Losing X and Y zero

[boom-shanka]

HiI'm trying to use my CNC to cut double sided pieces of wood with Mach3 & Win10.The problem is when after i run a job the X & Y zero point are marginally out in a negative direction. This is seen in the image attached; go to zero, i'd drill a hole - run a job, then go to zero & drill again.The holes are out of alignment by about 2+mm in both directions (-x & -y), the job was run in air so the wood is not(?) a factor.It seems to happen on all jobs & the longer the job, the more "out" it is. I've been trying double sided milling & got out of alignment results - thought it was my newbie skills but then i noticed this zero issue.Have disabled toolpaths in Mach3, tried stopping other apps in background, running Windows in safety mode...Any ideas appreciated

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[ger21]

Reduce your acceleration by 50%, and see if it helps.

[KH0UJ]

the longer the job, the more "out" it is

It`s a clear sign that your machine had a backlash, we have one unit in the shop that does the same thing on your output, unfortunately it`s not a ball screw, it had a 0.5mm back lash on all axis (X,Y,Z), the longer the program code the more offset it got. replaced it with a ball screw and all the offsetting issue vanished.

[boom-shanka]

It`s a clear sign that your machine had a backlash, we have one unit in the shop that does the same thing on your output, unfortunately it`s not a ball screw, it had a 0.5mm back lash on all axis (X,Y,Z), the longer the program code the more offset it got. replaced it with a ball screw and all the offsetting issue vanished.

Thanks for your help.Is there a way i can verify this is the case with the machine? The machine is quite new - less than a year old & hasn't been pushed very hard in that time.

[boom-shanka]

I've halved the feedrate & unfortunately it hasn't made a difference.

[awerby]

Ger was talking about reducing acceleration, not feedrate. That can help in case of lost steps. In Mach3, you can find the acceleration and velocity settings in the Motor Tuning dialogue. Acceleration is how hard it tries to go at the beginning of a move; it's where steps often are lost. Velocity is how fast it goes once it's up to speed, and is as fast as the machine will move, even if the commanded feedrate is higher.

But it's unclear whether your problem is from lost steps or something else. You say that you're cutting "double sided" pieces. Does that mean you're flipping them over and trying to hit the same hole from the other side? That could be due to improper registration on the flip.

As Khouj points out, you can also get errors like this from backlash. That's what happens when a screw starts to move, but before it engages the nut. It's usually seen when you change directions, and come back to a spot from the other side. To check if that's the problem, run back to the original zero point and approach the hole from the same direction as before. Backlash doesn't accumulate; if your nut is engaged when you hit the original zero point, then it should be accurate when it hits the hole.

[boom-shanka]

Ger was talking about reducing acceleration, not feedrate. That can help in case of lost steps. In Mach3, you can find the acceleration and velocity settings in the Motor Tuning dialogue.

Ahh, ok thanks - will try that & see how i get on.

But it's unclear whether your problem is from lost steps or something else. You say that you're cutting "double sided" pieces. Does that mean you're flipping them over and trying to hit the same hole from the other side? That could be due to improper registration on the flip.

The photo i posted initially is holes drilled from the same side into spoil board - i just mentioned double sided as i couldn't get anything aligned with whatever method i used which prompted me to see if there was an issue somewhere else.

As Khouj points out, you can also get errors like this from backlash. That's what happens when a screw starts to move, but before it engages the nut. It's usually seen when you change directions, and come back to a spot from the other side. To check if that's the problem, run back to the original zero point and approach the hole from the same direction as before. Backlash doesn't accumulate; if your nut is engaged when you hit the original zero point, then it should be accurate when it hits the hole.

Thanks again - i've just chatted to the CNC reseller & they have exactly the same machine which does not have an issue so will wait for them to get back to me.

[boom-shanka]

Just a follow up & closure for this...

After 3+ months of trying to get double sided milling aligned the reseller tried on their machine - with exactly the same problem.

They replaced the conduit(?) circuit board with a new version & everything was fixed; shipped one to me & i did the same, fixed.
X0Y0 not losing position & my first perfectly aligned double sided job in 3 months - finally.

The problem card was a eCut v2.0 B - the new card is AKZ250 v6 (A)

Yay