Drilling Large Holes
I have been drilling many 3/4" to 1 1/4" diameter, 2" to 5" deep holes with the mill and lathe. Some in tool steel and some in mild steel. I am getting chatter unless I drill in small increments at 60 rpm. Everything is tight. Any suggestions on doing this drilling in the most efficient way possible?
Have you tried taking it all at once at about 60 rpm? It works better sometimes. Use a lot of coolant such as the black sulphur thread cutting oil if you can find it. You might drill a pilot hole no larger than 1/4" but I would try it without a pilot first. Rigid set up is important of course. Constant heavy pressure with no let up. Pause just a second to break the chip now and then but don't come out of the hole.
Thanks Cass, I usually drill a 1/2" pilot but that didn't work so well. As I just got some split points, I can try biting it all off at once, sounds like a lot to chew though. Don't generally do production work so I am not very well versed in quickest methods. All education is appriceated.
I start a larger hole with a drill smaller than the finished size. Then I use the largest boring bar I have that will fit (while minimizing the overhang). You will get a better finish this way and I find it much, much faster.
You can eliminate chatter when drilling in a milling machine by using the power feed on the quill. (if the mill is equipped with one) Keep everything wet with dark thread cutting oil.
There's a simple trick for drilling large holes in machines having limited thrust.
Stepping the hole out in small increments is a common technique but problems frequently develop. It often results in the starting hole being "lobed" that is three or five sided. Subsequent drill makes the lobing worse not better. Orbital chatter is another common problem associated with large drilled holes especially in thin work.
Drilling with a 1/32" or 1/16" undersized drill a bit less than a diameter into the work and reaming or boring to the finish drill size gives the finish drill a clean start.
2/3 of the thrust required to feed a drill is needed to force the chisel point into the work. Relieve this load and you have only the feed thrust of the cutting edges to over come. Pre-drill the hole using a drill diameter about the width of the drill's chisel point. This gives the body drill a good start with little chance of lobing to develop. The small pre-drill helps center the finish drill and usually prevents lobing or orbital chatter.
There are hazards associated with stepping the hole diameter out in small increments although conditions may force this expedient on you. By far the biggest hazard is self-feeding where in ductile materials the lip of the drill hooks and tries to follow its own spiral into the metal. This happens too quickly for response by normal reaction times. Usually the drill is snatched from the quill or drill press spindle and the tapered shank's driving tang is mangled. Less often the drill shatters scattering low velocity shrapnel and scaring you half to death.
This tendancy may be eliminated by "backing off" the lip angle. Stone a small flat on the lip of the drill by holding the stone face parallel with the drill's axis. The flat doesn't have to be any wider than the drill's feed rate.
[This message has been edited by Forrest Addy (edited 08-28-2003).]
Thanks all. Tried feeding with no pilot hole, was not happening, tailstock slips. Still got chatter with 1/4" and 1/2" pilots. Chatter was dramatic unless I sped up the machine to the point where I was quite uncomfortable with the required (very agressive) feed. This is the only place I ever seem to have problems with chatter. Perhaps I need to understand the dynamics involved better, so as to avoid it in the future.
You might try a very slow rotation coupled with a moderate to high feed. It may chatter at first then settle in. Does for me.
I suggested drilling the full size with no pilot hole are a very small pilot hole. Forest suggested a very small pilot hole. My suggestion of taking it all at once applies to doing it with a drill press or mill and not a lathe. I think you will have trouble putting enough thrust on the drill with a small lathe tail stock even it the drill tang is locked in and doesn't spin. I run the drill 100 rpm or less. Use the black sulphurized pipe threading oil and a sharp drill. It does take some torque and you need a sharp drill. The comments about the problems of stepping out that Forest mentioned are real. The only stepping out I do is maybe 1/32" or so at the end. When you do that you need the drill in contact with the workpiece when you start the drill to avoid chatter caused by being slightly off center and having some runout of the drill. This is the reason for the drill hanging and spiralling through I think. It is hard to have the rotation speed too slow. Be sure the workpiece is strongly fixtured.
I have drilled quite a few holes at about 1" in steel with my SB9. Using low backgear it runs 40 rpm. I recently drilled a two inch deep hole in HRS at 1" dia in a weight for my telescope. I used a 1/4" pilot, then a 3/4" MT drill in the tailstock and then a 1". No problems at all with chatter. All nice and smooth.
Secret? I use a lubricoolant called Aerosawz. Made by Synthetic Lubricants Inc. I hate the stuff because it is a bitch to clean off the work but it sure works. Comes in a spray can with a tube like WD40. It is designed for power hacksaws, bandsaws and drilling. It sticks like glue, doesn't throw off and is not water soluble.
Another very effective cutting lube for drilling is beeswax. Buy a real beeswax candle, scrape it into the flutes and as the drill heats it melts and lubes, doesn't run away. That is what we always used in aircraft work.
[This message has been edited by Evan (edited 08-29-2003).]
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