Film-Tech Forum ARCHIVE: Screwcutting.

This is topic Screwcutting. in forum Film-Yak at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=8;t=006587

Posted by Stephen Furley (Member # 1277) on 01-01-2012, 06:11 AM:

I've done plenty of screwcutting in my time of just about every type, multi-start threads, left hand threads, imperial threads on metric lathes, module and D.P. worms etc., but there's something that's always puzzled me. A srewcutting lathe needs a leadscrew, which needs a screwcutting lathe to make it. You can guess what the question is, how do you produce a screw in the first place, without a leadscrew?

I can think of a number of ways to generate a screw manually, but most of them would not be very accurate. About the best that I can come up with would be to take a rod, and wrap around it in a helix two narrow strips of material side by side, with the width of each being equal to half the pitch of the screw required. One would be fixed in place on the cylinder, and the other one then removed. Was it done in this way? If not, has anybody got a better idea?

Posted by Rick Raskin (Member # 1561) on 01-01-2012, 08:28 AM:

I saw a show on how they make large bolts. The threads were pressed into the bolt after it had emerged from a furnace. However, that begs the question of how the press die was made. I could see a tap being used to cut the die, which then brings up another question... And so it goes.

Posted by Scott Norwood (Member # 30) on 01-01-2012, 10:08 AM:

Isn't the same problem present with gears? How does one make a very precise gear without a geared device to properly measure the distance between the teeth?

I'd love to know the answer to this.

Posted by Ian Parfrey (Member # 5122) on 01-01-2012, 02:12 PM:

It's my guess that the easiest and most accurate way to make an initial screw thread would be to lay flat the thread on a die of some sort. Think screw rolling here.
If my memory serves me correctly, a screw rolling die has straight female threads cut into the die block as straight channels which then become the male thread profile after rolling.
Straight grooves are MUCH easier to manufacture then helices of any sort - shaping machines were used widely in the manufacture of thread-rolling dies in days gone by.

The same would go for the initial profiling of gears athough a gear blank is very easy to profile on a horizontal milling machine, a mandrel and a dividing head.

Posted by Jim Cassedy (Member # 4115) on 01-01-2012, 03:16 PM:

quote: Stephen Furley
how do you produce a screw in the first place, without a leadscrew?

I seem to recall reading that the early tool & die makers originally
created screw patterns by wrapping a stiff paper right-triangle
around a dowel & then tracing the spirial pattern created by the
edge of the hypotnuse onto the wood and using that pattern
as a guide to cut (by hand) the screw threads into the dowel.

The "legs" of the triangle (and thus the length of the
hypotneuse and diamater of the dowel) determined the
thread pitch. This is how they created those big big wooden
screw-jack-thingys used in old paper making & printing
presses, and the lead-screws on early lathes.

Just FYI> A really entertaining examination of the history
of hand tools, especially screwdrivers, can be found in the
1999 book "One Good Turn - A History Of The Screw
and Screw Driver" by Witold Rybczynski.

I read it when it first came out, and then found a used copy
on Amazon last year for a couple of bux & re-read it last
summer. In fact,I just gave my used copy away for Chirstmas
to a friend who will be having sugery soon so he'll have something
to read while he's in the hosptial. The book is full of intersting
info and illustrations, but never boring!

Posted by Stephen Furley (Member # 1277) on 01-01-2012, 03:20 PM:

Generating a screw from a flat die is an interesting idea, but I've only ever seen it done for short threads of smallish diameter. Could it be scaled up to produce something like a lathe leadscrew?

Cutting gear teeth with a simple form cutter on a milling machine is ok for gears which do not run at high speeds or transmit large amounts of power, I've cut clock wheels and pinions myself this way, but the problem is that you need a dividing head, and that needs gears. How do you make those gears in the first place?

For high speed or high power applications the tooth shape really needs to be generated. Traditionally, there have been three main ways of doing this, gear planing with a rack cutter, gear shaping with basically a gear wheel with cutting edges, and hobbing. All of these machines normally use gears in their operation. The teeth on the rack cutter would be straight-sided, and would be the easiest to produce. I cannot think of a way to produce a gear shaping cutter without using pre-existing gears. To make a hob would require both a leadscrew and a relieving shaft on the lathe, both of which would require gears to drive them. Of course, screwcutting also requires gears to drive the leadscrew.

I can think of ways to make the large gears with inserted wooden teeth as were used in things like windmills and watermills with nothing more than simple hand tools to an accuracy which would probably be close enough for the purpose, but that's a long way short of modern gears.

Posted by Michael Voiland (Member # 5386) on 01-02-2012, 12:42 AM:

This is the best I could come up with which is nuts and bolts
http://www.youtube.com/watch?v=3kxcw08p_oY
This is a youtube link.

Posted by Simon Wyss (Member # 6218) on 01-02-2012, 08:31 AM:

It is easier to make a cog wheel than a screw. You need a dividing device that allows you to rotate something by a certain number of degrees. Its hole disk can be made by hand, I mean one makes scribes across a round flat with the help of a centering angle. Twelve equal sections give us 30 degrees for example. Cog wheels can be cast, forged or pro-forged, broached, milled, ground.

Once you have gears you can make a lathe which advances the slide or saddle in a certain ratio relative to the spindle rotation. Later on you add a mainscrew to the gear stock.

First class mainscrews are forged (rolled), then stress-relieved, milled, hardened, and ground.