From: Robert Bastow <firstname.lastname@example.org>
Subject: Re: Carbide Drill Wisdom?
Date: Thu, 21 Oct 1999 07:03:15 GMT
Carbide drills are for REALLY hard or abrasive materials Steve..I don't thing
you will have much joy useing them on something as soft as stainless steel.
The reason they chip is that they are too sharp!!
The tendency with a newly sharpened drill (or reamer for that matter) is to grab
and snatch a little bit, until the edge gets a microscopic amount of wear on
it...Kinda like a regular drill snatches in brass.
Normally, with a HSS drill, this doesn't present a problem..the drill is tough
enough to take the microscopic deformations etc without chipping.
Carbide, on the other hand, just isn't. The only way it can "Give" is to chip or
snap. You will find the same thing happens with carbide milling cutters too!
If you are in a position where you simply "Haveta, Gotsta" use those drills you
should try using a MINIMUM center hole...just enough to get the drill running
true. Better still, use a proper centering drill..it is what they are made for.
A "Center Drill" AKA a "Slocombe" drill is properly used for making the center
holes in which your lathe tailstock center runs. It is all the wrong shape for
starting drills!!. They tend to be used for that "bastard" purpose because a)
the name sounds about right, b) there is usually one close at hand. c) they are
short and stiff enough to start reasonably true!! ARRGHH!!
So use a pukkah centering drill..ground to the same point angle as your carbide
drill. You can get those cheap on ebay too..I just stocked up on them.
DO NOT use a pilot drill!! Go to full diameter, run about 150% the speed you
would use for HSS and use a fine feed. Don't let it rub..that will only work
harden the stainless, but don't try and "Horse" it through either..in other
words, use it just as you would a HSS drill, with a bit finer feed, but be more
wary of applying sudden loads.
If you have a quill lock, snug it up. Not so you can't push the drill, but
enough to dampen vibrations..Especially the RADIAL vibrations which are doing
most of the damage here.
Try "Busting" the edge of your new drill with a diamond hone..just a couple of
wipes to take that razor edge off it. It sounds cruel and unusual I know, but
it works like gangbusters in taming the chatter on gundrill bits and chambering
reamers. A "D" type gundrill bit will sing like a lark and fall to pieces on
you until you learn the art of "doctoring" the edge in the way I described.
One trick, maybe not applicable here..but useful to keep up your sleeve, is to
start the drill through a business card!! Obviously it spins with the drill and
only the very tips of the cutting edges protrude through the card. However, the
fibre that is trapped just back of the cutting edge acts as a microscopic
"Traveling steady" for just long enough to get the tool "Settled down" into the
cut, without starting to snatch and chatter.
This also will cure chattering on a morse taper, a rifle chamber, a taper pin or
a taper lede hand reamer..the trick in all these cases is to wrap the reamer in
a single thickness of typing paper and feed in firmly, again, damping the quill
against RADIAL movement.
Steve Lindsay wrote:
> I'm new at using carbide drills. I picked up a bunch of carbide
> twist drills from ebay. They look and are sharpened similar
> to a regular high speed twist drill. I'm drilling 303 stainless
> and am setting up in the turret like thus:
> 1st position is just a stop to help stock in the collet chuck in the right
> 2nd position is carbide .258 drill full depth (1.512")
> 3rd position is carbide .297 drill full depth (1.512")
> 4th position is highspeed steel 9/32 (.281")flat end drill to
> square up bottom of hole full depth (1.512").
> 5th position is 5/16" carbide reamer (the reamers I'm using have a step).
> Ream so the second step is a depth of 1.222" deep
> I am first center drilling in another lathe the end of 303 to help
> the carbide drill center. The center drill I am using
> is 5/16 dia and I'm almost taking it in to where the 5/16" dia starts.
> I then move it to the turret lathe and am having trouble with
> the first carbide drill. The carbide is chipping big
> time. At first I ran the lathe about the speed you would
> for a high speed drill thinking there would be less chatter
> until the drill gets in a ways and then I would speed it
> up. Well I guess it must be chattering. It really didn't sound
> like much of a chatter....about what it would if you were using
> a high speed drill. But I guess it was to much chatter as
> it sure made a mess of the carbide cutting edge. I finally
> just didn't worry about the chipping and speeded up the
> lathe and pushed. It went in pretty good even chipped.
> With the next (3rd) position I ran the speed and feed fast
> right away. It didn't chip this drill much but there are still
> tiny chips on the edge if you look close.
> What am I doing wrong? I'm wondering if I should skip the
> first carbide drill and just go directly to the larger one. Also
> maybe I should use a smaller centerdrill hole so the carbide
> drill doesn't have as much time to chatter around until it hits
> the bottom of the center drill hole. Any ideas?
> What about the 5/16 carbide reamer? Should I be running this
> slow with cutting oil or fast without the cutting oil.
> Thank you for any carbide wisdom!