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Relative newbie question on internal threading?

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  • Relative newbie question on internal threading?

    How many thousanths (.001's) infeed does it normally take to get from male piece starting in the internal almost a full turn to acheiving a better than hardware store fit but easily threaded in with fingers? My attempts seem to take more infeed than I feel like they should. I use full profile bits for both int/ext.
    I am troubleshooting my setup. But without knowing what the norm is Im a little behind the curve.
    To keep to my question I AM NOT asking about calculating infeeds. .708 x tpi or what not. Specifically asking about the amount of infeed from 1 turn in to finished thread. So I can compare to my situation and then evaluate if Im doing it right. Or if my set up is lacking. Thank you
    Last edited by Shawnba67; 12-10-2017, 09:57 AM.

  • #2
    Originally posted by Shawnba67 View Post
    How many thousanths (.001's) infeed does it normally take to get from male piece starting in the internal almost a full turn to acheiving a better than hardware store fit but easily threaded in with fingers? My attempts seem to take more infeed than I feel like they should. I use full profile bits for both int/ext.
    I am troubleshooting my setup. But without knowing what the norm is Im a little behind the curve.
    To keep to my question I AM NOT asking about calculating infeeds. .708 x tpi or what not. Specifically asking about the amount of infeed from 1 turn in to finished thread. So I can compare to my situation and then evaluate if Im doing it right. Or if my set up is lacking. Thank you
    its hard to know how close you are, one turn in is not a thing, i could take more passes than you might think or take one or two. Sneak up on it a thou DOC at a time until it threads the way you want it to.

    If you want something with more a correct procedure to it, say you need to make a part to tolerance make a part to fit some commercial part, you need to measure, and as always, there's a range of tolerances governing things.
    .

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    • #3
      Well, good set up +proper procedure=good part. My lathe is manual. I Think that I have my tools set up correctly (fishtail guage) however I have no better idea on how to criticaly QC than to see if my procedure matchs somekind of standard . The cut til it fits really isnt useful here. If my cutter is off perpindicular then my threads will need to be oversize to fit.The fit will feel good but the threads will be weaker. I have no way/knowledge of how to verify true(fishtail guage does not inspire confidence)oo perpindicular of my thread tool. However if my infeed matchs what most are ending up with my set up is likely correct.

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      • #4
        Obviously you are aware of the "double depth of thread" tables, that allow you to get to the general infeed amount.

        For inside threads there is no good simple way to measure other than a go-nogo gage for the thread type. You can also section a sample part, and check it on an optical comparator, but that is a bit impractical for most HSM uses. Usually HSM folks use a mating threaded part as a "go" gage, and stop when it enters the threads properly.

        As for getting the angle correct, you can set the boring bar or holder in line with the lathe axis, which should set the cutter in proper perpendicular position. If you are using a full form cutter, presumably it is a commercial one, and if used in its proper holder, is suitably aligned if the holder is. That you can measure with an indicator as you move the carriage.

        If that is not going to do it, maybe a picture of the setup is needed.

        The feed per pass is dependent on the thread size, the setup, the material, the machine itself, the cutter, etc. I do not think any number can be given. A large thread on a small machine takes more passes, just due to material removal.

        What size thread is your typical work? What is the machine? in what material?
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          I think you need to describe your method bit more. What ID and OD diameters you are using before you start threading? What do you use as a reference for setting the tool starting position? Light touch to workpiece turned to nominal diameter?
          Or do you start with ~0.05mm oversize stock and let the full form insert do its work and keep cutting until you end up with correct OD/ID ?

          I have never actually used full thread form inserts but AFAIK you can start with oversize stock and keep cutting until you end up with desired major diameter?

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          • #6
            To go from starting but jamming to a good low play easy running fit? It depends on the diameter in a big way. On a little 1/4 to 1/2" thread it may only take another couple of thou. And depending on the bar you're using it may just take another spring cut or two. But on a 2 inch part it may take another 10 thou. After all, what might be considered a good low clearance fit on a 2 inch diameter 8TPI would be disaster on a 6-32 size thread.

            I do know that when I hit that point where the thread starts but jams I only make cuts in 1 or 2 thou steps from that point on and take a lot of spring cuts when it's going on but is still binding. At that point of fitting but with binding it really is only a thou or three away from perfect depending on the size.
            Chilliwack BC, Canada

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            • #7
              I use taps and dies a lot to work to industrial standards. For example, a tap and split (adjustable) die of the same diameter and thread pitch, metric or inch.

              Drill and tap a hole in a piece of steel. You can harden it if you like, or leave it soft. This is the female thread gauge.

              Single point a steel bar a little oversize. Open the thread die a bit oversize and thread it on the bar. Check the fit of the tapped part. It should not quite fit. Close the die a little and rethread. Check the fit. Repeat until you like the fit of the parts threaded together.
              This is the male thread gauge. Leave the thread die at that setting. You can harden this gage or leave it soft.

              Sometimes you won't have a tap or die to use for a reference. In that case, you can single point the male part and use the 3 wire method to measure it. If you have an internal thread, you can use the male part as a plug gauge for the female.

              As for aligning the insert correctly, you can use a square from the chuck face to the insert holder. This will be plenty good for any job. If your plug gauge threads in one turn, you are probably very close to finish size. Advance in .001 increments until you get the desired fit. It's a good practice to make a couple of extra passes at the final setting to smooth out any minor high spots.

              Comment


              • #8
                The general advice of taking .001 at a time makes good sense, BUT it raises the danger that you might end up with them galling and stuck together forever. Been there, did that.

                If you can thread them together 1 turn and then jam, you have a problem. If I remember correctly, when both threads are the same pitch and cleanly cut, they should continue to thread on if they get by one full turn IF single pointed. The first few tapped threads may be tapered due to the grind of the tap but single point should have threads that are straight and parallel.

                Your use of full profile inserts should guarantee a good thread profile. That assumes that you are using the right insert for the TPI and that you are using a different insert for internal than the external thread. The fishtail gauge is for use with a V profile ground bit, not really meant for full profile inserts. You will have to align the insert by measuring against the insert holder's shaft.

                If I were you, I'd start with the external thread's major diameter exactly spot on and the internal major diameter just .001 less than the calculated dimension.

                Dan
                At the end of the project, there is a profound difference between spare parts and extra parts.

                Comment


                • #9
                  unless you are using full profile inserts, don't try to "test fit" or measure unless you debur. Cratex (rubber/abrasive) works wonders.

                  "hardware store bolts", are generally a very sloppy fit but can be all over the map.. Don't get caught by sizing to one then finding another is too tight!
                  Last edited by lakeside53; 12-10-2017, 01:52 PM.

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                  • #10
                    I think "single turn" is more a case of a poor description. Until I have at least a running fit I've never gotten a close but not quite fit to go on even a full turn. Usually it's about a 1/4 turn or slightly more until it wedges tight. And that makes sense if you think about how the thread engages when there's still an interference fit.

                    Good point on the deburring step. It's been a long time since my last internal thread. I recall using a triangle scraper as one deburring step and I might have used a patch of fine emery for the fine fitting stages. Emery then a cleanup before the test fit.

                    I really like the idea of a rubberized abrasive block for this. Lakeside, what sort of Cratex (or other brand) tool are you using for that operation?

                    Ya, know... For a TPI on a larger diameter than usual I wonder if a tap could be used as a deburring tool? Like when I do a 2 1/2" - 8TPI internal thread to fit my lathe spindle could I use a 1"-8TPI tap as a scraper to deburr the inside thread crests? This would be with hand power of course. I can imagine something like that catching and wobbling around in there with me on the other end....
                    Last edited by BCRider; 12-10-2017, 02:02 PM.
                    Chilliwack BC, Canada

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                    • #11
                      End grain on hardwood block works also for deburring. Baltic birch is what we have here, I'd imagine Oak would work same way.
                      Add some abrasive if you feel like.

                      Comment


                      • #12
                        He is using full profile cutters, so the burr issue is much reduced.

                        For any OTHERS reading, the burrs which WILL BE on the threads unless you deburr, or use a full profile cutter, can (will) fool you into thinking you do not have the thread deep enough. Then you cut further, so that when you finally deburr, the fit is like a pencil in an apple barrel.

                        Taps are obviously full profile cutters, and work well for finishing to size (or, I suppose, deburring) when it is closer tolerance. Pay attention to the "H" number to get the correct sizing. For large threads on bigger workpieces, that is less practical as each tap can cost $100 or more.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          The way you've phrased the question suggests that you're a newbie so the first thing we need to know is how
                          good the fit needs to be. As you gain experience you will soon figure out that in many cases "close enough"
                          really is good enough. There's a reason why nuts and bolts are made to the tolerances they are. Working to
                          tighter tolerances than are needed for a specific job is always a waste of time and money. As an amateur the
                          "cost" of your time may not be a big deal but you're still imposing stresses on yourself that may not be necessary.

                          The only practical way to measure internal threads is with a GO-NOGO gauge whether purchased or home made.
                          If you're working to a standard then the simplest--but most expensive--solution is to purchase a set of gauges for
                          the diameter and pitch you're working with. Making your own male gauge is pretty easy because you can measure
                          over wires to get the thread to a standard tolerance. You then use your male thread to fit the female.

                          As to finishing an internal thread if you're only making one or two pieces the only way to arrive at your final fit is to
                          "sneak up" on the dimension. As you gain experience it becomes easier because you learn not only how your lathe
                          performs but also how different materials respond to different feeds, speeds and DOC. Making multiple parts is easier
                          because by the time you've made a few you can figure out where to set your dials--or the readings on your DRO--to
                          hit the numbers every time. Once you reach that point then the only thing you have to worry about is tool wear....
                          Keith
                          __________________________
                          Just one project too many--that's what finally got him...

                          Comment


                          • #14
                            Originally posted by J Tiers View Post
                            He is using full profile cutters, so the burr issue is much reduced.

                            For any OTHERS reading, the burrs which WILL BE on the threads unless you deburr, or use a full profile cutter,
                            This should read "burrs which MAY be on the threads". Burrs are not automatic when cutting threads just as they are not automatic with any other machining process.

                            If you use sharp tools, the proper major and minor diameters and follow the recommended infeed steps you are likely to have a burr free thread in many materials.
                            At the end of the project, there is a profound difference between spare parts and extra parts.

                            Comment


                            • #15
                              Originally posted by danlb View Post
                              This should read "burrs which MAY be on the threads". Burrs are not automatic when cutting threads just as they are not automatic with any other machining process.

                              If you use sharp tools, the proper major and minor diameters and follow the recommended infeed steps you are likely to have a burr free thread in many materials.
                              Add 5 bucks and you can get a cheap coffee at starbucks with that......

                              There is always a burr. ALWAYS. it goes with the geometry of the cutter, cutting forces and the nature of ductile workpieces. Your comment is REALLY about whether or not the burr is large enough to make a difference.

                              With the typical materials the new guy works with, and the typical threading cutters (not full profile), there WILL BE a burr, and it will likely be significant. This is fact based from prior threads we have seen.
                              1601

                              Keep eye on ball.
                              Hashim Khan

                              Comment

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