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TB001 Rev3 – Out of Round Circles and Arcs 5 7 24
Question What is causing an inconsistent radius after milling a circle or arc?
Answer Mechanical gear ratio aka ‘overall turns ratio’ ‘motor revs per inch’ set incorrectly
and/or excessive mechanical lash in the machine or improper backlash compensation setting or loose pulleys loose
keyways set screws loose timing gears support bearings thrust bearings/nuts/shims and any other lost motion
from mechanical reasons
Solutions
• Set/adjust cnc12 backlash compensation value if mechanical lash is measured to be less than 0 001" See TB037 – proper
mechanical back lash measurement technique
• Verify proper axis Overall Turns Ratio axis motor revolutions per inch or mm linear movement per axis motor revolution
• Repair mechanical if lash is over 0 001"Info graphic below showing the effect of mechanical lash in the Y axis with a CW
cutter path with no measurable lash in the X axis
Info graphic below show exaggerated example of Mechanical lash in the Y axis with a CW cutter path and no measurable lash
in the X axis
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TB001 Rev3 – Out of Round Circles and Arcs 5 7 24
Elliptical circles
Verify proper axis Overall Turns Ratio axis motor revolutions per inch or mm linear movement per axis motor revolution
Verify proper Steps Per Revolution between the CNC12/Wizard settings and the actual axis motor DRIVE settings
Related reading for Acorn users is here https //centroidcncforum com/viewtopic php?f=63&t=1801
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TB001 Rev3 – Out of Round Circles and Arcs 5 7 24
Problem Steps Marks Divots appear on surface finish at the quadrants 0 90 180 270
Cause Overcompensating for backlash amount in software or excessive inconsistent mechanical
backlash
Solution Fix mechanical system to less that 001” backlash
Or Reduce the CNC12 software Backlash Compensation value for case where CNC12 is being commanded to over correct
for mechanical lash Set Lash Compensation value to zero and remeasure the mechanical lash and set CNC12 software
backlash compensation value to match
See Measuring Backlash TB37
https //www centroidcnc com/dealersupport/tech_bulletins/uploads/37 pdf
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TB001 Rev3 – Out of Round Circles and Arcs 5 7 24
Resources
Marc Leonard Measuring Backlash Method
“There are as many ways of measuring backlash as there are CNC technicians Each method will give you a different answer This is my
way which is reasonably representative of practical cutting speeds and conditions ”
https //centroidcncforum com/viewtopic php?p=46199#p46199
Back Lash compensation and screw compensation video by Alan
https //centroidcncforum com/viewtopic php?p=46692#p46692
Steps per rev don’t do it like Mach did it
https //centroidcncforum com/viewtopic php?f=63&t=1801&p=11936&hilit=lash#p11936
Related Topics
Screw compensation for better precision by Uwe
https //centroidcncforum com/viewtopic php?p=80095#p80095
Document History
Rev1 Created on 1997 10 01 by #277
Rev2 Created on 2017 03 17
Rev3 Created on 2024 05 07
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