backlash compensation questions

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dorin
Posts: 606
Joined: Thu Nov 28, 2002 5:24 am

backlash compensation questions

Post by dorin »

Hello,
I have been building a CNC router for a number of years and I am finally cutting boards.

I am using mach 3 and have enabled backlash compensation and the values I have are about 0.028" for x and y axis.
That seems really big to me..and it also seems odd as the x-axis has 1/2-10 threaded rod and the y-axis is 1/4-20.
(why are they both about the same?)

Problem is that when I want to cut a small circle, say .8 inch diameter, it ends up with 4 obvious 'starting points'.
(Little bumps on the circle)

Thoughts or suggestions?
Thanks,
Mike
www.chaski.com
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GlennW
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Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: backlash compensation questions

Post by GlennW »

Hello Mike,

How did you measure to arrive at .028" backlash?
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
dorin
Posts: 606
Joined: Thu Nov 28, 2002 5:24 am

Re: backlash compensation questions

Post by dorin »

-I put an engraving bit into the tool.
-Manually drove the device backward until the backlash was out. (With z-up)
-put z-down, turned on device
-Then I did a g0 x1
-Then I did a g0 y1
-Then measured how short an inch was engraved.

I can cut out a 1" x 2" block and only be about 0.004 when I have this set.
I have had it mark out points to drill with good accuracy.
Have been meaning to, but for some reason never get too...trying to cut a large circle.

Thanks,
-Mike
www.chaski.com
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GlennW
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Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: backlash compensation questions

Post by GlennW »

Hello Mike,

I don't know the first thing about Mach3 or what it displays, but to set backlash comp on my machines you would move the axis in one direction and stop. Then set up and zero an indicator against the spindle or tool shank. (spindle not running) Then incrementally jog the axis until the indicator needle moves. Subtract the indicator needle movement from the actual displayed axis movement of the control and you have the backlash amount. I have precision ball screws, so it's only a few tenths comp. but without it, it will leave a mark at 12:00, 3:00, 6:00, and 9:00 in the cut as you describe when the axes change direction.

Perhaps some of that procedure can be used on your machine.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
dorin
Posts: 606
Joined: Thu Nov 28, 2002 5:24 am

Re: backlash compensation questions

Post by dorin »

Now that you mention it, ages ago when I first too a stab at it...I took my dial indicator from my lathe with its magnetic mount/holder and a large cast iron frying pan from the kitchen...put it on the table for the x-axis and measured.
I should try that again!
www.chaski.com
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GlennW
Posts: 7284
Joined: Sun Apr 15, 2007 9:23 am
Location: Florida

Re: backlash compensation questions

Post by GlennW »

Ahhh, the frying pan technique!

Good one!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
JackF
Posts: 1616
Joined: Mon Jan 26, 2009 3:56 pm
Location: Caldwell, Idaho

Re: backlash compensation questions

Post by JackF »

Dorin, does your router use ball screws, acme, or? With .028 backlash it doesn't sound like a ball screw and it sounds like the .028 backlash is too much for the mach 3 to correct. Like Glenn mentioned, a quality ball screw will have backlash in the 1/10s range.

Jack.
dorin
Posts: 606
Joined: Thu Nov 28, 2002 5:24 am

Re: backlash compensation questions

Post by dorin »

x-axis is 1/2-10 acme thread.
y and z are 1/4-20 threaded rod.
-Mike
www.chaski.com
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