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Colt Tsmg’S - Finish And Roll Marking Questions

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#1 Digger



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Posted 06 November 2011 - 04:49 PM


I am new to the board although a long time observer. This is my first posting.

I am looking for any information on the roll markings, surface finish and bluing on the original 1921 Colt TSMG’s.

I am trying to reproduce these features as close as possible (except probably the bluing – a separate pursuit…). In particular, I am looking for any details on the finish eg what methods that may have been utilized to get the sanded(??) finish and (if sanded) what grit size and techniques may have used in the process etc etc. Regarding the roll marks, any estimates on the depth of the rolled text, logos and any other unique characteristics or information would be very helpful.

I do not have access to an actual ’21 so any assistance you can give would be much appreciated. I do have most of the Thompson books etc that have been available over the last 4 years and constantly trawl the internet for good quality (close up) photos of the ‘21’s and information off this board. These are my only means of reference at this stage.


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#2 darrylta


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Posted 06 November 2011 - 08:02 PM

Welcome Digger, I'll take a stab at two of your questions.

All the Colt SMG's received the same carbonia finish as the handguns of the period.

I've read that the sanding was done by hand with 220 grit paper. An amazing feat, done by real craftsman.
I've also noticed that the sides and the bottom of the nose end beyond the flat sides were not sanded
as coarse if any. The sanded finish on the early Colt 1911 Army's matches the Colt Tommy's.

The finish started out a very dark blue tone and even darker at the polished surfaces. With age fading,
you see the light blue tone on most of today's examples. Ron Guns and Turnbull do the carbonia finish well,
You can buy new examples of their work in 1911 form if your interested.

As far as the roll markings, they were very fine like most fine Colts of the period. No comment on the depth.

My 2 cents,

Edited by darrylta, 06 November 2011 - 08:55 PM.

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#3 reconbob


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Posted 06 November 2011 - 09:56 PM

Welcome to the board. If you go digging on the internet there is an article out there
from yesteryear describing the apparatus for doing the Colt Royal Blue finish. I do not
recall if "Carbonia" was a proprietary name (like "Parkerizing" was for the Parker Rust
Proof Company) but in a production setting special furnaces with conveyors were
manufactured specifically to carry the parts thru a heated atmosphere and imparting
to them the famous royal blue finish which is not as dark as "standard" immersion
black oxide process. Steel aquires a deep blue color at 550-600 degrees. I do not
remember if the atmosphere in the furnaces had something added to it to affect the
As an aside - when we machine here with tungsten carbide insert milling cutters,
the principle at work is for the cutting edge to create so much friction that the steel
is heated to the point (at a molecular level) that it becomes soft, and therefore easier
to cut. Most of the heat ends up in the chips and the general guideline for good
cutting is chips that are a deep blue. Grey chips = too much heat, silver steel colored
chips = not enough heat, neither of which is good for carbide.

As far as the engraving goes, the original guns were roll engraved, and to exactly
duplicate that process would cost many thousands of dollars for the dies. You can't
get a good copy with laser engraving, because while a laser would be relatively easy
to program, the laser overlaps and double-burns at the beginning and end of a letter
(such as an O) and also double-burns at intersections (such as crossing a T) the result
being little dots throughout the engraving.
Another possibility would be a pantograph, but it would be very time consuming to
setup the different size and font lettering, spacing, etc.
The other option is to machine-engrave - machine the engraving into the surface of
the steel with a tiny endmill. This is how we do it here and its probably as close as you
can get to roll engraving. Here is a photo of the patent dates for a M1921 machine

Posted Image

When a rolling die is used the die presses the characters into the surface and
leaves a slight raised edge around them which you dont get with milling. However,
it seems that the M1921s don't have this so they may have been polished after
engraving which would have smoothed off the edges.

The depth of the engraving is between 0.002"-0.004". The deeper you go, the
thicker the characters get and can actually blurr together on small lettering like
the patent dates. So I would say small letters like the patent dates would be 0.002"
deep and the big letters like COLTS PATENT FIREARMS etc. could be 0.003"-0.004"

The depth of the bullet logo is about 0.0005" deep. (Thats 1/2 of one thousanth
of an inch). Here again, if you go deeper - even if its roll engraving - the very tiny
lettering above and below the bullet as well as the THOMPSON signature inside the
bullet blurr together and become illegible.

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#4 mnshooter


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Posted 06 November 2011 - 11:42 PM

Excellent information Bob.
Is the sample lettering piece a test run, or is there enough depth that the final finishing
(to remove the existing milling/grinding marks) leaves the lettering at the correct depth?

Edited by mnshooter, 06 November 2011 - 11:45 PM.

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#5 reconbob


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Posted 07 November 2011 - 02:47 PM

The light striations in the photo are from the blanchard grinding which we
use to prep the receivers for machining. You can polish out the striations with
emery cloth wrapped around a steel block and the engraving is still legible. The
picture is a production receiver that was ordered with the M1921 Colt markings.

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#6 mnshooter


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Posted 07 November 2011 - 04:16 PM

All very interesting.
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