Hello Guest it is December 09, 2019, 10:06:31 PM

Author Topic: Another lost steps post...  (Read 8201 times)

0 Members and 1 Guest are viewing this topic.

Offline Sweep

*
  •  150 150
    • View Profile
    • pollz.co.uk
Re: Another lost steps post...
« Reply #20 on: May 26, 2010, 03:19:33 AM »
If I keep going positive I don't accumulate errors, the DTI will stay at 0.004" adrift. If I reset the DTI to "zero" after the first positive move, then any subsequent positive moves will not add to te error. The error is only introduced when I change direction.

Offline Hood

*
  •  25,856 25,856
  • Carnoustie, Scotland
    • View Profile
Re: Another lost steps post...
« Reply #21 on: May 26, 2010, 03:26:31 AM »
It sounds like you have mechanical backlash. This could be in the ballscrew itself but can also be in the angular contact bearings or even loose gibs can give your table a twist thus giving you an error.
If you can put the DTI on the end of the table and have the poiter on the end of the ballscrew shaft then step slowly one way then reverse in small steps and see if the DTI needle moves, if it does then you have float in the screw which means either flex in the brackets or bearings need adjustment.
Hood

Offline Sweep

*
  •  150 150
    • View Profile
    • pollz.co.uk
Re: Another lost steps post...
« Reply #22 on: May 26, 2010, 03:33:41 AM »
I am pretty sure that there is no mechanical backlash as have already pulled the table too and fro (well applied pressure in both directions) and the DTI needle never flinched. I think an experiment on the Z axis would prove or dispell your toughts as the weight of the whole spindle assembly will naturally remove any mechanical backlash. If there is 'electronic' backlash the error will still appear.

Offline Sweep

*
  •  150 150
    • View Profile
    • pollz.co.uk
Re: Another lost steps post...
« Reply #23 on: May 26, 2010, 03:37:26 AM »
Sorry,to fully answer your last post, i'll try move the axis by my smallest resolvable increment (200steps/mm) of 0.05mm and see what movement of the axis is measured on the DTI

Offline stirling

*
  • *
  •  2,188 2,188
  • UK
    • View Profile
    • www.razordance.co.uk
Re: Another lost steps post...
« Reply #24 on: May 26, 2010, 04:30:10 AM »
The error is only introduced when I change direction.
The only two reasons I can think of for this are lost step pulses on dir change or backlash. The former is usually a single step pulse lost but for your res and error you'd have to be losing around 20 pulses! That can't be right... Don't know about the rest of your system but do belts really have no backlash?

Cheers

Ian
« Last Edit: May 26, 2010, 11:34:55 AM by stirling »

Offline Sweep

*
  •  150 150
    • View Profile
    • pollz.co.uk
Re: Another lost steps post...
« Reply #25 on: May 26, 2010, 02:57:52 PM »
ah, looks like I wasn't paying attention at school during maths.
1 / 5mm/rev = 0.2rev/mm of ballscrew shaft
0.2rev/mm x 2.5 belt ratio = 0.5rev/mm of motor spindle
0.5rev/mm x 400step/rev = 200steps/mm of motor spindle
therefore 1 step = 1 / 200 = 0.005mm/step or 0.00019"

I would, therefore, agree that 20 steps would need to get forgotton about to realise such an error. I'll have to have a closer look at mechanical backlash as I would agree that this now seems like the obvious problem.

thanks for the lesson Sterling

...Sweep