Axis scaling errors - is this just to be expected with C7 screws?

[GND]

Thanks for your take on this, JAZZ - I certainly don't have a practical way to measure over a significant length of the screw travel to the accuracy I would need! I'd have to base it on short segments, which it sounds would be far from ideal.

I just heard from Marchant Dice and they confirm the ballscrews I have should be no worse then C7. So to me it just feels like my issues are being caused by something other than screw pitch errors. Prime target must be lack of rigidity. But how to prove this....?

Cheers
Graeme

[JAZZCNC]

Yes I agree on the screw. I think it will be mixture of screw end float ie: End bearings and/or structural weakness. Doesn't take much deflection and can easily be hidden.
Suggets you give the machine a good shake and going over.

[magicniner]

Sorry Graeme,
Ideally you play with step calibration only on a machine that is as mechanically perfect as you can make it and which runs repeatably but with scaling issues, I was working from the assumption that you'd been through the mechanics.

- Nick

[GND]

No worries Nick - no, alas mechanically it is far from perfect!! I suspect I just need to address that first....

JAZZ - I also suspected end float on the screws right from the start, but measured it at something around 0.04mm by putting the DTI on the end of the ballscrew and moving the gantry back and forwards. Seemed surprisingly small, considering it just uses twin-row angular contact bearings at the fixed end with no preload. But if I move one of the axes by 1.00mm using MDI and measure with a DTI, and then go back to zero, the gauge zeros repeatedly with very little lash indicated. All seems good. But is it different under a cutting load?

Cheers
Graeme

[JAZZCNC]

Ok well in that case give us some idea of electrics used ie: voltage, Motor size, etc

[GND]

No problem. Recently switched to the 4Nm NEMA-23 steppers from CNC4YOU, driven by some pretty basic drivers which I inherited - Motion Control Products MSD980. This is all run at 68v, which is a bit higher than ideal I know, but dates from when the machine was fitted with NEMA-32 "super bricks", which is how it came to me. I had resonance and speed problems, so swapped out to the smaller motors a little while back. They don't seem to run too hot, hence for now I'm assuming they are ok. Otherwise I will need to downgrade the toroidal!

Cheers
Graeme

[JAZZCNC]

68V is perfect voltage for 4Nm 23 so don't change. The MSD980 where not bad drive and can handle 80V so no issues there provided setup and wired correctly.
So provided setup correctly regards current etc then this shouldn't be your issue and you'll have plenty of power. So can rule out lack of power.

If your sure that you have no mechanical issues and Stiffness isn't so bad and all electricly sound then all thats left is the dreaded parallel port/PC. (I pressume your using PP.?)

First thing to do is change the Parallel cable. Not uncommon for parallel cable to give problems.
Next is control software, if Mach3 then I'd back up your XML file and do fresh install. Again I've seen mach3 do strange things and re-install cure them.
PC would be my next swap out. If possible I'd try another PC.

Best of all is to dump the PP and fit motion control card. Preferably Ethernet based. USB is better than PP but I'd avoid if can afford it.

If Still have problems and does turn out to be stiffness/weakness then I wouldn't try to make silk purse out of sows ear. Just sell it on and build another better machine.