Positioning accuracy and repeatability – achieving tolerance less than the accuracy
Here’s a question: A CNC turning center has a positioning accuracy of 0.010mm and repeatability of 0.002mm. Is it possible to machine a diameter with a tolerance of +/- 0.006mm on this machine ?
The answer: Yes.
With a pos. accuracy of 10 microns, a command of X100.0 will cause the tool to go to anywhere between X99.990 and X100.010. However it will repeat this error within 2 microns, every time. Let’s assume it goes to X100.07 in the first part – out of our tolerance range. Subsequent parts will be in the range X100.05 to X100.09. So just check the dia. on the first part, and fine tune the offset so you get 100.00 dia. Because the repeatability is 2 microns, subsequent parts will come within 100 +/- 2 microns.
Essentially, this CNC lathe makes a mistake in positioning, but it makes the mistake consistently enough that it is a behaviour. The ‘mistake’ is the positioning accuracy. The ‘consistency’ is the repeatability. This machine is making its mistake with a high degree of consistency, high enough for us to achieve close tolerances on parts. This applies to any machine – lathes, machining centers, grinding, etc. The idea for this post is from a question that Mr. Virendra Patil of Shree Spherotec Pvt Ltd asked in an earlier post. He also confirmed the logic through laser measurement.
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