How best to cut a hole in CNC.

[cameraman32]

I've just graduated to CNC, having retrofitted a cheap Rong Fu machine with motors, ballscrews, etc. And I'm getting quite familiar with Gcode and Mach3. It took a while to figure out that the Taiwan inch for the Z axis is only a metric approximate of a real inch, but I digress

In the part i'm building, I need to first put a 3" diameter hole in the middle of a 1" thick bar of aluminum. Before the CNC I would have chucked it up in the Lathe and bored it out, but now i've got to take a new approach, and since there are so many ways to program a hole I thought I'd ask this forum.

First off, it needs to be done with a 1/2" flat endmill.

The first time I programmed it, I would cut down 1/8" and make little circles in the center going progressivley outward continuously cutting. This worked well for cutting, but the chips would fall into my pocket.

Now i'm making a series of holes that go all the way through, starting small and slowly widening the hole, cutting only with the edge of the mill. This takes much longer.

I'm now wondering if I should cut a series of holes like I am now, but in a helical motion instead of in stair steps. I'm a little nervous about using the bottom of the endmill to cut down at the same time as around, but it would be a big timesaver. I'm curious how this would affect tool wear, since I'm using the bottom of the same bit to face off tops of the part in a finish pass.

Any thoughts would be appreciate. Thanks!

Stewart

[holbieone]

you can't push those little machines

you may want to cut a plug out first then come in with some finish passes

use G03 with a bottom cutting end mill

plunge .05 down and cut a circle with a wall tolerance of -.025

repeat the process until you cut the plug out

then get rid of the wall tolerance and run it again to size of pocket

use a shop-vac to remove the chips (don't use compress air, the chips will end up in places you don't want them)

[philbur]

I would:

Using a drill bit drill one large diameter hole in the center, for the chips to drop through. Then to open it to 3" use an end-mill together with the circular pocketing Wizard in Mach3.

Regards
Phil

Originally Posted by cameraman32 I've just graduated to CNC, having retrofitted a cheap Rong Fu machine with motors, ballscrews, etc. And I'm getting quite familiar with Gcode and Mach3. It took a while to figure out that the Taiwan inch for the Z axis is only a metric approximate of a real inch, but I digress In the part i'm building, I need to first put a 3" diameter hole in the middle of a 1" thick bar of aluminum. Before the CNC I would have chucked it up in the Lathe and bored it out, but now i've got to take a new approach, and since there are so many ways to program a hole I thought I'd ask this forum. First off, it needs to be done with a 1/2" flat endmill. The first time I programmed it, I would cut down 1/8" and make little circles in the center going progressivley outward continuously cutting. This worked well for cutting, but the chips would fall into my pocket. Now i'm making a series of holes that go all the way through, starting small and slowly widening the hole, cutting only with the edge of the mill. This takes much longer. I'm now wondering if I should cut a series of holes like I am now, but in a helical motion instead of in stair steps. I'm a little nervous about using the bottom of the endmill to cut down at the same time as around, but it would be a big timesaver. I'm curious how this would affect tool wear, since I'm using the bottom of the same bit to face off tops of the part in a finish pass. Any thoughts would be appreciate. Thanks! Stewart

[cameraman32]

Good ideas. I had previously used the circular pocket wizard, after boring a 1" hole for the chips to fall, but I found that the chips didn't fall well when the pockets got larger than 1.5".

Using a shop vac is a good idea to clear the chips, but i'm trying to avoid constant attendance of the machine when making the parts.

I really like the plug idea, that would save a lot of machining. I've been nervous about cutting plugs out because I've thought they might get jammed between the bit and the inside wall of the piece before falling through. Is this a good practice?

Thanks,
stew

[philbur]

If you are milling 3" holes in 1" plates you are going to very quickly have a pretty good pile of chips anyway. So without a vac you may need to frequently clear chips in any case. I recently milled an 83 mm hole in 30 mm aluminium plate and I was producing chips faster than I could manually clear them from under the the workpiece.

Regards
Phil

Originally Posted by cameraman32 Good ideas. I had previously used the circular pocket wizard, after boring a 1" hole for the chips to fall, but I found that the chips didn't fall well when the pockets got larger than 1.5". Using a shop vac is a good idea to clear the chips, but i'm trying to avoid constant attendance of the machine when making the parts. I really like the plug idea, that would save a lot of machining. I've been nervous about cutting plugs out because I've thought they might get jammed between the bit and the inside wall of the piece before falling through. Is this a good practice? Thanks, stew

[cameraman32]

Good Point. Maybe i can make a bracket to hold the shop vac nozzle near the part and plug it into the "spindle on" relay controlled by Mach3 and program in some periodic chip vaccuming.

thanks!

[S_J_H]

I really like the plug idea, that would save a lot of machining. I've been nervous about cutting plugs out because I've thought they might get jammed between the bit and the inside wall of the piece before falling through. Is this a good practice?

I first predrill a hole for end mill clearance. The predilled hole is larger than the end mill you will use to cut the part and large enough to allow for lead in/out. The clearance hole should be close to the outer edge of the larger plug you will cut out.
Start and stop each cut pass inside that predrilled hole.
Then the plug will just drop out and not bind up with the cutter at the end.
I have done small holes and up to 6" in diameter and it seems to work pretty well with this method.

Steve