Hello Guest it is May 24, 2019, 11:36:21 AM

Author Topic: X Axis slowly goes wrong, every time.  (Read 1862 times)

0 Members and 1 Guest are viewing this topic.

X Axis slowly goes wrong, every time.
« on: July 02, 2012, 01:07:22 PM »
Hi,

2 1/2 axis machine built up and everything runs ok in terms of keyboard movement. X Y Z all great.

Produced some G-Code from VisualMill and start the machine running.

But slowely, it drifts away on the Y Axis, in the same direction everytime.
As in, After milling around doing a curved handle shape the top edge has hardly no curve and the bottom has an increased curve.

A bit like, G01 Y100 and it moves 100, G01 Y0 and it moves 99, G01 Y100 and its now 101...?? Make sense?

The numbers I think are right on the screen but my Y axis is drifting further away all the time.

X is the same settings etc but it works lovely.

Any Ideas?

Offline Hood

*
  •  25,816 25,816
  • Carnoustie, Scotland
    • View Profile
Re: X Axis slowly goes wrong, every time.
« Reply #1 on: July 02, 2012, 01:20:34 PM »
Can think of a few things but widen the pulse for a start and also drop off the Vel and Accel a bit to see if it helps.
Hood
Re: X Axis slowly goes wrong, every time.
« Reply #2 on: July 02, 2012, 01:41:14 PM »
Should have posted fully,

I'm milling a handle, a basic rectangle with 4 dips in it to make it more finger comfortable.

I've tried:
Pulse width from 1 to 5
Dir width from 0 to 5
Changing decimal places from 4 to 2
Acc/Dec time from 0.5 to 2 seconds
Feed speed from 3500 to 1500 and up to 6600 mm/min (I'm milling an expanding foam type material, Hardly any resistance)
Checked mechanical fittings, all tight

Noticed:
I'm always cutting clockwise, so always cutting in to the material and the handle bumps as it mills from X- to X+ cause an angle to appear on the Y+ side. So every time around its more Y+ than it should be. And milling the "mountain" shapes which should be the same on both sides has a steep incline on the Y- side and a shallow incline on the Y+

I'll try to upload a photo to show it in detail.

Can it be backlash? I though this would cause errors on both the + and the - of the movement? not just on one side?

Offline Hood

*
  •  25,816 25,816
  • Carnoustie, Scotland
    • View Profile
Re: X Axis slowly goes wrong, every time.
« Reply #3 on: July 02, 2012, 01:46:31 PM »
Wondering if you could have the active state of the step pulse set wrong for your drive.
However from your description it does sound like it may be backlash, will see better if you can post a pic.

Hood
Re: X Axis slowly goes wrong, every time.
« Reply #4 on: July 05, 2012, 12:54:14 PM »
Been busy!

Wrote some GCode that is basically G0 X0 Y0, G0 X30 Y30 in a loop, 200 times.
At the end I move to 0, 0 and then actually move to 0, 0 to see how far the machine has walked.

After 200 moves the machine had walked 2.5mm on the X axis.

I calculated 0.0125 mm backlash but after a retest it was still 2.5mm out. So it's not backlash.

I had some wobble on the bench from inertia from the motors so I broke out the welder for some angle supports. Solid now but a retest shows 2.5mm out.

Why always 2.5mm after 200 moves??

Changed Look-ahead to 1 move (Read this somewhere) No change, 2.5mm.....

Modded my CnC board to give some breakout wires, put these in to a new m542 driver board and...

Perfect!, no idea how a board was giving me 2.5mm error yet move the motors what seems perfectly.

So... Thead closed... Faulty control gear.

Thanks for your help.