Backlash Compensation: When is it Applied?

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SteveHGraham
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Joined: Sat Jan 17, 2009 7:55 pm
Location: Florida

Backlash Compensation: When is it Applied?

Post by SteveHGraham »

I'm working on the CNC lathe today. I learned that I was having problems because the steppers were disabling after a timeout. When I turned the lathe back on, the steppers would jump as they initialized, making it impossible for me to start where I wanted to. I got that fixed.

Now I'm dealing with step count and backlash.

My screw moves 1/2" per turn, and the KStep board I use defaults to 16 microsteps, so I decided to try a figure of 6400 steps/inch in Mach3 (200 times 16 divided by 0.5). Seems to be okay. Not perfect, but it looks like it's off about 0.001" in 0.500".

I tried to calculate the backlash using the jog function in Mach3, set to move 0.001" per step. I moved the carriage in one direction and then moved it in the other, counting the times it failed to respond. I figured that was my backlash number.

Here is the problem: I don't know where KMotion (the program that applies the backlash correction) makes the compensation. If the screw is tight when I start a program, and the program adds 0.010" to make up for backlash, the carriage will go 0.010" too far. It will only work if the correction is only applied in one direction. So...what's the answer? I can sit here and try to guess, using the dial indicator, but I figured someone here would know how this works.
Every hard-fried egg began life sunny-side up.
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SteveHGraham
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Joined: Sat Jan 17, 2009 7:55 pm
Location: Florida

Re: Backlash Compensation: When is it Applied?

Post by SteveHGraham »

Well, I found it! It only applies when the motion is positive along the axis. I'll leave this thread up in case someone in the future has the problem.
Every hard-fried egg began life sunny-side up.
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Dave_C
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Re: Backlash Compensation: When is it Applied?

Post by Dave_C »

Steve,

Looks like you got it figured out. Mach 3 makes it easy to apply the backlash but only from one direction. If you cut from right to left all the time you can get around the backlash issue. If you want to cut in both directions then you may need the backlash comp on.

I've noticed that Bob-Cad Cam makes all the cuts from right to left when I run a program on the lathe. I have pre-loaded ball nuts so I have no backlash issue but good to know how it works if you need it.

Dave C.
I learn something new every day! Problem is I forget two.
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