Announcement

Collapse
No announcement yet.

Carbide inserts for pull-threading

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Carbide inserts for pull-threading

    Well, I'm sure you're all familiar with Joe Pieczynski's videos (and others') on threading away from the chuck. If not, they're here for external and here for internal.
    For better or worse, I like my carbide inserts for threading and I've gone the route of full-profile as there aren't really many different pitches in the range I'm likely to cut. I've got a reasonably good right-hand holder for ER16 inserts and, when I've remembered to switch the spindle to reverse, got good results with holding that upside-down. Problem came when I wanted to thread to a shoulder and couldn't because the point of my tool (once upside down) was on the right-hand side of the tool. So I'm trying to get a good understanding of the inserts and finding it difficult to get good information.

    What do I mean by that? Well, inserts come in internal or external (designated with I/E or N/E I'll use I here as it's clearer to read in this context even though less common) , right-hand or left-hand. On a basic level, you pick the combination of what you want and cut your thread. But if you're going off-piste, getting information on what each option means seems to be difficult. I originally thought that left or right was specific to the thread you wanted to cut - specifically that there would be relief on the cutting point appropriate for cutting a left-hand or right-hand helix. As far as I can tell, I'm wrong and L/R is only about which side of the triangular insert the cutting point is.
    I'm not certain on the internal/external designation though. I suspect there may be different clearances when it comes to full-profile. If so, it matters if you're trying to use an internal insert to cut an external thread and vice versa....although probably not enough that it wouldn't work. The best data I've found is this sheet from Widia which on page 55 and 56 has dimensions of the inserts. This seems to show minor differences (but important?) between internal and external but not between left and right. Is an ER the same as an IL and an EL the same as an IR or are they different? All four combinations are available but some are quite difficult to find - at least at sensible prices - so it would be helpful to know if I'm looking for something that I don't need.

    Try it and find out! Well, I did but I'm not sure if I just 'got away with it' because it was a fairly small pitch. I recently cut a pair of threads that worked. For the external, I used an IR threading bar with an IR insert which I successfully cut an external M24x1 thread from a shoulder, away from the chuck. This was with the spindle in reverse and the tool cutting at the back of the workpiece. I could have used an EL holder but I'd have needed a rear toolpost or to hold it upside-down at the front....which isn't great for seeing what you're doing.
    For the internal thread I used an IL threading bar with an IL insert; again with the spindle in reverse and cutting on the back side of the bore....which is better for visibility too.

    When it comes to shims under inserts for different helix angles, I'll confess that I have absolutely no idea and I'm just hoping it doesn't apply to any of the threads I'm cutting....but I'm aware it's a blind-spot at least.

    Can anyone point me at any definitive information that will lay this to rest?

  • #2
    My understanding is also that internal and external inserts don't mix and match with the holders if you want perfect/correct threadform.
    And thread handed-ness is set with the insert shim and tilted insert pocket.

    This also has some info:
    https://www.sandvik.coromant.com/sit...c-2920-031.pdf
    Location: Helsinki, Finland, Europe

    Comment


    • #3
      The difference between internal and external inserts has to do with the clearances and shapes of the crests and valleys. Usually, unless the threads are going to be thoroughly inspected you can get away with interchanging between internal and external inserts. There are also UNJ thread form inserts that have different crests and valley radii specs than conventional 60 degree standard threads.

      When I first bought holders for lay down triangular external threading inserts they required tapered insert seats to set the inserts at the correct angle (at that time I wasn't using full profile inserts). The later model holders don't have insert seats.

      When I've had a high pressure tooling salesperson promoting a brand of threading inserts I've used the internal/external questions test to their knowledge. Seems like only about 50% understand the differences.

      Comment


      • #4
        Originally posted by DR View Post
        ... high pressure tooling salesperson ...
        \o/
        Location: Helsinki, Finland, Europe

        Comment


        • #5
          Thanks guys. That's certainly some interesting reading from Sandvik there. Definitely explains the helix angle/shim issue.

          Seems like the difference between internal and external inserts is largely the radial clearance angle (p.30) from what I can tell. If so, on a bar, that could easily be compensated for - hell, I'm only setting it by eye off the flats so I'm probably more than that out already!

          Left and right inserts then are just which way corner of the triangle has the cutting point - ie the handedness is of the insert and not of the thread.

          Left and right holders, however, have a built-in flank clearance angle (p.26 - 28) and these are - at least for Sandvik holders - 1° either side of straight. This would mean a shim ought to be required to use a left handed tool holder for right-handed threads and vice versa.
          Interestingly, according to the formula that Sandvik gives, all of the standard ISO Coarse threads ought to require a 3° shim rather than the 1° they say is most common.

          Seems like my next step has to be to try and measure what tilt angle my holders have. Although, only the 20mm bar even has a shim so it could be just that they've set all the smaller ones to 3° out the box.

          Comment


          • #6
            If you are threading towards the chuck
            with the insert right-side up
            or threading away from the chuck
            with the insert upside down
            the helix angle is the same.
            Was that your question??

            -Doozer
            DZER

            Comment


            • #7
              Mainly I'm trying to get things straight in my head and hopefully have an answer that will come up if someone else searches for it.
              What I was trying to understand is what the difference between each variation of insert is. In otherwords what's different about an internal vs an external or a left vs a right.
              From the docs I'd found and the one that MattiJ linked to, it seems that left vs right insert is only whether the cutting points are point clockwise or anti-clockwise off the triangle - ie whichever fits the holder you want to use, it has nothing to do with the handedness of the thread.
              Internal vs external might have a 5° radial clearance angle difference (except the really large ones) that can be compensated for by turning the bar.
              Helix angle clearance is built into the holder, not the insert. It may be adjustable with shims (on larger/better holders) but probably means that a left-handed holder is not appropriate for cutting a right-handed thread unless it has a shim to compensate - although an EL insert in a IR bar should work fine.

              Very happy to be corrected if I've got any of this wrong....that is sort of the point

              Comment


              • #8
                I have found this calculator handy: http://www.mitsubishicarbide.com/en/...eading_formula

                Comment


                • #9
                  Originally posted by old mart View Post
                  I have found this calculator handy: http://www.mitsubishicarbide.com/en/...eading_formula
                  What's odd about all this shim business is my internal threading bars aren't big enough on the end to have room for a shim under the insert. I've never checked, they must have a pocket that's pre-cut at an angle. Some day maybe I'll check one to see.

                  It's kinda interesting to understand all this stuff......but in the real world sometimes just do what seems to work. As an example my newest holder for external threading does not have a shim under the insert. I've used that successfully to do 1/4"-20 and also 2"-20 with a 20 tpi full profile insert. Based on the difference of helix angles between the two threads ideally one of those threads should have had a shim correction, but it worked without a shim.

                  Comment


                  • #10
                    Originally posted by Cenedd View Post
                    Internal vs external might have a 5° radial clearance angle difference (except the really large ones) that can be compensated for by turning the bar.
                    Helix angle clearance is built into the holder, not the insert. It may be adjustable with shims (on larger/better holders) but probably means that a left-handed holder is not appropriate for cutting a right-handed thread unless it has a shim to compensate - although an EL insert in a IR bar should work fine.

                    Very happy to be corrected if I've got any of this wrong....that is sort of the point
                    AFAIK there could be a small error on flank angles if you use IR insert as EL, same as if you grind negative rake to threading tool. I think that what the Sandvik guide was also referring to.
                    Hard to say if its any sort of real problem ever, I tried to spin my head around the math but only got headache. Would need CAD or math genius to calculate the amount of the error.
                    Thread milling has also kind of related profile error, best estimate I have found was "not usually significant if you use enough small tool"
                    Location: Helsinki, Finland, Europe

                    Comment


                    • #11
                      In a recent thread, I made a custom internal IR16 threading bar to cope with 3/4 X 8 ACME http://bbs.homeshopmachinist.net/thr...utting-inserts
                      The helix angle for this was more than average, so the holder had the angle built in.
                      Some time ago, I had to produce a 1/2 X 8 acme twin start external thread for a Smart & Brown toolpost, and I solved the helix angle problem with a pair of custom shims which tilted the entire toolholder anticlockwise.
                      Helix angles, although inherently simple always cause me lots of confusion and headaches.
                      Doozer in #6 has the answer for cutting both ways.
                      Last edited by old mart; 10-23-2018, 01:19 PM.

                      Comment


                      • #12
                        DR: I'll have to agree with you that if it works, where's the problem?! I do seem to like over-complicating things though and prefer to understand how and why they work if possible. If I ever manage to catch up with the learning curve it should stand me in good stead!
                        Most of my holders also don't have space for a shim (which are a similar thickness to the insert by the way) so I'm guessing they've pre-set the angle with the holder to one that works for at least most of the threads in the range of that size. It's useful to have an idea when I'm about to go wildly off-piste though!
                        The only holder I have with a shim is a 20mm IR bar and as an import-special, it obviously comes with no information on what that shim is etc - listing doesn't even show a shim in the pictures. Ya pays yer money and rolls the dice!

                        MattiJ, OldMart: Definitely a cause of headaches trying to wrap my head around this. It's not even that helpful if you have some inserts sitting in front of you as the differences (at least on a 1mm pitch) are so tiny that you wouldn't spot them. I'm sure that just adds to Matt's "not usually significant if you use enough small tool" though!

                        Comment

                        Working...
                        X