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Noob: Turning tapers to the end, binding every time.

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  • Noob: Turning tapers to the end, binding every time.

    I know I've got to be doing something wrong here.

    I'm trying to turn a spike (long story.)

    I've adjusted the carriage to turn a 15 degree taper using the minor adjustment. So far so good. I'm using a simple HSS bit and some 1018, going slow.

    I can take slices out in decreasing radii as I get to the tail just fine. But at some point, probably 1/4" diameter, the piece grabs the tool, rocks the carriage, rips it under the piece and bends the piece's shank. (the other end is a 5/16-18 tap screwed in to a 'holder' stock, so it'll stay in the chuck.)

    I've switched to carbide and back, I've run it faster and slower, DOUSED the thing in coolant and always, BANG, start over.

    Seems I can't quite be gentle enough with the cut on that thin tail. Is this an operator error or should I be using a different tool (grinder? file?) for the fine end?
    Proud machining permanoob since September 2010

  • #2
    Couple ideas come to mind - the tail stock is not perfectly centered with the spindle - possibly higher, and the cutter is falling below the centerline on the pointy end. The ways are not parallel to the turning center.

    The pointy end needs to be at the tail stock end of the work and a traveling rest may be needed to back up the cutting forces.


    • #3
      It is a bit difficult to understand the setup that you describe, so a bit of clarification would help.

      What are the main dimensions - roughly. Big end, small end length etc.

      How is the big end held? Is it just threaded onto a tap held in a chuck?

      You say that you are using the 'minor adjustment'. I'm not sure what you mean. Can you elaborate?

      Is the thin end supported by anything?

      First guess is that you have an unsupported, fairly slender part sticking out a fair way from a dubious support at the chuck end. Using a tap to hold a workpiece is not recommended. The part is deflecting and you will need to work out a better setup. Could you start with a greater length of material directly in a chuck, with a small centre supported by the tailstock in the other end? You could cut off the unwanted parallel bit later.


      • #4
        As suggested above, you're getting deflection (pretty easy to do).

        Some things to check:

        - Are you using extremely sharp positive rake HSS tools? You want as little cutting pressure as possible.

        - Light cuts and feeds and the appropriate (faster) speed may do better.

        - Is the tool exactly on dead center (at the tailstock end) so it has little tendency to climb or dive?

        - Is there anything you can do to make the workpiece itself more rigid? As suggested above, a larger diameter held in the chuck might work. If the whole thing is pretty long, then a center rest ahead of the taper portion might help.

        - Is every gib, saddle clamp, etc. as tight as it can be and still slide freely?

        If the accuracy of the tapered end isn't a big deal, you might also consider rigging up a guide to use with a belt or disk sander to create your pointy ends.


        • #5
          Something that comes to mind is to have a better way to support the tailstock end. A center might not be good enough, although it should be. What you could do is chuck something in the tailstock, then drill into it for an inch or so using a drill held in the headstock chuck. That hole would then be concentric with the spindle axis. Don't disturb this jig in the tailstock until you're done, and don't crank the ram in or out either. Keep it snugged also.

          Then when preparing the workpiece, chuck it up close and turn an inch or so to a diameter that will closely fit that hole. Then remount the workpiece to give the length outside the chuck that you need to turn the taper. You'll be cutting off the stub you turned after the taper is finished. Apply some lube and support the end by bringing the tailstock in so the end of the workpiece inserts most of the way into the hole. That will act like a bearing and will help prevent the workpiece from flexing when you've got it turned down to as narrow as I think you're wanting it. This should also put that end at center height, regardless of whether the tailstock ram is at proper height already.

          As Pete and dp suggested, the tool height should be right. The thinner the piece you're turning, the more critical this becomes. The clearance angles on the cutting tool become more critical as well, and it must be sharp. You're shooting for no tool deflection either towards or away from you, and as easy of a cutting action as you can get. No hogging as the workpiece gets thinner-
          Last edited by darryl; 07-24-2011, 04:11 PM.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


          • #6
            stick an inch out of the chuck, turn the taper to a sharp point, bring it another inch out of the chuck, continue the taper to blend in with the point, bring it out some more and continue the taper and so on and so on. Try that. Peter
            The difficult done right away. the impossible takes a little time.


            • #7
              Iv found 0 rake tools can help reduce dig in, if you sharpen the hell outta them

              Other then that, you can finish a fine taper like that with a file.
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


              • #8
                Are you cutting/thinning towards the headstock or towards the tailstock?
                Does the cut go all the way to the chuck? Or is there a longer shaft before the taper?

                A 30 degree cone by itself shouldn't be that big of a deal to cut. The point shouldn't be more than 1.73 times the diameter away.


                • #9
                  Watch for Too WIDE a cut.... as in DOC, depth of cut along the edge round nose tools are a problem this way, with forces both in-line with the part, as well as radial.

                  The radial component is the big problem as the part gets thinner on later passes

                  Try a tool with a fine almost sharp very small point for the finish passes.

                  Ideally, a tool with ALL the cutting on a radial line (as close to zero cutting at any angle other than 90 deg to axis as possible), so feeding forces are axial, AND making the full depth cut in one pass might be best, IF you can do it.

                  making ONE cut to finish size means that you still have the full size of the stock at all points to the left of the tool, for maximum stiffness. Of course, the total depth of cut needed may be too great to do that.

                  In that case, do the minimum stock reduction, until you are at a size that you CAN do in one pass, and then do it.

                  I end up needing to make custom noses for counterbores. A number of my counterbores require a shank 0.093 diameter, and about an inch and 3/16 long.

                  No WAY could I do that unsupported in several passes, and there is no room for a center. Even if I had a female center, the shank isn't strong enough to be supported by the center.

                  but doing a cut as above, one pass to final size, I can reliably make counterbores with 0.093 shank and up to nearly 3/8" nose diameter.

                  I have never tried it for a taper, but I would imagine it should work so long as the unsupported length to diameter ratio is not any larger than around 12:1 as per the above.

                  Keep eye on ball.
                  Hashim Khan


                  • #10
                    I discovered yesterday that my 1/2 cordless drill with a socket on a cut-off extension made a good power feed for the top slide on my lathe for doing tapers. That may not solve your problem, but a very even feed might help things (and make your hand less tired, I was doing internal tapers over 5" in multiple tubes).



                    • #11
                      Wow. Thanks for the feedback.

                      This reminds me of my earliest couple posts. I suspect every thing you've mentioned is coming in to play.

                      - The piece is unsupported at the tail-end. (Starting over and cutting the taper mid-way through a blank that's well supported seems like a solid way to solve this.)
                      - The tool could probably be a bit sharper. (Time to unbox the grinder I got for my birthday *\o/*.)

                      So here's the problem once I iron out those things:

                      I have a taper that widens out to about 5/8" at the base, then immediately drops down to a 3/16x18 threaded shank. (From the side it looks like a christmas tree.)

                      Given the final dimensions of the piece, I'd thought the right thing to do was to thread the shank end ('tang' might be a better word) then screw that into a blank to hold it in the lathe for turning down the taper.

                      If I hold the bar stock and cut the taper first, how can I then hold the work from the taper side to thread the other end?
                      Proud machining permanoob since September 2010


                      • #12
                        OK your description makes it a bit clearer. I thought you were holding via a tap threaded into the big end.

                        I still think you need support at both ends, so use the tailstock if possible.

                        Why not make your Christmas tree in two parts. Drill and tap the big end and make the 3/16 threaded part separately. That way you can drill and tap say about 3/8 longer than necessary in the big end leaving you room to hold that end solidly in a chuck. When you finish your taper, part off carefully to length. That will leave you with a tapped hole the right length.


                        • #13
                          Without using the tailstock you could cut 3/4 of the taper (to the point) then reverse the piece and holding it by the unturned 1/4 drill and tap for your tang. Then reverse again and holding it by the tang you will machine the last 1/4 of the taper which is the thick part and will not be projecting much from the chuck. Polish while spinning in the chuck with emery to eliminate any ridge left.
                          "Patriotism is the last refuge of a scoundrel"


                          • #14
                            Originally posted by willmac
                            OK your description makes it a bit clearer. I thought you were holding via a tap threaded into the big end.

                            I still think you need support at both ends, so use the tailstock if possible.

                            Why not make your Christmas tree in two parts. Drill and tap the big end and make the 3/16 threaded part separately. That way you can drill and tap say about 3/8 longer than necessary in the big end leaving you room to hold that end solidly in a chuck. When you finish your taper, part off carefully to length. That will leave you with a tapped hole the right length.
                            I coulda had a V8.

                            I... I used to be smart. Yes. That is exactly the right thing to do. Coupled with the idea of turning the taper in the middle of the blank (not to mention sharpening my tools), I think I should be able to work this out. Thanks.

                            It's a totally frivolous little nonsense project. In fact it's a screw-in replacement tip for this piece of silliness.
                            Proud machining permanoob since September 2010


                            • #15
                              Ok, it's a short steep taper with a thread stud on one end.

                              Take shaft of correct size, turn one end down and thread as needed. Saw off piece as long as needed for taper plus 1/16".

                              Take shaft you just cut off and chuck in lathe, face, drill and tap the end one inch deeper than the stud on taper part. Remove shaft and go to vise and saw the shaft down the center of the end so the shaft is split longways about 3". Deburr the bore and face.

                              Now back to the lathe and chuck the shaft up loosely then screw the threaded part into the shaft flush to the face and tighten the chuck. Now cut the taper and finish it with a file and emery cloth. Loosen the chuck and wedge the slot open and remove the tapered part.

                              PRESTO, your done.

                              To cut a taper like that use a sharp cutter set on exact center as you can get with NO back relief. If you use back relief to work will tend to climb over the cutter.
                              Last edited by Carld; 07-25-2011, 11:27 AM.
                              It's only ink and paper