[watsonstudios]

This is a question geared towards building my custom CNC machine. What's the best way to drill large holes in aluminum (3/4" 1" etc.) using a 12" benchtop drill press, 1/2in. chuck? I'd like to do as much as I can myself so I don't have to go to a machine shop. The larger holes would be for bearings, motor mounts etc.

[Geof]

It depends a lot on the thickness of the material you are drilling. It is not likely you will get a nice 1" hole through 1/2" material. The reason for this is that the point of the drill breaks through before the full diameter is deep enough in the material to get support from the lands on the drill and run true. So it bangs and bumps and grabs and you finish up with a very non-round hole. And if you have not clamped your part very tightly badly bruised fingers or worse.

What to do? Make your workpiece thicker; bolt or clamp pieces of scrap on either side of your part so the drill has to pass through three thickness of material. This means it is cutting full diameter in the top piece before it starts to even enter your part. Then it has completely finished the hole through your part before the tip starts to exit the bottom piece. Actually you stop as soon as the full diameter is through your part.

Don't be shy about drilling extra holes in your part and the scrap to bolt the pieces topgether tightly. The most important thing is to get a good hole in the correct place; a few extra holes floating round are no problem.

Also never, never, never drill a pilot hole,at least not all the way through. It may be helpful to spot drill a shallow hole so the big drill does not wander but if you take it too deep the big drill will tend to grab and jump around.

You mention bearings so you need accurately sized and positioned holes; this is pretty well impossible just using a drill. Over a year ago I did a demonstrations showing how it is possible to bore a hole just using a drill press and get an accuracy of around 0.001".

http://www.cnczone.com/forums/showthread.php?t=16696

This takes a bit of fiddling and practice but it is doable.

Alternatively you could try 'potting' your bearings in metal filled epoxy.

Drill a slightly oversize hole in your bearing mounting plate and also drill a couple of small bolt holes adjacent.

Take another piece of material and drill a hole that is a suitable size to bolt your bearing to. This is your bearing potting plate.

Put the bearing in the correct place in the oversize hole and drill holes in the potting plate that line up with the holes in the mounting plate.

Bolt the potting plate firmly to the bearing mounting plate; now your bearing is positioned correctly in the mounting plate.

Trowel metal filled epoxy into the gap between the bearing and the hole. Don't trowel it into the bearing!!! It won't spin very well.

When the epoxy has cured you have your bearing firmly mounted. For extra security you could make side plates that just overlap the outer race of the bearing so preventing sideways movement does not just rely on the epoxy grip.

[Newby2]

Ever tried a Unibit? They seem to drill an extreemly accurate size, I have one that goes to 1/2" and another that goes to 3/4".
With a drill press, you may get results better than what you expect...these bits are not cheap but are well worth the money.
Steve

[mxtras]

Very true - the Unibits work very well but I have only seen them for thinner materials - the steps are not "tall" enough to let you drill one size through a thick plate on the Unibits I have. Not to mention, the resultant hole is not accurate enough for a bearing - but they are a good way to get a large hole in something with a smallish drill press.

Good write-ups, Geoff! (this one and the other one you linked to)

Scott

[watsonstudios]

Thanks for the info guys - I'm still unsure if I'm going to attempt this myself but we'll see. Thanks again.

[Mitsui Seiki]

Try to first drill the hole to 0,7", do a circular interpolation up to 0,95".
The fine bore it up to 1".

[Geof]

Try to first drill the hole to 0,7", do a circular interpolation up to 0,95".
The fine bore it up to 1".

Interpolation is really tricky on a 12" drill press. I find I can't hold my hands steady enough .

[Mitsui Seiki]

Oops,I guess I didn't read your message well enough.
Sorry

[Zumba]

It can't be done well on a drill press. You need a quality mill or lathe.

Drill bits don't drill round holes. Even if they did, the diameter would be wrong. A bearing that wobbles in its housing is useless. The bore housing tolerance for an ABEC 1 and 3 bearings (1" diameter) is typically around +0, -.0004". Larger bearings have looser tolerances, around -.0007" for a 6" bearing.

Best bet is to either get a machine shop to do it or get a quality mill and Criterion boring head.

[Mcgyver]

why not drill the hole 1/64 undersized and ream?

[Newby2]

Loctite makes a retaining compound, I believe RC630, that is used to mount bearings in a bore that is up to .015" oversize. It was made to take up the slop of a bearing in a bore.
Steve

[mugabe]

Alternatively you could try 'potting' your bearings in metal filled epoxy.

Drill a slightly oversize hole in your bearing mounting plate and also drill a couple of small bolt holes adjacent.

Take another piece of material and drill a hole that is a suitable size to bolt your bearing to. This is your bearing potting plate.

Put the bearing in the correct place in the oversize hole and drill holes in the potting plate that line up with the holes in the mounting plate.

Bolt the potting plate firmly to the bearing mounting plate; now your bearing is positioned correctly in the mounting plate.

Trowel metal filled epoxy into the gap between the bearing and the hole. Don't trowel it into the bearing!!! It won't spin very well.

I guess if there will be 2 side holes made straight to bearing hole, after mounting and proper positioning of bearing , epoxy can be injected into holes.

[Geof]

I guess if there will be 2 side holes made straight to bearing hole, after mounting and proper positioning of bearing , epoxy can be injected into holes.

Yes that would be much neater than spreading it all over the place by my method . You can get epoxy dispenser that mix and squirt the stuff out of two syringes through a tiny nozzle.

[kellogs72]

You could try a one inch annular cutter with a annular cutter holder that has a 1/2" shank and use your low speed on press. Other possibility is a deming drill that has a 1/2" shank. Again, very slow speed.

[digits]

Have you already bought all of your bearings yet? If not, you might be able to get some precision flanged bearing mounts that simply screw down to your plate. I am using them in my new machine, and while expensive, they do allow me to get away with the really ugly holes my X-1 interpollates. As long as the plate is flat and the bore hole is large enough even where it's not quite round, the bearings mount perfectly as long as their 4 securing screw holes are spot on.