I've never cut any type of threads before on my mini-lathe and I'm about
to embark on a project that will require a few threaded parts.
How difficult is it cutting these types of threads? Ant special things I
need to keep in mind?
According to SomeBody :
Yes -- you need a taper attachment for cutting the tapered (NPT)
thread mentioned in the "Subjet: " header -- as well as needing the
ability to cut the correct number of threads per inch.
Which Mini-Lathe is this, and what thread pitches can it cut?
What diameter pipe are you planning to thread? For any
reasonable length, you need to have a large enough lathe so the pipe
will pass through the spindle, so you can cut the threads close to the
chuck. This sounds out of the reach of the typical Mini-Lathe.
Also -- normal straight threads terminate in a groove to give
you time to disengage the half nuts (if any). Tapered pipe threads can
go on to terminate in the air.
Most people cut pipe threads with dies, which are a lot more
portable than a lathe -- even a Mini-Lathe.
Actually, I think it is possible to do so if one uses one of the NPT thread
chasers that are typically used in die heads. Die heads typically have 4 or
6 chasers and the operator would have to rig up a way to mount one chaser to
present to the work. That might be done by holding it in a QC-type tool
holder or turret tool post.
If you don't have a taper attachment for your mini lathe then the tail
stock has to be used. I don't know if your mini lathe tailstock has
the ability to be set off center. If it does, then moving it toward
you 3/8" per foot will give you the proper taper for NPT threads. The
taper is 3/4" inch per foot on the diameter. Or 1 degree, 47 minutes.
So you could take a piece of pipe and hold it between centers with the
tailstock set over and single point the threads. The tough part about
this is driving the pipe. Using a faceplate, a lathe dog, and a center
in the spindle is one way to drive the part. The lathe dog has a screw
which tightens against the part and a bent over tang that fits into
one of the slots in the faceplate. The tang will fit loose in the
faceplate. So use some binding wire to hold it against the side of the
slot that does the driving. This will keep it from bouncing around.
Remember that the taper per foot is 3/8" per side per foot. This means
that if you thread a piece 4" long the tail stock only needs to be set
over 1/8" If you don't know some of the terms I used then google for
them or go to the library and check out the book: How To Run A Lathe.
This book is old, and will assume you are using a different type of
cutting tool and tool post. But the lathe itself is little changed and
you will see good pictures of faceplates and driving dogs. I just got
the book off the shelf and checked for tapered thread cutting. And
there is a drawing of the setup needed to cut the tapered thread. It
shows an important part of the setup. This is making sure the
threading tool is square to the straight part of pipe. There is a tool
called a "fish tail" that is used for grinding threading tools and for
setting the tool square to the work. They are cheap. And if you are
gonna grind your own tools (and you should learn how) then get one.
They are very handy.
According to Eric R Snow :
With the proviso that the workpiece must be short enough to turn
And if it is longer, then my earlier requirement that the
spindle through hole must be large enough to accept the pipe through the
spindle still applies, along with the need for a taper attachment.
Granted, on a Unimat SL-1000, the headstock could be swiveled to
produce the taper -- but cutting threads on it is too much of an
exercise even with straight threads. The taper would make for other
problems with the threading attachment for that machine.