Need Help!- X axis over travelling during run.

[Senna]

If your math for the X axis steps per inch is correct... then I would guess that your X axis acceleration is set too low. Here's information I received from one of the 'guru's' with regard to acceleration when I was setting up my machine...

The acceleration setting effects how the steppers stop or decelerate. An acceleration setting that is to low will cause coasting down of the axis and the machine to overshoot where it should have stopped. It can also cause the machine to overrun home or limit switches. A general rule is to use between 10% and 15% of the velocity as a starting set point for the acceleration setting. A good visual indication would be to snap "rapidly release" an axis jog key during a jog... that axis should stop motion instantly.

It sounds to me you have the Y axis accel set in the correct range but your X axis accel is set too low and you are consistently coasting down and overshooting that 1/16".

Senna

[PaulRowntree]

Originally Posted by Senna The acceleration setting effects how the steppers stop or decelerate. An acceleration setting that is to low will cause coasting down of the axis and the machine to overshoot where it should have stopped. It can also cause the machine to overrun home or limit switches. Senna

Low acc will cause overshoot when manually jogging (and is scary to see when your gantry is rushing towards an immovable object...), because the controller (Mach3?) can't predict when you are going to release the jog button. I would have thought that limit switches would stop the movement as fast as gantry momentum allows. But when running gcode, the 'look-ahead' predictor knows exactly where it is going on each axis, and does its best to get to the desired position by starting to slow down when 'the end is near'. If you try to maintain constant velocity as well when going around a corner, then the controller has to start slowing down on one axis and speeding up on another before the actual corner.

The upshot of this is that the controller may cut corners short but should never overshoot.
If there is any mechanical slop available, then overshoot may happen because of gantry inertia.

[JayCop]

My plans to try higher accl and possibly turning of CV were thwarted by my controller computers decision to stop working abruptly.

Luckily I have another computer that I just had to load XP onto, and get the settings from the old hard drive on. So I will be able to test again tomorrow.

[ger21]

Originally Posted by Senna It sounds to me you have the Y axis accel set in the correct range but your X axis accel is set too low and you are consistently coasting down and overshooting that 1/16".

Lowering the accel will not cause overshooting. Say you want to move 5 inches. There is a fixed number of steps required to move those 5 inches. The control will send the exact number of steps required to travel those 5 inches. No more, no less.

Either the machine is losing or gaining steps, or it's a mechanical issue.

[PaulRowntree]

Originally Posted by JayCop My plans to try higher accl and possibly turning of CV were thwarted by my controller computers decision to stop working abruptly. Luckily I have another computer that I just had to load XP onto, and get the settings from the old hard drive on. So I will be able to test again tomorrow.

Bad break, and it always happens when you need it to work (as if there was any other possibility).

But seriously, is it possible that your computer/interface has been marginal and has been giving inconsistent pulse trains?

[JayCop]

At this point I hope it was the computer.

[JayCop]

Tested and it is better with the higher acceleration. Unfortunately it is still a little off.

Does any one have some screen shots of the settings they have in mach3 that you may provide so I can see if I have changed something I should not have?

[JayCop]

I had an epiphany today. I went back to the drawing board and looked at the G540 documentation. I had not used used the G540 xml because well I was lazy and didn't want to have to change all of my home and limit settings. Figuring the settings I had from a previous BOB were fine.

The proof was in the pudding. Now when I cut a 10"x10" square guess what? It is 10"x10"!

Now my next little problem I need to iron out is making the square have true 90 degree corners. I am out of square by about 1/64" on my 5" long machinist square. The corners also have a slight chamfer where the next axis starts to move before the other stops which I guess could be from the acceleration not being all that high ie 20.

Does anyone have a simple way of adjusting the squareness of the axis based on the amount it is out of alignment on the cut outs?

Is there anyway to get true square outside cuts without the slight chamfer while also not killing the speed of circles and arcs?

[JayCop]

Thanks to PaulRowntrees program warp drive I have resolved most of my inaccuracies. With Pauls warp drive program I was able to find that my machine is about .15 degrees out of being orthogonal.

I have a new problem now that I have not been able to get figured out. I am making custom computer cases and the fan grills that I make are half circular arcs. On one side the archs are perfect every time and on the other side they vary pretty substantially. Attached is a picture of what I am talking about. Left side good, right side bad

[ger21]

Is the cutter plunging on the right side? Try starting in the middle of the slot.

[JayCop]

Originally Posted by ger21 Is the cutter plunging on the right side? Try starting in the middle of the slot.

Yes it is. I am unsure how I could change that in my cam program CUT2D. I tried ramping the plunge of the cut over 100mm this last time and it did not seem to help.

I did cut a panel that had the same grill on it 90deg from the other ones and they look significantly better. Could this be backlash on one of the axis? I couldn't imagine that there would be so much backlash in the R&P setup but hey you never know. I have been chasing my tail with this machine.

[PaulRowntree]

Originally Posted by JayCop Thanks to PaulRowntrees program warp drive I have resolved most of my inaccuracies. With Pauls warp drive program I was able to find that my machine is about .15 degrees out of being orthogonal.

I am glad to hear that it helped you out!

EDIT : Does the warped G-code look correct on the right side of the cut? Did you try a previewer to make sure that the warping isn't messing things up? It never has for me, but some children (and some programs) sometimes misbehave when out on their own ...

Cheers!