Subject: deep hole question
From: "Robin S."
Date: 18/06/04 02:49 GMT Daylight Time
Message-id:
"Dave Baker" wrote in message
...
No one suggested predrilling with a smaller diameter drill.
Probably because it's usually a very bad idea.
That's an aggressive posture.
Not really. It just is a very bad idea. A 1/16" drill bends like buggery, they
snap like carrots and they only have the flute length for an inch or so of
drilling. It isn't the tool for starting a 5mm hole 2 inches deep. When I need
a second opinion on a machining matter I ask my mate who's a 30 year CNC
machinist making high volume complex items. We've looked at accurate hole
drilling many times. The answers are mainly common sense. Use the shortest
drill you can. Stub drills are stiffer than jobber length ones. Carbide stub
drills are stiffer than HSS ones. Slot drills are better than either. Centre
the hole first. Use the highest quality drills you can afford. Use the biggest
drill you can get away with if you're double drilling.
Rarely (never?) in machining are things black and white.
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.
Again, posturing. What's with the attitude?
Moi? Attitude ?
The 1/16" hole doesn't need to go all the way through, although if it does
it probably won't hurt. Once the drill has entered the material and is going
reasonably straight, it should hold its course. Mind you, all of this is up
in the air anyway. If he wants to put a 13/64" hardened shaft through the
hole, it almost certainly won't work as there will be some wander anyway.
But we've only estabilished that he's looking for *less* wander than he had
before.
What he is looking for is as good a job as he can get. Same as anyone else. The
advice should be to get him spot on if possible.
There is one good reason for double drilling and that's to get good
tolerance
on hole size.
Perhaps in a pinch. I'd be inclined to order the reamer, if you really want
to get technical. But in this instance, we're not getting that technical.
If you're after tenths then sure a reamer is essential. Double drilling can get
you within half a thou thou or a thou with a good quality drill and the right
technique.
In fact, lets reafirm our objectives he "Home shop machinist" Bill
Chernoff doesn't appriciate that his 13/64" drill wanders .020" through a
piece of 2" thick 6061 aluminum. He hasn't stated his feeds, speeds, machine
he's using, amount of experience he has, other situations that would lead
him to make assumptions, etc.
The above will help us later.
Putting a very small starting hole in a job is a no no.
I work by 15-30% (for a new drill the web is typically about 15% of the
diameter).
Don't understand that comment.
My solution basically mirrors a previous suggestion. Start by aligning the
machine as accurately as possible.
How accurately? Do you see notes on drawings saying "as accurately as
possible"?
Yep - In the UK it was deemed to mean within 1 thou in the celebrated law case
Arkell V Pressdram (1971).
Yes, I'm being a jackass. As you can see it's about posturing.
Ensure your drill chuck has no runout.
It certainly sounds like we have a profesisonal journeyman machinist here. I
should like to see this drill chuck, and I should like to pay very little
for it because Bill Chernoff probably doesn't want to spend $400+ on such a
chuck.
Remember our reafirmation of objectives above?
Jackass? Yes. It's all about posturing.
Make
a countersink to just over the finished hole size with a centre drill.
A countersink with a center drill? You have to choose.
Can't see your problem here. A countersink is what a centre drill is designed
to make.
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.
100RPM, .0003/rev? That's slow.
On a 5mm drill I'd say 250 rpm for a slow second pass.
Peck lightly and don't press the drill hard enough to bend it.
Do you frequently feel your drills bend?
No - I use mine with careful feel and technique.
Use a brand new
drill.
Most HSM's have unlimited budgets and time to buy new tooling whenever
required.
Struth. A 5mm HSS drill costs what? a dollar or two?
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.
Perhaps you need to resharpen more drills.
I've resharpened **** loads of em and never managed to get them as nice as a
brand new high quality one. That's even using a drill sharpening fixture. If
that's my own fault so be it but if I can't do it then so can't most other
people.
I can see the necessity for new tooling in a production setting but for the
HSM and frequently in tool shops, one drill could be sharpened ten times
before it shatters and another one is used.
A carbide drill has a higher modulus of elasticity than a steel one i.e.
it
bends less.
Ha! That's certainly something Bill Chernoff should go and buy tomorrow
morning. A carbide drill!
I'm not advocating that everyone needs to use carbide drills. Just pointing out
the physics of the thing. I only have a few myself and they were given to me by
a friend who'd got them free from someone else. When I'm drilling really tough
materials and need good concentricity (aluminium bronze valve guides) I start
with a 5mm carbide drill and then finish with the 8mm HSS one. Advantage is the
carbide drill never wears and stays sharp and takes the bulk of the material
out. The 8mm one follows what's there.
Still, a 5mm carbide drill isn't mega bucks either if the job merits it.
--
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 ****.