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Offline JimG

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Scale Problem?
« on: February 25, 2013, 06:13:28 AM »
I machined a load of parts on my Seig KX1 a couple of weeks ago and when I got around to assembling them last week I found that some of the parts were oversize - typically +0.35mm in 75mm on the X axis,  or just under +0.5%.   I've audited the CAD and GCode and all the values are correct.  Unfortunately I was producing the parts as fast as I could over two or three days and I can't remember any peculiar conditions that might have existed when the oversize parts were machined.   I re-machined one of the oversize parts yesterday with the same GCode files and it turned out dimensionally accurate.

I checked the motor settings in Mach3 and they seemed to be OK - Steps Per = 500; Velocity = 1000.2;  Acceleration = 150; Step Pulse = 2; Dir. Pulse = 0.

I remember having the same problem about a year ago when another part turned out oversize,  but I was able to re-machine it to correct dimensions using the same GCode files.

Any pointers to what might be happening?  At the moment I'm going to check the motor settings before I start any machining.  I never normally touch the scale settings on the MDI and always assume them to be 1,  but I will keep an eye on them as well.  I also checked the code from Cut2D for the possibility of any G50/51 entries,  but there are none.

The mach version is 3.043.022.

Jim.
Re: Scale Problem?
« Reply #1 on: February 25, 2013, 09:40:04 AM »
Jim,
If just the X, and only sometimes, you may just be operating at the ragged edge of your steppers ability.  Not knowing more about your part or machine, I recommend that you try lowering the velocity and aceleration by 25% (or any % you choose) to see if the problem goes away.

John Champlain
« Last Edit: February 25, 2013, 09:41:52 AM by Picengraver »

Offline JimG

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Re: Scale Problem?
« Reply #2 on: February 25, 2013, 11:26:20 AM »
Jim,
If just the X, and only sometimes, you may just be operating at the ragged edge of your steppers ability.  Not knowing more about your part or machine, I recommend that you try lowering the velocity and aceleration by 25% (or any % you choose) to see if the problem goes away.

John,

Last week I was more worried about the lengths of the parts so I've just checked the heights of the pieces which I hadn't done before,  and they are undersize!! - typically about 0.6mm in 30mm - approximately -2% error.   I've re-checked the piece I cut yesterday and both height and length are to size.

I don't think the machine is being worked too hard.  I'm cutting quite small detail in styrene with small diameter (1mm, 2mm) carbide cutters and the typical feed depth is 0.15mm,  with 25% stepover at 300mm/min and 3000 - 4000rpm.

Jim.
Re: Scale Problem?
« Reply #3 on: February 25, 2013, 09:39:19 PM »
Jim,

X too large, Y too small.  Seems odd.  What is your backlash?

John Champlain

Offline JimG

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Re: Scale Problem?
« Reply #4 on: February 26, 2013, 03:43:28 AM »
Jim,

X too large, Y too small.  Seems odd.  What is your backlash?

John,

I haven't checked the X and Y backlash recently since parts seemed to have been coming out the correct sizes and shapes until the problem parts of two weeks ago.  I'll get the dial test out this morning and have a look.

Jim.

Offline JimG

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Re: Scale Problem?
« Reply #5 on: February 26, 2013, 05:27:21 AM »
John,

Just checked the X and Y axis for backlash.  There is no backlash compensation set up for these axes in Mach - both backlash settings = 0.  Both axes were the same - at ~0.0008"/0.02mm.  My dial indicator is graduated in 0.0005",  so the 0.0008" was a (hopefully) intelligent guess at the small amount under one thou for the backlash.  The metric movement was a bit more of a guess working on an imperial dial.  ;)

Jim.
Re: Scale Problem?
« Reply #6 on: February 26, 2013, 02:10:05 PM »
Jim,
Sorry, but I'm out of suggestions/thoughts other than trying a reload of Mach3.  Perhaps one of the power users here will step in with help.

Good Luck,
John Champlain

Offline JimG

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Re: Scale Problem?
« Reply #7 on: February 26, 2013, 03:47:29 PM »
Jim,
Sorry, but I'm out of suggestions/thoughts other than trying a reload of Mach3.  Perhaps one of the power users here will step in with help.

John,

Thanks for your time in considering the problem.   I'll just keep making parts and checking each one after cutting so that if I hit the problem again,  I might be able to see what might have caused it in the machine setup at the time.

Jim.