full turn of the leadscrew crank,which would give a movement of 0.0005 inch for each full-step of a 200steps/rev motor, as opposed to something like 0.008 inch per step for thetypical dot-matrix printer's carriage assembly. I am now planning to useone of these commercially-available x-y milling tables when I build mysecond machine. I have purchased one, the cheapest ($99) model atwww.use-enco.com. But, I bought the same make and model, brand new,


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on eBay.com, for only $69 plus shipping (the seller happened to be fairlynear my area, though, so shipping was only about $15, for the 36-poundtable).

And, I also thought about using two old dot-matrix printers' mechanisms

to make the x-y positioning system. I didn't THINK that one printer'smechanism could be mounted directly onto another one'sprinthead-carriage assembly, because I assumed that the "bottom" printerwouldn't have enough power to still be able to move correctly, with theweight of the other one on it. So, originally, I considered having oneprinter carry the printed circuit board (PCB), with the other printersuspended above and perpendicular to it, to move the drill, giving thesecond axis of positioning. However, I still would have needed to find away to move the drill up and down (I was hoping I could figure out howto make the printer's paper-feed assembly, and/or motor, help performthat task.).

Recently, I happened to find a used-computer store where they haddozens or hundreds of old printers that they'd taken as parts ofcomputer-system trade-ins, for newer systems that they sold. So, I

bought some of the printers, for only $5 each! I figured (hoped) that theywould probably be worth more than that, just for their stepper motors(and power supplies, et al).

I found out that the IBM Proprinter models have pretty HEFTYprint-head-carriage stepper motors in them, at least compared to theEpson FX-850 and FX-1050 models. The IBMs' motors are 2.25"diameter, and about 2" long. (They are Sanyo Denki "Step-Syn" models,4.1V, 1.1A, 200 full-steps per revolution, unipolar (i.e. 6 wires).) TheIBM Proprinters also utilize a screw-type drive system for the printhead,as opposed to the cog-belt drive system that most of the other olderprinters use.

I also bought a couple of the biggest, heaviest, oldest "daisy-wheel" typeprinters that I could find, there. One of them, a CPT Corporation model


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A071, also had the big motors, with sizes just like those in the IBMProprinters: 2.25" diameter x 2" long (But it had TWO of them, insteadof just one: Minebea Co. Ltd. "Astrosyn" "Mini-Angle Stepper" models,6V, 0.85A, 200 full-steps per revolution, unipolar. (The IBMs each hadonly ONE of the bigger-size motors in them, with a second one that was

much smaller, and coarser-stepped, for the paper-feed roller's drive.)).The CPT A071 daisy-wheel printer also had very heavy, solidconstruction, with printhead-carriage slides that were a full 5" apart (atleast twice as far apart as those in any of the dot-matrix printers), and avery nice, heavy solid-metal printhead-frame/holder, between the slides,which even had roller bearings above and below the slide bar on the"little" side (the side that was farthest from the paper path) (See photos:).

Printhead Frame/Holder, CPT A071 Daisywheel Printer:

Roller Bearings on Slide, CPT A071 Daisywheel Printer:


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I stripped a couple of the Epson FX printers, and a couple of the IBMProprinters, one small (8.5" wide) one and one large (15") one of each

type, down to where I had only the carriage-slide assembly, with theprinthead-holder/frame, the stepper motor and drive assembly, and partof the frame that held it all together. I also did the same with the CPTdaisywheel printer (see photo:).

Printhead-Carriage Assembly of CPT A071 Daisywheel Printer:

After taking apart the printers and actually SEEING what their carriagemechanisms looked like, I began to think that the big CPT A071daisywheel printer "should" be able to have one of the small Epson or


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living selling surplus electronic test equipment, such as oscilloscopes,signal generators, spectrum analyzers, power supplies, and many otherinteresting kinds of equipment (Click the link at the top or bottom of thispage, to see all of the cool equipment that I have around here!).

I soon had the motors going, and played around with the printers'mechanisms. It was quite interesting to vary the frequency and the widthof the "step"-input pulses. At times, the motors seemed more likemusical instruments, with resonances only at certain frequencies. I wasalso able to note large differences in a motor's torque, speed,"smoothness", and sound/noise level, depending on the pulses' frequencyand widths.

I built a small "mount" assembly, on the CPT daisywheel printer'sprinthead assembly, to make a level base on which to try mounting theEpson FX-850 printer's entire carriage assembly, using small aluminumchannel and angle stock and 4-40-size brass machine screws and nutsthat I purchased at a local hardware store (see photos:).

Mounting Bases Constructed on Printhead Frame/Holder:

Mounting Bases Constructed on Printhead Frame/Holder:


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I put the smaller printer's assembly onto the mount, on the larger printer'sprinthead-holder/frame (see photo), and powered up the larger printer'sstepper motor. As I had hoped, the motor had NO TROUBLE at all,moving the WHOLE THING, reliably (see photo:).

Epson FX-850 Carriage Assembly, Mounted on CPT A071

Printhead Frame and Carriage:

Epson FX-850 Carriage Assembly, Mounted on CPT A071

Printhead Frame and Carriage:


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Epson FX-850 Carriage Assembly, Mounted on CPT A071Printhead Frame and Carriage:

So, right now (6/6/02), I am in the process of making a vertical axis, towork with the x-y axes I've already made. It will be similarly simple,easy, and inexpensive. I had a piece of countertop lying around, 36"x30",on which I will mount the completed x-y machine. Then, a third (8.5"dot-matrix) printer's carriage assembly will be mounted, on a type ofgantry, oriented vertically over the center of the x-y system. I was goingto make the stationary gantry out of a similar piece of countertop. But,


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for me, it seems quicker and easier to just use threaded metal pipe tomake an "L"-shaped support, over the x-y machine. (And, as can be seenin the photos, I still need to construct a platform on the smaller printer'sprinthead assembly, to hold the actual workpieces. It will be made ofsmall aluminum channel and bar stock, similar to that used on the larger

printer's printhead frame.)

IBM Proprinter (8.5-inch) Carriage Assembly, mounted on threaded

pipe support:

3-Axis Unipolar Stepper Driver Circuit, on proto-board:


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To be continued...

Questions or comments? Email Tom Gootee at [email protected].

New alternate email address: [email protected] (Use only if

you can't get through via [email protected].)

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Building Inexpensive CNC Machines