Threading a barrel, accuracy - Page 2
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  1. #21
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    A most interesting barrel /tenon thread is used in the Ross Model 1905/Military Mark II./Scottish Deerstalker/Military Target/Sporter/models.....the thread is a coarse 3- 1/4- tpi Left Hand ,and after tightening is locked into the tenon with an offset setscrew,which appears as a second hole next to the central securing screw hole.....To remove the barrel,unscrew the setscrew ,rap the barrel with a copper mallet,and it unscrews by hand.......The barrel thread is of a peculiar form ,entirely to accomodate the method of cutting with a milling cutter in a dedicated machine.....These guns were unbeatable in their day,and took 95% of the prizes at the 1913 Bisley Teams competition at 1000 yards......The Ross .280 was a 1905 version of the 7mm Remington Magnum ,and the flattest shooting round of its day.

  2. #22
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    Quote Originally Posted by tobnpr View Post
    Thread fitment of receiver/barrel is detrimental if too tight- you don't want tight threads.
    What's critical, is the facing of the receiver ring and barrel shoulder to be perfectly perpendicular to the threads.
    "Loose" threads, within reason, are perfectly fine. Threads bring the parts together- it's the shoulders that align the parts.
    I make some parts designed by engineers and concentricity is very important on them. The drawings specify a loose fit to improve repeatability when these pieces are changed out. All the years I made stuff for Kleinguenther he wanted barrels, muzzle breaks and screws in the pillars to drag when screwed in. When I first saw the drawings for these later assemblies and gauges I thought they were wrong, but after I straighten one of these assemblies, I can pull them out and screw them in again and again and they stay the same when checked with an indicator. Thousands of parts later the process just keeps on repeating. So I have to say tobnpr and the engineers are right. Shoulders square to the threads and a relatively loose fit, what is not to like.

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  4. #23
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    Quote Originally Posted by FredC View Post
    I make some parts designed by engineers and concentricity is very important on them. The drawings specify a loose fit to improve repeatability when these pieces are changed out. All the years I made stuff for Kleinguenther he wanted barrels, muzzle breaks and screws in the pillars to drag when screwed in. When I first saw the drawings for these later assemblies and gauges I thought they were wrong, but after I straighten one of these assemblies, I can pull them out and screw them in again and again and they stay the same when checked with an indicator. Thousands of parts later the process just keeps on repeating. So I have to say tobnpr and the engineers are right. Shoulders square to the threads and a relatively loose fit, what is not to like.
    I wonder if those same qualities were noted by the practical men of yore who designed and used threaded machine tool spindles.

    I wonder.... Well, no I don't wonder at all. That is why a tight fitting "register" on a lathe back plate is not required for accuracy and repeatability.

  5. #24
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    Yes. Shoulders must align, flanks\thread angle must match. Smooth running fit with no excess play or tight spots is the key. Take your time, sneak up on it, and get it right the first go. Threads are hard to fix.

  6. #25
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    Quote Originally Posted by CalG View Post
    I wonder if those same qualities were noted by the practical men of yore who designed and used threaded machine tool spindles.

    I wonder.... Well, no I don't wonder at all. That is why a tight fitting "register" on a lathe back plate is not required for accuracy and repeatability.
    Funny that you bring up machine spindles. The drawing note is for a machine spindle and I quote directly from the drawing:
    "THE OVERSIZED THREAD (7H) IS ESSENTIAL
    FOR ACHIEVING TARGET RUNOUT SPEC.

    M5 x .08-7H"

    The spindle supplier for this OEM has a 180 to 270 day lead time!
    In the attached photo is a threaded bushing that I hold in a 15mm accuracy collet with a threaded stud below it. A finished assembly in the middle and a gauge on the right to make sure the bushing is running less than .00005 TIR. The gauge was turned, faced, and thread milled in one set up for zero runout. Tried to make the gauge between centers and could not even get close to zero runout. The threaded stud has a 1/8 bore in the end opposite the threads all important features are done in on operation. the threads are milled behind the shoulder with a special cutter.

    When I straighten a batch of these assemblies I always check after the last part is finished and the runout after I finish is always the same as when I started. If it was off slightly (less than .0005) the high spot will still be in the same clock position.
    My business is not threading barrels but knowing what I do now, after getting a tight fit on the threads I would take a couple light cleanup or spring passes to make a little clearance on the threads then take a light pass on the shoulder to make everything as perfect as possible.

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  8. #26
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    It used to really bug me that reloading dies are so loose fit in the press bushings. What I really should have been concerned with is the squareness of the lock nut face to the threads. I bought a batch of nuts that clamp onto the threads instead of using a set screw, I need to mount them all on a single pointed 7/8 mandrel and face and mark one side!

  9. #27
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    In post #25 I forgot to attach the photo. concetricitygaugers.jpg

    "If it was off slightly the high spot will still be in the same clock position." Missed a zero here should have been the same (less than .00005)as stated earlier.
    I am getting old.

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  11. #28
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    Barrel tenon threads can take over 1,000 foot pounds of tightening torque, but typically only get 100 foot pounds....so I think we can get away with sloppy threads.


    I am really grateful to the Caterpillar engineer who, on this forum years ago, explained how to calculate my max scope base torque.


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