On Sun, 19 Mar 2017 18:38:43 -0400, Bob Engelhardt
Form taps make a stronger thread in materials that are fairly ductile.
So for example, wrought aluminum aloys, mild and stainless steels, and
some copper alloys. It's a little scary but I have form tapped
thousands of 10-32 holes in 304 SS. You need to pay attention to how
many holes have been tapped so that the tap can be changed before the
torque gets too high and the tap breaks. Free machining brass is not a
good material to form tap because it is kind of brittle, but silicon
bronze form taps well. When tapping blind holes form taps are nice
because there are no chips to worry about. They are especially good
when the max length thread is needed in a blind hole. I use form taps
as much as possible in my shop.
Dunno, but I've retired several #10-32 spiral point taps after 1000
holes tapped freehand with a Milwaukee reversible drill in 12 Ga CRS.
They were the panel mounting holes in custom welded relay-style racks,
~ 60 panel mounting holes per rail so 8 bays per tap.
I got a 6061 aluminum part back that the shop had form tapped in holes
that were apparently too large, so the tap left a second false thread
groove between the real ones that a screw could engage. They were
small screws, #3 or #4 IIRC, and the torque difference wasn't all that
noticeable at first.
Blind holes is one example, as they don't form chips which need
to be cleared out. Just make sure that the tapping head stops before
they bottom out. :-)
Your workpiece should be a fairly ductile metal, of course.
Forget about tapping hardened tool steel -- but then cutting taps don't
do to well there, either. :-)
I've even used them in Teflon, where the material expands to
grip the screw once it is in place.
Be sure that you use the right hole size. You can look it up in
a table in _Machinery's Handbook_, or in certain machinist's
On Monday, March 20, 2017 at 3:44:35 PM UTC-4, Bob La Londe wrote:
In interviews with many tap manufacturers, I've always heard them emphasize the importance of hole accuracy for thread-forming taps. It sounds unlikely that punching holes would be a reliable way to do it.
We have progressive dies at work that use them. The main advantage there
are no chips to deal with.
The punches are sized per tap requirements. This means we get less
engagement at the bottom (in the punch blowout). The bolts are not under
high stress so we are not worried about striping out.
On 25/03/2017 10:26 AM, firstname.lastname@example.org wrote:
Shouldn't be a big deal. Holes come out pretty much to the punch size,
so likely just need a custom decimal size punch, I'd think.
Pretty sure I watched a YouTube video of a punch press that could
produce a bit of an extrusion, offering more material for threading.
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Ummm ... these are Balax taps and this Balax page shows that a .177
drill will give a 70% thread.
I'm not used to thinking about threads in percentage terms, but 70%
sounds pretty good to me.
On Tue, 21 Mar 2017 00:58:18 -0400, Bob Engelhardt
70% will be plenty strong enough. If tapping punched holes in fairly
thin material you can get away with a slightly undersized hole because
the metal will just extrude away from the material surfaces. Remember
that a high pressure cutting fluid is good to use because the thread
is being formed.
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