Accurate drilling in a lathe (Or mill)?

Philipintexas · Jul 25, 2012

Something we've all done hundreds of times BUT, I've always wondered, and have never been told, what is the correct, most accurate way to drill a hole?
I usually use a center drill to start the hole and switch to the drill I need but, should I just start an indention, drill the length of the center-drill, maybe even partially countersink? Sometimes the next size drill-bit doesn't seem to be centered accurately on the previous hole. What is the correct way to get an accurate, properly sized hole assuming it requires several drill-bits?

tornitore45 · Jul 25, 2012

A twist drill bit is not a precision tool that guarantees a round hole of the right size on location size.
If a drilled hole is all you want a center drill or spotter drill leaving a cone wider than the pilot drill is a good start followed by a pilot hole as least as large of the web of the next drill.
Next drill undersize and finish with the right size bit.

Next level of refinement same as above but ream the last 1/64th diameter.

Best accuracy is a bored hole on location, round and accurately sized.

Center ponching is unnecessary, find edge with edge finder, locate by dials and bore away after an initial drilling.

Tin Falcon · Jul 25, 2012

First of all a caravat. hole drilling is a roughing operation. but often good enough for many applications such as tap drilling and clearance holes for fasteners.
IMHO the first step in an accurately located hole is a accurately symmetrical drill bit.

Assuming the drill is located where you want it in either the mill or lathe follow the above instructions.
if you are picking up a hole location from a layout the first step is the scibed line then prick punch , then center punch , 1/32 drill bit ,3/32 drill bit , center drill, center drill up to its shoulder , pilot drill then to size. Re Machine shop Practice ,K.H. Moltrecht pg411.
there are many methods referencing hole location . edge finder. pump center. tooling buttons. transfer buttons, transfer punches etc.
Tin

kvom · Jul 25, 2012

tornitore45 said:
A twist drill bit is not a precision tool that guarantees a round hole of the right size on location size.
If a drilled hole is all you want a center drill or spotter drill leaving a cone wider than the pilot drill is a good start followed by a pilot hole as least as large of the web of the next drill.
Next drill undersize and finish with the right size bit.

Next level of refinement same as above but ream the last 1/64th diameter.

Best accuracy is a bored hole on location, round and accurately sized.

Center ponching is unnecessary, find edge with edge finder, locate by dials and bore away after an initial drilling.

I'd agree with everything in this quote, except that when the hole is larger than 1/4" leave 1/32 for reaming rather than 1/64.

A spotting drill will be much stiffer than a center drill and less likely to deflect on contact (and less likely to break off the tip).

steamer · Jul 25, 2012

If your worried about location and straightness...I would drill undersize, bore to .010 under with a single point boring bar to correct the drill wander and then ream to size.

Drills just make non round non straight holes in metal...but don't hold size or geometry well..and reamers follow the hole you put them in

Dave

njl · Jul 26, 2012

If your having bother starting the hole exactly where you want it to be. I made myself a little stylus with a long tapper to a very fine point. I chuck this first and then I can lower the point and move the mill table to point the stylus to exactly where I want the hole to start. Then I raise the quill, remove the stylus, chuck the drill bit and start drilling. I find it is much easier to center the stylus accurately on a pop mark than it is to center a small drill bit or a milling cutter.

terrywerm · Jul 26, 2012

I have done the stylus trick, and find that it works quite well. The only difference is that my stylus was 1/2" in diameter, made from a piece of drill rod about 6" long. Put it in a collet in the lathe and turned a 30 degree taper near the end, finishing in a fine point. The larger diameter made the entire piece more resistant to accidental bending.

Philipintexas · Jul 27, 2012

I appreciate receiving the benefit of more experienced guys, I tried using some of the ideas today and liked the result. Never tried centerdrilling deep enough to produce a countersink, but it really helped center a drill bit I know is slightly bent.
On the idea of a stylus, I have this and find it's often easier to lower it into a centerpunch hole, apply enough pressure to hold the part and then clamp the part instead of trying to bring the part to the fixed stylus.
Again, thanks for the input.

Accurate drilling in a lathe (Or mill)?

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