I can try that, I've just run out of ideas and it doesn't help when 2 months ago I knew basically nothing about this.

I just tried one more thing hoping it's more accurate than what I tried before on calibration.  Using the .1181 bit, I set it about .02 into the material, then ran the calibration utility and told X to move 5"
Entered that it moved 5", so I could get access to jog, and moved the gantry out of the way and measured the cut less the width of the bit.  I got 5.108 less .1181 for a travel of 4.9899  I then reran the utility again and entered 4.9899.  I repeated the same process for Y which was a net of 5.009  Even then this isn't off that much.

So I ran your code again and got .168 and .999, at 45 degrees on the outside of the pocket was 1.0

I just ran the gcode you sent again at 15 ipm, 3x faster from the 5 ipm, didn't make any difference other than quicker.

If your axis is .010" out over a 5" distance then that is a long way out in cnc terms and will cause sizes to be inconsistent, you need a 5" command to move 5.000" before anything will work right.

Graham
 

Without having a dial indicator handly available, is the method I used accurate?  My thought was if I measure the cut with my digital calipers, less the dia of the bit, that should get me pretty close?  I orginally thought for what I'm doing it's close enough, but since I'm making really small stuff, it's probably more important to get this right.

It is probably best to calculate rather than try to measure, doing the calculate steps by the button will only ever be as accurate as you can measure where if you calculate by pitch/motor steps etc you should get good results, unless that is the screws are not very accurate and then if that is the case you wont get it accurate by any means.
Hood

Hi.

You could try to increase RPM, 500 is very slow for a .079 end mill.

Just for the fun, try it at your MAX RPM and try to keep material as cool as possible, put some water on the cut and look if it's make any difference.

Jeff

The rpm's means little, I do have a variable speed router, but only 15k to 30k, currently running at 25k.  I've had better results at this speed with chips.

I did the cut and measure and did it a couple of times and seems to be consistent, 5" gets me 5.002 to 5.004
Not sure on the calculate method how it's done. From the vendor, "The screw is 10 threads per inch, the stepper is 200 steps per revolution, you multiply one by the other and you get 2000. The controller can be set at 1/1, 1/2, 1/4, 1/8 and 1/16 step mode. Your controller is set at 1/2 step mode so you multiply 2000 by 2 and you get the the steps per inch"
So this means I'm at 4000 steps per inch?  1 step = .00025  So I just add or subtract however many steps to equal what I'm off?

I must have measured pretty good, and I learned a major lesson and alot about calibration today!  I just recut the file, and I got 1" outer on the pocket and .174 on the inner island!

Close isn't good enough, it has to be correct.  I thank everyone who helped me with this!  I'm going to cut some other stuff and verify that they come out correctly now as well.

Mark

What the H********* is going on??? 

Last night after recalibrating and cut the gcode correctly provided by Graham, I cut a couple others, everything was working great.  Shut everything off, went to bed.  Tonight when I get home, I can't cut anything correct again, not even Grahams gcode I cut last night.  The outside of the pocket was 1.020 all the way around and the center island supposed to be .175 was .153.  So based on my new knowledge, I figure if the circle is off by .018, then I'll manually change the steps in Motor Tuning accordingly.  I should be at .00025 per step, and since the circle is off by .018, I should modify X and Y by that amount, so .018/.00025 = 72.  I subtracted that amount from each X and Y and ran the job again.  Now my circle is 1.000 - 1.001, but the damm island has dropped to .138  How can that be?

Because nothing else makes sense, I went back to the original values and added 72 and recut, the circle as expected was now 1.0028 and the island is .149  I've set it back to what I had last night, but I don't get what is wrong with this setup.  Values are the same as what I had last night when it started cutting correctly.  So what's changing?  Extremely frustrated with this, been screwing around with this problem for a week now.  Everything I read or advice I get makes sense, yet nothing works.  Why can I adjust the outer pocket and yet cannot get the damm island to come out right?

Hi Mark sorry to hear your woes.

No one has mentioned your machine, if there are mechanical issues then that is what is going to determine the accuracy.

This is where a skilled operator on a manual machine comes in he/she can adjust on the fly.

Maybe some pics and specs of your machine would help

Phil

I'm not in the big league, just a home 'desktop' cnc.  I purchased it new from a guy off ebay, for the most part it seems built well enough and I wanted a plug and play system rather than build one, at least for my first system. 

Here's what specs I got:
24"x12"x 3" cutting area
- 282 oz/in steppers on all axis direct driving the lead screw
- 12 Precision linear ball bearings (4 per axis)
- Hardened shafts
- Anti-backlash wear compensating lead nuts
- 3 axis chopper microstepping controller

It came with a Rotozip Z1 router which I've replaced with a Rotozip Z20 because of runout and I wanted variable speed.