What is the best technique for getting the most accurate, round holes using a CNC end mill? (I cannot get my spindle slow enough for straight drilling, minimum is 10k rpm, so I am milling them.)
I am using my home-built 3-axis CNC router (gantry style mill) to bore some holes in aluminum, so that steel pins can be inserted and used to accurately position the stock for 2-sided ops.
I am finding that the holes are not coming out exactly round -- they are slightly ovalized, but not purely in X or Y. The long axis of the holes are at approximately NE / SW, maybe biased more towards Y (north/south). They also seem to be slightly tapered, with a smaller diameter at the bottom of the hole.
I've set up the toolpath to use a .125" 2-flute cutter to helicaly bore a .253" diameter hole, .350" deep, with a pitch of .020". According to my harbor freight calipers the pins are .248" diameter and 1.000" long. I'm using 15,000 rpm, 45 inches/min, which results in .0015" IPT.
On the first attempts, I tried climb-boring the holes using 2 passes with a .020" stepover, with a 3rd spring-pass. On the spring pass you can still hear it cutting, mostly as it gets to the bottom of the hole. Result: The pins go into the holes, but as the pins go further in, its more of a press-fit. There is a slight amount of play in the NE/SW direction, but tight in NW/SE.
On the 2nd attempts, I changed the toolpaths to do the first climb-boring pass leaving .006" of stock. and the 2nd climb-boring pass clearing that at the same speed. No spring pass. Result: Pins would not go in, but you could still see that the holes were ovalized in NE/SW. I could hammer them in, but that's not what I want.
This seems like the cutter is probably being pulled into the material at NE/SW due to different machine stiffness in X and Y. Or maybe its being deflected at NW/SE. Is there a different technique that would hopefully result in rounder, more accurate holes? Do I need to do a conventional pass and then a climb pass? Do I need to go faster to account for chip thinning? Slower to reduce deflection? Higher RPM? Full-depth finish pass? Please steer me in the right direction.
Other notes:
- I have checked the squareness of the X gantry to the Y rails by boring 3 holes a few inches apart in a triangular pattern, and pounding pins into them. Then, placing my machinist square on the pins, and checking for gaps with feeler gauges. A .0015" feeler (smallest I have) will not go between the square and any pin. It's as square as I can get it.
- I have trammed my spindle as good as I can get it using a dial-test indicator mounted in the spindle. It's within .002" over a ~4" span in both directions
- My spoilboard has been faced-milled to machine-level. No detectable sawtooth or trough pattern, which confirms tramming is good.
- I have adjusted the Z axis rails perpendicularity to the bed, by using the same dial-test indicator and running it up and down on the machinist square. It's within .003" over the entire 4" Z travel. Difficult to adjust and requires removing the carriage, re-assembling, and re-checking.
- There is no detectable backlash in any axis, checked with a dial indicator. It might have .001", hard to tell, the dial indicator was cheap. My machine has leadscrews in all axes, with delrin anti backlash leadnuts, and each leadscrew is clamped in a 2-bearing setup at the end opposite the motors, using delrin threadclamps against the bearings.
- Steps/unit is spot on. Calculated to be exactly 10,000 in all axes, with the microstepping and leadscrew pitch that I have. Checking it with a tape measure and a V-bit over 22" in X and 7" in Y confirms it.
- I tried turning on backlash compensation in Mach4 anyway, up to .005" in the Y axis, and it seemed to reduce the ovalization, but did not eliminate the it. I am hesitant to go any higher with that setting out of concern that it may cause other milled features to be inaccurate, especially in softer materials.