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Dinner time! Can of worms

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  • Dinner time! Can of worms

    I'd like to offer everyone this can of worms- I've been thinking about Sir Johns pivoting threading tool holder. Having just finished making my boring bar holder, which can double as a holder for other things, it seems apt that I could adapt a swinging tool holder to it. My lathe isn't particularly solid, so if I'm faced with making a fairly coarse thread, I have to use the angled compound, etc. I have nothing against that, if works great, but I can't set up an angle with the new holder. This isn't an error in the project- the intent was to eliminate as much flex as possible, and this mount takes the place of the compound, et al. It's designed for boring, not threading. But then I get to thinkin'-

    If I made a pivoting threading tool holder, the pivot action could also advance the tool at the 30 degree angle (29.5 or whatever). I was thinking that the insert holder could ride on a threaded rod, and as it rotated, it would advance ahead by the 30 odd degrees. The holder would have a clamshell design whereby I could adjust it for essentially zero play on the threaded rod. The rod itself would be tightly held and not rotate.

    The thread pitch would have to be very coarse- maybe even 1 thread per inch. I'm having trouble visualizing how to relate the desired thread pitch to the action required. Offhand, it seems that if I had a 60 degree helix angle, anything riding on that would advance by that angle as it's rotated. But I think that idea is flawed- in fact I'm sure it is.

    With a pivoting holder, there will be a certain radius dimension from the pivot axis to the cutting edge. If I mounted a cam at that radius, and it had the 60 degree angle, the edge of the tool holder would be forced to move left and enter the workpiece at a 30 degree angle as it's pivoted forward. I can see that working, and perhaps it's better because the back pressure from the cutter would go directly towards this cam- seems so anyway. There would be the usual adjustment of course for the depth that the cutter can go. It would not drive the cutter forward, but prevent it moving forward by more than is dialled in- this is the only way that you could retract the cutter quickly anyway- and of course that is the goal- to be able to retract the cutter for a reversal of the carriage, then bring it forward again for the next pass.

    I'm brainstorming on the forum- just trying to get some ideas, and maybe somebody has done this already in some way. All ideas welcome.

    Oh, and enjoy the can of worms
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Ok, here's the parameters. I'll simplify it- a guide rod 1 inch diameter will have a close-fitting tube slipped onto it. The end of the tube has a helix angle milled onto it. An arm rides the rod, and has a matching helix angle. As the arm is rotated on the guide rod, the interaction of the helices moves the arm sideways. On the arm, at a radius of 1 inch is the cutter. The cutter must move sideways to trace out a 30 degree angle. What is the angle of the helix?

    It's the math I'm having a problem with. The helix angle will have to be more than the angle described by the cutter. If you take a piece of cardboard 6.28 inches long and draw a line on it at 30 degrees to one edge, form it into a loop, then place the loop concentric to the guide rod, the line you drew is the line the cutter must follow as the arm is rotated.

    Just with some rough mental gymnastic calculations, I'm estimating that the helix angle would transform to something like one thread per two inches. If I get it close to correct, then the adjustment to get the swept angle just right could be done by altering the position of the cutter. If the radius of the cutter is less, it will describe a larger angle, and with the cutter out further it will describe a lesser angle.

    Yes, I am trying to re-invent the wheel
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      You said, "If I made a pivoting threading tool holder, the pivot action could also advance the tool at the 30 degree angle (29.5 or whatever). I was thinking that the insert holder could ride on a threaded rod, and as it rotated, it would advance ahead by the 30 odd degrees. The holder would have a clamshell design whereby I could adjust it for essentially zero play on the threaded rod. The rod itself would be tightly held and not rotate."


      I am having trouble understanding your concern. The swing pivot axis needs to be parallel to the machine axis. Here is one that I built to use a vertical threading insert holder. The hole in the holder is a very close fit to the shoulder bolt.

      I advance the compound at 29° to put on the cut while the carriage is returning. I don't need the ability to do left hand threads so I have not keyed the holder to the block. It would be easy to add it if the need arose.
      The can of worms for the swing threader is that you need a lathe with fast reverse. The threader works great. If I were starting from scratch, I would use a die type insert as John wound up using. Second best would be a full thread form laydown insert.
      Byron Boucher
      Burnet, TX

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      • #4
        Originally posted by darryl View Post
        My lathe isn't particularly solid, so if I'm faced with making a fairly coarse thread, I have to use the angled compound, etc. I have nothing against that, if works great, but I can't set up an angle with the new holder.
        Try advancing the top-slide 50% of the infeed instead of angling it.
        Paul Compton
        www.morini-mania.co.uk
        http://www.youtube.com/user/EVguru

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        • #5
          I must have missed the original post on this tool, can someone please post a link to it.

          Comment


          • #6
            This is where it started.
            http://bbs.homeshopmachinist.net/thr...threading-tool.

            Google it and there is a lot of interesting reading on various forums.
            Byron Boucher
            Burnet, TX

            Comment


            • #7
              Part of my reason to figure this out is that I want to eliminate using the top slide entirely- but I still want the tool to advance at the 30 degree angle. The cross slide will also be fixed at one position, with the tool being advanced solely by the degree of rotation of the arm that holds the tool. Thus it must move to towards the headstock as it advances. It is not enough that it can swing up or down to clear the groove.

              The tool will swing through an arc which would intersect the spindle axis, and thus the cutter will present the same rake angle regardless of the diameter of the workpiece, or the degree of depth to which the thread has been cut. What will change slightly is the point at which the cutter touches the workpiece. It won't be straight in from the front, as is usual, but instead it would be more towards the bottom. The cutting forces would be more towards the back than straight down. There is benefit to me from this as well because the carriage will not be caused to lift at the rear- the front way will be taking the pressure. But this all is incidental to my 'problem', which is to figure out the helix angle to make on the shaft that the cutter arm mounts onto.

              I think this is simply a mathematical formula, but I can also work it out mechanically. I would turn a groove in some scrap so the right side of the groove is tapered at 30 degrees. That would match the standard thread angle. Then I'd mount my arm with the cutter attached onto the pivot rod. With that adjusted to place the tip of the cutter just touch the beginning of the taper, I'd measure the position of the arm on the pivot rod. Then I'd adjust it so the cutter is near the full depth but still just touching the side of the taper. Measure both the axial and rotational position of the arm again, then draw an angled line between the two points. That would be my helix angle.

              I could also do it by trial and error. Because the arm would only need to rotate a degree or three to go from the beginning to the full depth of a thread being cut, I could temporarily fasten a small tab to the pivot rod. I could angle the tab and keep changing that until the cutter slides nicely along the tapered test piece as the arm rotates. Once I get that angle, I would duplicate it in the final cam.

              I suspect that all this is confusing. It will be a while yet before I can test this out, so maybe a simple way of achieving my goal will yet present itself.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

              Comment


              • #8
                Ive read and read and re-read this thread a few times and im lost (not that it takes much to confuse me)
                Im more of a picture person, words do not always sink in...

                So....
                Originally posted by darryl View Post
                .... Having just finished making my boring bar holder, which can double as a holder for other things, it seems apt that I could adapt a swinging tool holder to it....
                Pretty sure i seen a thread here where a person took off there compound and mounted a block in its place with a hole through it which holds tools, you (cant find thread, can you link us) ?

                Is this correct (trying to get a general/basic visual picture)...
                You want to mount a thread bar (of a sort) in a "block" that's mounted to your cross-slide which has a bore through it that runs parallel to your 'Z' axis (ways) ?
                Now you want to be able to move the tools head at a 30deg "in-feed" while the tools "shank" is mounted/held at a 90deg to the chuck face ?
                And you want to also incorporate a flip up/pivoting thread head ?

                _
                ~ What was once an Opinion, became a Fact, to be later proven Wrong ~
                http://site.thisisjusthowidoit.com
                https://www.youtube.com/user/thisisjusthowidoit

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                • #9
                  Hmm- lost my last post-

                  Anyway, here's a drawing of my proposal.




                  The pivot bar mounts in place of the boring bar in my new holder. It's parallel to the spindle axis and at the same height. That's all that's important with this bar for now. In the side view, the handle can be rotated up or down, and it will be seen that the cutting point (shown touching the workpiece) can be moved up into the workpiece, or down and away from it. The top view shows the rough setup for the helical angle. As the handle is made to go down, the arm is made to move left at the same time. So, yes- the cutting point moves left as it enters the workpiece, and it is disengaged by lifting the handle. I've drawn the helical angle very crudely, but in the final project the arm will be totally constrained by the 'side-shift' mechanism. There will be no free play left or right.

                  The cross slide is cranked to the point where the cutter would swing through the spindle axis if there were no workpiece mounted. The slide is then fixed in that position. To cut a thread, the usual gearing is set, an adjustment under the handle is set to put the cutter just touching the workpiece, and the feed engaged. At the end of the thread, the handle is lifted and the carriage returned to the start. The handle stop is adjusted again and you take another pass. The handle can be lifted manually, but cannot be lowered past the adjustment point. It is simply held down while the pass is being made. I will probably rely on a spring of sorts to keep it in the raised position when it's not being held in contact with the adjustment.
                  Last edited by darryl; 02-26-2013, 05:54 AM.
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                  Comment


                  • #10
                    ok, think i follow now.

                    Not knowing your lathe, your trying to "lock down" as much as possible (cross-slide and no compound) to gain rigidity ?
                    Your looking to cut threads by only advancing the tools head and not using the cross-slide and no compound ?

                    Talking this step by step, would the following work ?
                    For the time being - ignoring the swivel and just work on the head "travel".
                    Make the thread tool like a boring bar head for a mill, but instead of having the dove tails cut at a 90 to the shank, cut it to 30deg so it slides as if you where cranking the handle of a compound. You would just have to make the head big enough to maintain full contact/support between the two mating surfaces while sliding to cut the deepest DOC of the coursest thread your lathe can cut.

                    Hope that made sense, its late...
                    ~ What was once an Opinion, became a Fact, to be later proven Wrong ~
                    http://site.thisisjusthowidoit.com
                    https://www.youtube.com/user/thisisjusthowidoit

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                    • #11
                      Ok, I think what you're saying is make a mounting block to fit my cross slide, and put a dovetail on the top at the correct angle. The tool would go on the slide part.

                      Makes sense. With one solid block between the cross slide and the tool slide, I'd still be ahead of the game in terms of rigidity, though there is still the play in the slide to consider.

                      A couple other ideas have come to me now. I could make another mount to fit the cross slide, then put the pivot bar at the 30 degree angle. I could still swing the tool out of the groove quickly, plus swing it into the work and up against the adjustable stop. The 30 degree entry into the groove is then fixed and automatic. I should be able to achieve this will virtually zero play, and the cutting forces would be transferred in a straight line directly to the cross slide table. This seems like a very workable idea.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                      Comment


                      • #12
                        Originally posted by darryl View Post
                        ...and put a dovetail on the top at the correct angle
                        I was thinking more on the "side" (1-2 pics). But your idea seams to be more solid (if i understand) and easyer to implement the swivel/flip up (3-4 pics).
                        I did not draw the adjustment screw or dove tails... Maybe a knurled knob with some lines in it to make it easyer to get the DOC.

                        Just rough ideas/drawings...

                        Thread bar for existing holder...




                        Thread block...


                        3/4 shank tool with 5/8 pivot.
                        ~ What was once an Opinion, became a Fact, to be later proven Wrong ~
                        http://site.thisisjusthowidoit.com
                        https://www.youtube.com/user/thisisjusthowidoit

                        Comment


                        • #13
                          Nice drawings, thanks for that. That is of course the essential idea. I've played around with the basic idea tonight, and have come up with an idea which doesn't use my boring bar holder at all. It's a dedicated mount which fits my cross slide and combines some essential features. I'm not going to try drawing it as I don't think I can depict it properly. It is still a pivoting design, but because the depth of any thread I'm likely to cut will be no more than about .1 inch, the small arc that the cutter would go through is not likely to cause any problems with rake, clearance, etc. The downward pressures will be carried pretty much directly to the cross slide surface, and side play will be virtually zero. It will all be aligned and clamped while the holes are drilled, so essentially there will be no gap between any sliding parts. I'll have to allow enough 'relaxation' so that once assembled, the mechanism doesn't bind.

                          If I did my math correctly for the height, I'll be able to mill a slot to hold a 1/4 inch sq. toolbit in perfect alignment and at center height. The 30 degree offset will be automatically set any time the mount is set into the t-slots in the cross slide, so this should be fast and easy to set up for use, and fast, easy, and solid in use. I'll be able to remove the cutter for sharpening, and re-mount it without having to align anything.

                          I have two options for the infeed dial. One is to mount it on the movable part, which means that either I make it a left hand thread so the infeed goes deeper with a right hand turn of the dial, or I stay with a normal thread and turn the dial to the left for infeed. The other option is to mount the infeed lead screw to the mount itself, which means a normal thread will allow turning the dial to the right for infeed. I'm favoring the first option because I think it will be simpler, but I don't have any left hand taps. I'm going to consider getting used to the idea of turning the dial left for infeed. This should be ok, as the dial numerical markings can be punched in a clockwise direction. It should become intuitive- as you rotate the dial to higher numbers, the cutter goes inwards.

                          Anyway, this is a lot of talk for something which you can't yet see. I'm confident this is going to work to my satisfaction, so I'll just go ahead and build it, then show some pics.
                          Last edited by darryl; 02-27-2013, 06:15 AM.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            Looking forward to seeing it done.
                            ~ What was once an Opinion, became a Fact, to be later proven Wrong ~
                            http://site.thisisjusthowidoit.com
                            https://www.youtube.com/user/thisisjusthowidoit

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                            • #15
                              Worked on it today. As usual, there's a lot of prep to do in order to have flat surfaces to work with and from. One piece of steel went in the de-rusting solution, others had their best sides sanded to give a reasonably flat surface to jig the part up with so other sides could be skimmed. Lots of hand-held bandsawing of 3/8 thick material in this job.

                              Because my lathe has the T-slots in the table, I had to make strips that would slide closely into the slots to use as hold-down nuts. Each strip is caught by two bolts in the mount, so they don't turn around and make pains out of themselves like single T-nuts do. This is the third time I've used this method, and it works well, so I'm making some extra 'nut strips' this time.

                              Seems like every project I do requires 500 or more operations to be done.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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