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axis problem
« on: August 24, 2008, 08:37:07 AM »
i have a bridgeport knee mill i converted over with mach 3 i use the calibrate axis botton to calculate steps. i can do the calibrate axis over and over and come up with the same but when i type on the mdi line y8.375 it will move .691 to far. i can type in y-8.375 it will go right back to were i started y8.375 and it will move .691 to far it is consestent on what it moves it will go the same over every time. i can go back to calb. axis and every thing stays the same. has anybody had this problem? thanks

Offline Chaoticone

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Re: axis problem
« Reply #1 on: August 24, 2008, 08:59:36 AM »
Make sure when you calibrate the axis you have given a move prior to that to take out any backlash. I prefer to do the math to get the steps per but the calibrate works great if you take backlash into account. Have you checked your backlash? I seriously doubt it is anywhere near .691 though.

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.

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 other 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. With that being said, you really need to know the pitch of your ballscrew first. I would say your steps per will be 10,000. You can check this 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. ;D

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