Re: Speed of Tapping Head? - tap styles

John,

Yes, for a thread-forming tap the drill size is larger, slightly over
the thread's pitch diameter. Manufacturers of the taps have charts.
If you can't find a chart start with the upper pitch diameter limit
and work up.

The actual hole size is more sensitive than for a cutting tap since a
drill that runs a few thousands oversize will give a threaded hole
with a significantly lower percent of thread. We start with a chart
value and run a test piece to make sure the thread percent is
acceptable. 65% is about what we shoot for, higher for thin or soft
material. If you try to go 75% and beyond your chances of tap
breakage go up exponentially. 65% is a good number since the thread
is cold formed and probably stronger than a 75% cut thread.

Getting the correct drill size is a problem. For instance, 1/4-20,
the recommendation is a #1 drill. I find that's a little on the big
side. #2 is a bit too small so we use the next smaller commonly
available drill which is a metric one. In some rare cases, to get
the correct hole size, it has to be reamed to exact size.

The taps work best at speeds way beyond those of cutting taps and the
torque needed can be high. An extreme-pressure additive cutting
fluid is a good idea. Try form tapping 1/2-13 in stainless. The
torque value is so high it's beyond most people's equipment.

The taps will last almost forever is used correctly. My record is
near 5000 blind holes 3/4" deep in leaded steel (finally broke that
one, didn't set tool offset correctly).

Doug

--- In CAD_CAM_EDM_DRO@y..., "jmkasunich" <jmkasunich@y...> wrote:

>
> I've been thinking about trying thread-forming taps.
> I read somewhere that you have to use larger tap drills
> with forming taps to make up for the fact that the tap
> doesn't remove any metal. How do I select the right tap
> drill size - info for thread-forming taps isn't in
> Machinery's Handbook or any of the other books I have.
>
> John Kasunich