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Double Rifles, Single Shots & Combinations >> Building Double Rifles & Gunsmithing
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Crusader68
.224 member


Reged: 25/11/11
Posts: 41
Loc: Baton Rouge, LA
Monoblock/barrel thread size
      #254257 - 29/09/14 08:43 PM

For those who used threading(with or without solder) to attach barrels to a monoblock, which thread form and size did you use? On bolt action, and others, where the barrels are threaded into the receivers the thread pitch is alot more coarse than the suggested choices found in Mr Brown's book. For 12ga the suggested size and pitch is 7/8-20, I'm curious to know if you get a tighter fit with finer threads?

--------------------
Erik S.
"... mais épargnez le visage"

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aromakr
.375 member


Reged: 20/04/11
Posts: 849
Loc: Hamilton, Montana
Re: Monoblock/barrel thread size [Re: Crusader68]
      #254287 - 30/09/14 12:02 AM

You can make the threads as tight/loose as you want when turning the barrel. Course threads just give you a deeper bite. I believe the size of the mono block and cartridge being used will dictate what thread you can use.
Bob

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Dumprat
.300 member


Reged: 20/02/14
Posts: 205
Loc: Vancouver island bc.
Re: Monoblock/barrel thread size [Re: aromakr]
      #254306 - 30/09/14 03:12 AM

Fine threads have more strength. Larger surface area of contact within the thread itself. Machine threads have specs for fit and will need to be held in order to have the strength that you are wanting. You can google the sizes for 7/8-20tpi and you can measure them with wires and a mic to get it right.

Loose threads are probably not a good idea....

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Waidmannsheil
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Reged: 19/04/13
Posts: 2443
Loc: Melbourne Australia
Re: Monoblock/barrel thread size [Re: Dumprat]
      #254319 - 30/09/14 07:00 AM

Fine threads can be torqued to a greater amount than course threads when good material is used. They also have a greater ability to stay tight when subject to vibration or shock loads. The automotive industry uses mostly fine threads for these reasons. In soft materials however, fine threads will pull out and therefore a course thread must be used. Older bolt action rifles such as a Mauser use course threads as they are case hardened all over. If you case harden a fine thread, the thread will be all case and no tough core and will tear out. A course thread will have a hard skin but still have a tough core. Modern bolt action rifle designs such as the Browning A-Bolt (Howa) have a fine thread as they are not case hardened but locally heat treated where required.

Waidmannsheil.

--------------------
There is nothing wrong with vegetarian food, so long as there is meat with it.

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Shotgunlover
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Reged: 24/08/13
Posts: 45
Loc: Greece
Re: Monoblock/barrel thread size [Re: Waidmannsheil]
      #254353 - 30/09/14 07:43 PM

Interesting subject. I wonder if the threading process on barrel and monobloc, due to the extended surface scoring involved, weaken the metal more than a smooth surface.

There is the alternative of the fretted barrel used by the French (Cannon Frette) where the monobloc is heat shrunk and sodlered onto the barrel. Ron Vella used liquid nitrogen to achieve a fretted barrel fit in his last project the 6.5mm.

I am wondering, not offering an opinion. Is there any risk of the barrels loosening and coming off the monobloc?

--------------------
Shotgunlover

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Dumprat
.300 member


Reged: 20/02/14
Posts: 205
Loc: Vancouver island bc.
Re: Monoblock/barrel thread size [Re: Shotgunlover]
      #254363 - 30/09/14 11:28 PM

Heat shrunk fits have huge holding power. Depending on the amount of interference and the thickness of the surrounding metal.

I used to shrink sprocket hubs on shafts for sawmill drag chains. 4.437 shaft with .010 shrink and a 24" sprocket pulling several hundred yards of heavy chain and bark,dust and debris. Easily serveral tons on some occasions.

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Crusader68
.224 member


Reged: 25/11/11
Posts: 41
Loc: Baton Rouge, LA
Re: Monoblock/barrel thread size [Re: Dumprat]
      #254372 - 01/10/14 02:35 AM

My monoblock is based off of the 12ga set from the factory, caliber is going to be .500 ne or more likely .450 3-1/4" because of the cheaper and more wide availability of components. Is there any negative aspects to using even finer threads than the 7/8-20 as Brown suggested? Looking in my reference books I do see specs for 7/8-24, but with each increase in thread pitch there is also the need to ream a slightly larger hole but I guess it really goes by the starting thickness of the 12ga chamber walls.
I would be concerned with warping if the barrels have to be heated along with the monoblock. I'm not familiar with heat fitting but I do know some materials have different rates of thermal expansion which would have to be taken into account, or at least I think they would.

--------------------
Erik S.
"... mais épargnez le visage"

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Waidmannsheil
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Reged: 19/04/13
Posts: 2443
Loc: Melbourne Australia
Re: Monoblock/barrel thread size [Re: Crusader68]
      #254382 - 01/10/14 06:52 AM

With a shrink fit there would be no distortion of the barrels as the monoblock is heated and the barrel is cooled in liquid nitrogen. This provides a very strong fit which if the machining finish on the mating components is good, ie. a ground finish and they are both parallel then there will be a strong bond. Often components joined in such a manner are impossible to press apart without deforming one or both components, and require machining to separate. Fine threads pose no problems, most clevis joints on hydraulic cylinders use very fine threads and look at the forces that they are dealing with. Gun making which dates back hundreds of years traditionally use solder as they did not have the materials to make decent cutting tools such as taps. Parts were soldered and keyed. Today's materials are infinitely better than even 80 years ago and not only are fine threads usable, they are desirable, providing of course on doesn't go ridiculously fine. Beretta even used a Loctite type adhesive at some stage to join barrels into the monoblock of its shotguns. Merkel proudly advertised for many years that their demi-block barrels were joined by soft solder, they were however also keyed behind the ejectors with two long Scotch keys.

Waidmannsheil.

--------------------
There is nothing wrong with vegetarian food, so long as there is meat with it.

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Shotgunlover
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Reged: 24/08/13
Posts: 45
Loc: Greece
Re: Monoblock/barrel thread size [Re: Waidmannsheil]
      #254514 - 03/10/14 06:02 AM

Ron Vella used EDM wire cutting for his monobloc for his latest 6.5 project. Using wire cutting is probably more costly than conventional machining, but beig force free it needs no holding jigs. Check out his thread on the 6.5 build, it is informative.

--------------------
Shotgunlover

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dpcd67
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Reged: 08/07/13
Posts: 25
Loc: USA
Re: Monoblock/barrel thread size [Re: Shotgunlover]
      #256228 - 03/11/14 05:54 AM

If you are worried about heating the barrels, you should not be building doubles. You will have to heat them to put the ribs on and to regulate them; remember they will be bent in the regulation process. And to attach the quarter rib. More heat to solder them to the monoblock won't hurt anything.
I would never consider a heat shrunk barrel to be safe in a home built double rifle as the holding strength of thin walled monoblocks is not much. I use threads, with or without solder; doesn't seem to matter. There is not much thrust on those threads so the pitch doesn't matter.
And coarse meaning, not fine, is spelled, coarse.

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Crusader68
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Reged: 25/11/11
Posts: 41
Loc: Baton Rouge, LA
Re: Monoblock/barrel thread size [Re: dpcd67]
      #256892 - 17/11/14 11:57 AM

Yes of course one must heat a barrel to solder ribs and other parts, but the choice of which soldering alloy is very important as some silver solder(like the stuff I use in jewelry work) doesn't flow until the 1500 F mark which is not a good idea for use on ribs. Barrels are heat treated and stress relieved/tempered to very specific tolerances by the manufacturer and when you heat them too hot they warp, which is one reason why full on welding isn't used(I think). One can get extremely good bonds from shrink fitting, I(along with many others) used that to attach a drill chuck to a tapered arbor, but the arbor and chuck are produced to extremely tight tolerances from the factory so not much heat is needed(or desired) to do the job, app 300 F. I don't know how much heat barrels can withstand in a concentrated area before it becomes detrimental, and my point above was to say as much and express caution to anyone who should like to try. In Mr. Brown's book he shows a good illustration of what too much heat does to barrels, so proceed with caution.

P.s. Please excuse me if I misspelled any terms, I'm from south Louisiana and we speak differently and have a notoriously poor public school system.

--------------------
Erik S.
"... mais épargnez le visage"

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Ron_Vella
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Reged: 29/04/05
Posts: 432
Loc: Oshawa, Ontario, Canada
Re: Monoblock/barrel thread size [Re: Crusader68]
      #257010 - 20/11/14 12:25 PM

In my opinion, "fine" means 32 threads per inch and that is what I've used on my builds where the barrels were threaded into the mono-bloc, eg; 3/4"-32, 13/16"-32, and 7/8"-32. Such taps are available from any large supplier of machine tools but will cost you north of $100.00 each.

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Shotgunlover
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Reged: 24/08/13
Posts: 45
Loc: Greece
Re: Monoblock/barrel thread size [Re: Ron_Vella]
      #257053 - 21/11/14 10:04 PM

Beretta stated in its old catalogs that it smonobloc solders had a low melting point, the figure was, if i remember rightly, about 250 degrees centigrade. Hardly hot enough to induce warpage in steel.

--------------------
Shotgunlover

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Ron_Vella
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Reged: 29/04/05
Posts: 432
Loc: Oshawa, Ontario, Canada
Re: Monoblock/barrel thread size [Re: Shotgunlover]
      #257058 - 22/11/14 03:42 AM

Shotgunlover,

I'd also be willing to bet that they used a solder which was ground to a powder, and mixed to a paste with its' proper flux. I'd also bet that the barrels were held in jigs and placed in an oven, in batches, where the heating and cooling was strictly controlled as to amount and duration.

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