Hi group, I want to make a 1" BSPT taper pipe thread tap. Okay I can
cut the pitch on my lathe okay and that would be fine for a straight
thread but is there any way of cutting a tapered thread without a
taper turning attachment?
Thanks in advance.
"is there any way of cutting a tapered thread without a
taper turning attachment?"
" Set the compound at the desired taper angle and
make "x" movements with it."
Thats am impressive sounding tactic Bob.
Id enjoy watching it done.
That procedure is used for omnidirectional variable pitch threading.
You have to cut the full thread depth in a single pass.
It might be possible on a CNC lathe or a chucker with power
feed available to the compound.
In my youth I saw such a setup using a drive from the end of the lead screw
that ran through gears above the bed to tailstock height. A long drive
shaft with universal joints and a sliding joint in the middle put drive to
where the compound handle had been removed.
It was a special setup put on one lathe and used for the single
You can chase the tapered thread by running between centers and offsetting
the tailstock such that you get ¾" taper per foot. On short items that
isn't much of a problem, but it can be challenging to get enough movement of
the tailstock on longer items.
Your only other options if you don't have a CNC would be to use a rapid
threading attachment, or a tracer. A special stylus is required for
threading when using the tracer.
I haven't tried it, so I'm not sure if it's effective or practical, but it's
been mentioned here before that a tap of the same thread pitch (even if it's
not the same diameter or tapered) can be used like a form cutter.
If the tap were held at an angle to the lathe's center axis to match the
desired taper(and supported to keep from snapping), then the correct feed
would move the side of the tap into the workpiece as it advances toward the
If you had the appropriate cutters from the threading die, this
probably/might've already occurred to you.
Probably would be difficult with a wimpy lathe though.
Hmm ... I don't think that he is going to be cutting threads
with the compound, unless there is some interesting linkage to the hand
As for the offset -- it would be pretty close, but it would
introduce some slight error into the thread pitch, as you would be
cutting the length on the angle instead of parallel to the axis of the
pipe. The error would probably be little enough to not be a problem, as
the angle is fairly shallow and the length of the thread is fairly
But -- really, other than the taper turning attachment, perhaps
a pipe threading die would be the other way to get a perfect thread -- or
close enough, at least.
Because it's not a normal tap but part of a special threading set up
and will have a hole right through the centre and a hole through the
driving shank to drive it.
Thanks everyone for the ideas. I dont have CNC, a tap or die of the
correct pitch, no way could I use the compound so it's the tail stock
idea or buying a tap and getting the holes spark eroded.
Thanks for the offer but I am 12,000 miles away in sunny Australia
enjoying beautiful summer days. Short sleaved shirts and shorts are
the order of the day so you will understand that I want to stay away
from the freezing northern hemisphere at the moment.
As a matter of interest how much would it cost?
While I haven't made a tapered tap, I have made a tap
and I have made a tapered thread. The thread application was a on-of so
I wasn't particularly concerned about how long it took.
I always thread away from the headstock as that let's me just run off
the end of the work piece. I use the usual compound at 29.5 degrees for
advancing the cut and zero the cross feed so I can make a cut, move the
cutter out of the way, run back to the starting point, re-set the cross
feed, advance the cut and make another pass.
For the pipe thread, I simply started half a turn further out on each
pass and advanced the cut by the calculated amount (e.g. 0.0015" for a
20tpi thread). On completion I dressed the thread very lightly with a
triangular file and put the piece in service. No leaks.