Good morning/afternoon/evening gents, got a stupid question for those more knowledgeable than I.
When it comes to fitting a barrel into a trunnion, one usually uses a press but I find myself asking if it is really necessary for a home builder. We all know that heat causes things to swell and I'm sure most of us have used a torch to loosen a stubborn part.
Now I know we cannot heat the trunnion for fear of ruining the heat treatment, but what about cooling the barrel? The Ak sees wide use in the Saharan region in Africa so the trunnion must be good to 150 or so. Likewise it is used in Siberia so -50 is probably safe for the barrel. That's a 200F difference we can exploit.
That is, the final diameter of a tube is equal to the initial diameter times the product of the temperature difference and the coefficient of expansion plus one. Given the coefficient of expansion of steel is 9.4uin/inF this means for a two hundred degree difference we can count on about .002inches of play.
That doesn't seem like much, but two thou is the difference between needng a press and not.
I'd really like someone with more experience than me to slap me about the face and remind me that a little knowledge is a dangerous thing because I'm thinking a heating lamp can warm without heating and that dry ice and rubbing alcohol are available at the grocery store.
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Posted: July 28 2016 at 10:25pm | IP Logged
One issue since you mention AK is production tolerance. You might still be pressing with .002 or .004 shrinkage.
In general (not barrels)have done .003 interference fit with a kitchen freezer and camp stove and LIGHT hammer taps. Halfway in is when you know if you measured right and let the parts reach temp.It really sux when something stops.
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Posted: July 29 2016 at 8:05am | IP Logged
I have frozen a AK barrel before in the home freezer.
Just a couple hours and it aided in pressing when i had a problem with pressing without freezing.
The only thing i have done manually was freeze plugs for an engine, freeze em then tap in to the engine block.
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Posted: July 29 2016 at 2:33pm | IP Logged
I do it all the time when fitting bearings I shrink the bearing by freezing it overnight and then spray it with plumbers freeze spray for freezing pipes.
Also rather than heating the hole it is being pressed into I have also frozen that, the metal shrinks and the hole does appear to increase in size, it sounds counter intuitive but seems to work.
The plumbers freeze spray works great, and doesn't tend to make things brittle when pressing into cast iron.
I recently repaired the head on my Victoria milling machine after the upper bearing seat broke out.
I had it bored up and a top hat pressed in to take the bearing, I froze both the top hat and the cast iron of the head then pressed in the top hat which took very little pressure on my workshop press
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Posted: July 30 2016 at 1:01am | IP Logged
A very heavily insulated box filled with dry ice, will chill the part down to -79C, at that temperature you get better shrinkage than you can get with a domestic freezer. You can also use an small electric oven that can heat to 250C, for the female part.
Using that method I've achieved interference fit of 0.2mm or 0.007". That was used for shrink fitted gundrill tips to their shafts, by the way.
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