receiver Threads help

L

Leeroy

Machinest / Gunsmith
G'day all

Just wanting get some advice from the more Knoledgable smiths here.
I was truing up one of my old sako actions this weekend, the usual set up in a spider chuck with a mandrel down the raceway to clock off.
Faced the front face and the lugs no problem, but made a huge stuffup skimming the threads..
I correctly set the gearbox to cut the required 16 TPI but i didn't check the change wheels which were set for metric thread.:(
I am normaly very thorough with this type of thing but i was in too much damn hurry.:mad:
Anyway the result was badly mangled threads..
After resetting the machine and re-cutting the threads to clean up , the receiver now looks pretty thin. Actualy it's 0.140 from the top of the threads to the outside of the receiver.. See attached pic. The front 2 threads have be machined out(the step you can see).
Is this too thin now to be used safely?? The rifle was to be chambered in .308 Win.


Thanks

Leeroy
 

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Leeroy said:
G'day all


Is this too thin now to be used safely?? The rifle was to be chambered in .308 Win.


Thanks

Leeroy
Click to expand...
,,,,,,,,,,,,,,,,,

The answer is no. Just on a calculated estimate and guessing the steel strength it should still be able hold some 20 000Kg of static tensile strength.

Shoot better
Peter
 
Probably OK

A Remington 700 is about 1.370 to 1.375 on the OD of the action. It has a major thread diameter of 1.062. That is a difference of .313, which leaves a wall thickness of about .155.
That is .015 lmore than what you have now. I would amagine that the Sako is manufactured from at least as fine a quality steel as the Rem, with the same level of strength.
I would find it difficult to believe that .015 would kill it.

Here is a little tip from a machinist. When using a small lathe to re-catch and chase threads in order to just true them, DO NOT use power. After catching the thread, engage the half nut and turn the chuck by hand. The most you will ever, (probably), have to take out is .010 or so, the extra time it takes to just turn the chuck by hand is worth not having to worry about the malady you have now.
I am a Machinist. That is the way I do it........jackie
 
Leeroy

This time you'r are safe and for now lucky as there is no danger of blowing up the ring. If you'r careful, you can utilise the "step" in the receiver to fit a 0.01MM clearance tennon shank and control the centering of the barrel. I know that Jackie and many others would disagree, but that's what I always do.

I somehow remember thay you've had some other Sako/Tikka? bolt related problem. How did you go with it?

Shoot better
Peter
 
Leeroy said:
G'day all

Just wanting get some advice from the more Knoledgable smiths here.
I was truing up one of my old sako actions this weekend, the usual set up in a spider chuck with a mandrel down the raceway to clock off.
Faced the front face and the lugs no problem, but made a huge stuffup skimming the threads..
I correctly set the gearbox to cut the required 16 TPI but i didn't check the change wheels which were set for metric thread.:(
I am normaly very thorough with this type of thing but i was in too much damn hurry.:mad:
Anyway the result was badly mangled threads..
After resetting the machine and re-cutting the threads to clean up , the receiver now looks pretty thin. Actualy it's 0.140 from the top of the threads to the outside of the receiver.. See attached pic. The front 2 threads have be machined out(the step you can see).
Is this too thin now to be used safely?? The rifle was to be chambered in .308 Win.


Thanks

Leeroy
Click to expand...

Heck no!

For every bit of thinned receiver you have added the same thickness of barrel material to the diameter. The difference in the receiver and barrel steel properties is so minimal that is down in the noise levels................Don
 
Something else to mention here is

That the only critical area here is the barrel end/receiver thread inner end junction and its strength is limited by/to the ultimate tensile strength (stress) of the receiver steel itself. In this case it's at least five times the margin of what a .308W can generate.

Should we all be wrong, the first thing that would show up would be a hard case extraction due to the receiver stretch upon firing. Safe in this case, thought.

Shoot better
Peter
 
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Thanks everyone!

G'day all.

I feel much better now..:eek:

Jackie, Thanks for the tip.. I chased the original thread by hand after the first disaster and i will do them by hand from now on..:D

Peter, just to be sure..
I used a generic 60 deg threading insert to cut the thread, which have a minimum tip radus(Sharp point). Also to get all the threads to clean up the front threads were very sharply pointed on top when i had finished. I then took the tops off with a boring bar and a quick hit with the emery cloth to tidy up. Given that, by my calculations using my thread diagrams for UNF threads, the thread is arround 0.050 deep. This then leaves only 0.095 of solid steel arround the thread.. Is this still strong enough??

Oh and yes it is the same sako action as before. It had a hell of a lot of slop in the bolt/reveiver so I have rebored the bolt raceways and had the bolt metal sprayed and ground to size.. Still some work to do on the bolt, have to machine some new inlets for the port cover stop piece, and re-contour the port cover strip inner for the larger diamiter bolt..
Will post some pics when it's all done..


Cheers

Leeroy
 
Leeroy- if we calculate in the Round figures only

We end up with a solid "meat" of about 200 sq.MM in the weakest receiver thread area.

If the steel has estimated hardness of 36HRC it directly corresponds to an ultimate tensile strength (UTS) of abouts 120kg/square mm. Without knowing the exact yield strength (YTS) one can use "the rule of the thumb" and "safely" say the yield strength (YTS) it's 80% or a 0.8 of the UTS.

120x0.8 = 96Kg./sq.MM (YTS)
200x120= 24 000Kg UTS - of the receiver
200x 96= 19 200Kg YTS - of the receiver

.308W at 65000psi is about 4555/sq. centimeter old Atm.
The botom area of the .308W is about 0.785/sq.centimeter
4555 x 0.785=3575Kg of a total pressure. Not including any case grip at all.

19200 / 3575= 5.37 times the margin of the .308W at 65 000 psi.

Should the steel be harder the figures will also be higher.

If you'r not feeling safe on the calculations just don't do it. It's that simple.

Sorry guys, Leeroy is from a "Metric" country.

Leeroy I hope I've made it clear enough for you to understand. You can use the exact measurement to be more precise.

Shoot better
Peter
 
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Thanks..

Thanks for the indepth reply Peter..

I started to do the calculations you have done but couldn't find any ref of hardness of receiver or UTS/mm2 for 4140.
The numbers clearly add up and that totaly reassures me now!:D
BTW thanks for the conversions to metric.. Much easier for me to picture.. :D

Cheers

Leeroy
 
Keep in mind, the Sako is NOT like a Remington receiver in that the remington is cylindrical with constant o.d.

The Sako or Tikka actions have two nice dovetails cut into the reciever which will act as notches and stress concentration areas. Hopefully they are tangential to the solid ring of receiver you have calculated. If you haven't considered the notches you should consider them to be safe. Yes the barrel od will increase to handle the pressure, but barrel movement and axial shear loads are handled by the receiver. The thinner it gets the more likely it will flex and the higher the stresses. Consequently although it may work now, I suspect its fatigue cycles to failure has been reduced.

I don't know what factor of safety Sako uses in their designs but there normally is a generous one.

Try emailing Varmint Al to see if he can model something for you.
 
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Leeroy-if you go to ASSAB or Bohler they may have

A HRC testing machine and they would be able to measure the exact hardness and give you a hardness/strength reference chart. Toolmakers and heat-treaters are also able to provide you with the exact info that you want.

Just for your sake to be sure and feel safe this is what you should do. If I was you I'ld have done exactly that. Because if in your mind you'll feel unsafe everytime you pull the trigger you'll never shoot the rifle well.

Also keep in mind that a Sako receiver may be carburised and that would change the things a little. I'm not a gunsmith and I never had nothing to do with any Sako, but being made in Europe there is always that possibility.

An estimate is one thing and knowing for sure is the other.

Shoot better
Peter
 
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