On Sat, 22 Nov 2008 22:29:19 -0500, "Ed Huntress"
wrote:


wrote in message
...
On Sat, 22 Nov 2008 20:36:49 -0500, "Ed Huntress"
wrote:


"axolotl" wrote in message
...
Ed Huntress wrote:

From memory, not from sources: I believe old optical threads were
Imperial, not metric, and that they were based on the 55-degree
Whitworth
form, without the fancy roots. The fine ones were usually hand-chased,
not cut on a screwcutting lathe.

My uncle did a lot of that work


Ed,

How about a lesson in thread chasing?


Kevin Gallimore

Sure. Teach away. I'd be glad to learn. d8-)

My uncle was very good at it. 'Made all his own chasing tools. I tried it
on
a piece of aluminum barstock, soon after I got my lathe, and I finally got
one to come out good -- after three or four tries.

You need to have a good sense of timing. If you're too slow, you wind up
with a few closely-spaced, shallow grooves. If you're too fast, you can
wind
up with a double-lead thread.

Do you understand the basic idea? The tool is a freehand turning tool,
which
you hold over a tool support much like the one on a wood-turning lathe.
The
tool looks like a flat chisel on the end, but with four or five teeth,
each
shaped like a single-point threading tool, only it can be simpler, without
any rake.

You start moving the tool along the work, in the direction of the thread
you
want to cut, and then you plunge the tool into the work while you're
moving
it lengthwise along the lathe bed. If you get it right, it cuts a
single-lead thread and it self-feeds after the first revolution of the
work.
You'll need to make a few passes to cut the thread to full depth, but
after
the first pass, the tool follows the previous cut.

It's a bit of an art but people who specialized in it got it right every
time.

--
Ed Huntress


Greetings Ed,
I've cut thousands of threads of all sorts of different pitches and
design. But I have never hand chased a thread. Horoligists apparently
hand chase threads sometimes. The smallest thread I have single
pointed is 0-80. I tried 00-96 but couldn't make it work. Too much
deflection of the screw. Now, after reading your post, I just have to
learn how to hand chase threads. I know it will come in handy for tiny
threads.
Thanks,
Eric

I see that there's a lot of info on hand-chasing threads in wood on the Web,
and some on the subject in metal. I'll bet that with some of that info and
some practice you'd find that it isn't hard.

The good thing is that there is some latitude in the starting speed with
which you move the tool, because the second and following teeth tend to
follow the start, and they push the first tooth along once they get a grip.
So it's not as magical as it may sound. It just takes some practice.

My uncle (a shop teacher) worked with a big astronomy club full of telescope
makers back in the '30s, helping them with machining for their hobby, and he
got quite good at it. He told me that I should learn because it would come
in handy. I can't say it has, but I don't work with optics or clocks.

I thought a lot about the business of making the cutters for extremely fine
threads, and it seems to me that's the hard part. If you have the right
pitch available on your lathe you might be able to rig a boring-bar-type
flycutter, between centers, with the blank tool mounted in the toolholder,
and just take tiny nicks with the flycutter to mark the positions for filing
in the chasing tool. Then finish it off with a file. It probably requires
the very finest of Swiss-pattern jeweler's files for the fine ones, working
with magnification.

My uncle told me that the keepers (they must have a name but I don't know
what it is) for individual elements in a refractive telescope are typically
mounted on a mandrel for machining and threading, because they're very thin
and fairly large in diameter relative to the thread length. He also
mentioned that a lot of threads on old telescopes were *******s, because the
machinist would cut the outside thread and then bend the end of the tool
over to cut the matching internal thread. And they worried more about
getting a proper fit than about actual thread pitch.

That must have taken a lot of finesse. I got interested in making clock jigs
at one time, after I met Dick Moore and read his early books, and my
interests went in the direction of precision positioning rather than making
threads.

Good luck and give us a report when you get a chance to try it.

--
Ed Huntress


Thanks Ed. I will post results. Hand chased threads have started to
really rouse my curiosity.
Eric