Subject: deep hole question
From: "Robin S."
Date: 18/06/04 00:51 GMT Daylight Time
Message-id:


"Bill Chernoff" wrote in message
news:%q3Ac.732915$Pk3.3341@pd7tw1no...
I am drilling 13/64 holes 2 inches deep in 6061. The bottom of the hole
wanders as as much as 0.020-0.030 off center. What is the best
straightness
I can hope for on a hole that size, that deep?

Any drill sharpening, etc, hints?

As others have said, center or spot drill first.

No one suggested predrilling with a smaller diameter drill.

Probably because it's usually a very bad idea.

Something like a
1/16" drill would be good.

The thinner the drill the more it flexes and wanders around. Once you have a
hole that's on the **** the larger drill tends to follow it and the job is
****ed. If you seriously think you can drill an accurate 2 inch deep hole with
a 1/16" drill then I have to suggest you've never tried it.

There is one good reason for double drilling and that's to get good tolerance
on hole size. You drill once at about 0.2mm to 0.5mm undersize and then do the
final pass at a slow speed and feed with the finishing drill which will then
cut pretty much to size.

Putting a very small starting hole in a job is a no no.

My solution basically mirrors a previous suggestion. Start by aligning the
machine as accurately as possible. Ensure your drill chuck has no runout. Make
a countersink to just over the finished hole size with a centre drill. Go as
deep as you can with a 2 flute slot drill (milling cutter) in a collet that's a
tad under finished size. You'll get almost zero wander with a slot drill. Then
either single or double drill to final size at a slow speed and feed.

Peck lightly and don't press the drill hard enough to bend it. Use a brand new
drill. I find you never achieve such evenness of flute width and accuracy of
cut on a resharpened drill as you do with a new high quality one.

A carbide drill has a higher modulus of elasticity than a steel one i.e. it
bends less.


--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
Next time someone insults you remember it takes 12 muscles to smile politely
and try to pass it off but only 4 to reach out your arm and slap the ****.