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Author Topic: Parts are being cut smaller than the gcode #s  (Read 5241 times)

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Offline BWP

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Parts are being cut smaller than the gcode #s
« on: February 14, 2008, 02:15:53 PM »
Hello Mach Users,  I am new to cnc, and just learning my way around Mach3. I drew some parts in Autocad and sent them to the machine and after the parts were cut they varied in sizes. A cut that was supposed to finish out at 12' measured  11.948, the next cut was supposed to finish out at 39.125 and finished at 39.0625 (appx.) and a parts that was to finish out at 46.625, finished at 46.5 appx. Can someone please help me with this problem ? The machines motors are tuned to have 1917.812366 steps per inch, if that helps.

Offline Chaoticone

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Re: Parts are being cut smaller than the gcode #s
« Reply #1 on: February 14, 2008, 02:46:52 PM »
Here is how you do the math. I will give you mine as an example. What we need to know is, How many pulses from Mach will it take to move the machine 1 of your units? (inch,mm) Mine is in inches. 1 inch = 10,000 pulses on mine. This is how I came up with the number.
My steppers have 200 pulses per revolution. ( 360 degrees divided by 1.8 degrees = 200 )
Micro stepping = 10. So 200 X 10=2,000
Motor to screw gearing is 1-1. So 2,000 X 1=2,000
Screw lead is .200 of an inch. My screw will have to turn 5 rounds to move the nut 1". 1.0 divided by 0.200=5. So 2,000 X 5=10,000.

Once you check to make sure your steps per are correct, check your backlash.  Checking your backlash should be no problem if you have an indicator. Put the indicator on something solid. Jog your axis until you move the indicator to a reading of about half of its travel. Write down the reading on the indicator. Jog the axis in the opposite direction by a set amount, say .010" and see how much difference the indicator shows. If the indicator says you moved .005 and Mach says it has moved .010 then you have .005" backlash. You can check your steps per with your indicator also. Set up as before., only this time run the axis in until it just starts to move the needle on the indicator. Write down your reading. Going in the same direction give the axis a move with Mach. If Mach thinks it moved it .010" and your indicator says it moved .010", your steps per are right.

Brett


;D If you could see the things I have in my head, you would be laughing too. ;D

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Offline BWP

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Re: Parts are being cut smaller than the gcode #s
« Reply #2 on: February 14, 2008, 03:41:48 PM »
Thanks Brett,
where do I find out how many steps my motors per rev?

Offline Chaoticone

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Re: Parts are being cut smaller than the gcode #s
« Reply #3 on: February 14, 2008, 03:54:44 PM »
Most stepper motors are 200 pulses per rev. Are you using steppers or servos?

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline BWP

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Re: Parts are being cut smaller than the gcode #s
« Reply #4 on: February 14, 2008, 03:58:45 PM »
steppers.

Offline BWP

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Re: Parts are being cut smaller than the gcode #s
« Reply #5 on: February 14, 2008, 04:27:32 PM »
I tried 2000 an dnow my parts are growing in size.

Offline Chaoticone

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Re: Parts are being cut smaller than the gcode #s
« Reply #6 on: February 14, 2008, 04:48:46 PM »
Have you had a look at the manuals for Mach?

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline BWP

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Re: Parts are being cut smaller than the gcode #s
« Reply #7 on: February 14, 2008, 04:58:32 PM »
No, the tech guy came out and set my machine up. that is my next step.

Offline Sam

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Re: Parts are being cut smaller than the gcode #s
« Reply #8 on: February 14, 2008, 07:40:24 PM »
If you have an indicator, mach can figure out the math and set your axis for you automatically. In the settings page, there is an axis calibration feature. Move the axis into the indicator a small amount, take note of the number on the indicator, or zero the dial. At the bottom left of mach there is an axis calibration button. Click it. Select the axis to calibrate. Click the pop-up window to make it active (bug?), then type in the distance you want it to travel. Be careful not to over-travel your indicator. Mach will then ask how far the axis actually traveled. Type the actual distance in, and you should be good to go!
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."
Re: Parts are being cut smaller than the gcode #s
« Reply #9 on: February 15, 2008, 12:37:34 PM »
Hey Sam,
That's really nifty, never saw it in the docs. anywhere.
 Thanks a bunch,
RC

Looks like you started "Hot Roding" at an early age.  8)
« Last Edit: February 15, 2008, 12:41:23 PM by Overloaded »