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A funny for IOWOLF - rookie cutting threads

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  • A funny for IOWOLF - rookie cutting threads

    I thought a bit before posting, and I even tried a few things, but I'm stuck.

    I've been practicing on my lathe a bit more now that I have the head of my Bridgeport taken apart. One of the things I've made is a little pill-jar sized container out of tool steel. I've picked up some good hands-on knowledge of cutting tools, lubricants/coolants, and getting a nice shiny finish. I even got to practice grinding my own stuff to get nice sharp corners in bored holes.
    I threaded the tops of a couple of containers to 20 tpi. I had to use more primitive methods in some cases because I don't yet have a proper thread-chasing dial. All that is ok, though - I've managed to cut just one thread and not bung it up too badly. I just go slow and make small feed increments on the compound. (Yes, it's set to 29 degrees, just like the books say.)
    I cut the top first, including threads. I make a rough cut of the opening taper of the thread with an indexed lathe tool. Then I cover all the ways and things, and then I use the Dremel with a grinder to clean up the taper. The only area I work in this step is just the very start of the thread, to help get the thread started. Part it off and put it aside, remove covers carefully, vacuum the whole area with the shop vac.
    I then make the body of the container, and begin to cut threads. I measure everything a lot using the calipers, and when I get close, I start "test fitting" the top. Prior to a test fit, I make sure that the threads aren't "overcut" or otherwise rough.
    Twice now, it's gone from "still just a little to tight to start" to "starts fine, but it's now a bit over, so it's sloppy." The last cuts are .001" at a time, but the slop I end up with feels like more than that.
    This is all just an excercise for me, but I'd like to be able to get a nice precise fit. I'm looking for suggestions on how to improve the fit, so let it fly, fellas. Thanks.
    The curse of having precise measuring tools is being able to actually see how imperfect everything is.

  • #2
    > Twice now, it's gone from "still just a >little to tight to start" to "starts fine, but >it's now a bit over, so it's sloppy."

    its tough to get a good fitup with threads..
    testing with the cap/top isn't really a good way to get 'precise' threads, but i know what you mean-- you're looking for a good solid feel when you screw that top on.

    depeding on the condition of your tooltip, threadcutting almost always kicks up some burrs. those burrs will make the thread feel tight and you'll want to cut off some more.. but then you'll be cutting out the root too, and the root might be okay.

    when it feels "almost right", instead of cutting the next thou off, try hitting it with some light sanding.. or a fine file. clean off the dust and then see how it goes.



    • #3
      What Tony said about raising a burr is dead on. Depending on the material being cut, threading often raises a burr on the top edges and that will make the thread fit tight or even not start. That's why precision threads are measured with the three wire method. The wires rest on the middle of the flanks, avoiding any burrs, and give a true reading. The burrs need to be rumoved. I generally make a light pass with 200 - 400 grit emery cloth before checking for fit.

      Another tool that is absolutely necessary in my shop is a magnifier. It is almost impossible to see defects on a small thread with the naked or even properly corrected eye. I keep several sets of inexpensive magnifiers handy in the shop - 2X, 5X, and 10X. I also have two really good lenses in my pockets at all times, 10X and 20X Hastings Triplets. I have carried these for over 40 years now and couldn't begin to count the times I have used them. The original black finishes are completely worn off to shinny brass. Absolute necessities.

      Just an observation/suggestion: I keep a couple of rolls of 1" wide emery cloth by the lathe (180 and 320 grit). It only takes an instant to tear off a small piece and use it to clean up the burrs and tool marks on the work. I apply a few drops of cutting oil to improve the finish and to help control the spread of any abrasive dust. They can be used on a finger tip or "shoe shine cloth" fashion on stationary or rotating work. This is much faster than setting up the Dremel or a tool post grinder and produces a very nice finish. But, of course, it is not as precise as a tool post grinder. Most parts don't require extreme precision. Wipe the oil/grit mixture off of the part immediately.

      Paul A.
      Paul A.
      SE Texas

      Make it fit.
      You can't win and there is a penalty for trying!


      • #4
        You've received some excellent suggestions regards your threads. Polishing them with abrasive cloth before fitting is always a good idea, but be sure to clean them well when you do, otherwise the abrasive can be the source of erroneous fitting, or even the cause of galling. I generally wash the thread with an acid brush dipped in solvent, then blow it off with an air hose, though that may not work well for guys with their machines in their basements because of the smell and mess.

        Your threading tool. Learn to grind it without any top rake. If you alter the top, it changes how the tool addresses the cut, altering the included angle of the thread. It also prevents a plunge pass for cleaning up the thread, a process that will often improve the surface finish drastically, especially in free machining materials such as 303 stainless.

        Be aware, not all lathes are marked such that the compound should be set at 29آ°. Depending on the orientation of the 0آ° point, you may have to use the 61آ° mark instead. To insure you've set yours at the right mark, it's a good idea to set the compound parallel to the cross slide, then turn it 29آ° if there's any doubt.

        Remember, the whole idea behind feeding at the angle is to create the vast majority of the cut on the side of the thread, yet still wiping the back side slightly, assuring good form and finish. This allows for better chip flow, and lowers, considerably, the pressure at the tip of the tool, especially after the first pass. Setting the compound at the wrong angle (greater than 30آ°)will truncate the thread, plus leave steps on the back side.

        One other thing to remember when chasing threads. It's important that the compound feed your threading tool such that the cut loads the carriage against the propelling side of the lead screw. That prevents generating a drunken thread. When chasing an external thread towards the headstock, the handle of the compound should point towards the tailstock. When you switch to the internal thread, if you thread towards the headstock instead of towards the tailstock, the compound should be moved so the handle aims towards the headstock, assuming you keep it on the front side of the lathe. You can accomplish the same thing by placing it on the back side, with the handle then pointing towards the tailstock. It's very important to adhere to this process, especially on small machines, where the carriage has little mass and is easily influenced.

        Good luck with your threads.



        • #5
          You might try making some spring cuts also. The burr thing is very true, but if you make another pass at the same infeed setting, and the tool is cutting, it means you had enough deflection to keep the cutter from cutting to the intended depth.

          Since there is more deflection further away from the chuck, by the time you get the far end to where it works, you have probably cut more than necessary at the part nearer the chuck.
          Lynn S.


          • #6
            I use a small 60 degree 3 sided jewelers file to knock the burrs off and shine up the threads, works nice for threads that are just a pinch too tight but you dont want to make 1 more pass and screw them up.

            realy threads are better a pinch loose than too tight.
            on tight threads one spec of dirt or a small ding will make them gaul up and be trashed.

            Also when I file I like to put the spindle in reverse and use the file backwards and the file tracks away from the chuck,a much safer way to file on the lathe.

            [This message has been edited by tattoomike68 (edited 09-07-2005).]


            • #7
              Sometimes if you up the spindle speed and hit the threads with a wire brush, that helps.


              • #8
                I'm not sure i followed the part about the dremel and cutting a taper. For a workman like job of it, undercut the ends to the major or minor dia (depending if its internal/external). This creates a start to the thread that won't get dinged if the end of the piece gets dinged. Is this what you were describing?

                the picture below (for an angular contact bear leadscrew piece i'm making) shows this undercut on both pieces.

                I don't know that I've every used an abrasive on threads i've cut. Its not going improve the accuracy and adds a bunch of cleanup work, both to lathe and work piece. Not that there's anything wrong with it, just don't think you need to.

                remember the 29 1/2 degrees is simply to avoid cutting on both sides of the threading tool. The final cut should be at the same compound setting with a the cross feed a tiny bit past zero so you get the true 60 form, not 58 or 59. you can chase (go over it) one more time at this setting and it will sometimes take bit more off as it works any spring in the set up – even on one side it’s a relatively big cut the tool is taking.

                Obviousl alignment of the 60 degree tool to the work is very important as is having tools ground to 60 - but you can easily get them very close with hand, eye and a thread guage and possibly a maginify glass/or loupe.

                The last few passes are at a thou or so – it’s a decent size width of cut, and needs a corresponding small depth to keep the cutting forces reasonable. With the compound set at 29.5 to cut a 60 degree thread, advance the compound in a total amount equal to 0.750 divided by the threads per inch. I usually leave a little bit of clearance as well, ie the bore a is thou bigger than root dia and OD is a thou less than major dia. Always cut the male first, as you are doing, to use as a gauge on the female. .

                with a bit of practice you can turn nice (not to a ground standard obviously) fitting threads without the bother of abrasives. It also helps to use a good material like a free cutting steel

                [This message has been edited by Mcgyver (edited 09-07-2005).]


                • #9
                  As always, you all come through in fine fashion. Thanks for all the useful info.

                  Mcgyver, that is almost exactly the mirror image of what I'm doing. I was cutting the female side first, though. That's the kind of end result I'm trying to get - nice work.

                  I think my problem is probably burrs. I may have a tiny bit of rake on the top of the cutting tool, but I was using a 60 degree gauge and magnification to grind the cutter. Paul, you are absolutely right about magnification. I've even been able to use the surgical telescopes to get a cutter back in the correct spot after disengaging the power feed. (See previous post - "First Lathe Project") I didn't have to do this on this particular project, though. Also, I don't use them all the time, but I probably need to use them more. I've been using optivisor type magnification.

                  Harold_V, I'll have to stand in front of the lathe to properly visualize your suggestions about direction of feed and such. I have been doing repeated cuts when finishing up without changing the crossfeed position (other than to stop, back up, and make another pass) It helps, but as I mentioned, it's not quite getting me there.

                  I'll have to try cutting the male first - which I haven't been doing. Getting a nice finish on the female side is a lot tougher, and that's why I'd been trying to do it first. I have had great results on the finish of tool steel using sucessively higher grits of wet-dry paper, 320-400-600-1200. (with the ways and everything covered up, of course) It may be time to make some kind of tool or finishing device to make this process easier on threads.

                  So, I'll head back to the lathe and cut some more. When I get good results, I'll report back with photos. Thanks again, all.

                  The curse of having precise measuring tools is being able to actually see how imperfect everything is.


                  • #10
                    I keep handy wire brushes of the type that are about the size of tooth brushes. I have brass and SS ones. The brass ones are good for a quick swipe on aluminum to knock off any slight burrs and the SS are good for steel.

                    Free software for calculating bolt circles and similar: Click Here


                    • #11
                      <font face="Verdana, Arial" size="2">Originally posted by Evan:

                      Nice work there, Evan. Now you're just rubbing it in, aren't you?
                      The curse of having precise measuring tools is being able to actually see how imperfect everything is.


                      • #12
                        I did this about 5 years ago....

                        The tame Wolf !


                        • #13
                          I did this about 23 years ago:

                          Free software for calculating bolt circles and similar: Click Here


                          • #14
                            yea, But did the nut fit?

                            The tame Wolf !


                            • #15
                              Yep, 6-32. First thing I ever made on my SB9, as a test.
                              Free software for calculating bolt circles and similar: Click Here