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  • Originally posted by txfireguy2003 View Post
    Wow, sorry guys, I didn't realize I was going to start such a debate. Clearly, I need to go back and study, as I'm just a dumb fireman trying to learn I new skill to save me some money on gunsmith labor, and maybe make a few bucks to help support my rifle addiction at some point in the distant future.

    Guess I need to buy some training videos so I can learn this stuff. I've pretty much taught myself to do just about everything I ever wanted to do, but machining has so many variables and moving parts and 15 different ways to skin the same cat, I'm clearly going to need some professional education.

    Sent from my SAMSUNG-SM-G891A using Tapatalk
    I wouldn't take it that way. It was a valid question and I'm sure you've learned a bunch about threading as a result Don't worry about the old farts, they just like arguing for the sake of it. I tend to tune them out, although there are pearls of wisdom scattered amongst their writings so it can be worth the effort sometimes.

    From what I've learned starting probably around the same place (or even further back than you) is to keep it simple. Figure out how to do stuff the old reliable boring way first, then once you have that down, try a different method if it takes your fancy. Keep asking questions. Then, without even realising it, you may be in the position to start offering advice to others

    Oh, and always practice on scrap first!

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    • Originally posted by txfireguy2003 View Post
      Wow, sorry guys, I didn't realize I was going to start such a debate...
      Look at it this way: if you'd asked an easy question, you'd have got a simple answer. Granted, some of it is working out what you're doing, how and with what but fundamentally there are two opposing views here...and neither of them is wrong from their own perspective. My trouble is I'm in both camps and it's not an easy reconciliation.

      There's the practical: you turn a set diameter, feed in a set amount and it's done.

      Then there's the theoretical: everything you thought was a fundamental basic is actually a side-effect of how it really works and that's a pain in the backside to measure.

      Neither viewpoint is wrong, they're just different. I really wanted the nice straight-forward view to be correct and if it works for you, that's great. Me, I have to understand how it works and why and there's a ton of confusing and conflicting information and half-truths out there. It does work if you look at it from PD being a starting point from a theoretical point of view. Trouble is that it's arse-about-face when it comes to actually cutting it. It'll help you diagnose the problem though if you understand the theory and that'll help you develop a practice that is probably based on OD and infeed. Work out which one or combination of the viewpoints works for your brain (hey, we're all wired differently!) and go from there.
      The commercial nut and bolt by mail-order does sound like the quickest way of ruling out which item is the problem and qualifying your end product...if it's possible to get them, of course.
      Good luck.

      Comment


      • Originally posted by txfireguy2003 View Post
        Wow, sorry guys, I didn't realize I was going to start such a debate. Clearly, I need to go back and study, as I'm just a dumb fireman trying to learn I new skill to save me some money on gunsmith labor, and maybe make a few bucks to help support my rifle addiction at some point in the distant future.

        Guess I need to buy some training videos so I can learn this stuff. I've pretty much taught myself to do just about everything I ever wanted to do, but machining has so many variables and moving parts and 15 different ways to skin the same cat, I'm clearly going to need some professional education.
        Probably all you need is someone looking over your shoulder and pointing out what's going wrong. Sometimes it's a single thing, sometimes it's multiple.

        Just remember some of the precepts of machining: no instrument reads exactly right, everything is made of various hardnesses of rubber, no tool is really sharp.

        Comment


        • Okay, so I went back and started over with standard threading technique and have been able to make several threads that accept commercial nuts with a very good fit. The only issue now is terrible surface finish, especially in steels. These were done with carbide inserts, and I'd read that HSS gives a better finish than carbide at low speeds, and that it's tougher, as in it doesn't chip as bad (which I haven't had problems with). So tonight, I tried a HSS threading insert from AR Warner, and it left terrible surface finish too, and the tip broke on a pass with 0.005" in feed on the compound (set to 29 degrees). It broke mid pass on a 1/2-28 thread in what I think is 4140. I know 4140 isn't the easiest steel to machine, but it's a fairly common steel for rifle barrels, which is my ultimate goal. In fact, this was a junk rifle barrel I was practicing on. I guess I'll stick to the carbide inserts, but I sure wish I could get a better finish.

          Sent from my SAMSUNG-SM-G891A using Tapatalk

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          • Has your 4140 been hardened? "pre-hard" at RC28-32 threads very well with lay down ER16 carbide at low (by cnc standards) speeds.

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            • Originally posted by lakeside53 View Post
              Has your 4140 been hardened? "pre-hard" at RC28-32 threads very well with lay down ER16 carbide at low (by cnc standards) speeds.
              Rifle barrels are sort of pre-hard and usually (very) easy to get nice looking threads compared to say 1018.

              Carbide works as good as HSS if not better on barrel steels.

              Txfireguy: how much cut of depth you were trying? If the surface is tearing up try less cut of depth and more passes. Start with 0.004" DOC and reduce to 0.002" DOC for the last 4 passes.
              Location: Helsinki, Finland, Europe

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              • What brand of carbide inserts are you using? What tool holder? What lathe?
                Location: The Black Forest in Germany

                How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                • While I am no expert I have cut a fair number of threads, internal and external, with HSS too lbits I ground, leaving the top flat and stoned smooth with a fine stone. I haven't read the pages and pages of posts you got in response, but if you haven't tried it already I would try a good HSS tool bit ground to 60 deg. (or whatever your thread is) with a good cutting oil thick enough not to drip off and don't try and cut the thread in one pass, no cut more than .005, and use the compound to feed in at 29 deg. And don't forget the flat or radius on the end of the tool.

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                  • [QUOTE=J Tiers;1168291...The 5/16" is 0.312" 0.311 is a bit large for that, the 1/8 H flat is not accounted for in that... [/QUOTE]

                    The 1/8H flat does not need to be accounted for in the OD. The theoretical thread has an OD of .3125 AT THE FLATS. The flats are NOT created by taking the OD down.

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                    • Originally posted by J Tiers View Post
                      ...using Dan's 0.0361 thread height (H), there should be H/8 taken off the tops of the threads as the removed sharp crest. that amounts to .036/8, or 0.0045" reduction in radius, or about 9 thou off the diameter. So a theoretical 5/16 is 0.3125". Taking 9 thou off it, leaves a true adjusted OD of 0.303", which should be the target OD on the blank...
                      You have a fundamental misunderstanding of the thread form. The tops of the flats on a nominal thread are AT the nominal OD. You don't remove material from the OD to get the flats at the crests of an external thread.

                      Comment


                      • Originally posted by danlb View Post
                        Just to clarify a bit... The OD should be .3125 or a few thou less, not .303. The thread FORM is based on what you'd end up with if you used a triangular cutter and cut a sawtooth thread. In the picture below you see that sharp pointed crest outlined. The Height (H) is based on that sawtooth too. The measurements are defined as if the peak of that sawtooth male thread is 1/8 H beyond the surface of the major diameter. The major diameter is the nominal size of the screw.


                        How do we apply that? A 5/16 screw will have a diameter of 5/16 for full engagement, slightly less for partial engagement. The minor diameter is 5/8 H deeper than the major diameter and that's the root of the screw thread. It's supposed to be flat or flat with a radius. If you don't have a tool with a flat tip, you can go an additional 1/4H deeper, resulting in a thread that bottoms out in a V 7/8 H below the Major Diameter.

                        With the properly shaped tool, the crosshatched area in the picture below remains after cutting a screw. Using a sharp pointed tool, the greenish bottom of the thread is cut away too. In both cases, the pitch diameter is the same. The pitch diameter is defined as the width of the thread at a point 1/2 H from the top of that sharp thread that does not exist. It's easier to measure 3/8 H from the Major Diameter.



                        NOTE: If you want less thread engagement and turn the OD to be smaller than the Major Diameter you still base your depth of cut, pitch on the Major, not the diameter.


                        Dan
                        Dan, you are one of the few who "get" this....

                        Comment


                        • Originally posted by andywander View Post
                          You have a fundamental misunderstanding of the thread form. The tops of the flats on a nominal thread are AT the nominal OD. You don't remove material from the OD to get the flats at the crests of an external thread.
                          The PD defines the thread. The nominal MD is just that, nominal

                          Example:

                          I have two HoloKrome allen head screws. The PDs are essentially identical, one measures 0.2607, the other 0.2610.

                          The MDs are quite a bit more different. One is at 0.3045, the other at 0.3097. You can see the difference in the crest flats

                          PD and MD vary independently within the tolerance.

                          The suggestion I made was for the OP to deliberately make a thread of the right size but with the minimum MD, because that would do as much as possible toward eliminating interference of the MD. he had a fit problem, and that would help diagnose it. It's not uncommon for folks to make a sharp V thread and find that it will not screw into an internal thread, because the crest flat is not done.

                          Yes, you CAN turn to the OD and then cut to the depth of thread. If done right, you can get to an acceptable PD and never measure a thing. Many folks do that.

                          If you really want the thread right, you cut to the PD. THTA is what the QC folks will be sure to check. (at home, QC is you, in which case anything you can stand goes)

                          If you use a "topping insert", the form is automatically correct

                          The PD of a thread is the PD whether the MD is at max or min. It sets the fit.
                          Last edited by J Tiers; 04-29-2018, 12:03 PM.
                          CNC machines only go through the motions

                          Comment


                          • Originally posted by J Tiers View Post
                            The PD defines the thread. The nominal MD is just that, nominal

                            Example:

                            I have two HoloKrome allen head screws. The PDs are essentially identical, one measures 0.2607, the other 0.2610.

                            The MDs are quite a bit more different. One is at 0.3045, the other at 0.3097. You can see the difference in the crest flats

                            PD and MD vary independently within the tolerance.

                            The suggestion I made was for the OP to deliberately make a thread of the right size but with the minimum MD, because that would do as much as possible toward eliminating interference of the MD. he had a fit problem, and that would help diagnose it. It's not uncommon for folks to make a sharp V thread and find that it will not screw into an internal thread, because the crest flat is not done.

                            Yes, you CAN turn to the OD and then cut to the depth of thread. If done right, you can get to an acceptable PD and never measure a thing. Many folks do that.

                            If you really want the thread right, you cut to the PD. THTA is what the QC folks will be sure to check. (at home, QC is you, in which case anything you can stand goes)

                            If you use a "topping insert", the form is automatically correct

                            The PD of a thread is the PD whether the MD is at max or min. It sets the fit.
                            I read your posts as suggesting that the nominal OD was located at the top of the theoretical sharp V thread, instead of at the flats. If I misunderstood, I apologize.

                            Comment


                            • Originally posted by MattiJ View Post
                              Rifle barrels are sort of pre-hard and usually (very) easy to get nice looking threads compared to say 1018.

                              Carbide works as good as HSS if not better on barrel steels.

                              Txfireguy: how much cut of depth you were trying? If the surface is tearing up try less cut of depth and more passes. Start with 0.004" DOC and reduce to 0.002" DOC for the last 4 passes.
                              My DOC was only .005" on the compound, set to 29 degrees. Actual DOC, I'd have to do some math, but somewhere less than 5 thou.

                              Sent from my SAMSUNG-SM-G891A using Tapatalk

                              Comment


                              • Originally posted by J Tiers View Post
                                The PD defines the thread. The nominal MD is just that, nominal

                                Example:

                                I have two HoloKrome allen head screws. The PDs are essentially identical, one measures 0.2607, the other 0.2610.

                                The MDs are quite a bit more different. One is at 0.3045, the other at 0.3097. You can see the difference in the crest flats

                                PD and MD vary independently within the tolerance.

                                The suggestion I made was for the OP to deliberately make a thread of the right size but with the minimum MD, because that would do as much as possible toward eliminating interference of the MD. he had a fit problem, and that would help diagnose it. It's not uncommon for folks to make a sharp V thread and find that it will not screw into an internal thread, because the crest flat is not done.

                                Yes, you CAN turn to the OD and then cut to the depth of thread. If done right, you can get to an acceptable PD and never measure a thing. Many folks do that.

                                If you really want the thread right, you cut to the PD. THTA is what the QC folks will be sure to check. (at home, QC is you, in which case anything you can stand goes)

                                If you use a "topping insert", the form is automatically correct

                                The PD of a thread is the PD whether the MD is at max or min. It sets the fit.
                                The PD does not "set the fit" by itself. It has a tolerance range , just as the major diameter does.

                                It would be just as true as your last two sentences if one wrote

                                "The MD of a thread is the MD whether the PD is at max or min. It sets the fit."

                                A thread does not meet the specs for its class unless both the PD and the major diameter are within the tolerances for that class.
                                Last edited by cameron; 04-29-2018, 02:16 PM.

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