From: "Kurt Laughlin" <fleeta@sgi.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Anyone built the "BIG 50" Rifle Action by Gunmetal Designs?
Date: Sat, 19 Dec 1998 11:37:59 -0500

>How difficult would it be to make something like the "artillery
>thread" used in many field pieces? Basically a large interrupted thread
>that matches a similar thread in the breech?


I think it would be quite a chore - In fact, I've always wondered how they
do it!

The thread is a buttress form - like a sawtooth.  One flank is almost
vertical, the other at 45 degrees.  The thread can only handle load in one
direction, obviously.  The near-vertical flank minimizes the radial force
component under load that can cause thread dilation.  (A normal symmetrical
thread actually tends to expand the ID of the nut under load.  With a thin
walled nut, it can expand enough to let the screw pop out, or at least
enough so that the remaining thread engagement is too little to prevent
shearing off the tops of the thread.)

If that was all there was to it, there would be no problem:  Turn the
threads and mill away 50% of each in some pattern so they can be pushed
together and engaged in a quarter turn.  Unfortunately, you don't want to
lose half of your thread strength, so they use 2 to 4 different thread
diameters on the same piece!  The breech screw looks something like this:
from 12 o'clock to 1 o'clock the thread is eight inches in diameter.  From 1
to 2, it is six inches in diameter.  From 2 to 3 it is cut away (no thread)
but a cylinder five inches in diameter.  At 3  o'clock it is back to 8
inches, and so on, four times around.  There is a cutter relief between the
different diameters, but the sides are straight, along a radial line.

To close the breech, you line up the 8 inch sections with the cut away
sections of the breech ring (the nut), push it in, turn it 1/12 of a turn
and you are locked.  The advantage is a) only need to rotate 30 degrees, and
b) you have 66% of the threads engaged.  The more diameters, the shorter the
throw and the higher percentage of thread engagement.  With three different
diameters, you are up to 75%.

Anyways, it looks like they might have been cut with a hob or thread mill,
with the part on a rotating table.  I'd be interested to find out how they
did it for sure, especially with turn-of-the-century technology.

Note that screw breeches are generally used on crew-served field pieces (the
traditional-looking "cannon") while tank, anti-tank, and anti-aircraft guns
that require rapid reloading use sliding breeches.  The Germans made a 60 cm
(23-5/8 inches bore) siege mortar (the Karl Morser) that used a sliding
breech.  One other reason may be that a screw breech requires some distance
behind the breech ring to pull the breechblock out and swing it out of the
way.  A sliding breech doesn't.

KL

From: Robert Bastow <Tubal_cain@hotmail.com>
Subject: Re: Anyone built the "BIG 50" Rifle Action by Gunmetal Designs?
Date: 20 Dec 1998
Newsgroups: rec.crafts.metalworking

Kurt Laughlin wrote:

> >How difficult would it be to make something like the "artillery
> >thread" used in many field pieces? Basically a large interrupted thread
> >that matches a similar thread in the breech?
>
> I think it would be quite a chore - In fact, I've always wondered how they
> do it!

Good description Kurt.

There is an excellent picture of a "Welin" screw breech at:

http://www.hkkk.fi/~yrjola/war/kuivasaa/gun.html

In full size practice the male and female threads are SCREWCUT (not Milled) on
specially adapted, relieving lathes with an interrupted indexing motion.  This
usually takes the form of a special, heavy duty, "geneva" type motion.
Contemporary accounts speak of "great crashings and bangings and sudden
shuddering stops" accompanied by a tool point that moves in and out, under cam
control, to cut the various thread diameters.

Size for size, the Welin Breech is far lighter, stronger and cheaper to
manufacture than the monsterous sliding blocks favoured by the Germans.  It is
also more gas tight..using the "DeBange Pad"...and easier and cheaper to
load...it uses silk bags of powder rather than huge brass cartridge cases.

No wonder we won the war!!

Robert Bastow

From: Robert Bastow <Tubal_cain@hotmail.com>
Subject: Re: Anyone built the "BIG 50" Rifle Action by Gunmetal Designs?
Date: 20 Dec 1998
Newsgroups: rec.crafts.metalworking

To make a simple, one step, four segment "interrupted thread" is easy.  However
the "Welin Thread' used on artillery pieces is a whole different animal.  Ther
you have two, sometimes three or four different thread diameters over twelve to
sixteen segments...It is a bear to make in smaller sizes.  I know because I am
tooling up to make one for a 1/8 scale madel of a WW1 9.2" Howitzer.

A single stage thread would have little advantage over a conventional bolt
head...which is in fact a form of single turn thread.

Robert Bastow

From: "Kurt Laughlin" <fleeta@sgi.net>
Subject: Re: Anyone built the "BIG 50" Rifle Action by Gunmetal Designs?
Date: 19 Dec 1998
Newsgroups: rec.crafts.metalworking

>Size for size, the Welin Breech is far lighter, stronger and cheaper to
>manufacture than the monsterous sliding blocks favoured by the Germans.
>It is also more gas tight..using the "DeBange Pad"...and easier and
>cheaper to load...it uses silk bags of powder rather than huge brass
>cartridge cases.
>
>No wonder we won the war!!


Yes, I should've thought of that.  Sliding breeches always require a
cartridge case, or at least a stub.  I just got out my copy of "German
Artillery of World War II". . . I never noticed that practically ALL the
German-designed guns used a sliding block - even the 80cm K(E) Gustav!  One
that didn't is the 42cm Gamma Haubitze, built by Krupp around 1906.  The
book, incidentally, says it is a sliding breech (probably out of habit!),
but you can see clearly in the photo that it is a Welin type. . . so much
for expert authors.

Why we won the war is pretty much evident to anyone who makes models of WW
II aircraft or military equipment.  In fact, that's what makes German
subjects so popular - they had fifteen dozen types of everything!  If you
look at American tanks, f'rinstance, we had, for the most part, the M3/M5
lights and the M4 Sherman mediums.  The Russians had the T-34.  The Germans
had 8 major types, with umpteen variations of each.  This is not good in a
global war.  There were more T-34s as well as more M4s made in 1942 -1945
than all types of German tanks, combined, made from 1934 - 1945!

KL

Newsgroups: rec.crafts.metalworking
Subject: Re: Interrupted Screw Threads?
From: Robert Bastow <nil_carborundum@hotmail.com>
Date: Mon, 28 Sep 1998 10:02:20 GMT

> OK, may I muddy the waters a little here :)
>
>  The process described works fine for *single* interrupted threads.
> But look more closely at that artillery piece (especially a large
> one). Chances are, it's not a single thread cut at 90 degree points,
> but *two* interleaved threads cut out at 60 degrees.
>  So moving around the circumference, one sees:
>
>   1. Large diameter thread
>   2. Smaller diameter thread
>   3. Cut clean away
>   4. Repeat from [1]
>
>  When disengaged, the small (male) thread can pass clean through the
> large female, while the cut-away section passes clean through the
> small thread.
>  The closed breech now has engaged threads over 66% of its
> circumference, not 50%. Result: a stronger gun.
>
>  Now please, how to cut those threads (esp. the interior one)?
>
> --  Dave Brooks    <http://www.iinet.net.au/~daveb>
> PGP public key via <http://www.iinet.net.au/~daveb/crypto.html>, or servers

Dead on description Dave of what is called a "Welin" breech screw.
Some of the larger ones have three or even four steps.

For a good picture of one go to your search engine, enter" breech" and it
will take you to a site in Finland that has as good a picture as I've
seen.  I do have the URL but it doesn't seem to transmit well.

Now as to how they are made.  The article in the ME was not a very good
description. In real life they are done on special machines that combine
the operation of a relieving lathe (used also for turning relieved form
tools such as gear hobs) and an intermitant drive (probably some form of
"geneva" motion.

I am currently building a 1/8th scale model of a WW I 9.2" howitzer which
has a two step Welin screw. Here's how I plan to do it.

First job is to slow the lathe down from minimum low speed of 55 rev/min
to a tenth of that.  I have rigged a temporary drive from a DC motor to
an auxilliary pully on the main drive motor with a 10 to 1 reduction.
The main drive acts only as a jockey shaft. Now I get variable speed from
zero to five rpm..Lotsa torque!!

Fitted to the chuck back plate is a cam that controls the cross slide
movement via a direct push rod. The cam has four quadrants at 90 degrees,
each quadrant has a full dia (controls major thread dia on the female
bush..opposite on the male).  then there is a sharp drop off (square edge
follower) to relieve from major dia to minor dia.  The minor dia is not
controlled by the cam but independently by an adjustable back stop on the
cross slide. This gives me independent control of the two thread
diameters..relative to each other. The final step of each quadrant is
ramp back up to major dia.

In operation the feed screw is removed from the cross slide just as in
taper turning and cross slide is controlled by a strong spring in one
direction and and adjustable stop in the other.  Segments are planed out
of the bush and breech block , obviously, before threading.

Not quite as simple as some people think but quite straight forward after
you have thought about it for 35 years as I have done with this
particular model!!!

Robert Bastow

Don't force it..Use a bigger hammer!!