Quote Originally Posted by Bryan Lisowski View Post
David, take this from someone who has never ran a CNC before. If you are cutting the outside of the rectangle or circles first, could something be shifting or could the file not be taking some measurement accurately post cut of the outside.
Thanks, Bryan, but the board is securely screwed down to the spoilboard. It's not going anywhere.

Quote Originally Posted by Gary Campbell View Post
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.
I checked the bit - it is 0.250" on the nose. If I really work at it I can make it be 0.2495" at the very tip of the bit but for all practical purposes, and for this test, it's a 1/4" bit.

I know using hardwood or acrylic would be better but until I get this a lot closer I don't want to waste good materials. Because the X and Y are very close now I need to figure out what's going on with the inside dimensions. I can slow the feed rate but I don't really think that's the issue. And going from conventional to climb (or vice versa) won't amount to the 0.020" and greater errors I'm seeing. And if the X and Y are as close as they are I don't think any error in the R&P factors in on the inside dimensions.

I have Forstner bits of the size holes I cut and I can't get those bits into the holes. I can accurately measure the Forstner bits and most are about 0.010" undersize but even at that they won't fit into the holes I'm boring, so that tells me my inside measurements aren't too far off.

David