Announcement

Collapse
No announcement yet.

lop-sided threads

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

  • lop-sided threads

    Well,

    I was cutting threads yesterday and discovered that the thread "V's" are lopsided. Rather than being nice isosceles triangles, they're shaped more like buttress threads.

    I'm assuming the protractor on my saddle is off and when I put it over to 29 degrees, it isn't actually going far enough. I'm using triangle inserts and know those are a perfect 60 degrees. I also made sure the inserts were correctly possitioned relative to the work surface.

    So..., the only thing I can figure out is that with the protractor out of whack, the tool is feeding-in too much toward the chuck and too little into the work. I'll try testing this tonight. But, I was just wondering if anyone else had this problem and if you think my hypothesis is a probable one...

  • #2
    hard to imagine protractor being wrong. easy to check. is 0 or 90 correct? the marks would have to visably messed up.

    some protractors are screwed to the base of the compound, perhaps yours has shifted?

    forgive me, but did you set it to 29.5 degrees from paralell? (rather than from perpindicular) protractors on different lathes number in different diretions.

    is your tool post squared to the axis?

    Comment


    • #3
      I had the same problem. I finally figured out that I was swinging the cross slide the wrong way.

      Comment


      • #4
        Set your compound at 90 degrees (perpendicular). Then swing it 29 degrees to the right.
        ----------
        Try to make a living, not a killing. -- Utah Phillips
        Don't believe everything you know. -- Bumper sticker
        Everybody is ignorant, only on different subjects. -- Will Rogers
        There are lots of people who mistake their imagination for their memory. - Josh Billings
        Law of Logical Argument - Anything is possible if you don't know what you are talking about.
        Don't own anything you have to feed or paint. - Hood River Blackie

        Comment


        • #5
          chuck up a really big bolt. bigger the better. swing your toolpost over and see if you can't manage to take cuts only on the left side of the existing thread. (once you've already set your toolpoint with respect to the work). when you withdrawl, it shouldn't cut on tailstock side of the thread lands.

          is the insert secure in the toolholder? some of the cheapies aren't that good / dont last too long. might look good when standing still, but when loaded behind cutting forces, it might twist back enough to give you odd looking threads.

          anyrate, the real curiosity behind my 2cents, how well do those thread cutting inserts work? (this case aside).. i've gotten all of my toolholders second hand.. and have only ordered new inserts for tools i had (maybe thats stating the obvious).. but some of those good thread cutters (insert) are quite spendy.

          are they all positive rake? would it make sense to have negative rake inserts for thread cutting?

          i've always used HSS. the next toolholder i get will be a threadcutter. pros/cons?

          -tony

          Comment


          • #6
            A few considerations before digging into your compound rest protractor / angle indicator:

            1. Your tool shape. Hand ground or insert type. If it is hand ground, re-check the angles. For triangle inserts, as you mentioned you have, there is another possibility mentioned in #3

            2. The tool point setting to toe part itself. Was the tool set at the included angle properly? By this meaning the 60 degree tool point set at 30 to each side (equal?). Did you use a "center gauge" to check the tool set of tip and angle to the part diameter.

            3. Tip of tool cracking. Check your tool. Often the tool tip will get a chip in it, or wear. Tools that chip in threading tend to chip not at the front, but from the very tip to the back side. Little flaky chips off the insert or HSS tool tip that go on the bak sode, and make that "buttress" shape because the chip tends to break off at more angle than less angle. (say making the point 30 degrees on the lead side, and 45 to 60 degrees on the other side off of perpendicular, but only for that chiped off length).

            4. Is your insert tight in the holder?

            Triangular inserts tend to work "less better" than actual HSS ground tools or actual thred inserts because the "rakes" are made for cutting diameters on tri. inserts, and not threads. The nose radius being more of less also has effect. Thred inserts have bit of rake and less "relief" or rake on the back edge over the front edge to prevent chipping.

            Before I can get into fixing compound "angle indicators, which I have done before, I need to know if your compound reads "0" or "90" when your compound is set so it travels along the part centerline (turning a diameter with the compound using the compound rest dial as the feed).

            CCBW, MAH

            Comment


            • #7
              The others are right, compound must be 29.5 degrees from perpendicular not parallel to the spindle axis.(some lathes, including mine, will read 60.5 degrees on the compound when set correctly). Also adjust your tool to the work (on center vertically and perpendicular to the surface) AFTER you set the compound angle, NOT before.

              [This message has been edited by Carl (edited 09-21-2004).]
              THAT OLD GANG 'O MINE

              Comment


              • #8
                Thanks for all the replys, guys. I'm going to mull this over, do a little tinkering and let you know what I find. Meanwhile, keep 'em comin'!

                Comment


                • #9
                  Another thing you might check Tony, the other day I cut a thread and noticed the same problem you described. Double checked my setup and tool and finally discovered my dial was just loose enough to move when I would pull it away from the thread at the end of the cut. Just something else to check.
                  Michael

                  Comment


                  • #10
                    I has a similar problem setting the compound at the proper angle. A mark on the crosslide, and a 0 on the dial- line them up and it's off by 15 degrees. I don't know why, except error in manufacture. I made a setting gauge to use the edge of the crosslide as a guide, and problem gone. I still have to set the bit at the proper angle, so there's two more adjustments to make anyway, mounting of the v bit, and orienting the toolpost. I chuck a bolt, and use the threads to align the v bit.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                    Comment


                    • #11
                      OK, I'm an idiot!

                      My cheap Asian import lathe has a protractor that only goes to 45 degrees on either side of parallel to the work. Hence, I was setting the 29 degree angle from parallel. Tomorrow I'll get the proper angle set-up and do it right.

                      Thanks for all the replies!

                      Comment


                      • #12
                        "...protractor that only goes to 45 degrees on either side of parallel to the work. "
                        Do you mean the markings only go to 45deg, or the compound will physically only rotate that much?
                        If the latter, it sounds like a case of "Can't get there from here." Or else you'll have to feed straight in w/the crossfeed, so the thread is cut equally on both flanks simultaneously.
                        That's certainly do-able. One of my old lathe booklets (e.g. South Bend's "How to Run a Lathe", or one of the others) describes a technique where you make one pass, then intentionally miss the thread with the next pass, then the third pass splits the first two, then a final pass cleans it up. This was a case where the same guy was cutting the same thread daily for a long time (years maybe). So he had perfected this over time with lots of practice.

                        Comment


                        • #13
                          <font face="Verdana, Arial" size="2">Originally posted by lynnl:
                          "...One of my old lathe booklets (e.g. South Bend's "How to Run a Lathe", or one of the others) describes a technique where you make one pass, then intentionally miss the thread with the next pass, then the third pass splits the first two, then a final pass cleans it up. This was a case where the same guy was cutting the same thread daily for a long time (years maybe). So he had perfected this over time with lots of practice.</font>
                          There's always more than one way to skin a cat. Could you describe this in more detail?

                          Paul A.
                          Paul A.

                          Make it fit.
                          You can't win and there is a penalty for trying!

                          Comment


                          • #14
                            Sorry, should have been more clear.

                            The markings only go out 45 degrees. (See, told you I'm an idiot! )

                            Comment


                            • #15
                              LOL, that is the same as mine. I got it set up right and put a punch mark so I can find it again.

                              Comment

                              Working...
                              X