Want to give that some more thought, Roger?
Plug taps are generally used when power tapping, where starting the thread
isn't an issue. There is almost no need nor reason to use a taper tap in a
production environment. Fact is, they're pretty much relegated to hand
Through holes are generally tapped with spiral pointed gun taps, and
anything less is a stupid idea. You can tap a hole and be back out of it
in something like 5 seconds by using that kind of tap.
There's lots more to this issue than how simple it appears, depending on
Most of my tapping is performed using a Bridgeport or lathe.
(sometimes an electric drill in control panels).
The most crooked hole I have ever made was with a 5/8 NC taper tap by
hand. I got it a bit crooked and it kept going and going off axis.
My curiosity was peaked by thinking, all these taps cut one thread per
revolution. To some degree, the torque required should be the same
unless the depth of cut is a major factor since Bottom > plug > taper.
Elucidation on the subject will be well received.
On Feb 15, 10:56 am, firstname.lastname@example.org wrote:
I won't claim any expertise here, but I think that is a bad
Each of the taps - unless it is a tap whose taper length exceeds the
thickness of the material - will remove one full thread's worth of
material with every revolution. The torque necessary to remove that
material depends on the amount of material removed, but it also
depends on the width of the cut. A bottoming tap without any taper
might remove all of the material with one tooth. A taper tap with the
taper ground for six threads would cut over at least six teeth, or six
times the number of flutes. I'd have to think about that one a bit -
I suppose the accuracy of the tap would determine how many teeth
actually do any cutting.
I think a long taper tap might be like a taper reamer. You take off
very little material with each turn of a taper reamer but, because of
the long length of cut, it takes a lot of torque to turn.
The problem with bottoming taps is the point load, or load per tooth. It's
dead easy to break a tooth in tough materials---whereas with plug or taper
taps, the chip load per tooth is much lighter. Load on the shank may or may
not be the same. As a result, those of us with experience tend to avoid
bottoming taps, but instead use a modified plug tap that has been somewhat
shortened, and go in a tapped hole to remove just the small amount needed to
establish full depth. The shortened tap usually allows for a couple more
turns. Just like a bottoming tap, they don't start well, but are good for
cleaning out the tapered portion of an existing tapped hole.
It's usually desirable, if you're trying for a full thread depth, to use
taps progressively (when hand tapping), so each tap removes a little more of
the material, yielding less for the bottom tap to remove. That lowers the
You likely realize that there are variables, and what works one time may not
another. Doesn't hurt to have a background in tapping, so you have
experience to call on to address questions that may arise.
Harold and Susan Vordos wrote:
.......those of us with experience tend to avoid
I always did this. Once a coworker asked for some flat-bottom taps. I
told him just to grind off the tip of a spiral point tap leaving two or
three tapered threads and he looked at me like I was an idiot (in a
BTW, isn't that point just for holding the tap between centers for
Allow me to explain my thinking on this, and perhaps you will still disabuse
me of my ignorance, but I will graciously stand corrected if that is the
I was thinking that since the load on each cutting tooth on a taper tap
would be lower then the tool life would be extended. Consider if you will a
circumstance where multiple through holes are to be tapped. For the sake of
the example let us assume that the plate to be tapped is about 1 diameter
A tap would have to be cycled into the hole until the first full thread
cleared the bottom of the hole. If a taper tap had to make 15 complete
revolutions to a plug tap making only 7 complete revolutions then the time
difference per hole would be a little over double. If the tap were running
at 100 rpm, 14 revolutions would be 14/100 of a minute and 30 revolutions
would 30/100 of a minute so the net effect on the cycle time would be 10
seconds, but if by having smaller size chips and longer tool life more
cycles could be squeezed out of the tap before it needed to be replaced,
then those factors might be greater than the cycle time loss.
Also if the quality of the thread is better by taking a smaller bite per
revolution, then that may be a factor also.
About the time I had mastered getting the toothpaste back in the tube, then
Chip load plays little to no role in power tap life, particularly with
modern cutter geometry. Taps such as the spiral pointed variety cut
beautiful threads and will do so for a much longer time than you might
expect, particularly when they're run at proper speed with proper
lubrication, in a tapping head. Material plays a role, but it always does,
regardless of the nature of the tap. If you've ever tapped mild steel
with a spiral pointed tap, using the old Tap Magic (1,1,1, trichlor.), you
may have noticed the shiny surface of the thread flanks. That material is
difficult to machine with a good finish, but those taps accomplish it well.
There is nothing to be gained by using a taper tap under normal
circumstances, and plenty to lose.
A taper tap would see more miles per thread, so, if anything, it would wear
prematurely, especially considering you have to back the tap every quarter
turn, and remove it from the hole to clear chips on a regular basis. Because
the chip accumulates in the flute, where they ball up if you don't clear
them often enough, occasionally resulting in a chipped tooth when you back
the tap, and usually a degraded thread. Unless a thread is very shallow
in depth (1/4 diameter or less), if you attempt power tapping with hand
taps, the chip load usually fills the flute and results in torn threads, or
a broken tap. It gets worse. Assuming you're using a taper tap, when you
go back in the hole, even by hand, you run the risk of screwing up the
threads. One shot tapping with the proper tap, run quickly and withdrawn
even faster, has the potential to produce threads of exceedingly high
If you've never tapped with power, none of this may make much sense, Roger.
Hand tapping is a whole different world. Taper taps are very desirable in
the hand craft world, but, like buggy whips, they've outlived their
usefulness in the modern shop.
On Feb 14, 4:41 pm, email@example.com wrote:
I use spiral-point plug taps exclusively. I think hand-taps are a
*complete* waste of time, even if taping by hand. Spiral-point taps do
not require any reversing until the hole is completely threaded. I
don't see any drawbacks other than taper hand taps are perhaps easier
to square by hand than plug taps.
If it is pre-tapped - use a plug. If there is a bottom hole issue - plug.
If it is a new tap - use a taper and then a plug if needed. Tapers start nice.
Simple as that.
Then, there are different taps that have ? H(number) ? and that indicates
the effective depth of cut or % of cut.....
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
IF the tap is not started perfectly straight to the hole, it will become
increasingly hard to turn as it goes deeper into the hole. Unless the
tap flexes. it will cut progressively deeper threads on one side of the
hole until either the tap breaks or the tape self aligns in the hole.
A tapered tap is easier to start straight in the hole. If you start any
tap straight it will be a lot easier to turn it and not break it off.
Close-----but no cigar. If there's a bottom issue, use a bottom tap.
That's what they're for. Almost no lead, and difficult to use, but
effective for getting that last couple turns. It's nearly impossible to
start a hole with one.
Only by hand--------and never, ever, power tap with hand taps unless the
material in question is very thin. It's an instant recipe for a broken tap.
There is no need for taper taps when power tapping.
No-----it indicates the theoretical pitch diameter of the tapped hole.
Drill size dictates the depth of cut.
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