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Response To Jimfromfl's "my Blish Locks"


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#1 Bob B

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Posted 23 August 2004 - 03:33 AM

Although intended as a response, I'm posting this as a new topic because the images in the original post are so big they take forever to load and run the text way off the page.

Despite having no practical experience in the phenomenon of Blish lock wear, the images do hint that the locking mechanism may not be operating ideally. I came across the following surprisingly detailed description of lock design and operation, excerpted from an evaluation by Philip B. Sharpe published in the journal American Institute of Criminal Law, 23 (1932-33): 1098. (Complete article available at http://www.saf.org/L...s/PSharpe1.html ) While most of this description won't be anything new to veteran Thompson folks, it may help shed some light on JimFromFL's question.

QUOTE:

A wedge-shaped piece of metal weighing one ounce locks the entire mechanism purely through friction on sliding surfaces created by the breech pressure while the bullet remains in the barrel. When this pressure is relieved, as happens when the projectile passes to the open air, this wedge is released and unlocks the mechanism.

The idea is not new. Naval men discovered many years ago that their heavy fixed pieces played them pranks. They noticed that in firing the ten and twelve-inch rifles with service shot or target projectiles, the. guns behaved according to the handbook. But when blank salutes or light practice charges were fired, the heavy interrupted-screw type breach block cammed itself open automatically. Naval ordnance experts set forth to solve the puzzle mathematically¾and did. Commander John Blish is credited with arriving at the correct solution, and his principles were applied to several pieces of ordnance during the late war¾most common of which was the three-inch anti-aircraft rifle which unlocks its breech and ejects the empty cartridge case as the projectile starts skyward.


(jump)

The lock is perhaps the most interesting piece of mechanism, since it is around this item that the entire mechanism functions.

First of all, its appearance is outstanding. Shining like a piece of gold, it is in direct contrast with the polished steel bolt, or the heavily blued actuator into which it fits. It is formed of titanium aluminum bronze, an alloy selected for the obvious reasons of coefficient of friction, strength, toughness, resistance to wear, and others.

This lock is designed like an "H" with oddly shaped lugs on either side. The sides of the "H" engage with grooves in the bolt, the bar with the actuator, and the lugs with the receiver grooves.

The surfaces between the bolt and lock are at an angle of 70 degrees to the axis of the gun. Those between the lock and receiver are at an angle of 45 degrees to the axis, so that there is formed on the lock an intercepted angle of 25 degrees between the bolt bearing and the receiver bearing surfaces of the lock.

The chamber pressure is transmitted to the lock through the [Page 1105] bolt, and the angle of the receiver and bolt slots with the corresponding lugs of the "H" wedge have been figured so that the coefficient of friction of the alloy plus the breech pressure of the cartridge causes the adhesion of the lugs to their corresponding slot surfaces in the receiver walls. At the proper time this pressure falls to a point wherein it is just sufficient to recoil the bolt to the rear and operate the mechanism. This reduced pressure will not permit the locking surfaces to adhere, the wedge slides upward, releases its locking effect, and the action functions by reason of the energy of recoil still remaining.

Should any police departments have a Thompson gun in which the lock has been "altered" or "smoothed up" by some over-energetic gunner or repairman, it would be wise to replace this lock with a new one. The various angles have been carefully determined, and must not be changed. Should they be filed down too much, the gun might open early, with considerable surprise to the gunner. However, Thompson engineers state that the action is built sufficiently strong, to resist the slam of the bolt even though the locking surfaces were removed entirely.

Design of the weapon forestalls damage of the lock while within the action; should it become damaged while outside, replacement rather than repair would be the best policy.


Sharpe states that adhesion due to the titanium aluminum bronze alloy's coefficient of friction exerts a positive locking action during the time before the bullet leaves the barrel, and allows movement only when pressure reduces to the point where it "is just sufficient to recoil the bolt to the rear and operate the mechanism." The type of wear and scoring evident in the photos suggests (if Sharpe's statement is to be taken literally) that positive locking isn't being sustained long enough (if at all) and instead is allowing operation before blowback pressures are appreciably reduced.

Certainly this must be the case for lock #4 (furthest to the right), which shows grease deposits that must interfere with adhesion. If the (relatively) non-precision lock surfaces are forced to operate at high pressure - even in a layer of grease - they will score and release metallic particles that can only add to the abrasion. On the other hand, it's never made much sense to me that a system designed to exploit the frictional coefficient of a particular alloy should operate well in an oil-saturated environment such as the upper receiver of a Thompson.

The other two used locks show a different sort of wear, including what appears to be some galling and maybe even a little looseness or misalignment evidenced by the asymmetrical upper wear surfaces. Almost looks like there may be some "cocking" of the lock as it disengages, but I'll let others with hands-on experience speak to that.


Sorry I haven't answered the question as to whether the depicted wear is "normal", but maybe this will help get discussion going.

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#2 TSMGguy

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Posted 23 August 2004 - 03:00 PM

The Blish lock on my gun (a M1928A1) has worn in exactly the same way. When I first noticed this wear, I laid in a couple of spare Blish locks, but haven't been tempted to replace the part as the gun seems to function normally, firing at a precise 750 rpm with full power ammo. I thank BillinVA for posting the very detailed photos.

Highly unscientific tests on my part with the gun involving dry receiver Blish lock slots v. lubed slots seems to produce no change in the rate of fire.

I read where the concept of the Blish locking principle was proven to be inapplicable in the Thompson because of the low pressures involved. The bolt is never locked closed by the Blish lock. As a practical matter, the lock serves mainly to mechanically connect the actuator and bolt, and also serves to slow the cyclic rate. The gun is basically blow-back operated, as the British proved during WWII when they locally replaced the lock with a simple nut and bolt (not recommended!). The redesign of the gun into the M1 dispensed altogether with the unnecessary lock and separate actuator.

Gen. Thompson may have even known this, when his tests of a .30-06 Blish principle infantry rifle failed. Could the unneeded lock have been more about the desired patents surrounding his then-new Sub Machine Gun? After all, the blow-back principle had been around and in use for years, and he might not have been able to protect it under patent.

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#3 JimFromFL

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Posted 23 August 2004 - 08:26 PM

Interesting article. Thanks for the post.

I was expecting to hear that everyones Blish Lock looked this way, but after maybe 20K or 30K rounds. Not after 2-4K.

As the article stated, the lock looks like a shiny piece of gold. It made me think that since mine is wear quickly maybe it was really made of gold. tongue.gif cool.gif (i can only wish).

To TSMGguy,

You said your locks looked like this as well. About how many rounds before you noticed the wear?
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#4 TSMGguy

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Posted 25 August 2004 - 04:42 PM

Hello, JimFrom FL,

When I bought the gun, I noticed very slight wear on the ears of the lock the first time I field stripped the gun. Usage since then has been maybe 2K rounds (if that much: I don't shoot the beast very often), and the wear is now much more pronounced, looking like the AOC example second from left in your excellent photos.

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#5 AZDoug

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Posted 25 August 2004 - 05:03 PM

Titanium aluminum bronze?

I thought titanium was not produced commercially until the 1950's? All those Blish locks floating around out there were made between 1920 and 1943. Aluminum bronze sounds mroe correct.

Doug
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#6 Bob B

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Posted 26 August 2004 - 01:39 AM

AZDoug,

The reference to "titanium aluminum bronze" is quoted directly from the article by Phillip Sharpe published in the 1932-33 journal of the American Institute of Criminal Law. While I don't doubt that some of the TSMG's use of the Blish locking principle was hyped and perhaps even to some degree misrepresented , this early reference leaves no doubt that such an alloy existed (in fact or in theory) at that time. Titanium was discovered in 1791 and isolated as an elemental metal around 1910. Titanium Dioxide was discovered in 1821 and saw limited use as an artist's pigment until it was first mass produced in 1916. It's certainly possible some form of titanium was used as an alloying element before solid shapes were introduced in 1953.

That said, the original Blish patents place no reliance on special alloys or the frictional coefficients of different metals. In the patents, pressure/release curves detail the precise relationship between breech pressure and the degree of intercepted angle - the presumption being that all parts are of hardened steel. Different breech pressures in different guns are accommodated by variations in lock angle and nothing more. All steel parts in the case of the TSMG would probably have resulted in an unacceptable rate of receiver wear, so it seems likely a bronze alloy Blish lock was chosen to limit the wear to this replaceable part. If this is the case, then it's clear any change in critical angle from the pressure/release curves disclosed in the patent was simply to compensate for the different frictional coefficient of bronze. It's easy to see how marketing might have woven a mystique around the "magical" adhesion properties of a "special" alloy, but the facts - and the patents - seem to demonstrate this had nothing to do with it.

Your observation about the commercial availability of titanium caught my interest, so I went searching for an answer. I didn't find one exactly, at least as to date, but titanium IS sometimes specified as an alloying component in aluminum bronze. However, it does nothing to increase strength or wear resistance as most would assume. Small amounts of titanium (0.2-0.5%) serve to minimize porosity in Al/Bronze castings and alloy welds, and also to slightly improve corrosion resistance (for example in seawater heat exchanger tubes and the like). The addition doesn't make the alloy mechanically stronger or more durable. I can only imagine (if it was used at all) that the addition of titanium might have made for higher integrity of parts by minimizing porosity.

Any metallurgists out there?

Bob

Edited by Bob B, 26 August 2004 - 01:46 AM.

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#7 PK.

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Posted 26 August 2004 - 10:50 AM

No time to discuss in detail.

1919 AO print rev 7 lists material as “red aluminum bronze”.

Principle of adhesion not applicable to low pressure 45acp.

‘21/28 guns should not be fired without the lock in spite of what the British did in W.W.II to counteract sand- the lock acts to delay the opening of the bolt and slow and time its travel. This is not a “blowback” action but a “delayed (or retarded) blowback” design; there is a big difference.

The images do not appear in the post now (for me at least) and I have not seen them. The make of gun is not indicated. Many WH made guns have locking slots in the receiver that are in the wrong position and of the wrong shape and will contribute to accelerated wear to both receiver and lock.

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#8 Bob B

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Posted 26 August 2004 - 12:07 PM

PK,

Here's a copy of the original message and a consolidated reduced image of the two huge closeups. Hope this helps.

QUOTE
JimFromFL Posted: Aug 21 2004, 05:45 PM 
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These images contain 4 blish locks.
The left most one is unused. The remaining 3 are each have a few thousand rounds (less than 10,000 each).

Although the wear on the 3 may not look like much via the pictures, seeing them in person seems a bit excessive esspecially only after a few thousand rounds. Maybe after 20K rounds or so, it would look normal, but not after just a few thousand rounds.

I have run my fingers along the rails in the upper and they are smooth.

Is this what your blish lock looks like after a few thousand rounds?


Image Hosted by ImageShack.us


The original post and pictures are here (GREAT detail, but takes forever to load): http://www.machinegu...t=ST&f=3&t=3028

Bob
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#9 PK.

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Posted 26 August 2004 - 08:44 PM

Bob B, thanks for re-posting the photos.

For some reason, not all locks have the proper angle on the top (as shown in the photo) of the ear as the left one appears to have. The right one also appears to be of the proper angle as well. If you look at the two middle locks in the photo, you can see that they sloped down towards the left from the start, leaving an edge to wear quickly away. None of these are unusual in my experience and once seated, the wear will slow appreciably. You will likely see much more wear over the life of the part on the rear face where the lock engages the slot in the receiver.

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