floppy CNC.

Hurro...

I've just created a folder called "Chris's Odd machines" in the files area, and in there are my two experiments I've built on the way to building a router.

The first, and most likely to incriminate me is the floppy CNC machine. I built it in about 5 hours flat from three 5.25 floppy disk drives.

I scored a bunch of them when stripping down some old computer gear, and found they had the cutest little screw and guide assembly. Out with the hacksaw, and I made the wee X-Y bed by taping the plastic "head" holder carriage and screwing what was left of another drive to it, at 90 degrees (give or take 5 degrees).

The Z axis was a bit more tricky, and I used more of the third drive for that..

The plotter pen and holder is off my sekonic A2 plotter. I build a pretty little sprung loaded mechanism from PVC sheet, as are the vertical sides.

The whole thing gives me a 21x21x20mm available work area, although all I ever did with it was plot silly pictures onto bits of "post it's" stuck to the bed. I was going to make a baby spindle and cut wax with it, but I nicked one of the stepper drives for the lathe.. read on.

This floppy CNC let me do a heap of playing with software though, I used this setup to test stepster, desknc, cncpro, kcam, and some other's I can't remember now. Taught me a bit about Gcodes as well. I think it was on this list I posted the results of that testing? The of the good things about a machine like this is that nothing it does will damage itself, the steppers are just too piss weak. :-).

The stepper drives are just PIC's driving four mosfets, no current limit, I just played with the supply voltage until I could still touch the steppers after 10 minutes of no movement! Very scientific.

The other photo's are of my CNC lathe. I think I mentioned somewhere on this list in another post that my lathe is a bit of a toy? Well, here it is, warts and all.

I retro-fitted it so that I could test my stepper drive circuit on the large stepper that's on the 'X' axis (the bigger of the two steppers). The circuit is just the standard L298/297, running off 32V or there abouts. (unregulated linear supply). The cross slide is running off one of the drivers from the floppy CNC machine, and uses a couple of big current limit resistors off the 32V rail. They are fan cooled (They need it!!!)

The timing belts came from model helicopters, I scored some old stock from a model shop ages ago. The down side there is the funny ratio, which gives me .003968(etc) mm per half step. Realistically I can only get about .1mm repeateability, and about .025mm resolution out of it, as long as I take small cuts. The step resolution is immaterial when the lathe chatters taking a .1mm cut from brass...

The mount for the cross slide motor is PVC plate, as per the floppy CNC, and the 'X' axis one is the mount the motors came with, wood screwed to the MDF base board.

The steppers are ripped from a sekoshia BP5420 dot matrix printer, if you can get your hands on one (I got four...) they have two good steppers, a couple of good ground shafts and a good heavy transformer. I got two of mine for $20 scrap value.. You need a good back to lift them though, as they are a good 30-40kg. From the days when printers were made properly. WANG also rebadged them as their own. I used to service these printers so I knew what I was looking for when I was visiting the local computer junk store (we have one... Oh for surplus stores like the US has...).

I have also uploaded a picture of some parts cut on the machine, they are all from 15mm stock.

If you've never retro-fitted a lathe like this you have no idea how cool it is to be able to make something with a ball nose on it with a machine like this. I was in mad inventor heaven when I cut that... :-).

The only real problem it has is getting an accurate 'zero' for the cross slide. I build a block that sat on the rails with an insulated "touch pad" which is on the centerline (in theory). I jog the tool up to it, and when the circuit is made an LED comes on. Should work, but somewhere the practice doesn't quite match the theory as I can't get very good repeatability out of it. before I use the lathe I normally skim some stock, and then set the zero using a micrometer measurement from the diameter of the stock. I then try not to turn things off until I've finished cutting whatever it is I wanted. Lost steps has not been a problem, as both steppers can brute force they way around this light weight machine.

All laughing aside, this machine has made just over 100 prop adaptors for electric model aircraft, and they are consistent enough for my needs, and I've managed to flog them off. And I've had a good play with tool path generation software with this lathe, and written a couple of simple post processors to clean up code from converted DXF's. It's all part of the learning process I figure.

Boy, do I drone on...

Cheers, Chris.

* Design engineer, Assembly worker, Cleaner.
* Ohmark Electronics. PO Box 5302, Christchurch, New Zealand.
* http://www.ohmark.co.nz


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