On 2009-12-02, Stupendous Man wrote:
Am I the only one who has trouble tapping blind 1/4-20 holes? What a pain in
the ass!
I am doing better counting the thread rotations and reversing than setting
the depth stop.
Any tips from tap experts? I have a 500 or so holes to tap.
Machine is a Bridgeport mill with a Tap-Matic 50X

Hmm ... how shallow are the blind holes?

Anyway -- what I do to set a Tap-Matic (of various sizes and
models) is to first measure how deep into the hole the tap will go, and
make a spacer bar to sit on the table as a reference for that depth.

Then, I pull down on the tap holder (with the tap mounted, to
disengage the dog clutch, rotate it a bit and release to let it sit on
top of the teeth of the dog clutch. This is as far down as the tap can
go under power.

Then I feed the spindle down and set the stop so it stops just
before the tip of the tap touches the reference spacer. (I usually do
this on a drill press, not a mill, but the mill probably has a more
precise depth stop.)

Then rotate the tap a little while still at the stop position,
to make sure that it retracts again. If it doesn't, or you can't rotate
it at all, then you probably actually have let the dog clutch re-engage,
so back out and try again. :-)

Now -- the one which I have which is in the size range of the
50X is a "Model A", which does not have the torque limit adjustment, but
which *does* have an adjustment for how far the dog clutch must travel
before it releases -- useful for self starting in shallow blind holes.

So -- first off -- how much chip clearance do you have (if you
are using spiral point (gun) taps?) There must be extra depth in blind
holes to collect the chips.

The spiral *flute* taps will pull the chips up instead of push
them forward, so they may offer better behavior in shallow blind holes.
I've not tried them, as most of my tapping is through holes, so get your
own experience with them and report it to the newsgroup.

And then there are thread forming taps, which produce no chips,
but which need greater torque to drive them. Whether the thread forming
taps will work for you is in part a function of what material you are
tapping.

The really nice thing for normal taps with the heads which have
the torque limit is you start with a new tap, adjust until it just
barely does not slip throughout the tapping cycle, and when it *does*
start slipping again, it is time to move to a new tap -- the one you
have is getting too dull to trust with power tapping.

I hope that this is some help,
DoN.

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