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What's wrong with my threading technique?

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  • #76
    The force is pulling the insert away from the pocket, not pushing it against.

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    • #77
      Originally posted by J Tiers View Post

      So... 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.
      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
      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.

      Comment


      • #78
        Originally posted by JohnMartin View Post
        Yes, insignificant deflection. Of the workpiece. But if that boring bar is sticking out six inches or so.... And .010" is a pretty decent cut.
        It's not sticking out very many diameters of the bar, not from the last pic.

        Neither is the work, so if anything flexed by 0.060", it would have bent. That leaves either a mystery miracle, or a shift of the toolpost etc to explain how there was 0.060 movement of the compound and yet the diameter did not change by at least 50 thou.
        CNC machines only go through the motions

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        • #79
          The only way I can account for the growth is if the thread is being formed, not cut. This can happen if the spindle is turning the wrong way and the dull side of the insert is being forced into the softer aluminum. Then it acts like a knurling tool, displacing the aluminum away from the tool.

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

          Location: SF East Bay.

          Comment


          • #80
            Originally posted by danlb View Post
            Just to clarify a bit... The OD should be .3125 or a few thou less, not .303. ....
            Not relevant to the original comment

            If you turn the part to the 0.303, then you will be left with the correct flat on top when you have cut to the correct pitch diameter and cleaned up the burrs.

            You do NOT base the advance on the part diameter, the RIGHT way is to check the pitch diameter and cut for that.

            If you like, you CAN just take the 1/8H off of the theoretical advance amount that you would use for a sharp thread, and you will still theoretically come out right at the pitch diameter if you start at the 0.303 But it is better to measure, or check against your go-gage.... which may simply be the nut you want to use!

            If you use a topping insert, that cuts to correct form, then you will automatically have the correct OD when you hit the correct pitch diameter, because the insert will cut down to it

            https://www.sandvik.coromant.com/en-...s/insert-types

            He has a V-form insert/ Note the comment under that heading:

            V-profile inserts – threading with minimum tool inventory

            These inserts do not top the thread crests. Therefore, the outer diameter for screws and inner diameter for nuts must be machined to the right diameter prior to threading.


            trying to hit the right pitch diameter simply flying blind by the advance of the tool is fine if you are a CNC machine, or have a very loose thread to cut, but that method is subject to errors when threading on a manual machine.

            If you stay at the 0.311 or so OD, then you will have to turn down the finished thread to get the correct form, because that will not allow for the 1/8 H flattening of the crests. The result may not fit the nut just on account of the excess thread crest running into the root flat of the nut.
            Last edited by J Tiers; 03-29-2018, 12:24 PM.
            CNC machines only go through the motions

            Comment


            • #81
              Originally posted by txfireguy2003 View Post
              I took a couple more photos this morning before leaving for work, and measured the part I cut last night. Now, you all saw my micrometer measurements last night, 0.312, no taper at all. After threading, it measures 0.325!

              On my next attempt, I will start by turning the shaft down smaller, say to .302 to account for the flats on the crests, as that may be the problem, but I don't know how to account for the major diameter growing.
              NO.. don't reduce your diameter to account for burrs! Burrs happen on gummy materials. Set your starting diameter correctly (not technically correct I usually go a couple of thou less than max, but often just start at max and even have been known to feed the tool like a cutter to reduce the od after.), go to the correct depth (measure with wires or whatever), knock the burrs and a tad more off off with a file, clean up with cratex.

              Threading external is very simple. You are fighting it way to much.
              Last edited by lakeside53; 03-29-2018, 01:27 PM.

              Comment


              • #82
                Originally posted by J Tiers View Post
                Not relevant to the original comment

                If you turn the part to the 0.303, then you will be left with the correct flat on top when you have cut to the correct pitch diameter and cleaned up the burrs.

                You do NOT base the advance on the part diameter, the RIGHT way is to check the pitch diameter and cut for that.

                If you like, you CAN just take the 1/8H off of the theoretical advance amount that you would use for a sharp thread, and you will still theoretically come out right at the pitch diameter if you start at the 0.303 But it is better to measure, or check against your go-gage.... which may simply be the nut you want to use!

                Jerry, give it up.

                You admitted in another thread that you don't do single point threading and when you do you finish with a die or file to form the threads. Your ignorance shows when you are talking about a standard size (which 5/16 UNF is) and you start telling folks to make it even smaller to create the flats.

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

                Location: SF East Bay.

                Comment


                • #83
                  Originally posted by danlb View Post
                  The only way I can account for the growth is if the thread is being formed, not cut. This can happen if the spindle is turning the wrong way and the dull side of the insert is being forced into the softer aluminum. Then it acts like a knurling tool, displacing the aluminum away from the tool.

                  Dan
                  Exactly.

                  Comment


                  • #84
                    That may be true, but gummy does burr. The posters' burrs are what.. 6 thou (12 diameter)? Happens to me all the time with "slow speed threading" in 1018/A36 junk, "unknown AL", but they are easy to deal with. 12L14, 416, 7075T651, 6061T651...etc. are tiny.

                    I try to use full form inserts, but don't always have them.
                    Last edited by lakeside53; 03-29-2018, 01:28 PM.

                    Comment


                    • #85
                      Originally posted by danlb View Post
                      Jerry, give it up.

                      You admitted in another thread that you don't do single point threading and when you do you finish with a die or file to form the threads. Your ignorance shows when you are talking about a standard size (which 5/16 UNF is) and you start telling folks to make it even smaller to create the flats.

                      Dan
                      I have NO IDEA what you are talking about....
                      '
                      I DO single point threading, you are thinking of someone else. I have in fact followed up with a die, but only a couple times. I got that idea right here.

                      So perhaps you need to be looking in the mirror when you are typeing.

                      I am getting kind of tired of you dogging me all over the forum with this crap, putting words in my mouth and then extrapolating....

                      As far as what I am suggesting.... if I had not seen it happen, I would not talk about it. I am not perfect like you think you are, I have made mistakes and learned from them.

                      You seem to want to change the PD.

                      Why don't you just concentrate on what the OP is having as a problem? SOME of us are trying to solve his problem.
                      Last edited by J Tiers; 03-29-2018, 01:52 PM.
                      CNC machines only go through the motions

                      Comment


                      • #86
                        It's got to be burrs, because I'm seeing chips forming and cutting away from the part as expected, sometimes more than expected. The aluminum is cutting, I'm running into the sharp edge of the tool, not the dull edge as if turning the wrong direction. I'll pick up a piece of steel soon and try that. I did thread a section of 1/2-28 a while back in steel, in three normal fashion towards the chuck and it worked fine. It might just be the junk unknown aluminum I'm working with.

                        One other reason I was using aluminum is because I may end up pouting together an anodizing tank to anodize small parts, like these bolt knobs. The only two materials that can go in the anodized bath are aluminum and titanium, and I thought it might be worthwhile to have aluminium threaded rods to accept my knobs instead of hanging them by wires, since anodizing can cause parts to grow a small amount, and create interference with threaded parts..... having a threaded rod already there would, theoretically, prevent the solution from contacting the threads and therefore minimize that problem.

                        Here's the final product: obviously not the same knob, but I cant find a picture of the black one installed.

                        Sent from my SAMSUNG-SM-G891A using Tapatalk

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                        • #87
                          Maybe you already said it but are you using something like WD40 as a cutting oil on the AL? Helps.

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                          • #88
                            Responses in red.

                            Originally posted by J Tiers View Post
                            I have NO IDEA what you are talking about....
                            '
                            Yep. I agree.

                            I am getting kind of tired of you dogging me all over the forum with this crap, putting words in my mouth and then extrapolating....

                            I'm not dogging you. I'll stop responding to your posts when you stop giving bad/incorrect advice.

                            Why don't you just concentrate on what the OP is having as a problem?
                            I did. Now you have him reducing the diameter based on the idea that you said that it will help. That will screw him up if he does not understand the relationship between the major diameter and the depth.
                            At the end of the project, there is a profound difference between spare parts and left over parts.

                            Location: SF East Bay.

                            Comment


                            • #89
                              I can't find the insert designation for the insert that you are using. Would you mind posting it again? That will tell if it's full profile or partial.

                              Also if you could confirm that either
                              a) the spindle is running backward and the insert is right side up ... or
                              b) The spindle is running normally and the insert is upside down.
                              c) neither of the above.

                              Also, where were the chips? On top (as in a) or on the bottom (as in b)?


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

                              Location: SF East Bay.

                              Comment


                              • #90
                                I am not surprised that your OD was larger after cutting the thread. In fact I would have been surprised if it wasn't.

                                It is called BURRS! Even a dead sharp threading tool can produce burrs on the top of a thread which is supposed to be flat. That increases the apparent OD. This is normal in the thread cutting process. We deburr everything else that we cut so why would we expect threads to be exempt. Form threading tools and threading dies are two ways of preventing this.

                                With normal tooling, BEFORE measuring a freshly cut thread, you MUST deburr it. This is not optional. And you have to do it before every measurement if the threading tool has made a pass or two on the thread.

                                File. Brass brush. And you can inspect it with a good quality 10X magnifier. I carry three magnifiers in my pockets at all times, even to the grocery and to church. I always have them ready for use.



                                Originally posted by danlb View Post
                                The only way I can account for the growth is if the thread is being formed, not cut. This can happen if the spindle is turning the wrong way and the dull side of the insert is being forced into the softer aluminum. Then it acts like a knurling tool, displacing the aluminum away from the tool.

                                Dan
                                Paul A.
                                SE Texas

                                And if you look REAL close at an analog signal,
                                You will find that it has discrete steps.

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