Wobblycogs CNC Mk2

[Wobblycogs]

Well it's been a busy day for the credit card. The drivers, steppers, ballscrews and nuts, bearings and mounts, spindle and vfd have all been bought mostly from China. I think I might have accidentally bought a collet set as well - I asked for a spindle with a ER20 collet and I think he's read that as "collet set". Not a problem though as I was going to buy a set anyway, I hadn't realized Chai had started sell them. If anyone needs drag chain he seems to be doing some nice looking stuff at a reasonable price.

I've also been tempted over to the dark side, I installed LinuxCNC on one of the old machines I have kicking about last night and I have to say I like what I see. There were a couple of minor complications but over all it seems to be a nicely put together system. I ran the latency tests for a couple of hours with a fully stressed machine and achieved ~7,000ns max jitter so I think it should work quite well. The parallel port supports SPP, EPP, ECP and ECP + EPP modes. I've seen someone mention that one of EPP or ECP is better but I can't find the thread again.

I was looking closely at the electronics in Jonathans "sufficiently strong" build and I can't see a BOB anywhere in the control box. In fact it looks like the drivers are connected directly to the parallel port with the cable being wrapped in a bit of foil. Is that correct? I've not been able to find all that much mention of people running steppers direct from the parallel port, just the odd post here and there but I wouldn't mind giving it a go. As has been mentioned elsewhere I can always move onto using an ESS + BOB if necessary but I might as well give it a shot.

[Jonathan]

7000ns is exceptionally good latency, what motherboard etc are you using?

I was looking closely at the electronics in Jonathans "sufficiently strong" build and I can't see a BOB anywhere in the control box. In fact it looks like the drivers are connected directly to the parallel port with the cable being wrapped in a bit of foil. Is that correct? [...] but I might as well give it a shot.

I left Sasha to wire up the control box and he had problems with the breakoutboard, so for now the stepper drivers are indeed connected directly to the port. The output voltage of the port is sufficient so switch the driver inputs reliably and the driver inputs are opto-isolated, so there's no chance of damaging the port. To get isolated inputs it's easiest to just get a breakout board. In short, it works... but that doesn't mean it the best idea. So by all means give it a shot, but I wouldn't advise using this setup permanently.

Not sure why the foil is there, maybe he ran out of shielded cable but I thought I lent him a 100m reel!

[Wobblycogs]

Yes, I was quite pleased with the latency. Looking at the machine spec I'd forgotten quite how good it was, massive overkill for a control box I know but it was sitting around not doing anything. In fact it's been switched off for so long the BIOS battery needs replacing. I'll replace that and re-run the latency test to confirm the result.

The machine is:

Motherboard: GA-K8NXP-SLI
Processor: Athlon 64 3000+ (not sure which revision, I think it's a 130nm ClawHammer as I bought it quite soon after they were released)
Memory: 3GB DDR400
Video Card: some old nVidia card, can look it up if you're interested.

Initially I was getting a latency of about 50,000ns but I noticed that was caused by a spike when I first played sound through the on-board sound card. Disabling the on-board sound in the BIOS gave me a 7000ns. After performing all the package updates the system required I re-ran the tests and got around 6500ns. I also tried the nVidia binary drivers to see if that would improve video performance - bad idea, whenever anything using OpenGL started it caused a 250,000ns spike in jitter!

I hear what you are saying regarding running direct from the parallel port. Would you also recommend the PMDX-126 as a nice BOB?

[Wobblycogs]

I've been a bit busy with work for the last couple of months hence the lack of posts but the build has moved forward a bit. I've now got the steppers, drivers, screws, nuts, bob, nut brackets, spindle, VFD and a few other bits and bobs as well.

I met up with Neil the other night to swap parts from a joint buy and he kindly gave me a bit of 10mm plate to have a practice with. When I first saw it my heart sank - it looked distinctly too thin to build a machine from. I thought I'd give it a crack anyway and build one of the 8 brackets that I plan on using to support the X-axis and here's the results:


The angle from the base to the back is 89.7 deg which I'm fairly pleased with considering this is my first attempt at metal work. It's surprisingly strong but is it strong enough? The the parts we first roughly jigsawed (or hand sawn once the jigsaw blade gave up) and then cut to exact size on the mitre saw. The accuracy is ok but not exactly the perfection I had envisioned. I'll have to decide now whether to get some kind soul to mill the parts for me or to press on with trying to cut the pieces myself.