3 axis engraver / light mill from scrap project



Fired with enthusiasm after finding the basic ideas on Luberth Dijkman's excellent motion control site - and having been saving stepper motors etc. from scrap printers - with a couple of pc's literally exhumed from the skip (our IT guys are now better trained, they drop off their scrap in my office and any 'leavings' I return to the skip) I thought it might be fun to have a go.

Basic is the keyword - the program is writen in QBasic, it operates via the parallel port and outputs any HPGL plot file to the hardware, the structure of which is about as simple as you could get.

First thing was to get the motors running under PC control so the circuit board (all details on Luberth's site) with it's three ULN2803 (8 sets of darlington pairs, two of each being run in parallel to give about 500 mA output) was etched and assembled - power courtesey of an old PC PSU - (it's 486 is no more - it really was scrap !) and the box of stepper motors dug out, there were three 'almost pairs' of 5 / 6 wire bipolar motors and after a trip to the Minebea site (thanks to Richard W for finding that) for some information, one was hooked up, QBasic - which I have not used for years dusted off and a little program put together for outputting to the // data port.

Surprisingly this, after a bit of fiddling, resulted in forward and reverse rotation, not fast and not much power so the concept of driving a screw with the motor rather than belt driving the heads came about. Stainless M6 studding was used as a) it was cheap from work by the metre b) had a nice clean finish and c) with a 1 mm pitch per revolution giving a nice simple conversion factor to scale the steps output to give millimetres of travel. Further development had two motors running together under control so the x and y axes were catered for control wise.

After a lot of thought it was decided to make something similar to my old hand cranked shaper, a work table running on rails for the x axis, a bridge over this for the y axis and mounted on this like a gantry crane, a vertical z axis, initially something which would take an A4 sheet was envisaged but after finding a pair of ball bushings and 30" 3/4" ground rods from B W Electronics surplus page this grew to A3 (ish) - the full travel is now 450mm x 350 mm.

The ball bushings came as one bush each end of a short aluminium bracket, so, to get best mechanical advantage these brackets were put through the bandsaw and separated, mounted at either end of a bit of angle which was in turn bolted to the underside of a piece of 8mm plate which gave much more stability. Blocks to mount the rails were rigged up and the whole lot bolted onto a substantial chunk of chipboard flooring - left over from a mezzanine floor installed at work some years ago and too good to burn ! The 'Draper Dremel' seen clamped in place was for a test run before the Z axis drive was fitted.

The y axis was actually built first and had the carriage chugging up and down it for several days to prove the effectiveness (or otherwise) of using a home made PTFE nut for the 6mm studding to run in (thanks members of the modelengineers list for your input on this) - so far this has shown no objection to running dry, no sign of backlash and is a cheap and quick item to replace when it eventually wears out. As seen above the y rail is cramped to a couple of vertical spacers, when the working position for these is finalised they will have 45 degree butresses welded on for better stability - the y travel is guided by 4 (more components recovered from scrap) Hepco vee wheels running on a ground Vee bar bolted to an Aluminium transom bar - another transom bar will be fitted for enhanced beam strength. The table will have a sacrificial MDF surface fitted so the cutter can be run right through the work if 'light milling' rather than engraving.

Control of the Z axis proved to be another matter entirely, the four outputs to run this being the control bits from the // port - these are not so straightforward as three are inverted outputs and outputting from this port appears to have a knock on effect on outputting from the data port when operating under Windows, however after a lot of experimentation a third motor was eventually tamed. X and Y axis homing / limit switches were added to give a repeatable starting position and prevent overtravel and a program written to make the operation of the thing a little less fraught - finally after several months of tinkering, the three axes worked together and shoved a cutter about for the first time but only 140mm in any one direction. This prompted much head scratching, code checking and finally another visit to Luberth's site where an updated bit of code was found to fix a bug which seemed to explain this problem, the principles of this fix was incorporated into my program and hey presto we had full control in any of the three directions from limit switch to limit switch.

Obviously a test piece was now needed and as a near neighbour has long harboured an ambition to built a replica of Harrisons wooden clock requiring some 'interesting' numbers of teeth on it's gears, co-ordinate drilling a dividing / indexing plate seemed like a good test of repeatability, A drawing was made incorporating from 365 down to 52 equally spaced holes in 32 concentric circles and sent out to a plot file, this was uploaded on a floppy disk to the machine in the workshop and the machine set running one Friday evening about 18.30. With a sheet of polycarbonate taped to the bed, using a 1.6mm drill in a pin chuck secured in a woodworking router which was tie wrapped to the Z carriage, drilling commenced and continued - slowly, very slowly for hours, approaching midnight calculations indicated a likely finish time of 04.00 am so 'it' was left to it's own devices and the alarm clock set for 03.30.

At 03.40 Saturday morning, after more than 9 hours running and through very bleary eyes, the last hole, No.3286, was seen safely plonked on the pre-scribed cross hairs at the centre of the plate and here is the result (if you don't believe me count them for yourself), the three outer rings of 365, 360 and 300 holes are so closely spaced there was a degree of 'melt' due to heat from drilling, the inner rings of holes did not suffer this problem. Neighbour is naturally delighted with this new addition to his tooling, although he will now have to find another excuse not to start work on the clock !

Once the vagaries of running QBASIC programs have been fully overcome with a better user interface and the hardware finalised this should provide a useful little addition to the workshop for engraving plates and routing patterns and who knows maybe a bit of light milling - it has ended up somewhat larger than anticipated and somewhere to store / operate it provides the next problem on the list......

Hopefully this account may amuse some and perhaps prompt others to have a bash - as first foray into computer control for this steam'n'spanner man and it has been most enjoyable (and highly frustrating in parts) and shows just what can be made from scrap or re cycled materials !

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