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Lathe work, cutting a semicircle?

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  • Lathe work, cutting a semicircle?

    I decided to cut some aluminium dies for a pipe bender. If it works, and I wear out the aluminium, then I'll cut them from steel. I want to bend 5/8" OD pipe and have a rough idea of how I'd like the bender to work. Nothing fancy, just some circular dies and leverage.

    That's the plan. Problem one: now that I've got the blanks turned (mostly), how do I cut that 5/8" semicircle in the circumference? If I made a 5/8" forming tool, I expect it would just chatter like crazy on my little lathe.

    Do I make forming tools, left and right with side rake, that are 1/2 the diameter of the semicircle I want to cut, then work them in to a roughed out slot?

    Do I CAD out the depths and just plunge in a parting tool, leaving steps I can file out?

    Do I give up on the lathe and mill it with a rotary table?

    Do I get better advice on what to do from the people here?


  • #2
    Small single point cutter, and a radius attachment, somewhat the same as a "ball turner", except made so you can extend the cutter past the pivot point and cut a concave form. Some ball turners allow that easily.

    Keep eye on ball.
    Hashim Khan


    • #3
      A ball Turner would be best, but I've roughed out and then filed a groove up to 1/2".


      • #4
        Your suggestion of 2 half form tools is probably best, just work them in gradually, in a little, sideways a little, a bit at a time. You don't need a super precise groove, just keep checking with a piece of pipe, as you form the groove till you get something that the pipe seats nicely into.
        'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger


        • #5
          sometimes you can clamp a bar to the toolpost and use that for radius turning.


          • #6
            Use a ball end mill held in a boring tool holder.


            • #7
              For one off I plunge to depth in center with a cut off tool going a bit side to side. Then a series of steps. I used a cad
              program to calculate the steps. Finish with radiused tool bit. This for a small not rigid lathe.


              • #8
                You could set the rotary table up in vertical mode, fix the blank to the rotab, then cut the groove with a 5/8" ball end mill. You may want to rough it out first with a 1/2" ball end mill, then finish with the 5/8. Or, just make several passes with the 5/8, going from shallow to finish depth.


                • #9
                  Having slotted with a 5/8 ball mill I can tell you it screams in aluminum and chatters unless you have a really rigid machine. It just so happens I have to make 5/8 slots every day (I actually go .630 now). Its the standard spru size for hand injection soft plastic bait molds. Now I use a 3D CNC operation to make them with a 1/4 inch ball end mill. Its about as fast and I can even do it on my lighter faster machines.

                  For the guys who suggested taking a step wise approach on the lathe they are dead on. Cut it a little at a time. Then finish it up. You might even be able to use some hand turning methods (wood lathe techniques) to get close. I've seen a guy make aluminum crank handles that way.

                  A ball turner would also work.

                  The tool post swing method would only work if your tool post is single bolt to the compound, and not on a T-slot. I think you might have some trouble turning a small enough radius that way though.

                  A ball mill would not work very well on the lathe unless you have live tooling. Fixed in any kind of holder you would be cutting on one side and rubbing in reverse on the other side. In the interest of being fair I suppose you could use one if you made the bending die in two pieces, and machined half a round in each piece then bolted them together. It would probably still chatter and scream at the end of the cut.

                  Nobody seems to mention them anymore, but a simple set of radius gages will help you get really close even if you are free hand turning. I have a very old set that came from my grandfather. Other than that, nothing special about them, but I do use them from time to time.
                  *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.


                  • #10
                    I used a homemade radius cutter like a ball turner, 1/2 shaft, relieved to around .43 in the middle. Round hole for toolbit setscrew from side to hold it. Works good but needs a bit of polishing.
                    Of course you use a radius guage to check.
                    I did a die In steel, I think 2 inch radius..


                    • #11
                      Do as you have suggested and use your CAD to determine tool position and plunge out the material with a parting tool. If you plot enough points then there might not even be a need for a file.


                      • #12
                        On a smaller lathe that is chatter prone I'd suggest a 3/8 round ended form tool which you use to hog out the material and work the tool back and forth until it's "close enough". There's no need for half thou accuracy with a half cutout of this sort. Get it close with small forming cuts then blend out the high ridges with file and later by hand out of the lathe, with files or half inch stock wrapped with emery cloth.

                        You WILL get some distortion of the tube with this setup so don't go full half depth on the groove. Or if you want to go half or slightly more than make it a slightly wider groove with a slight oval shape so the tube won't lock into place after bending. with a templ


                        • #13
                          If you have more than one to do you might consider making a rig like this, I did a small one for bending dies also, but I made the dies of UHMW.


                          • #14
                            Thanks for all the ideas

                            I'm really having a hard time visualising a pivoting, ball-turning, kind of rig that would do the inside of a 5/8" diam. I suppose a 1/2" tool holder, rounded nose, and a hole drilled right near the tip. I could then make a some kind of stud to fit in that hole and somehow hold a length of 1/8" HSS. Something on the other end of the HSS to control the pivot. It too sounds like quite the project.

                            But, it never occurred to me to use the actual pipe I want to bend as a form tool.

                            Along that line, what if I cut an inch section off the pipe and welded it vertically to something I could clamp in the toolpost? That wouldn't take much effort. Then, a die-grinder on the top-center open end, sort of like making a simple ball-turning cutter, would make for a nice relief all around, leave me a nice cutting angle. It wouldn't have any relief on the outside... might rub some unless I kept it below center. Might have to carefully grind a bit of relief, though that would be a pain. Lathe it before welding? Also, if necessary, I could grind one side away so I was only cutting half at a time. If I did it smart, I could then flip the tool upside down and it would do the other side. Think it would work?

                            But, yeah, removing most of the waste with a parting tool or something makes sense. And, after that, maybe I'll try a bit of hand turning to see how I do. I hadn't thought of that either. Sounds fun. Might not need any custom tool at all.

                            Oh, and radius gage... just going to use the pipe

                            Thanks for all the replies,



                            • #15
                              AFAIK radius turning attachments don't work for small internal radii. You need a set up that uses some sort of tangential tool. I'd set up a tool bit vertically and use the compound for rotation.

                              The other way, perhaps more exact and solid is to use a boring head. You need to make something to hold it on the cross slid and be able to rotate it - such that its axis is parallel to the X. Something mounted in the tool post works so you adjust height. boring head adjustment becomes radius adjustment. Again, it uses a tangential tool