Month: August 2005

Rifling benches

Posted by on August 7, 2005

I’ve somewhat lost touch of what I said earlier about barrel making.  I recall having mentioned the very simple methods used 200 years ago.  It amounted to a simple bench, not much more than a heavy wood plank to which simple fixtures were attached which carried the barrel and the rifling tool.  I have a partial photo of such a bench in an old black powder magazine, which shows a rifling pull rod made of wood.  I’ve just seen, in a recent magazine, a photo of an exact reproduction of an old bench currently used by a US custom BP rifle maker to rifle his barrels.  He does it so that his rifles will be exact replicas in every respect.I knew the old benches were simple, but I was surprised just how simple.  As near as I can describe in words, it is a heavy plank, much the same size as a scaffold plank, maybe 250mm wide x 50 or 75mm thick, and about 2.50m long.  It stands on a pair of trestles.  The drilled barrel is secured in a pair of wooden blocks which are fixed to the base plank.  Another pair of larger blocks is positioned at the other end of the base plank, and carries the pull rod.  The pull rod is a wood rod about 75mm thick x 1m long with a spiral groove cut in it’s side for most of it’s length.  One of the blocks carries a pin that engages the spiral groove, so that the pull rod rotates as it is pulled through the blocks.  The spiral is whatever rifling twist is wanted.Attached to the front end ( the end that faces the muzzle ) of the pull rod is a steel rod slightly smaller than bore diameter by about a metre long.  This rod traverses the bore back and forth.  It carries the rifling tool which is a simple but precisely made short rod of bore diameter with a cutting tooth of adjustable height.  The only other item is an indexing disc.Could hardly be simpler.  These days a much more accurate bench could be made from steel.  The pull rod would best be made from turned or precision ground rod.  The spiral groove would best be cut by a machine shop on a universal mill which is about the only practical way to do it.  I haven’t enquired what that would cost but I shouldn’t think it would be prohibitive.[Originally posted to SATalkGuns -- Admin]

Thread dimensions

Posted by on August 3, 2005

Rolly G wrote :

“US thread terminology ignores flank angle and just quotes OD and TPI. TheAmericans apparently have always ignored the angle of the thread and justcut the thread to the closest TPI until the two will screw into each other.I have seen this stated in at least one US gun book. The 17 TPI comes fromconverting 1.5 mm pitch into inches by dividing 1.5 into 25.4 which equals16.93. Apparently it has been done satisfactorily, according to the bookshave read on the subject, ever since WW1 when they started rebarrelingGerman made Mausers to 30/06.”

Yeah, that’s all true, Rolly. But it depends what is consideredsatisfactory. Such approximations might work for some threads but they areanything but satisfactory. It works in a fashion for the 98 simply becausethe 98 thread is pure Whitworth ie 12 TPI x 55 degree flank angle. AnAmerican thread would also be 12 TPI. The only difference therefore is theflank angle, the US threads being 60 degrees. The barrel will therefore fitthe action, except for the flank angle. It should be noted, however, thatthe important contact is at the flanks, but a US thread fitted to aWhitworth thread will have no flank contact, only point contact which isanything but satisfactory. It escapes me why a gunsmith would not grind athreading tool to the correct angle when it is so easy to do.It should also be understood that for any given pitch, the depth will bedifferent for 55 Deg and 60 Deg angles, so that either the major or minordiameters will differ. The standard major dia of the 98 is 1.10 inches.But if it is cut to that dia with a 60 deg flank angle the minor diameterwill be too big. Conversely, if the minor dia is correct for the action, a60 deg flank angle will result in a major dia less than the standard 1.10inches. Thread engagement will only be the crest of the female contactingthe bottom of the V in the male. Very poor and not to be recommended.Making a US thread fit a metric thread is another matter. It should beunderstood that there is no point of similarity. Consequently it can bedone with some threads but only by making them a very loose fit which isalso not satisfactory. The proper method is to cut the barrel thread to ahand tight fit in the action. In that case a 17 TPI thread probablywouldn’t fit a 1.50mm pitch thread. 17 TPI is 1.49mm. The Sako barrelshank is about 22mm long, depending on the model. That’s more than 14turns. With a hand tight fit that would amount to 0.14mm error at thefourteenth turn. That would definitely not work. It could only work witha loose fit, in which the error would be zero at one end and 0.14mm at theother, so that only one or two turns of the thread would turn tight evenallowing for the crush factor.Rifle barrels are the last place for badly fitting threads. There’s onlyone way to do it, the right way. If there were real difficulties in gettingit right I might understand the approximations. But there aren’t. One ofthe reasons the centre lathe is called the king of tools is it’s ability tocut any thread. So all that’s needed is to grind the tool to the rightflank angle and set the lathe gears to the exact pitch. OK, gearboxequipped lathes can usually thread only in one language or the other andwould need extra translation gears for the opposite language.Lathes without gearboxes like my old Myford are less convenient to use butwill cut metric and imperial threads. My Myford is in imperial language butwill cut all metric threads with some approximation. Not the sort ofapproximation referred to above, much more precise, within one in onethousand error in the worst case, less than one in eight thousand on somethreads.I suspect that US thread terminolgy ignores flank angle because Americansthink no other angle exists than 60 degrees. Astonishing considering theMauser 98 is 55 degree whitworth. American resistance to anything nonAmerican is such that Brownells sells a tap for converting the Mauser actionthread from Whitworth to the US 60 degrees. That’s probably becauseaftermarket barrels are threaded 60 degrees. Considering the sheer numberof 98 actions in the US the unwillingness to thread the barrels to suit iscarrying obstinacy to the extreme. To be fair, however, Jack Mitchell inhis book “Riflesmithing” says the Mauser thread is 12 TPI x 55 degreeWhitworth, and shows one being so cut in the lathe, so not all US gunsmithsare unwilling to get it right.[Originally posted to SATalkGuns -- Admin]