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Powered threadcutting!

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  • Powered threadcutting!

    I had to do up thirty pieces with a coarse, short internal thread with a shoulder. Standard threading with a short bar was becoming time-consuming (and wasteful) since turning too slow (so I didn't over-run) left a poor, ratty finish, and turning faster gave me little or no reaction time- I'd either over-run, crashing the part, or I'd under-run, so the threads wouldn't be deep enough.

    So, in one of those cases where making a time-saving tool cost more time than it saved, I made up a "powered cutterhead" so I could cut the threads to nearly full depth in a single pass, slowly and carefully.

    First, I made a mount for my wedge-type toolpost:

    That holds an air die-grinder like so:

    I removed the hand lever, and the "button" is just tucked inside one of the clamping rings. The input air now has a flow control (quick and dirty speed regulation) and a 1/4-turn shutoff.

    I turned myself a 1/4" shank threadmill out of 3/8" 01 drill rod and just gave it a quick hardening with a torch:

    It all fits together like so:

    I found the best system was to run the carrige up, lock in the threading lever, and just turn the chuck by hand. (It's a small lathe, and easy to do.) I turned it at probably four to six RPM, and got nearly-full-depth threads in a single pass. I then used a sharp bottoming tap to give each one a very light cleanup.

    Six or seven hours to design and make the mount, the cutter and misc. plumbing (plus a run to the hardware store for the valve and throttle) and less than twenty minutes to run all thirty parts, and another ten to cleanup with the tap. Talk about a time-saver!

    The head can also be used as a toolpost grinder (something else I've needed) and more importantly, with a set of the metric interpolation gears I got off Ebay a while back, I should be able to do metric threads in a single pass, and not have to worry as much about the threading dial.

    Doc's Machine. (Probably not what you expect.)

  • #2
    Pretty slick Doc! Nice pictures too.


    • #3
      Nice Doc. I did something similar a while back. I tried cutting outside threads using a slitting saw which I ground 30 degrees on each side, then ground clearance on each tooth. I know I didn't get the geometry right, since that was all done by eye, but it worked. My spindle mount can rock a few degrees each way, so I can set it to match the angle of the thread being cut. The cutter itself strays from center height, but the thread itself still comes out fine. It looks to me from your pix that the depth of cut is adjusted vertically, with the cutting action being in the bottom of the tube in the chuck, and not at center height. If the cutter's angle is set by rotating the mount on your toolpost, then the cutter centered front to back in the bore, it would seem an easy way to get the angles needed. Maybe not so easy to feed in the cutter without a vertical leadscrew, though. Just wondering if any of what I'm saying happens to jive with how you set it up.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #4
        South Bend model A? Nice trick. Ground threads. I wouldn't have thunk that. Cool.
        Free software for calculating bolt circles and similar: Click Here


        • #5
          Actually, no, the cutting takes place at 9 o'clock (looking at the chuck) just like usual.

          I didn't think about having to angle the cutter to match the thread pitch until I'd already built the mount. I toyed with remilling the dovetail to match the necessary angle, but again, this was a customer run and time was getting short.

          I "built in" the center height to the mounting block, there is no adjustment at the moment. When cutting, I simply left the depth-of-cut a tad shallow, and let the cleanup tap make sure the thread form was correct.

          I had thought about tweaking the toolpost itself a bit, and either cutting at the top or bottom, but due to the slack in the geartrain, that recut a partial thread on the backout (I experimented with a piece of scrap) and ruined it.

          It wasn't perfect, but it worked, and the threads are correct as near as I can tell. I may whip up another one eventually, with a bit of angle adjustment for the thread pitch, or even put some sort of adjustment into this one. In any case, the job got done and I can ship in the morning.

          Doc's Machine. (Probably not what you expect.)


          • #6
            I misspoke when I said ground threads. I did wonder about the helix angle, but if it gets the job done then it's good.
            Free software for calculating bolt circles and similar: Click Here


            • #7
              <font face="Verdana, Arial" size="2">Originally posted by Evan:
              South Bend model A?</font>
              11" Logan, actually. Model 950 I believe? Used to be a "production" turret lathe, and it's got the nicks and scars to prove it. Works great! Came with the Buck "Set Tru" six-jaw, which has thoroughly spoiled me with it's concentricity (when adjusted) and holding power.

              Doc's Machine. (Probably not what you expect.)


              • #8
                Yup. That's called "thread milling" and you can do it on internal threads and external.

                Thread milling on machining centers is common. The cutter looks like a tap but the teeth are annular instead of following a lead. The machined positions, infeed to thread depth, circular interpolates the tool path as the axial feed follows the thread lead. The cutter is over traveled slightly to pick up the lap, dials clear and retracted for the next operation. Dazzling to watch but don't blink or you'll miss it.

                Ideally you have the cutter tilted to the helix angle but most common threads have a small helix angle (less than 5 degrees) so the generated flank angle error is slight.

                You done good.


                • #9
                  ..........Way cool Doc! Nice work. Did you bore the clamp holes in your fixture with a boring head. I have to make one of these for my ownself :-).

                  "11" Logan, actually. Model 950 I believe? Used to be a "production" turret lathe, "

                  When I saw the photo with the compound and the ways I thought "Logan" to myself. That's what I have, a Logan/Powermatic Model 1111011-L00H, made in 1981. It was used to turn plastic and had the crossbed dual toolpost and a bed mounted turret.

                  Great work and nice sharp clear photo's. Is this place and inspiration or what?

                  Son of the silver stream ..... Bullet caster.


                  • #10
                    No, I don't have a boring head for either the mill nor one large enough for the lathe. I did the large holes with the plates in the 4-jaw, stacked together.

                    Forrest says "I done good"? I'll take that as a compliment. I wonder if I can put that on a resume`...?

                    Tomorrow, I turn a new cutter, and see if I can have the same success with a fine-pitch double lead internal thread that's also kind of a pain...

                    Doc's Machine. (Probably not what you expect.)


                    • #11
                      Damn Doc thats brilliant. I gotta make one.

                      Paul G.
                      Paul G.


                      • #12

                        Nice job.

                        As other have said this is "thread milling" with a slight modification of the technique we use on CNC mills.

                        If you do some web searches of manufacturers of thread mill cutters you'll find a chart giving the minimum inside diameter thread versus cutter diameter without causing unacceptable flank error and other problems.

                        For the double lead thread, you'll need the smallest diameter cutter you're comfortable running since the pitch will be double and more likely to have flank issues.

                        Maybe someday you'll want to spring for a ready made cutter. The smallest I have is from Micro100 Company, a TM-250, meaning thread mill with a .250" diameter. Solid carbide, very sharp and fragile, but does a nice job. The carbide should work well in the high speed die grinder. Moon Cutter Company makes HSS versions which aren't quite so brittle.

                        Can I assume you're doing conventional milling as opposed to climb milling? We have better luck with climb milling in the CNC's. If you aren't climb milling you may want to think about trying it. Start at the back of the thread and feed out. Normally climb milling is not done in manual machines, in this case the cutting forces are so low and with the mass of the chuck and gear train it's unlikely to grab and self feed.


                        • #13
                          Exellent technique! Thanks for sharing it Doc.
                          To invent, you need a good imagination - and a pile of junk. Thomas A. Edison


                          • #14
                            For internal threading, it's conventional milling, for external threading it's climb milling. The die-grinder is not reversible, so to change that, I'd have to swap sides and turn the chuck the other way.

                            When I did experiment with an external thread, the finish was very poor- the cuts were highly faceted, notchy. I suspect the air-motor bearings aren't as rigid as they probably should be, and the climb milling let it chatter. Internal threading was far smoother.

                            I did look up ready-made cutters in MSC, but there was nothing in that shank size or with a small enough cutterhead. I'm sure they're available, but hey, it took me less than an hour to make this one.

                            The pitch of the double-lead should be even less of a problem for the helix angle; the bore to be threaded is larger, and it'll be a 16 pitch (32 tpi) rather than a 14 as the first one.

                            Doc's Machine. (Probably not what you expect.)


                            • #15
                              Why not just run the lathe in reverse and cut the threads on the back side of the hole?

                              And to avoid the problems related to offsetting the compound to the right jus feed straight into the work. good sharp tooling and it should work fine. This way you can run the tool into the bore with a stop set on the ways and have no problems with overshooting on the way out.

                              [This message has been edited by Spin Doctor (edited 05-24-2004).]
                              Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.