Technical: Locking Shoulder-heating/freezing to install?

[Mycroft]

I installed a locking shoulder in a rifle a few weeks ago, using the method that was suggested here: I let the receiver soak up the abundant heat out in the detached garage, and popped the locking shoulder into the freezer for an hour or so.

Then I positioned the rifle on the workbench and laid the hammer and punch nearby. With a dab of oil on the receiver to help things go smoothly, I went and fished the locking shoulder out of the freezer and dashed to the workbench. But between the house and the garage, the damned locking shoulder iced up! Water vapor in the air was encasing the thing in ice, and the ice reappeared as quickly as I could rub it away with my thumbnail. Finally, I dropped the part in place and - it just needed one light tap with the hammer to seat it flush. What an effort-saver.

[ August 08, 2001: Message edited by: gary.jeter ]

[ August 08, 2001: Message edited by: EMDII ]

[LooseCannon]

Glad to hear that system works.
Think I'll go that route next time.

Dave


[This message has been edited by LooseCannon (edited September 22, 2000).]

[Mildot]

Mycroft, did you also freeze the barrel before installing? Just wondering.

mildot

[Mycroft]

Mildot:

No, I had already filed back the barrel shoulder and done the trial fitting at "room temperature" before someone suggested the freezer method, but I may try it on the next one. If freezing the barrel causes it to time diffently *before* filing on it, I'll pass along the information. I suspect that it'll be less effective there than with fitting an interference-fit pin to it's bore, as with the locking shoulder. I've used the technique for other similar tasks before - most notably assembling auto engine piston and rod assemblies. Some don't use a floating wrist pin and you've got to heat the small end of the rod, freeze the pin, then align the piston on the rod and pop the pin in place. Just like a great big FAL locking shoulder.

But it'll be interesting to see if freezing the barrel aids in assembly.

[Mfg Eng]

The coefficient of linear thermal expansion for steel is .00000633 inches per inch per degree Farenheit. Temperature change required equals size change desired divided by .00000633 divided by size of the part. for example, to grow a .250 hole by .001, its .001/.00000633/.250=632 degree change in temp required. Not to be critical, but the difference between the freezer and garage is about 70 degrees. 70*.00000633*.250=.0001, not enough to be of practical use. Drop the pin in liquid nitrogen (-320) and heat the receiver to 300 degrees and you should get .001 on .250 diameter.

[ByronF]

Mfg Eng:

Wouldn't .001 change on a .25 tight tolerance fit (such as a locking shoulder) result in a loose-as-a-goose fit? I'm not arguing your engineering, I'm just wondering if a 70 F change might significantly affect the seating force for the shoulder even though dimensional changes are quite small.

Byron

[Mfg Eng]

ByronF, I didn't quite understand your comment about the loose fit, because the heating and cooling are temporary conditions for the purpose of assembly. When the temperature returns to nominal the fit will return also. Unfortunately I don't have experience with installing the locking shoulder. (I'm still working on my receiver vice). From observation I would say it is a press fit of about .0005 which you can drive. Even if you get .0001 less interference by heating / cooling, that will be lost the instant the two parts contact, thru conduction of heat. When using this technique you need to make enough clearance for the parts to fall together without much contact. In practice, .002 clearance will give you enough time to slide two parts together before they lock up (referring to press fits such as a bearing race on a shaft or in a bore). Then you need to have the oven and nitrogen bath right next to each other, as time is of the essence. You want to pull the heated part out, cooled part out, and drop them together, using gravity and a shoulder or fixturing for vertical location, within seconds. I do step on my dick sometimes because I don't know enough about the specific application. Just trying to show the physics and math involved so you can make better judgements. I deal quite a bit with metallurgy, and have great reference materials at my disposal. I like to pass information on when I see an opportunity, to help others understand what is going on. I do appreciate the comments, it helps me understand others' interpretation of what I say.

[Pete2]

I heated my reciever in the oven and froze my locking shoulder and still had to pound them together.
The headspace was too small so I took it apart (pounded it out) and the next time just pounded it back in without bothering to freeze etc. I didn't notice any difference in the amount of pounding I had to do.

[W.E.G.]

Here's a post that would have been fun to find by topic heading. I found it under "It's not the heat, it's the humidity..."

moved to FAQ