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Posted: 6/24/2011 8:33:30 PM EDT
| I've been seeing Colt M4's with small dimples in the take down and pivot pins lately.I've also seen small dimples on FN pins as well.I'm guessing this proves the early dimpled ones served no purpose and this is just the way they machine them sometimes.BTW the Colt M4's were pretty new.I normally don't see them on most M4's that come in |
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I think the dimples are a marker of industrial processes. In the early guns, the dimples were used to center the parts for metal finishing (probably grinding, which was used to bring the parts to exact size and concentricity). Later, a centerless grinding system was probably used for faster throughput and/or higher quality. Now, a new centered system lets some vendor beat the previous price.
Here are the two systems explained, centerless and cylindrical centered grinding. Note the shapes of the centers. Look familiar?: http://www.efunda.com/processes/machining/grind_centerless.cfm http://www.efunda.com/processes/machining/grind_centered.cfm?search_string=centered%20grinding As far as the difficulty of drilling the dimples goes, in production, the parts are hardened after machining is complete (and the dimpling would be a fairly early machining process, if dimples were used to center the parts for further machining). If you want to drill them, anneal the parts, then drill, then re-heat-treat. Same same if you want to neuter an FA selector or make a prototype with different positions (safe on center). In the real industrial world, it's always machine first, harden later, for reasons which should be self-evident. Here is some generic information on annealing steel. Does what he says about ruining tools on hardened steel parts sound familiar? To have a better idea than this what process to use, we'd have to know what alloy and process was used in the factory parts. Then you could simply look the annealing process up. http://hagerson.livejournal.com/96622.html |
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B Rodgers; Yep hard stuff! Go get yourself a small carbide center drill. Thats your weapon of choice, not a HSS or cobalt drill. The cobalt would probably work if your speeds and feeds were just perfect, but plain old HSS doesn't stand a chance.
Hognose: while I am a relative newcomer to industrial processes, and of course the weapon of choice nowadays would be a CNC lathe. I respectfully dissagree about your centeless grinding as a requirement. I still think that a lathe might have been used to produce these parts. But like I said, I don't know a lot about the older processes. I just think you can get a decent finish and hold these tolerances just as well on a lathe. I know with my manual lathe I can easily, and repeatedly hold tolerances of plus or minus .0005" on a production basis. If I am pressed to hold tighter tolerances, that too is possible. Now that I have shot my mouth off, I will read your links to see if I am wrong. Or just how wrong I might be! 
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I don't have your experience machining, for sure. And my experience in the machine tool industry is quite out of date. I worked for a company that sold the only American machine tools that were used inside Saturn's Spring Hill, TN plant –– long enough ago that the plant is abandoned (I believe), the workers idles, and the marque dead. Some day someone will write the whole tragedy of Roger Smith's idea for a company that would infect GM with Japanese-style (Deming-style, really) quality excellence. Instead, Smith hit the bricks and the new regime ensured that Saturn instead was infected with all of GM's pathologies. RIP. But all the machining centers, CNC stuff, tool and part handling conveyors, laser inspection jigs, you name it, were Japanese. Only some of the grinders were American. At the time, the Japanese couldn't hold our tolerances (now they can, Toshiba particularly).
Anyway, the whole idea of Armalite was to adapt aerospace, automotive and screw machine technology to the whole craft of gunmaking. Ideally, it would make it possible to make guns better, faster and cheaper. These three are the Holy Grail of industrial engineering and young engineers are told that for any given problem, they can pick any two. Only a technological breakthrough lets you improve all three at once (CNC is one example of a technological breakthrough). I think the history of the black rifle proves that they did indeed do it. I absolutely guarantee the solid pins were centerless ground to final size and parallel or taper, in an automatic-feed arrangement of some kind. I think centerless grinding was used for the safety and push pins, but don't know that. I don't have any good explanation for the dimples other than for centering the parts, and the only process I can think of that could be done either with dimples or without is centered vs centerless grinding. Can you explain how the dimples would be used sometimes in lathe production, and not other times? I suppose you could chuck the part (maybe in a special fixture that makes room, say, for the thumb lever of the safety) and do one half of it and then chuck that other end and do the rest. But I don't know how that would work, to be honest with you. Five tenths on a lathe –– that's really pretty good. Centerless is used extensively in the automotive and gun industry and can hold even tighter tolerances where needed –– one or two tenths (as ten-thousandths are known, of course. A tenth of an inch a tolerance for something you sawed with a WalMart miter box!) Very exact tolerances are not required everywhere, of course. Talk about hard materials, we had processes for titanium and Inconel parts for the turbine engine industry. Given that we could cut Inconel, grinding could process already-hardened steel parts, using diamond wheels, but usually that made absolutely no sense. Of course you're right that you can drill hardened HSS with tungsten carbide –– the problem is solved by adjusting the relative hardness of work and tool, and certainly most folks might be more comfortable buying a tougher drill bit than throwing gun parts into glowing coals! Your approach is much simpler. I'm extremely curious about how the production line was and is laid out and the changes it's been through in the last 50 years. Because those industrial processes leave their visible marks on parts –– marks like the dimples in 601 parts. |
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