Jon wrote:
"John R. Carroll" wrote in
message ...
Ed Huntress wrote:
"Terry Coombs" wrote in message
.. .
Terry Coombs wrote:
I've seen conversations about alum to dissolve broken taps ,
screws , etc in aluminum ... how about in stainless ? Last step
in making an indexable carbide end mill was to tap the screw hole
to 6-32 . Got about a quarter inch of tap broken off in the hole
now , and really don't want to start all over . I'm not sure
just what alloy the SS is , it was a gift , but likely an alloy
used in the medical field , since it was that kind of shop it
came from . We are NOT amused ...
An update ...
I remembered I had purchased a small assortment of solid carbide
drills / mills , HF specials , and all very small . I managed to
face the end of the tap square with a small carbide mill , and only
broke three bits center drilling the tap (all in the .030-.039
range) . Got the tap broken up and removed , reground the end of
the broken one , and promptly broke it again . I've never had good
luck tapping stainless ... and that piece is now back in the
material bin .
Stainless is a bugger to tap. It's gummy; it produces a lot of
friction against a tool; and, because of its poor conductivity, the
hole actually can shrink in diameter as you tap it (or drill it, for
that matter), depending on how much mass surrounds the hole and
constrains expansion, and thus whether local heating at the cut
interface expands the hole or shrinks it.
I hate working with it. But there are some experts around who know
exactly how to handle it.
Roll tapping SS with oil is reliable and removes a lot of the issues
that people confront when working with this material.
As for drilling:
http://www.maford.com/new.asp
I've heard of roll form tapping in stainless steel.
I'm scared, and I'd guess others in here are, as well.
Please share some details.
SS chips are a huge problem when drilling, milling turning or tapping. They
are work hardened and "sticky". They do the same thing to a cutting tool
that they do to human flesh - slice it up. Ever notice how easily Stainless
chips cut your hands? It's because the edges are work hardened and don't
round off from the heat the way tool steels will.
300 series Stainless (Invar as well) is extremly ductile and form taps are
very strong by their very nature. The flutes are very shallow.
The coating on the tap is important.
ALTiN - C20 is a coating specifically developed for stainless.
It's different than the C-30 grade and if you buy OSG coated form taps, for
example, be sure and tell your tool vendor that you are going to be tapping
SS or A286 with them and that you want them coated appropriately.The C-20/30
desognation is brand specific and I don't know the chemistry.
Hole preparation is critical.
First, carbide drills in Stainless really aren't that useful without being
properly coated. They both gall and chip and the edge also isn't really
sharp. Cobalt is best unless you want to buy something like the MA Ford
drills I linked to and the drill geometry I've had the best luck with is a
142 degree split point.Titex and Guerhing make similar products and they are
terrific. They last a long long time and make a nice clean hole. They are
also pretty expensive. A tap drill for a 1/4-28 form tap from Ford in their
XT series in 80 bucks or so. Cobalt is about $15.00, maybe less.
The hole should be to the proper size, depth, and be STRAIGHT.
You want the hole to be on location so that when you tap, you are entering
with a balanced load. Otherwise you rub one side of the hole hard and things
go down hill from there - the tap will push towards the soft side of the
hole and bind.
I'll normally spot drill - not center drill - with a 90 degree tool to a
diameter that's ten percent larger than the OD of the tap. That is important
also. Don't try and tap a hole in any material without chamfering the hole
first.
I've done it in sheet steel, used to production tap electrical boxes.
Thousands of them.
I've heard it is great in Aluminum, but I've never tried it.
The only reason to use a form or roll tap in Aluminum is chip control and
that's a very good reason.
Cut taps, except for taps like the OSG chip extracting type, force the chip
in front of them and pack the hole.
You have to go slow enough to allow for proper chip formation and evacuation
and you must also be certain not to approach the bottom of the hole to
closely. The problem with the chip extracting tap geometry is that it is
weak dur to the deep fluting required to force the chips up the tap. I use
them a lot in aluminum because I perfer cut threads to formed but my real
preference is to thread mill unles there is either a requirement or
specification precluding it.
Ther - metalworking. Bet you thought I couldn't do it!
LOL
--
John R. Carroll
www.machiningsolution.com