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Grinding threading tool for small internal threads

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  • Grinding threading tool for small internal threads

    I'm fixing up an old tractor, and the tie rod is hopelessly bent and was kludged up on one tie-rod end. It's simply a length of 1" diameter pipe, with threaded end plugs welded inside. I plan to make up a new tie rod out of 3/4" ID black pipe and turn some threaded ends to fit, & weld the ends in with my TIG welder.

    I first figured I would simply turn some steel stock to the proper size and tap out the holes. That is, until I measured the threads. Problem is, the threaded ends are M17 X 1.5 thread. Nobody carries such a size tap (except a few places that sell specialty taps for big bux...)

    My lathe can turn 1.5 mm pitch, but I've never turned an inside thread this small. (I usually tap small holes.) Machinery's Handbook says to use a 15.5 mm drill (.610") I've ground HSS thread bits before, but only to put in my boring bar.

    Any tips for this? I was thinking of taking a 1/2 bar of HSS and just grinding the profile on the side of it, taking off the lower half or so, maybe just enough to thread into the stock 3/4" or so. Any tips or advice is appreciated.

  • #2
    I've done 14mm x 1.5 (I think). At least 1.5 mm is quite a shallow thread, so there's not too much to take off. You're right on about taking off enough to thread 3/4 inch. If you try to avoid grinding off a lot of stock by using an angled HSS rod as the bit, then the angle restricts the depth you can thread to, so there's no win either way.

    I much prefer to thread upside down - with the cutter upside down on the far side of the work - so I can see what I'm doing. Theoretically, the forces on the topslide are reversed, but this never seems to cause me any problems, even with heavy cuts.

    If you're going to weld in an insert, you can arrange it so you thread right through, or at least you can counterbore from the other end first, so you're not bottomming your thread cut. That's really the only headache with internal threading - not crashing into the bottom of the thread, jarring the cutter into moving, and having to reposition or mess up the thread.

    Good luck.
    Richard - SW London, UK, EU.

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    • #3
      I've done small internal threads with the bar shown below. It is offset and turned from drill rod with a disk left on the end. The sides of this disk are angled using the cross slide to make a 60 degree threading tool. The top of the disk is ground off, then the end is hardened (I didn't bother to polish it so you can see the colors), then the top is stoned to sharpen it.

      The bit shown is from 1/4" drill rod but the idea should work for your situation by using larger drill rod.

      Click to enlarge:


      John

      Edit: the bottom of the disk is ground to provide sufficient relief.
      Last edited by GadgetBuilder; 04-06-2009, 10:21 PM.

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      • #4
        Along the lines of the boring bar style tool, I made one by drilling a hole through the diameter of the rod, maybe 3/16" from the end of it, to hold a short round HSS tool bit. Then drill/tap the center of the rod to add a set screw. Might also grind a flat on the tool bit to make sure it doesn't rotate under load, but make sure to account for proper helix angle.

        Here's a picture of it: http://img.photobucket.com/albums/v1...g/IMG_5496.jpg

        The first one of these I made was much uglier, but worked. I didn't have any round HSS blanks so I used a square one and roughly ground it to just fit in the hole. I now use that occasionally for cutting keyways on the lathe.

        -Matt
        Last edited by Ryobiguy; 04-06-2009, 10:35 PM.

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        • #5
          http://www.victornet.com/cgi-bin/victor/index.html

          Victor Machinery at the above link has the tap you want M17 X 1.5 for $20.50. Look under meteric taps and then down to the size you want. If you are going to use this tractor for any farm work I would use smoething better than black iron pipe for steering linkage. These things seem to break at the most inopportune time.

          Norm

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          • #6
            I'm curious- what make of old tractor would use metric?

            t
            rusting in Seattle

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            • #7
              Another way is to chuck up a bit of drill rod, reduce the dia for the "bar" part leaving a narrow full dia "disk" on the end. Now use a regular threading tool to "chamfer" each side of the disk to form a 60* V. Simply grind flat on top, flat at rear, and relieve the cutting edge. Harden & temper, sharpen w/ stone.

              Do be careful to grind off the correct side, or it will be a LH threading bar!

              uute

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              • #8
                Originally posted by Tobias-B
                I'm curious- what make of old tractor would use metric?

                t
                I guess "old" is relative; it's an early 1980's John Deere 950 compact, made by Yanmar (Japanese).

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                • #9
                  I'd try an M10 tap and grind away all threads but one. Wouldn't that make a working tool for all internal 1.5 mm metric threads over M10?

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                  • #10
                    Check Napa they should have the tap. Or grinding the tool it's not that hard to do... once you get over the idea of wasting all that HSS. I've made up a few bars from a 28 tpi to 8 tpi V threads... square and acme. The lead is what you real have to pay attention to.
                    Wow... where did the time go. I could of swore I was only out there for an hour.

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                    • #11
                      I guess I don't understand the problem here. Why not use a regular boring bar with either a threading insert or a small chunk of HSS ground to the right shape? I don't see any reason to grind the entire bar from a piece of HSS.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        Threads

                        A good way to make a small internal threading tool is to use an old tap. Grind the tap down to leave one tooth out on the end. Grind off enough of the old teeth to provide clearance in the hole. Excellent thread form and good material. I have made several using old 3/8"-16 taps.

                        JRW

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                        • #13
                          Originally posted by Evan
                          I guess I don't understand the problem here. Why not use a regular boring bar with either a threading insert or a small chunk of HSS ground to the right shape? I don't see any reason to grind the entire bar from a piece of HSS.
                          Simple; a boring bar wont fit in the hole......

                          I have several boring bars, the smallest is 9/16" diameter. It won't fit in the hole, (which is .610 diameter) when you have a stickout for the cutting bit....

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                          • #14
                            Originally posted by J. R. Williams
                            A good way to make a small internal threading tool is to use an old tap. Grind the tap down to leave one tooth out on the end. Grind off enough of the old teeth to provide clearance in the hole. Excellent thread form and good material. I have made several using old 3/8"-16 taps.

                            JRW
                            This is a great idea, thanks!

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                            • #15
                              I have several boring bars, the smallest is 9/16" diameter. It won't fit in the hole, (which is .610 diameter) when you have a stickout for the cutting bit....
                              Take some 1/2" bar stock, cross drill at the end and drill and tap a hole in the end for a set screw. Use a piece of drill bit and grind it to the right shape. Could take 10 minutes, tops.

                              Forgot to say any size will do, just turn down the end so it fits.
                              Free software for calculating bolt circles and similar: Click Here

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