Precision barrel fitting, part six: more details of thread turning and headspacing are introduced as the barrel and receiver get closer to becoming a complete rifle.
Gone are the days when interested individuals taking a tour of a major rifle manufacturer's plant would find row after row of milling machines, surface grinders, drill presses, and engine lathes, each manned by masters of their craft intent on producing excellence in a firearm. Sadly, also gone are many of the schools they attended much earlier in life to begin acquiring the skills they eventually came to apply. These amassed craft masters, a vanishing breed, have scattered to become treasured fixtures with companies much smaller than the ones they previously spent countless cumulative years. It should surprise no one that the barrel makers presently employing them are the most widely recognized for the consistent quality of their products.
Due to the facts just stated there will continue to be no significant mention of aftermarket barrel making. As I have up to now, I will continue to concentrate on methodology to be applied with OEM barrels and barreled actions. Certain practices and techniques, including chamber and headspace check-outs, you may opt to apply when a customer has provided a replacement barrel from an aftermarket maker to be fit to an existing receiver.
OEM barreled actions excepted, determining the internal receiver threads with a thread pitch gauge must, habitually, become the first measurement taken when an aftermarket barrel is involved. This is to be followed, habitually, with any threads present on the barrel tenon. Unless the maker of the barrel has been provided with specific technical data on the internal receiver threads, the barrel tenon will likely be left unthreaded. With an OEM barrel ordered for a specified caliber and model there is far less risk of a mismatch so the tenon will be threaded accordingly. Check it out regardless. Any discrepancy between the threaded areas on any barrel and those inside the receiver should be followed, habitually, by returning the barrel to its source with sufficient reason why clearly stated along with a request for corrective measures as clearly made, at no charge of course.
I'll assume the measurement passed without complication and you'll be able to move ahead without further interruption or concerns.
Among the non-conforming thread problems, one can be ignored but the other cannot. In the easy department are degrees of angle differences in some commercial actions. Most use 60 degree angle threads. Military Mauser 98 actions and their assorted commercial clones use 55 degree angle threads. These angles are so closely related a 60 degree angle tool can be used for turning both. They are so closely related you may be tempted to have a go at uniting the two, dissimilarly threaded major components. Please don't. Why will become obvious in the next three paragraphs. The key words are "loose fits."
Maybe I've made some of you uncomfortable by mentioning the depth of threads and not turning them overly deep. Perform some practice runs with an 18" length of galvanized pipe. Turn a set of 60 degree angle threads to 0.20" depth on one end of the pipe. Reverse ends and make another set of threads exactly as long and deep as the first. Exactly.
Now for the can't-be-ignored department. It exists within the Enfield M14, M17 and Springfield Actions: Square Head Threads. The tool used for cutting these is like a cutting tool, only shorter. The width of the point has to be accurately ground to size. The grinding has to occur off center and to the left on a tool blank.
More needs to be said about threads for "square heads." They aren't as easy to cut as 60 degree. The compound rest cannot be used to advance cutting depth, but the cross-feed can. All cuts must match the finished thread tenon diameter at 90 degrees. After every third pass at no more than 0.0030.005 per pass, perform a clean up before making another set of passes. Always remember that if you go too deep you will assuredly end up with loose fitting threads. Loose fits have no place in a mechanism subject to 40,000-55,0900 psi.
Before attempting the marriage between a barrel and a receiver, thoroughly clean the threads of both components. Be positive they are free of fouling or rust. Use a toothbrush dipped in a quality solvent for the cleaning, then throw away the brush. While there are a number of very good solvents for this purpose, they all taste lousy.
Maybe you need a little good news on thread turning about now. Unlike squares, 60 degree angle threads can be cut using the compound rest as your depth guide. The first three or four passes may be 0.020 per pass. After that, reduce the depth by 1/2, then 1/2 again as the thread nears completion, or only 0.001 per pass until thread tenon depth is met. That's met, not exceeded. It also means you will have to perform an increase in cleanups and checking barrel-to-receiver lit between passes. So be it.
Breech End Thread Relief
In most receivers internal threads have been cut with a tap. This being the case, it's not unusual to discover the last thread or two haven't been cut to the proper finish depth. There's no way the barrel/receiver fit will end up flush. Don't attempt a resolution of this by applying major force to turn in the barrel. Tenon threads were never intended to serve as a tap.
Now what? Carefully hand tighten the receiver onto the barrel. With a feeler gauge, measure the gap between the face of the receiver and the barrel shoulder. The gauge is telling you how much has to be turned off the shoulder, thereby "setting it back," to achieve the perfect fit. Do as the gauge dictates, plus 0.015. Try the fit again and obey what the gauge says.
You may need to make still another reduction ranging from 0.005-0.015 inches before the gauge can take some time off. When you are convinced the barrel shoulder/ receiver interface is optimum, check it again. Is it too narrow? Is it too shallow? In both cases, the problem is usually a single thread (just one measly thread) at either end. Locate it. Relieve it. Try the fit. If no further adjustment rears its head, chambering and headspacing can be pursued.
No, I haven't overlooked chambering or headspace. Not everyone agrees whether a barrel should be chambered before or after tenon threading. It is no surprise I stand in agreement with John Hinnant. Rough chamber reaming should happen before threading. Why so? A smooth tenon doesn't abrade or wear steady rest jaws as quickly as a threaded tenon. Worn steady rest jaws or even steady rest jaws undergoing the process of being worn lead to an earlier loss of concentric alignment. In addition, after rough chambering, a live center is used to support the barrel breech. The live center maintains thread cutting that is concentric to the chamber.
I'm making the same assumption I made earlier. To wit, you are working with an OEM provided tube and not a barrel blank. I maintain a belief than any human is capable of error and any machine can develop a temporary glitch. Ergo, no hostility or disrespect implied or intended, I practice caution in defense of my good reputation and a deep desire to continue steering clear of litigation by checking out any chamber delivered unto me.
Correct Chamber Depth
As you know, strictly adhered to specifications for all American cartridges and chambers have been established by the Sporting Arms and Ammunition Institute (SAAMI), Newtown, CT (203/426-4358, saami.org). You can purchase drawings and supporting data for all handguns and shoulder arms or, perhaps after a bit of pleading, only the chamber specs for the cartridge to be fired from the rifle at hand. For the long haul you're probably better off going the full monty.
First off, remove the extractor(s) from the bolt. When more than one is involved keep track of which is which. Reinstalling them incorrectly will lead to extraction/ejection problems that will tick off the rifle owner who will, rightfully, finger you as the cause. Now install a GO headspace gauge in the chamber. The gauge will prevent the receiver from fitting tightly against the barrel. That doesn't mean you've done something wrong. Maybe someone, or something, has though. Measure the gap between the receiver face and he barrel shoulder with your feeler gauge. You've just discovered how much more the chamber has to be deepened. What is required to accomplish this is a finishing reamer. What isn't required is turning on the lathe. Finish reaming is all done by hand power.
I recommend you devote extra time to study of the Hinnant book. Make a slew of notes to serve as step-by-step tips. They'll help a lot to avoid frustrating, and costly, errors. This advice should be emphasized in approaching and executing the chambering and headspacing phases of barrel fitting.
Much more will be said of chambering in Part Seven. Until then, consider the emphasis given.
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|Date:||Sep 1, 2014|
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