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Threading tools with top rake.

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  • Threading tools with top rake.

    Not wanting to hi-jack the other threading discussion, I would like to discuss the comments that basically said that the thread profile was incorrect if you used top rake angles. There is a lot of difference between 4” H 90 threads cut on drill collars and 32 tpi threads cut on muzzle brakes.
    Rod Henrickson aka speerchucker30x3 over on the PM site makes and installs muzzle brakes which tend to have fine threads. He shared his technique for grinding threading tools using HSS parting blades which uses about a 30° side rake. I was surprised at how well this worked with very light cuts. I hasten to add that it is best to use the 29° angled compound in this case.
    Not a very good photo but a parting/threading tool with a lot of side rake.

    A parting / threading tool with back rake.

    This application is used to produce a good snug Class 3 fit on the mating parts. I am never in a rush and I hone all my HSS tools to a very sharp edge. The extreme rakes produce the effect of a keener cutting edge which cuts better particularly in some of the less friendly alloys.
    What is fundamentally wrong with the tool if it fits the fishtail and cuts good threads?
    Last edited by Boucher; 12-25-2012, 11:07 PM. Reason: pictures not showing
    Byron Boucher
    Burnet, TX

  • #2
    The laydown inserts I use all have very postive rake. If that's compensated for to generate the same effective profile as a a flat 60 V tool, then it works fine.

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    • #3
      I can't make things out in the first pic, but the bottom one seems to have a lot of back rake - it has 30 side rake as well?
      .

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      • #4
        Originally posted by Boucher View Post
        What is fundamentally wrong with the tool if it fits the fishtail and cuts good threads?
        The main reason is the from tool loses its integrity. With 20 degrees back rake (a guess of what's in your pic) the edge the work sees is 57.7 degrees...I promise if you stone an edge to 57.7 degrees you will not be happy with the fit in a fishtail. This is a pretty big fail of the thread profile which means to get a good fit you will have to both cut the thread slightly deeper (reducing root strength) and that the out extremity of the thread will take the load rather than across the flank. may or may not matter to you or the application, but its why it not considered the right technique in most instances

        Back rake also will have a lessened benefit in the sense the wins come from rake perpendicular to the cut line . I can see the side rake ground perpendicular to the 30 degree face having benefits to hogging, Maybe there are certain materials you need it for, but it's going to play havoc with the profile when you take the last cut(s) in feeding from the cross feed where you skill both flanks. I've never had a problem cutting them I've not had a problem with flat top tools.

        It's also very easy to grind and stone a 60 point that is dead on. Not so easy stoning a perfect fit in the fish tail with back and side rake. Most materials can be machined with zero rake quite well; you just can't take as big a cut....that doesn't bother me for threading. Don't care as I don't try to hog it out when threading.

        If you're making say a bolt for reflector on a kids scooter all kinds of things will work....but what one can get away with is different that what the instructions should be to do it properly. That a off thread profile still holds isn't a reason to adopt rakes on threading tools and seems to me solution looking for a problem in most instances.

        There may be materials needing that grind and not needing an accurate thread in which case it might be a creative approach that gets a difficult job done. I've not had to thread such materials, ie that didn't work perfectly well with zero rake. For regular work it's not the correct way to cut a thread for the reasons mentioned above, namely it doesn't give the intended profile.

        If it works for you, it's not an issue for me, but you asked for the reasons
        Last edited by Mcgyver; 12-25-2012, 11:54 PM.
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        • #5
          Originally posted by Boucher View Post
          What is fundamentally wrong with the tool if it fits the fishtail and cuts good threads?
          Nothing really as long as the thread form is maintained. When I went to the Navy's Machinery Repairman school we were taught to grind a positive rake to compensate for the old fashioned rocker type tool holders we used. As soon as I started using a QC tool holder the top rake went bye-bye.

          Call me lazy if you want, but I've switched to insert tooling for threading and the ones I use have no top rake. Sure speeds up my threading.

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          • #6
            Originally posted by lakeside53 View Post
            If that's compensated for to generate the same effective profile as a a flat 60 V tool, then it works fine.
            Yes I can see that BUT with the clearances and the top rake it makes re-sharpening almost impossible (maybe completely) to keep the 60deg
            angle.
            ...lew...

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            • #7
              That's one reason I use laydown inserts. The other is I acquired a bunch in an auction lot Just as well, or I'd go broke buying them.

              The other thread has evolved into a good discussion on these issues.
              Last edited by lakeside53; 12-26-2012, 01:39 AM.

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              • #8
                In my books it is just easier to buy the threading inser, proper anvils and a proper insert holder. An insert has usually three cutting edges on it and costs around 10-15 EUR per insert and usually comes in either 5 or 10 piece packages.

                Also, if you get inserts that are full form, for basic threading operations you don't need to measure the thread. Just turn the part OD/ID to what the thread spec says and then thread until the insert just scrapes on that OD/ID.
                Amount of experience is in direct proportion to the value of broken equipment.

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                • #9
                  Have been reading one of my old college texted books about threading on a lathe. I was surprised they made it clear a zero rake angle on a threading tool.
                  The book says if using a lantern post toolholder with the rake angle built in to grind the threading tool to have zero rake angle. They went on to say if the toolholder has no rake angle no grinding on the top of the tool is necessary.

                  However, when I was a machine shop student some 43 years ago I remember the instructor had us grind a rake angle on threading tools. I thought the idea of advancing the threading tool at 29.5 degrees was to make use of the rake angle. If you are going the thread with no rake angle why not just feed the tool straight into the workpiece and forget about advancing at 29.5 degrees.

                  Some say when turning stainless steel to increase the rake angle. What about rake angle when threading stainless. Are some people saying when changing
                  from turning to threading the fundamentals of machining all change.

                  My turning tools are all precision machine ground


                  I have been threading on lathes for 43 years and always used tooling with rake angle. If using rake angle when threading is so wrong it seems like the reasons should have surfaced after 43 years. In the last couple of weeks I purchased carbide tooling for theading
                  both internal and external threads on my CNC lathe. Not sure what rake angle, if any, are used with the carbide tooling.
                  Jim
                  So much to learn, so little time

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                  • #10
                    Mcgyver -

                    That is a good summary of the reasons for using zero rake tools in screwcutting. Just to extend your point about what happens with a positive rake tool on the final straight in cut; the positive rake on the leading edge means that the trailing edge will have a negative rake. This is very unlikley to cut well and may just raise bur.
                    Bill

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                    • #11
                      thanks Bill.

                      Originally posted by outback View Post
                      I thought the idea of advancing the threading tool at 29.5 degrees was to make use of the rake angle. If you are going the thread with no rake angle why not just feed the tool straight into the workpiece and forget about advancing at 29.5 degrees.
                      I do a little of both. For most of the thread its advanced with the compound....you're cutting the cut width in half so an be a little more aggressive. The final few cuts are very light, a thou or two, straight in, and are for turning to final dimension and thread profile. I put some effort into getting thread tools dead on (once done they last a very long time) so like to do this final skim or even spring pass on both flanks so the profile is good.

                      I agree that for a lot of the threads done the 29.5 degree business won't matter, heck for a lot threads a lot of this won't matter, but it is I believe the best way to come at it, is no more work (less than grinding rake) and gives you a fighting chance when doing something more challenging or that has be dead on.

                      I have been threading on lathes for 43 years and always used tooling with rake angle.
                      which is fine, but that doesn't mean zero rake doesn't work. Like you, all my regular cutting tools, 95% of them, have top rake. Where I use zero frequently is with various form tools. Zero rake will cut steel and leave a beautiful finish, is far easier to grind and sharpen and is the only way to maintain a correct profile with a form tool. The price you pay is a higher cutting forces so for the same set up you're not going to have the same removal rates....doesn't matter imo as the use of form tools is a small portion of the total time spent at the lathe
                      Last edited by Mcgyver; 12-26-2012, 10:34 AM.
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                      • #12
                        For fine threads you can plunge straight in without much detriment, and I frequently do this for fine internal threads. Once the TPI decreases and depth increases the 29.5 degree in feed becomes more and more important. If you don't use this method, you are effectively working with a form tool with a long length of cut; often on an longer workpiece. You can easily get into the chatter region, and the quality of cut and finish suffers way short of that. The angled infeed gives a much smoother cut with less strain.
                        Bill

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                        • #13
                          The form change comes from tipping the tool in the holder relative to the direction of infeed, not from cutting top rake. When viewed from a line 90؛ perpendicular to the direction of feed above the cutter/work touch point, a 60 degree form is 60؛ with or with out top or side rake. Just keep the tool at exactly centered on the work and parallel with the direction of feed. If side rake is used then there is a leading edge one one side of the cutter so plunge cutting the thread won't work well. One side of the point will have positive top rake and one side will have negative top rake

                          You can demonstrate this with a block of wood and a disk sander very quickly.

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                          • #14
                            Originally posted by dp View Post
                            When viewed from a line 90؛ perpendicular to the direction of feed above the cutter/work touch point, a 60 degree form is 60؛ with or with out top or side rake. .
                            that is just not so. The V will only be 60 degrees 90؛ perpendicular to the direction of the feed IF the 60؛ V is in the same plane as the direction of the feed. A 60 degree V cutter ground with 20؛ back rake has a plane of contact 20؛ different than the plane of feed and will result in 57.7؛ V in the work.
                            .

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                            • #15
                              Accurate resharpening

                              Originally posted by Lew Hartswick View Post
                              Yes I can see that BUT with the clearances and the top rake it makes re-sharpening almost impossible (maybe completely) to keep the 60deg
                              angle.
                              ...lew...
                              The 60° profile is accurately produced using this setup.
                              [IMG]http://i109.photobucket.com/albums/n50/boucherbyron/IMG_0267Small.jpg[/IMG]
                              That is a CBN wheel and it takes just a second to touch up several threading tools. Note the feeler gauge under the front of the blade that sets the clearance. It is removed and the blade mounted normally for use.
                              Several vertical threading tools originally came with my lathe and that has expanded to include right side up and upside down and both 5 and 10° positive inserts. A. R. Warner even makes these in HSS.
                              [IMG]http://i109.photobucket.com/albums/n50/boucherbyron/IMG_0269Small.jpg[/IMG]
                              I know that I need to try the full profile lay down inserts and that If I had I would have save a lot of experimentation if I had gone there first.
                              For the fine threads in the gummy material the Warner inserts and the high side rake HSS are producing the best fit and finish.
                              Thank you guys for your inputs, this has been very helpful for me.

                              I have been having problems posting pictures. Anyone know what is wrong?
                              Last edited by Boucher; 12-26-2012, 12:16 PM. Reason: pictures not showing
                              Byron Boucher
                              Burnet, TX

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