Calibration question

[David Falkner]

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.

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

[Mark Bolton]

I hate guessing at responses but could backlash be an issue? Im no machine builder but when I saw all the varied measures on your test piece it just to me spoke to something transient/random-ish. This makes me want to run one of these on our machine.

I regularly machine fixture pockets and then parts for a snug/press fit and its always johnny on the spot. Part of me would be scared to run one of these as I'd likely wind up flipping out.

[David Falkner]

I hate guessing at responses but could backlash be an issue? I'm no machine builder but when I saw all the varied measures on your test piece it just to me spoke to something transient/random-ish. This makes me want to run one of these on our machine.

I regularly machine fixture pockets and then parts for a snug/press fit and its always johnny on the spot. Part of me would be scared to run one of these as I'd likely wind up flipping out.

I haven't seen any indication of backlash, Mark. The machine obviously has a little - I don't think you can build a R&P machine without a tiny bit. But it hasn't shown up anywhere else except when I try to do pockets and inlays. I can go to zero and run any of the cuts again and the bit doesn't remove any new material. That tells me the backlash, if any, is very tiny and not a factor and that the machine repeatability is very good. Even the inside cuts - no new material removed. If it kept removing material then I would just run the finish cut twice but that also tells me there's no deflection on the finish cut. But I didn't expect any, after all I'm only removing 0.005" in the finishing pass so I doubt the machine would deflect on a cut that light in softer material.

Did you see the Command Chief Stripes plaque I did last week? That's what brought all this up; I have another one to do and the inserts or inlay pieces didn't fit precisely on the first one. I want to avoid the handwork again because I'll have more of these to do. I also want to do inlay work on guitars, like the one I just finished in the photo below, and if the inlay pieces don't fit then I might as well cut them by hand. But that's one reason I built the CNC is to do inlay work.

036 - Command Chief Stripes, Walnut, Maple.jpg

037 - Command Chief Stripes, size comparison.jpg

The fact that the outside cuts are within a couple of thousandths to me is more the softer material I'm using rather than the machine. I may be in denial but I don't want to waste good hardwoods or acrylic until I get this much closer on the inside circles. Then I'll switch to some Walnut or Maple and do similar tests to really fine tune.

David

[Mark Bolton]

Now I want to run one of these and measure lol

[Gary Campbell]

David...
You appear to be falling into the "denial trap". Which means, You are assuming everything that has been accomplished prior was and even worse, is still right on the money. No amount of parameter adjustment will do any good if the machine isn't right and tight. To measure if there is backlash, use a dial indicator affixed to the table and bumping all axes, one at a time.

You also say the bit is ".250 on the nose" Did you measure the slot that the bit was cutting as I instructed, or did you mistakenly measure the bit?

[David Falkner]

David...
You appear to be falling into the "denial trap". Which means, You are assuming everything that has been accomplished prior was and even worse, is still right on the money. No amount of parameter adjustment will do any good if the machine isn't right and tight. To measure if there is backlash, use a dial indicator affixed to the table and bumping all axes, one at a time.

You also say the bit is ".250 on the nose" Did you measure the slot that the bit was cutting as I instructed, or did you mistakenly measure the bit?

Haha! Denial trap is an easy one in which to fall, Gary! Part of that is because this is Sunday and I've been at church most of the day, no time to get out to the shop. And part of that is because the repeatability is so good. So to check backlash I can certainly affix a dial indicator to the table but what am I looking for when I bump each axis? I assume it should be run very slow and just bump it... I guess a better question is can you point me to a good procedure for checking backlash?

No, I only measured the bit itself. Again, because I've been at church most of the day. I'll have to do that tomorrow and see what size slot it cuts.

Thanks!
David