T
tasy_ted
Guest
hi all
jerry thanks for picking that up was a little pissed on my home made ginger beer
regards tasy_ted
hi all
jerry thanks for picking that up was a little pissed on my home made ginger beer
regards tasy_ted
Hi All:
I'm late to this thread. I ran across it because I'm rebarreling a 22 PPC USA target model L461 myself. The barrel thread has been a mystery to me until I ran across this thread. It all makes sense now. A big thanks to Joe for the drawing information.
But I am REALLY curious about how you folks can call this situation an interference fit, and also how you can assume that it will improve accuracy. Unfortunately, my lathe does not have a 1.6 metric thread capability. So I am trying to make myself comfy with the idea of doing a 16 pitch thread. I`m just not there.
By my analysis, as the barrel is tightened, the so called interference will build up at the two ends of the tenon with good thread surface to surface fit and be loose in the middle of the threads with a tiny gap between faces, or worse be tight to the barrel shoulder and tight only at one end of the threads getting progressively looser toward the other end.
This is exactly what you DON'T want for accuracy. Its called binding, not interference. Binding creates the potential for the action and barrel to move between shots differently such that consistent vibration is lost. When consistent vibration is lost, so is accuracy. Action to stock binding is a very well known problem. I would think that action to barrel binding would be even worse.
Can you folks help me see why my concerns are unfounded?
I'm late to this thread. I ran across it because I'm rebarreling a 22 PPC USA target model L461 myself. The barrel thread has been a mystery to me until I ran across this thread. It all makes sense now. A big thanks to Joe for the drawing information.
But I am REALLY curious about how you folks can call this situation an interference fit, and also how you can assume that it will improve accuracy. Unfortunately, my lathe does not have a 1.6 metric thread capability. So I am trying to make myself comfy with the idea of doing a 16 pitch thread. I`m just not there.
By my analysis, as the barrel is tightened, the so called interference will build up at the two ends of the tenon with good thread surface to surface fit and be loose in the middle of the threads with a tiny gap between faces, or worse be tight to the barrel shoulder and tight only at one end of the threads getting progressively looser toward the other end.
This is exactly what you DON'T want for accuracy. Its called binding, not interference. Binding creates the potential for the action and barrel to move between shots differently such that consistent vibration is lost. When consistent vibration is lost, so is accuracy. Action to stock binding is a very well known problem. I would think that action to barrel binding would be even worse.
Can you folks help me see why my concerns are unfounded?
Mike Bryant
Active member
There have been an awful lot of Sako actions barreled with 16 tpi threaded barrels. Never particularly heard of an accuracy problem with them that could be attributed to the barrel thread fit. I'd say it's not a problem and probably not much different from fitting a barrel to some of the custom actions that use a slightly tapered thread to help with loading the barrel threads when they tighten. I looked at my lathe to see if would even cut a 1.6mm thread and it will do 1.5 and 1.75, but not a 1.6.
Butch Lambert
Active member
Mike,
My pages that I saved with barrel threads say the L46 is about .855X16TPI. It does not say 55 or 60 deg. thread. I know it shoots real well.
My pages that I saved with barrel threads say the L46 is about .855X16TPI. It does not say 55 or 60 deg. thread. I know it shoots real well.
martin zuck
Member
Some times I think we make things a little more complicated than necessary. During the last fifty years I have cut more than a few mauser threads at, heaven forbid, sixty degrees. Did these guns shoot any less accurately? Were they any less safe to shoot? Not to my knowledge. Also have had the same results with L461 sako's at 16 tpi. A few years ago I got hold of Harold Vaughn's "rifle accuracy facts" book and read with great interest about the "ramp thread". It made a lot of sense to me so I ground up a high speed tool and threaded up several 6ppc LV barrels. As I said, the idea of equally loading all of the threads seemed like a really good idea and it probably is in other applications but we failed to see any advantage when it came to short barrel shanks.
I have Harold's book too. I will read that section again. Who knows, it may shed some light on the 1.6mm pitch vs 16tpi.
I can see how it may or may not make any real difference. But given the plethora of other things we do that we "think matter", I'm not willing to just ignore it because its easier. My own casual analysis suggests its not the right thing to do. It seems rather intuitive to me that it is better to have threads that match up as well as possible so that they load up equally across the whole length of the tenon as the barrel shoulder is tightened against the receiver.
FWIW, I don't think that the thread angle is as important as correct pitch. The angle is likely to distort the steel a bit at each mating surface and perhaps it could even be called an "interference" fit. But I think it would still spread the load across the whole length of the tenon. If it just so happened to yield the steel a bit on the face of the barrel or receiver thread surface - so much the better.
Your comment also reminds me that I know some fastener engineers in the fastener world and also that I already have a good engineering text book on fastener threads that may also be helpful. I have used that stuff to calculate proper torques in the past and it has served me very well. I just didn't think about the tenon as a fastener till you mentioned it! duhhhh...... LOL!!!
Cheers!
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Butch Lambert
Active member
Is that 1.6MMto a millimeter? How does that actually compare to 16tpi?
I didn't read V's book, maybe there is a different meaning in his world but 1.6mm and 16tpi are almost the same http://www.newmantools.com/tech/pitchconversions.htm
Metric and sae thread specifications differ in how they are designated
SAE threads are described in terms of how many threads there are per inch of thread shaft. So 16 tpi (threads per inch) means that there are 16 threads per inch of shaft.
In metric the thread is described in terms of how far it is between individual threads. So a 1.6mm thread means that it it is 1.6 mm from one thread to the next.
Since 1 inch is exactly the same as 25.4 mm, a metric thread designation can be converted to SAE simply by dividing the metric thread pitch into 25.4. Therefore a 1.6mm metric thread is the same as 15.875 threads per inch.
Its that 0.125 difference over 1 inch, which is the same as 1/8 of a turn or 45 degrees over 1 inch, between metric 1.6 and SAE 16tpi that has me worried. Note that since the tenon is really .855 inches long, not 1 inch, then its really .125 x .855 = .1069 threads or 38.475 degrees of turn.
I do not think it is trivial at all!
But I'll be back later with a full engineering analysis......
Butch Lambert
Active member
I'm so excited! I hope this doesn't mean that my Sako that has shot many hundreds of rounds is unsafe? inaccurate? or what? I don't care about index as I have no previous lettering on the barrel to align or iron sights for that matter.
You are funny Butch! Your sarcasm is very well done.
But just in case.....
Index doesn't matter cuz you are rebarreling and even if you are just re-cutting the chamber with a barrel that has letters, you just cut the shoulder till they align and then cut the tenon to the right length and then chamber till it headspaces right.
Safety is not the issue either. The Safety Factor built into a rifle is WAY more than any thread fit at this level would impact.
The only issue of concern to me is ultimate rifle accuracy potential. If I want a reasonable shot at being able to produce consistent 0.1 to 0.2 moa groups then I have to do everything right and leave no stone unturned. I'm sure very few customers would complain at 1/2 MOA or even 1/3, but a fussy customer looking for the most is a whole nuther matter.
To achieve the best results, smiths lap lugs, square receivers, align bolt and barrel, cut precision crowns, custom select match barrels, precision cut threads and chambers, glass bed actions, etc etc etc. The list is a mile long.
The Sako is no Remington diamond in the rough. But the old girl I am working on was a good one. She produced 0.2" groups all day long in good conditions. Then the owner went away on vacation, lost power, and the bore got rusty and pitted. If I put a select match barrel on it with appropriate load development that does less, I am gunna look pretty sloppy.
So I don't want to leave this alone just cuz "everyone does it that way". I wanna know whether it matters or not at the performance level that I am trying to achieve. If I can do that at 16tpi - FANTASTIC. If not, well the job just got much harder to do..... I'm willing to do some analysis to find the answer and hopefully learn something in the bargain.
PS - the only reason I mentioned turns is to point out how much the thread misfit really means in a way that others would understand better. Any who has ever screwed a barreled action together will quickly agree that an eighth of a turn is a lot!
Butch Lambert
Active member
You know I would let you humor me a little until you said 0.1-0.2 groups consistently. Tell me what 1/8 turn means in a barrel with no sights? If you are rebarreling, it really doesn't matter. Put the sights and lettering where you want it. Refresh my memory, what does lapping the lugs have to do with accuracy? If you have to lap the lugs you have not done your machine work correctly. or your bolt is loose in the bore and lapping will not help at all.
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Wow, where do I begin......
Lapping the lugs has very little to do with your machining. If your machining is perfect, and assuming that you face the mating lug surfaces in the receiver, and assuming that the bolt is factory, then the bolt lugs will not usually be perfectly square to their matching surfaces in the receiver and will usually exhibit uneven or patchy wear patterns. When the rifle is fired, the uneven support of the lugs against the receiver faces causes the receiver to bend which imparts additional vibration which affects accuracy. Lapping corrects this by making sure that the rear of the bolt lugs and their matching receiver lug faces are a perfect fit. If your bolt is that loose that this doesn't matter, throw the whole rifle away or give it to someone who thinks 1 MOA is a good accuracy objective for a target rifle.
To be truthful, I expect that lug lapping will not be required on this rifle. After all, it was a tack driver before, and both lugs and receiver lug surfaces show very even contact across their entire surface. That said, lug lapping is standard procedure when building a precision rifle.
I seem to have totally confused you on this 1/8 turn thing. I am sorry for that. This was certainly not my intent. The issue is that the last thread on the tenon needs 1/8 more/less turn to reach design holding friction than the first thread on the tenon does. Ideally, the torque and resulting friction in any threaded connection should be the same across all the threads so that each of them bears their own equal share of the torque and holding friction load. They should not be tight at one end and loose at the other. It is this uneven sharing of the load across the threads that concerns me.
I would actually like to think that it doesn't matter. But I fear otherwise. In fact my fear is that the stress on the thread metal will be so bad that it will result in a few threads seeing four or five times their design stress while some see none. This will no doubt cause the thread metal stress to exceed its yield strength and cause permanent damage. The damage is inconsequential in that it will probably only take a permanent set. Its the resulting vibration and its effect on accuracy that worries me.
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Read Harold Vaughn 'Rifle Accuracy Facts' chapter 6 'Barrel-Receiver Threaded Joint Motion' to see that with your "perfectly executed threads" the first couple t'reads on the shank take 60% of the load whilst the last few nearest the boltface are just wagging in the air completely unsupported
John Kielly
Shari's fan club
Wished I could find my old thread gauges so I could run a 16TPI & a 1.6 mm together & see how they meshed.
They would appear to mesh just fine. Thread gauges are not long enough to demonstrate the problem. It would be better to put two bolts together to see it. Thread gauges are used to determine what the thread is assuming that you already know if it is metric or SAE. They don't work really well for the tweeners that quasi-fit.
oooops!
I think we have all done that before!!! I am too new on this forum to have goofed that way, but with time it will happen...... LOL!!!!
Butch Lambert
Active member
Why would you lap the bolt lugs and lug abutments on a factory bolt? My thread gauges show the 1.6 and 16 to mesh perfectly. Is the thread depth different?
mwezell
Mike Ezell
The question was asked if it would make a difference in accuracy potential of the gun if threaded 16tpi...At least that's what I gather. If so, then my answer in no, it will not. But don't EXPECT your Sako to shoot in the 1's and 2's all day long, as you said it did before, as there are many br rifles that don't meet that standard and there are many variables that individually are more critical to that happening than the very slight thread pitch difference.
That's my 2 cents worth after reading all of this. TIFWIW.
That's my 2 cents worth after reading all of this. TIFWIW.