Buffer: Circle vs. Square

[buzz]

There are four basic things that determine if a fatigue crack is going to form:

 

1. The level of stress in the part.

 

2. The number of load cycles.

 

3. The way the part is detailed. Like a sharp corner instead of a rounded corner, forged vs cast, etc.

 

4. The type of metal used.

 

If you address the above items properly, you can design something to have an infinite fatigue life, an infinite number of load cycles before the part fails.

 

But most products are not designed that way. It would be overkill, needlessly expensive and heavy.

 

S&W did not design their k frame revolvers for infinite fatigue life. Like most products, they traded durability for compactness and lighter weight.

 

If you fire 357 mag rounds through a S&W k frame every single day, it will eventually form a fatigue crack in the frame at the rear bottom of the cutout for for the cylinder.

 

But that would happen at a round count 20 times higher than any typical person would fire in their lifetime.

 

I would be interested in learning from those rental ranges when their guns start cracking, but I'll bet that the Thompsons would pass the 250,000 round mark before it happens.

 

If you took your Thompson to the range and fired 4 drums, every week for 10 years, you'd only hit the 100,000 round mark, which would cost you around $30,000 in ammo.

 

Your lifetime usage of your Thompson is probably more like 25,000 rounds, which won't even wear out the barrel.

[buzz]

I believe the bolt does actually contact the buffer. We occasionally get guns

that are not functioning due to low power/short recoil. One of the low tech checks

for full recoil is we put a tiny dab of grease on the front face of the buffer or buffer

pilot. When a properly functioning gun is fired the grease gets completely squashed.

 

Bob

 

 

Yeah, I think I did the scotch tape idea already and the bolt handle nailed it.

 

Setting aside all the imponderables, I think that it's a good idea to use a neoprene buffer instead of the phenolic resin washer.

 

You can hardly ever find a case where using a softer cushion on an impact load doesn't result in lower peak load.

 

Like the way a rubber recoil pad makes a hard kicking gun thump your shoulder less.

 

If you decrease the spring rate of a spring, make the spring softer, you always decrease the peak force in the spring.

 

Like for example, If you drop a 100 lb weight from a 5 foot height onto a spring with a spring rate of 20 lbs per inch, the peak load in the spring will be:

 

x = (100 x 5 x 12 x 2 / 20 )^.5 = 24.49 inches of compression, Force = k x = 20 x 24.49 = 489.9 lbs

 

But if you change the spring rate to 10 lbs per inch, then the max force in the spring is

 

x = (100 x 5 x 12 x 2 / 10 )^.5 = 34.64 inches of compression, Force = k x = 10 x 34.64 = 346.4 lbs

 

unless I fubared the math, the change in spring rate from 20 to 10 increases the spring travel by 41% but reduces the peak load about 30%.

 

so, in general, more softer = more gooder

Edited January 19, 2018 by buzz

[Piep]

"More softer=More gooder" sound good to me.. lol

[RChapman]

thats what i say, wasnt a spring going to absorb energy better than the actual buffer, softening recoil (not that recoil is a huge issue on the Thompson) and avoiding the bolt to slam back on the receiver?

 

well i guess if they havent made it its because a spring would compress and release energy, while the resin probably just absorb part of the kick... right?

Edited January 19, 2018 by RChapman

[Big Al]

The M1 Thompsons at The Gun Store range in Las Vegas appear to have cracked and been sloppily repaired. Note the vertical line at the very back of the receiver of the gun in the middle of the photo. Both of the guns they have exhibit this characteristic. I meant to ask about it when I was there shooting the gun, but forgot to in the rush through. I don't know for a fact if that is a repaired crack. I am just offering this information for your consideration.

2011-gun-store-nv-4.jpg

One more thing to note regarding the Thompsons at The Gun Store. The ammunition they had to shoot was loaded with powder-coated bullets and it didn't generate a recoil impulse of the strength I am familiar with in the Thompson. The rate of fire also seemed sluggish.

[Big Al]

thats what i say, wasnt a spring going to absorb energy better than the actual buffer, softening recoil (not that recoil is a huge issue on the Thompson) and avoiding the bolt to slam back on the receiver?

 

well i guess if they havent made it its because a spring would compress and release energy, while the resin probably just absorb part of the kick... right?

A wire buffer spring likely would have driven up the unit cost. That is just my speculation, however.

[buzz]

The 1921 Thompson is the most over-designed gun I can think of. There is literally no engineering refinement that they left out of the gun.

 

Not to mention the super-deluxe fit and finish by Colt.

 

I highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

 

The reason that they did not come up with a spring gizmo for a buffer is the same reason there isn't one in an UZI: because it's not needed.

 

This is an interesting discussion, but I think it's incorrect to assume that some special gizmo is needed to save the receiver from being battered.

 

The bolt is powered by a 45acp round and the impact on the buffer happens at the end of the recoil spring compression. How much velocity do you think is left in the bolt at that point?

[Big Al]

I highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

The gun may have retailed for roughly that amount, but the cost to make it was much lower. The agreed upon price in the 1920 contract with Colt for the 15,000 gun production run was $44.56 per gun with one magazine. That would be $541.14 today.

[RChapman]

The 1921 Thompson is the most over-designed gun I can think of. There is literally no engineering refinement that they left out of the gun.

 

Not to mention the super-deluxe fit and finish by Colt.

 

I highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

 

The reason that they did not come up with a spring gizmo for a buffer is the same reason there isn't one in an UZI: because it's not needed.

 

This is an interesting discussion, but I think it's incorrect to assume that some special gizmo is needed to save the receiver from being battered.

 

The bolt is powered by a 45acp round and the impact on the buffer happens at the end of the recoil spring compression. How much velocity do you think is left in the bolt at that point?

 

with the blish lock opening the action when the pressure is reduced, i think it wont crush your finger if you leave it in between... better not try anyway

Edited January 19, 2018 by RChapman

[ron_brock]

 

 

I highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

The gun may have retailed for roughly that amount, but the cost to make it was much lower. The agreed upon price in the 1920 contract with Colt for the 15,000 gun production run was $44.56 per gun with one magazine. That would be $541.14 today.

Where did the 1921A retail for $2500? What is your source for this data Buzz?

 

Ron

[reconbob]

Buzz - this may be one of those counter intuitive things, but if you lighten

the recoil spring, would that not increase the rearward velocity of the bolt

(less power needed to compress spring) which would increase the force

with which the bolt strikes the back of the receiver?

If you cut the recoil spring in half the bolt would recoil very fast.

Yes? No?

 

Bob

[Big Al]

 

 

 

I highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

The gun may have retailed for roughly that amount, but the cost to make it was much lower. The agreed upon price in the 1920 contract with Colt for the 15,000 gun production run was $44.56 per gun with one magazine. That would be $541.14 today.

Where did the 1921A retail for $2500? What is your source for this data Buzz?

 

Ron

He means the original retail price adjusted for inflation, which comes in just under $2,500.00.

Edited January 20, 2018 by Big Al

[buzz]

Buzz - this may be one of those counter intuitive things, but if you lighten

the recoil spring, would that not increase the rearward velocity of the bolt

(less power needed to compress spring) which would increase the force

with which the bolt strikes the back of the receiver?

If you cut the recoil spring in half the bolt would recoil very fast.

Yes? No?

 

Bob

 

 

Yes, you are correct.

 

The bolt will not stop going backwards until all the backwards kinetic energy gets sucked out of it. So if the recoil spring does less work, then the bolt will be moving faster when it hits the buffer.

 

These guys are talking about some kind of 1/2 inch long spring gizmo to replace the buffer. Like if the last 1/2 inch of bolt travel it hit a supplementary spring instead of thumping against the resin washer.

 

Technically, something like that would reduce the force on the rear plate. The longer distance that you take to slow down, the less acceleration is needed.

 

I was just saying that something like that wasn't really needed or they would have stuck it in the gun in 1921.

[buzz]

Anything that deflects elastically under load is technically a "spring".

 

You would use the same energy equations for a rubber band as a steel coil spring.

 

So if you want to come up with a fancy buffer that reduces recoil impulse on the rear plate, then you could use some kind of elastic material like rubber to make a big fat buffer.

 

The most ideal way to lower the thump of the bolt on the rear plate of the receiver would be to use a recoil spring that stopped the bolt 1 millimeter short of the buffer.

 

Let's say that the recoil spring soaks up 95% of the bolt energy and thumping against the buffer uses the last 5%.

 

Since you know that thumping against the buffer is a short, hard deceleration and the recoil spring provides a long, smooth deceleration, having the recoil spring do 100% would be better.

 

Whenever you take a longer time or longer distance to slow something down, that means the force is lower.

Edited January 20, 2018 by buzz

[Big Al]

highly doubt that they left a 1/2 inch long spring out of a $2,500 gun because of cost.

Some more on the spring vs washer. What goes into making a spring compared to a phenolic resin washer? With a spring, you need to get wire of a specific diameter and wind the wire around a mandrel at a specific rate (that might be varying if you tighten the coil spacing at each end). The wound wire is clipped at specific intervals and the bits are heat-treated to give it the desired characteristics. With a phenolic resin washer, you just pump molten phenolic resin into a mould, let it cool, and pop them out. Which one sounds less expensive?

Edited January 20, 2018 by Big Al

[buzz]

This thread presumes that the bolt delivers some sort of brutal battering to the rear plate of the receiver that needs to be addressed.

 

Before insisting that a spring buffer gizmo was not used due to cost, you first need to establish that such a thing was actually needed.

 

Can you point to all the fancy spring buffers found in other SMGs?

 

Or point to some failure in the Thompson that the spring gizmo would cure?

[benedw60]

I vote for a motion to ban scrambles from the forum board. Taking cheap shots at experts and dragging on long jibberish posts that have absolutely value is annoying

[buzz]

 

 

I think the problem was that the tone of your post was a flat declaration of truth about a topic that you are merely speculating about.

 

People find that a tad annoying.

Would you like to address how this doesn't relate directly to you..?

 

 

Your post is cringe inducing. But I'll answer this, it's a fair question.

 

I get paid to solve problems just like this one.

 

I have a BS and MS in engineering and I studied kinematics at the graduate level. One of the things I learned in grad school is how solve complicated dynamics problems, both by closed form solutions and by numerical integration with a computer.

 

The way these problems are solved is by mathematically representing the system as a series of lumped masses that are attached to each other by springs and dampers. Then the equations of motion are set up using D'Alembert's Principle and the resulting differential equations are solved using a numerical integration method such as the Wilson-theta method.

 

Those are my qualifications for talking on this subject. Six years of training.

 

A Thompson bolt doesn't have to be modelled as a lumped-mass spring system, it literally IS one. I can't think of anything that matches the model better. It wouldn't matter if it wasn't such a close match, the physics involved and the solution method are general and would work on anything from a car exhaust pipe to a skyscraper.

Edited January 21, 2018 by buzz

[buzz]

 

 

"This Shouldn't be Legal" - TD

Yeah that's really going to promote your hobby.. Quit being selfish you fools!! I won't forget your words, or shake your hand, EVER.

I've never met a higher-regarded and more destructive member of a closed community.

 

There is so much more to think about here..

 

 

For what it's worth, my experience with the guys on this forum is the exact opposite of your sad tale of woe.

 

This forum is an amazing storehouse of Thompson knowledge and these guys will go out of their way to inform and assist the new guys.

 

Your arch nemesis TD called me on the phone and gave me a detailed tutorial on how to collect data from official sources. He was extremely helpful to me, a guy he never met.

 

And your other tormentor, Reconbob, who manufactures these guns professionally, was kind enough to give me tour of his shop and answer about 100 questions on metalworking.

 

Maybe if you were to switch to decaf you might fare a little better.

Edited January 21, 2018 by buzz

[mnshooter]

I vote for a motion to ban scrambles from the forum board. Taking cheap shots at experts and dragging on long jibberish posts that have absolutely value is annoying

 

Tempted to 2nd this, but it is a helpful revelation once in a while to expose how certain minds function.

And why I don't work in customer service.

Edited January 21, 2018 by mnshooter

[reconbob]

One of the things I always wanted to try in. Thompsons the very thick, short
buffer spring out of a M1918A2 BAR buffer. Seems to me it would be a good shock
absorber...

Bob

[Motorcar]

Quit being selfish you fools!! I won't forget your words, or shake your hand, EVER.

 

Scrambles, This Board has been around for years and years. The one thing it hasn't evolved into is a flame site. Have you seen the vulgar language used on other gun site pages? Not here. The vast majority of members here are about helping and sharing of knowledge among the Thompson owners, would be owners, and those just interested. Now we aren't above a short, curt, and to the point answer, and everyone has there good and bad days, in the end it's about helping and that's where we usually get even if we disagree. Your last post was probably the most disrespectful I've ever read here in 15 years. I invite you to use the "Edit" feature and remove the things that aren't appropriate. (I don't care who "started it", not children here.)

[buzz]

One of the things I always wanted to try in. Thompsonis the very thick, short

buffer spring out of a M1918A2 BAR buffer. Seems to me it would be a good shock

absorber...

 

Bob

 

 

Every time I look at this thread I end up sitting and thinking for 20 minutes.

 

I'm trying to puzzle this out qualitatively because I don't have any data.

 

I found this slow-motion video of an M1A1 firing, you can see that the bolt has some decent amount of velocity when it thumps against the buffer.

 

It looks like it's hitting the buffer fairly hard. But is it hard enough to care about?

 

 

Think about it, the Garand operates with huge force, the op rod handle comes back so hard that it will split your hand open.

 

If that doesn't bother the Garand receiver at all, would 1/10th or 1/20th of that amount of force bother a Thompson?

[RChapman]

Well, here is another video, at 06.30 it shows the 1921 operating at slow motion, it can be further slowed down by youtube settings

it definitely hit the buffer pushing the gun back with a sharp kick, which definitely helps the muzzle climb

Of course, this is not a fatal problem as we all know, and if they probably not further improved the buffer back in the days, its because it wasnt necessary.

also there is another thing to point out  and please dont ban me for my poor example lol

but if you bash the urethane disk with an hammer you will probably break it in pieces, this doesnt happen when the bolt crush the buffer against it, hence, the energy its just enough to kick the gun back.

even if not needed, it could be interesting trying to address the kick, but considering the given space in the receiver, (for curiosity i have tried making some measurements) it appears there are only 2.5mm of space you

 

can use to absorb the remaining energy, unless you want to destroy the oiler pads, or you want the gun to not catch the second notch, the only solution would be to design a telescopic buffer like the mp40.

i'll tell you what, i dont mind keeping it like it is, otherwise its not a Thompson. but a Vollmer or Schmeisser Thompson

Edited January 21, 2018 by RChapman

[reconbob]

I think we have reached the point where when you arguing with

an idiot no one can tell the difference. And Brandon was already

thrown off the board once and has resurfaced as Scrambles.

I have to laugh when he takes a shot at Buzz. I am the first to

admit I get into with Buzz sometimes but I also have the highest

respect for his Engineering credentials.

Let's not forget that this all started back in the summer when

Brandon/Scrambles out of no where posted that Doug had "no

excuse" because he did not give Brandon a job.

Brandon - to have friend you have to be a friend.

 

Bob