Originally Posted by
Gary Campbell
David...
Before you can calibrate via your methods you will have to eliminate some variables first. The most overlooked are machine deflection and bit diameter. Cut outside of a 4" square, 60 ipm 1/8" deep, 15k rpm with a 1/4" bit. Cut it climb and conventional. Measure the dimensions. Conventional should be slightly smaller, but whats important is that HALF the difference is your deflection at .002 chipload. Now cut it at 180ipm and see if there is a difference at .006 chipload. I'll have to try this.
Next measure the width of the slot cut by the bit for each case. That is the diameter the bit cuts. May in most cases, be different that the advertised diameter.
These two tests allow you to be aware of the environment, not the accuracy. To check and verify if the step resolution is accurate you need a good eyeball, a sharp V bit and a precision rule. If you have one, like me, I like to use a USB microscope with crosshairs and a precision steel rule, but a vbit and a tape measure will work fine. Stretch the tape out in the X axis, align the vbit EXACTLY to a line for 1 or 2" and tape it down. Zero that axis. For a 4' machine I will command X to move 44" and see if I am exactly on the line for 44" plus what had been cut for zero. I'll have to see which tape 'appears' to be the most accurate.
If not exact, using the keyboard in incremental jog mode, adjust the vbit to the proper mark with care and record the X position on the DRO. To adjust Divide the value in the DRO by the actual distance (tape) and multiply that by the current resolution number. Edit the rez number, reboot if needed and retest. Repeat for the Y axis.
You want to do this over as long of a distance as is possible to minimize any human eyeball errors. .005 in 1" is vastly different than .005 in 40 inches. About 6 months ago I did something similar for measuring longer distance but must have missed something. I realize 0.005" over 40 inches is far better than 0.005" in 1 inch and that's what I'd love to see but I question how accurate my tapes are. I'll pull them all out to 48" and see how they compare.