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  • Fillets

    I have a part for my school's Mini Baja team, a torsen differential case, that I need to make roughly a .5" radius between several faces. There is not enough room for even a small ball turner as the space between the two faces is a little more than the two half inch fillets. I did two of the fillets with a .5 HSS tool with a .5" radius ground in, and had roughed in the shape with a standard RH cutter. Even with the precautions there is chatter marks and there was a lot of vibration. The lathe I am using couldn't get much more rigid, an old 16" Monarch C with 5/8" capacity QC toolholder. I still have two more fillets to do with easier access but I don't want these chatter marks. I was hoping for some advise. Oh yeah, the material is 7075 AL.
    -Rob Comer

  • #2
    Any chance of doing it on a milling machine with a ball end mill and a rotary table?

    If I were to attempt it on my lathe -- a lot more lightweight than yours! -- I'd probably compute a series of steps (carriage travel, infeed) every 0.05" or so of carriage travel, rough out the radius with a rounded cutoff tool, then blend the steps with a round file.
    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


    • #3
      Hey Rob,

      Any chance you could set up a rotatory table vertically on your mill and use a 1" ball end? (SGW outpaced me there!) That said I'm sure some folks on here can give you some advice on using your form tool.

      Real question though: What race(s) are you going to? I'm no longer on the team, but plan to volunteer at the Tennessee Tech event as that was my team. Hope to make it to the Montreal race as well.



      • #4
        A 16" lathe is hardly stout enough to do a 1/2" raidus without chatter. You will have to use maybe 25 rpm or lower and at least a 24" lathe. You are talking about a 1" square tool bit to get a good 1/2" radius ground on it.

        You are way beyond the capacity of a 16" lathe.

        When you are making a radius tool it takes a tool bit double the size of the radius. 1" raduis=2" sq tool, 1/2" raduis=1" sq tool, 1/4" radius=1/2" sq tool.
        Last edited by Carld; 02-25-2008, 05:30 PM.
        It's only ink and paper


        • #5
          If you have to work within the constraints of what you have, I would suggest you might try using a method that can be used for turning spheres. You can use a parting blade and make successive plunge cuts that cut steps. The more and finer these steps, the less to remove later. This could remove the bulk of what would have to be taken off with the radius cutter. However, at the very end of the cut with your form tool, its still going to have contact across the full face which is part of your problem. However, since this is a final finish cut, it could be very light.

          Work for minimal overhang with your tool bit. Even a relatively large chunk of tool steel will wag under force.

          Paul Carpenter
          Mapleton, IL


          • #6
            I have cut a lot of 1/2" radius' and nothing replaces a big lathe. When you are making a full face contact with a 1/2" raduis ground on a 1" square HSS tool bit it takes a VERY rigid large lathe and it still chatters. You have to file and polish the surface to get a nice finish.

            Doing it in a mill with a rotary table may give a better finish if you can rigidly mount the housing on the rotary table and the table is BIG and STRONG.
            It's only ink and paper


            • #7
              3/8" radius

              Use a 3/8" or even a 1/4" radius tool, minimum speed, maximum rigidity withminimum front clearance and back rake. Hone the cutting edge with a fine diamond stone. Use good cutting/tapping oil.

              Make a radius gauge from a 1" diameter washer or similar.

              Take the cuts and keep width of cutting edge as narrow as possible. Position the tool using combined cross and top slide - with carriage locked/fastened to lathe bed.

              You will be surprised at how good a job your eye and that tool and the 1/2" radius guage will do - with little or no chatter. It may take a bit of practice.

              You can "rough it in" anyway that suits, but finish it off with the radius cutter. Minimise tool "rubbing" - keep it cutting - to avoid work-hardening.

              Its surprising how often a good job is done quicker by going at it a bit slower.


              • #8
                Turn the tool up side down and run the lathe in reverse. I have had this trick work for me more times than I can remember. . its worth a try. I dont know why this works but it does a lot of times.
                Every Mans Work Is A Portrait of Him Self


                • #9

                  It would help to see pic of part.


                  • #10
                    Originally posted by lane
                    Turn the tool up side down and run the lathe in reverse. I have had this trick work for me more times than I can remember. . its worth a try. I dont know why this works but it does a lot of times.

                    Make sure your spindle does not use screw-on chucks or else you will have a lathe-chuck in your lap. I imagine that a 14" lathe will probably have something other than a screw-mount, but it's far better to be safe than sorry.


                    • #11
                      Keep in mind its not just the lathe that has to be rigid - you have to keep your work piece rigid too. Are you using a steady rest or something to support it?

                      We've done larger radiuses here on our cinci for formula SAE vehicles in aluminum, no prob. Just keep the tool super sharp, slightly above center and a low rpm. Flipping it over and running in reverse works wonders on harder material too.

                      Edit - just saw the above post. Our Cinci is a taper-key spindle. Its threaded but has a key to hold everything on. The threaded part is just for a collar to snug the chuck onto the taper


                      • #12
                        Thats a big radius, but in aluminum with a sharp bit and plenty of oil I dont see why that would be a problem, you didnt say what rpm you were running, but I would go with the slowest rpm you have and rough out most of the meat with other tooling before cleaning it up.


                        • #13
                          Thanks for all the input,
                          I'm going to have to do a step method and finish file it. I don't have my digital camera working but I have a photo of the lathe. The actual finish radius isn't that critical. To give you and idea of the part its 6" Billet 7075 about 5" long in a four jaw with a live center. I was under the impression the lathe is really rigid, but I imagine it is the 1/2" tool thats causing most of the problems with vibration with so much surface area in contact.
                          Jeff, our team (Clarkson Univ.) is attending tenn and montreal.

                          -Thanks, Rob


                          • #14
                            NOT THE PART I'M WORKING ON, JUST THE LATHE


                            • #15
                              Yeah...thanks for posting the picture. It speaks a thousand words. I was going to point out that the 16" C series Monarchs are hell for stout....way more massive than average as compared to a typical lathe of the same envelope. I have a friend with a 12CK x30" (really a 14" swing over the ways) and it weighs something like 5000# plus.

                              Its not the lathe that's chattering if I were guessing...rather the tool. I would still remove as much as possible without the form tool as I described above. I would then grind the form tool to form *half* of your half inch radius at one time. This cuts your surface area in half. You will end up with a tool just as tall, but half as wide.

                              Paul--who has always wanted one of those old Monarch's
                              Paul Carpenter
                              Mapleton, IL