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Author Topic: Out of round holes  (Read 2975 times)

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Offline ajl53

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Out of round holes
« on: September 23, 2012, 02:01:12 PM »
Hello,
I am new to CNC and have built my machine from a combination of various designs. I am having trouble making round holes...
I have tried changing motor settings both in the x and y axis, even increasing the steps per inch on the x. I keep getting the
same result. I have attached a photo of the results of a simple 1" hole with a .125 milling bit.
Any suggestions as to where to look for a solution would be greatly appreciated.
Thank you
Art

Offline Hood

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Re: Out of round holes
« Reply #1 on: September 23, 2012, 02:57:16 PM »
Backlash on your X axis I would say.
Hood

Offline ajl53

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Re: Out of round holes
« Reply #2 on: September 23, 2012, 03:01:28 PM »
Thank you!!!
After I posted and I just "TRIED" adding backlash to the x axis and it made the hole more round....
I'm now looking to replace chain setup with a ballscrew for better accuracy...

Offline Hood

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Re: Out of round holes
« Reply #3 on: September 23, 2012, 03:03:12 PM »
No probs, chains are definitely not the best, ballscrew or a good rack/pinion system would be much better or even a belt system would be better.
Hood
Re: Out of round holes
« Reply #4 on: September 23, 2012, 07:48:42 PM »
Actually, roller chain is zero backlash as long as sprockets are tight to the shafts and chains are slightly tensioned.  Belts, unless use as a primary (motor to final drive), have too much flex.  You can have backlash just from belts stretching.  The commercial plasma machines that use belt drive use something that looks like a Harley drive belt.  With the holes he posted a photo of, the backlash should be easily detected by tugging on the axes with the machine turned on.

We just serviced a router with rack and pinion that had the same issue.  A set screw had come loose on the Y axis pinion.  You could feel the backlash by tugging on the Y axis.

Looks like a router.  Ball screw would be the best option.  Ball screws are hard to use on plasma machines because of the cutting speeds needed. 
« Last Edit: September 23, 2012, 07:50:59 PM by rrc1962 »

Offline Hood

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Re: Out of round holes
« Reply #5 on: September 24, 2012, 03:16:31 AM »
What does a Harley drive belt look like?
Hood

Offline RICH

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Re: Out of round holes
« Reply #6 on: September 24, 2012, 06:52:25 AM »
Quote
I have tried changing motor settings both in the x and y axis, even increasing the steps per inch on the x

Don't do that. Use the calculated values  and just refine them based on actual testing ( should be minor  adjustment).  The backlash can change and even the tested steps per unit may vary over the total travel, but it is due to what the axis components are and as such you have what you have.
Now not to confuse .......changing the actual steps per unit for some operating range to make the circle concentric can be done, but,
it not practical in use and you really need to know exactly why you doing it....but that is for "cutting hairs so to speak" and I am not going to elaborate on it.

RICH
Re: Out of round holes
« Reply #7 on: September 24, 2012, 09:19:42 AM »
What does a Harley drive belt look like?
Hood

It's about 1-1/2 wide and 3/8" thick....and very expensive when you have one custom made that is 20+ feet long.  It's the drive belt Harley Davidson used on their motorcycles.  Koike is using a flat roller drive with a stainless steel band on their smaller waterjet machines.  I would have thought that the drive rollers would slip, but they said they don't slip until they exceed 6 ft/lbs.  I suppose if you hit 6 lb/lbs of torque something has gone wrong anyway.