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Help with circles
« on: September 12, 2011, 04:16:16 AM »
Hello Folks

Need your assistance again.


Having problem getting a perfect or even near perfect circle - tried everything I know (which isn't much)


Made a circle on Cut2D - exported manually to Mach3 (the auto system does not work for me..) cutting the piece I get a circle that has in two opposite locations a "tip" that looks like the machine did some straight lines instead of an arc.

Looking at the circle on the tool path in Mach3 - it seems that the top part of the circle (north/south) is bigger then the other half (west east) but I don't know if this has anything to do here.

I set the I J mode tab to Inc, in Mach3 config page, and also checked the exact stop tab.

Tried to do a separate circle from the wizard in Mach3 - looks much better but still can see the pointers.

Checked my axes, can't see any mechanical fault. Did 3 passes from Zero point - the tool went exactly to the same position

To try and show the problem - I drew an exaggerated circle on CorelDraw.

Hope you can give me a hand here.

Cheers


Isaac

Offline Tweakie.CNC

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Re: Help with circles
« Reply #1 on: September 12, 2011, 06:31:20 AM »
Isaac,

That effect can be caused by backlash on your X axis. Have you checked the axis for any slack or lost movement ??

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline RICH

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Re: Help with circles
« Reply #2 on: September 12, 2011, 06:34:03 AM »
Isaac,
Post the gcode file and your xml file.
Check spindle runout at the end of the cutter. Also check for backlash and the steps per unit.
RICH
Re: Help with circles
« Reply #3 on: September 12, 2011, 07:32:11 AM »
Thanks guys

I think this is a backslash issue :-(

Did some tests with a dial indicator - have observed funny thing.... suppose I jog the x axis to the left - zero mach x axis and zero the dial - when jogging to the right - the dial initially shows movement to the LEFT and then it starts to move to the right- backlash is about 0.45 mm ???


I have Kerk motion leadscrews with anti backlash nut ??  have no idea how this is happening - when power is OFF - I can see no slack movement between the motor shaft-coupler and leadscrew.

Tried to fiddle with the backlash config - shows better results though


Any ideas how to get rid on minimize the backlash?


Cheers


Isaac

Offline Tweakie.CNC

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Re: Help with circles
« Reply #4 on: September 12, 2011, 08:11:02 AM »
Isaac,

You probably need to check all the Kerk data sheets etc. for setting and adjustment of their screws / nuts.  http://www.haydonkerk.com/Portals/0/files/lead_screw_technology_engineering_data.pdf

Also check your machine for rigidity / tightness of all components and also for any lateral movement at the tool tip when stationary (and disconnected from the supply  ;) )

The problem with backlash is that the total amount is very dependant on tool loading. You can set up a machine for zero backlash with a D.T.I but as soon as it is put into operation, the deeper the cut or greater the tool loading then the greater the backlash that starts to appear. Another important factor here is having good quality and sharp tooling (dull cutters are best consigned to the bin, unless they can be resharpened cheaply enough).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline RICH

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Re: Help with circles
« Reply #5 on: September 12, 2011, 11:47:51 AM »
Check and adjust the preload on the the thrust bearings since they can be a source of backlash even though there may be no practical
backlash in the nuts / screw.

RICH
Re: Help with circles
« Reply #6 on: September 12, 2011, 12:07:58 PM »
Thanks fellas


I am REALLY new at this - so some of the terms are un-familliar to me:

What is D.T.I and how do I use it to set the machine to zero backlash - is this the config/baclash page on MAch3 ?

What are "thrust bearings" and how do I adjust the pre-load on them ?


Cheers


Isaac

Offline Tweakie.CNC

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Re: Help with circles
« Reply #7 on: September 12, 2011, 12:28:17 PM »
Sorry Isaac my bad  :(

D.T.I is "Dial Test Indicator".

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: Help with circles
« Reply #8 on: September 12, 2011, 05:47:06 PM »
Thanks Tweakie

Problem is I have only the screw and the nut - and have no idea how can I "adjust" anything.

My screw goes from the stepper via an aluminium connector straight to the unti-backlash nut which is a simple flange housing spring and the collar. The flange is screwed to the bottom of the gantry, and the other end of the screw is rotating freely in a hole that is pretty tight (no interference though)

My tool movement is minimal thanks to the Kress machine collet. I will look for a mechanical movement just to make sure. Other than that I am out of ideas, and how anything can be "adjusted"

Many thanks


Isaac

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Re: Help with circles
« Reply #9 on: September 13, 2011, 03:01:51 AM »
Hi Isaac,

As I am sure you already know - If the tool loading or feed rate causes any compression of your 'anti backlash nut spring' then 'backlash' will be present and could most likely cause the problem you are describing.

Something to perhaps try is to decrease the tool loading by reducing the feed rate and / or depth of cut and then use 'multi pass'.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.