Re: [CAD_CAM_EDM_DRO] Re: New guy with questions

> Thanks guys for all the information you people provided!
> I had no idea how to tap and now I know how to. I have searched
> around for taps and they seem to come in a bundle of 12 or so which
> makes it very expensive. I'll have to look some more.
>
> The next obvious question is say I have a 1/4" rod and I want to
> thread the end to be 1/4 - 20, how do I do it? A long time ago I
> bought a mountain bike fork and the tube wasn't threaded enough to
> fit through. I went to a bike shop and the guy threaded it in a few
> minutes. I have been wondering since what he used to do it. Do we use
> some cutting nut (opposite of a tap) of some sort?
>

Yes, the 'cutting nut' is called a die and can be purchased in almost
any hardware store, along with an appropriate handle. In use, you
twist it a half or quarter turn, then back off, much as with a tap.

The problem though is that it is very hard to get it started straight
and concentric to the shaft, and hard to keep it that way. If a lathe
is available, one can cut a taper on the end to help.

There is also a more accurate way of making threads with a lathe. A
v-shaped tool is used, and a chain of gears and a threaded leadscrew
are used to move the tool as the work spins, cutting a spiral that
in skilled hands will become a screw thread after a few passes.

My personal preference is to start by cutting such a 'single point'
thread in the lathe and then finish by running a die over these rough
threads to bring them useable close to the right shape and dimensions
without having to do any carefull measuring or fit testing.

You should be aware that cut threads are not as strong as commercially
produced screw threads, which are made on a lathe with a tool that
actually rolls grooves, forcing metal out of the troughs and in to the
peaks, rather than cutting it. If you carefully measure a commercial
bolt, you will find that the thread major diameter is slightly larger
than the diameter of the unthreaded portion - if you try to extend the
thread on such a bolt with a cutting die you will end up with
slightly flattened off thread peaks.

Chris Stratton