Announcement

Collapse
No announcement yet.

Making and using a form cutter

Collapse
This topic is closed.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Making and using a form cutter

    I had a "real" machining job this weekend. I have a customer that needs a particular specialized part made in low quantity. It's a bushing used on log debarking machines. The original part is made from bronze and is no longer available. Making a set of new ones (10 per set) would be very expensive in the original material.

    The bushing looks like this:



    It isn't a difficult part to make but because of the expense I suggested that the part be made from acetal instead. Acetal has excellent impact resistance which is what this part requires in service. It isn't a bearing and doesn't rotate in use. It serves as a hanger for a very strong bungee.

    While I could have just turned up a short run of these on the lathe I decided to take the time to make a form tool instead. There is a small chance that I will need to make more of these in the future. This is a perfect application for a full width form tool and once made the tool can be used to turn out these parts at very low time cost per unit. It also insures that the parts are identical. It's the next best thing to CNC. I also wanted to see how my "new" horizontal mill will handle a job like this.

    I started with a scrap piece of planer blade. It is high speed steel and just the right size and material for such a tool. In the following set of photos I have removed the compound on my SB9 and have replaced it with a temporary work holder that allows me to use the largest CBN wheel in my collection (CBN=Cubic Boron Nitride). CBN is the best for grinding steel as steel will destroy a diamond wheel.

    Pics 1 and 2 show the 9" wheel (just barely fits the lathe). I am using it to dress the part by creep feeding it with the electric lead screw drive. The depth of cut is a much as .050" but the feed rate is about 0.3 inch per minute. A little WD40 is used to cool the work and also to keep down the abrasive HSS dust generated. The wheel itself doesn't shed any tangible amount of abrasive but HSS dust is just as bad. The part is set above center so the wheel grinds the edge with the correct relief.

    After finishing cutting the profile I switch to a slightly smaller wheel to hollow grind what will be the top surface of the cutter. In pics 3 and 4 the cutter is set so that the center of the wheel is below the cutting edge which produces a small rake angle on the tool.




    This is the finished cutter. Note that the various radii are intentionally different to facilitate installation and retention of the bungees.



    The acetal has been supplied to me in near net diameter so all I have to do is cut to length, face the ends and core the part to 1" ID. It is then mounted on a custom mandrel on the horizontal mill and the cutter is plunge fed into the work. The new mill makes short work of this and had no trouble handling the load. Total time to cut the part is around 2 minutes and that can be sped up if I change the oil in the variable transmission.

    The parts have been intentionally made oversize to compensate for the lower overall strength of the acetal. In the application the exact size of the part isn't critical with the exception of the internal bore diameter.

    Free software for calculating bolt circles and similar: Click Here

  • #2
    Evan,
    Seems a lot of work to go to when a simple bull nosed lathe tool and two bed stops could do the same job.

    Also you haven't made it clear that although this works fine in very soft acetal anyone spending time to copy your arrangement and wishing to cut bronze or steel is in for a rude awakening due to excessive and dangerous chatter on a tool this wide.

    This really should contain this warning.

    .
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



    Comment


    • #3
      No need to warn anyone. I felt sure that you or someone else would. I don't know about other people but I can tell the difference between the machining properties of acetal and bronze or steel. It's why I don't use my wood bandsaw to cut steel but it works fine on acetal.

      I know that I could have just turned it up. Didn't I mention that already? I wanted to try a form cutter this size on the mill which I recently acquired. I also might need to make more. Keep in mind that I am NOT running a business.
      Free software for calculating bolt circles and similar: Click Here

      Comment


      • #4
        Originally posted by Evan
        No need to warn anyone. I felt sure that you or someone else would. I don't know about other people but I can tell the difference between the machining properties of acetal and bronze or steel.
        True but there must be many people on this board who have never plunged into ferrous or non ferrous metals with a wide tool.
        Granted you are not running a businees but you do have more experiance than many on this board and a warning would have been nice and not have to rely on someone else.

        BTW what program do you use to save the multiple images into. ?

        .
        .

        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



        Comment


        • #5
          John,

          Machining is full of techniques that work with one material but not another. It seems to me that it is self evident that acetal plastic might be just a bit different than steel. I think that even the new people to the sport are smart enough to realize they can't cut steel this way without at least 20 or 30 horsepower.

          I use Paint Shop Pro for most of the graphics work.
          Free software for calculating bolt circles and similar: Click Here

          Comment


          • #6
            Originally posted by Evan
            The original part is made from bronze and is no longer available.
            Switching from bronze to plastic is a pretty major change in materials properties, especially strength.

            Why didn't you want to use bronze?
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

            Comment


            • #7
              Probably cost.
              I just priced some pieces for split bushes, 2-1/4" diameter x 2-1/4" long, £31 EACH, that's about $60 your end and I may need 20 -

              .
              .

              Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



              Comment


              • #8
                Nice work Evan, and you got to use that grand old horizontal machine as well.

                Where does one stop with warnings, perhaps we should make up special and colorful warning icons to stick in each post, kinda like all those seen on a ladder.

                Here are a few
                1) Be sure machine is grounded.
                2) Secure long hair
                3) Button shirt sleeves
                4) If operating lathe be sure and remove T-wrench from chuck before starting.
                5) Do not grip or touch moving cutters
                6) Stop spindle before removing part.

                Comment


                • #9
                  I don't think it's a matter of where do you stop with warnings but the odd note will stop someone not so much getting hurt as wasting a lot of time.

                  Some newby is going to see that, not know the difference between delrin and brass and decide he's going to make some candle sticks.

                  So after he's spent probably 4 or 5 hours [ newby remember ] he plows into his stick of brass on his little C3 / Atlas / grizzly import and finishes up wearing it.

                  Lets face it we are seeing much more basic questions than this every day.

                  .
                  .

                  Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                  Comment


                  • #10
                    A few months back at our model club, Bill Huxhold, a craftsmen of note, showed how he made form cutters for a lampshade for his scale model Hardinge lathe that was made from bar stock.

                    It was exactly as Evan showed it, with one exeption - no relief on the top. He says he never relieves the top of the cutter so there is no loss of shape when resharpening. The shade by the way was extemely thin walled.

                    One club member asked how he was able to eliminate chatter marks. "Ah" he says, "even my Hardinge HLV will chatter with this size cutter. You just turn the lathe by hand for the final cleanup cut".

                    Geoff

                    Comment


                    • #11
                      Form cutting

                      Originally posted by ammcoman2
                      A few months back at our model club, Bill Huxhold, a craftsmen of note, showed how he made form cutters for a lampshade for his scale model Hardinge lathe that was made from bar stock.

                      It was exactly as Evan showed it, with one exeption - no relief on the top. He says he never relieves the top of the cutter so there is no loss of shape when resharpening. The shade by the way was extemely thin walled.

                      One club member asked how he was able to eliminate chatter marks. "Ah" he says, "even my Hardinge HLV will chatter with this size cutter. You just turn the lathe by hand for the final cleanup cut".

                      Geoff

                      Right on Geoff.

                      Exactly the way to eliminate "chatter" when thread/screw cutting - works just about every time - as this is form cutting as well!!

                      Comment


                      • #12
                        Originally posted by Evan
                        I had a "real" machining job this weekend. I have a customer that needs a particular specialized part made in low quantity. It's a bushing used on log debarking machines. The original part is made from bronze and is no longer available. Making a set of new ones (10 per set) would be very expensive in the original material.

                        The bushing looks like this:

                        It isn't a difficult part to make but because of the expense I suggested that the part be made from acetal instead. Acetal has excellent impact resistance which is what this part requires in service. It isn't a bearing and doesn't rotate in use. It serves as a hanger for a very strong bungee.

                        While I could have just turned up a short run of these on the lathe I decided to take the time to make a form tool instead. There is a small chance that I will need to make more of these in the future. This is a perfect application for a full width form tool and once made the tool can be used to turn out these parts at very low time cost per unit. It also insures that the parts are identical. It's the next best thing to CNC. I also wanted to see how my "new" horizontal mill will handle a job like this.

                        I started with a scrap piece of planer blade. It is high speed steel and just the right size and material for such a tool. In the following set of photos I have removed the compound on my SB9 and have replaced it with a temporary work holder that allows me to use the largest CBN wheel in my collection (CBN=Cubic Boron Nitride). CBN is the best for grinding steel as steel will destroy a diamond wheel.

                        Pics 1 and 2 show the 9" wheel (just barely fits the lathe). I am using it to dress the part by creep feeding it with the electric lead screw drive. The depth of cut is a much as .050" but the feed rate is about 0.3 inch per minute. A little WD40 is used to cool the work and also to keep down the abrasive HSS dust generated. The wheel itself doesn't shed any tangible amount of abrasive but HSS dust is just as bad. The part is set above center so the wheel grinds the edge with the correct relief.

                        After finishing cutting the profile I switch to a slightly smaller wheel to hollow grind what will be the top surface of the cutter. In pics 3 and 4 the cutter is set so that the center of the wheel is below the cutting edge which produces a small rake angle on the tool.

                        This is the finished cutter. Note that the various radii are intentionally different to facilitate installation and retention of the bungees.

                        The acetal has been supplied to me in near net diameter so all I have to do is cut to length, face the ends and core the part to 1" ID. It is then mounted on a custom mandrel on the horizontal mill and the cutter is plunge fed into the work. The new mill makes short work of this and had no trouble handling the load. Total time to cut the part is around 2 minutes and that can be sped up if I change the oil in the variable transmission.

                        The parts have been intentionally made oversize to compensate for the lower overall strength of the acetal. In the application the exact size of the part isn't critical with the exception of the internal bore diameter.
                        Very creative and a very nice job Evan.

                        That would be a nice job as a "Turning" job with a single-point tool on your CNC mill using it as a lathe.

                        Comment


                        • #13
                          Originally posted by oldtiffie
                          Very creative and a very nice job Evan.

                          That would be a nice job as a "Turning" job with a single-point tool on your CNC mill using it as a lathe.
                          Or as a trace cut on a lathe.

                          Comment


                          • #14
                            How zat?

                            Originally Posted by oldtiffie
                            Very creative and a very nice job Evan.

                            That would be a nice job as a "Turning" job with a single-point tool on your CNC mill using it as a lathe.
                            Originally posted by dp
                            Or as a trace cut on a lathe.
                            Interesting post Dennis.

                            I think I am missing something.

                            How is that achieved?

                            Comment


                            • #15
                              For a trace cut disconnect the crossfeed lead screw and mount the original on the tailstock spindle. Use a tool holder with a tracer probe that follows the original to regulate the depth of cut of the tool on the new part.

                              Wouldn't work for this job as I changed the size and profile of the part. Acetal will hold up in this application. Acetal is often better than metal in applications that experience a lot of hammering as it has excellent impact resistance, better than mild steel. That is why the original bushings were bronze as it will work harden under impact and last longer that way.
                              Free software for calculating bolt circles and similar: Click Here

                              Comment

                              Working...
                              X