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Wide radius cutting tool for lathe

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  • Wide radius cutting tool for lathe

    I'm seriously considering installing Rapid Aire Maxline air compressor line in my shop. The tubing is aluminum with a polymer cladding inside and out, and comes on a spool.

    The company sells a tool for straightening the stuff, but $150 for a tool I'll likely use one time is a non-starter for me. The tool is simply a series of 7 rollers with concave faces, that works like a brake line straightener. You push/pull the curved stuff through and it comes out the other end straight (more or less).

    So I'm thinking about fabricating this straightener for nearly no cost from parts on-hand.

    I have plenty of leftover stock on Delrin rod from an old project, and am thinking I could make my own rollers on my lathe similar to the pic below, but am wondering about the the tool to cut the approx 1in diameter and how would it bolt up in my BXA tool holders, which can accommodate 5/8in tools. Also wondering if a plunge cut, even on Delrin, is practical and if the chatter would be excessive.

    I suppose I could buy a cheap 1in wood gouge (if they exist) and grab the edge of it with my tool holder but that strikes me as rife with drama.

    Anyone have any pearls of wisdom?



    Click image for larger version  Name:	Concave_OD.jpg Views:	0 Size:	33.9 KB ID:	1930083

  • #2
    Ideal job for a good Ball/Radius turning tool .
    ...lew...

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    • #3
      All of your questions are dependent on the size of your machine. A big monkeywrench in the works from the start in terms of plunge cutting the radius is that your tool holders are too small. You could conceivably make two form cutters and plunge one side at a time, which might be better for your (assumedly) small lathe. I wouldn't hesitate to do this in one cut on a good solid machine like an American or Monarch.

      A swing radius tool will work too but may be a little more time consuming. Especially if you don't already have one.

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      • #4
        Originally posted by eKretz View Post
        All of your questions are dependent on the size of your machine. A big monkeywrench in the works from the start in terms of plunge cutting the radius is that your tool holders are too small. You could conceivably make two form cutters and plunge one side at a time, which might be better for your (assumedly) small lathe. I wouldn't hesitate to do this in one cut on a good solid machine like an American or Monarch.

        A swing radius tool will work too but may be a little more time consuming. Especially if you don't already have one.
        My lathe is a 12 x 36 Grizzly G4003G gunsmithing lathe.

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        • #5
          Your cutter does not need to be high speed steel for this. Case hardened mild steel will work. May not even need to be hardened. A lot easier to make compared to grinding HSS from a blank.

          Would a vee roller work as well?

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          • #6
            The simplest ball turning tool without any tooling is your Lathe Compound.
            If you have a Lantern tool post, it is really simple.
            Mount your tool say 1/2 " out from the center of your compound and then rotate the compound by hand turning it to produce a 1" diameter cut

            Rich
            Green Bay, WI

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            • #7
              how many do you have to do?

              typical radius cutters can't cut full profile along the work (to my knowledge) because to do so their motion is going to hit the work. The overthe top boring head style can....but that requires a build and also needs a Y axis feed on the tool.

              Easiest way imo if the quantity is low, is to grind a radius tool, say 1/4 or 3/8 and make up an excel sheet of X/Z lathe moves to generate the profile. There'll be some scallops on the surface but I doubt that would matter, still going to get lots of support to the tube around is radius
              in Toronto Ontario - where are you?

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              • #8
                Originally posted by Mcgyver View Post
                how many do you have to do?

                Easiest way imo if the quantity is low, is to grind a radius tool, say 1/4 or 3/8 and make up an excel sheet of X/Z lathe moves to generate the profile. There'll be some scallops on the surface but I doubt that would matter, still going to get lots of support to the tube around is radius
                This works especially well if you have a DRO. I wouldn't suggest trying to do this with dials. If you don't have a DRO, set up dial indicators, one registering on the carriage and one on the back of the tool or toolpost.

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                • #9
                  Originally posted by eKretz View Post

                  This works especially well if you have a DRO. I wouldn't suggest trying to do this with dials. If you don't have a DRO, set up dial indicators, one registering on the carriage and one on the back of the tool or toolpost.
                  dial, dro, imo makes no difference...its just numbers. I've never bothered making a radius tool and after 25 years have done every radius this way with a DRO. But i get the DRO is slightly easier to read.

                  I like idea of an indicator on the bed for the Z movement, vs slewing the compound around
                  in Toronto Ontario - where are you?

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                  • #10
                    Indicator or DRO are both better than dials for me because they're easier to read quickly. I tend to focus on efficiency and getting things done quickly - a bad back will do that to you. The less time I need to spend stooped over (bad back and 6'3", wonderful combo) the better.

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                    • #11
                      My son installed Rapid Aire plastic aluminum tube in my shop for me, and he just straightened it by hand. Ours did not come on a spool It came in large coils. I think we went with 3/4 ID so maximize air flow. Its big stuff. We installed just shy of 200 feet of the stuff. We used 90s in inside corners in the machine room, but in the large shop areas we just curved it around the corner support beams and laid it on the horizontal wall purlins. Then we threw in a T and a drop line anyplace we wanted a manifold block. We bought some blocks, but towards the end of the project we just made our own.
                      *** 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.

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                      • #12
                        Here's what I made for this purpose of making delrin rollers. Fits a 1/2" axa holder. Set the cutter to the radius and take nice light cuts. A 'plastic only' tool.

                        Click image for larger version

Name:	inside radius cutter.JPG
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ID:	1930131
                        Location: Jersey City NJ USA

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                        • #13
                          I've had good luck making pretty nice "look and feel" grooves like that using a round nose form cutter that is smaller than the final radius. Using a smaller cutter to hog out and then poke at the ridges avoids almost all risk of chatter.

                          The amount smaller for the cutter vs final size only needs to be a small amount to make this trick work. For making the sort of rollers you show with a 1/2" radius I'd probably make up a roughly 0.45 radius cutter. As you get close to final depth it would only need a few touches on the ridges to get a pretty decent "close enough" size for the groove. And with a slight pull back to check the shape you're looking for a nice even .05 wide gap. It should be pretty easy to see where it isn't even just by eye and ease a little away.

                          Such a cutter could likely be done in anything even a slight bit better than mild steel. You're only looking at cutting Delrin or similar and it only has to last a while. If you made the "cutter" up on the lathe as a fairly thick washer of around 3/16 thick with a relief angle cut into the edge it would be just fine. Then screw that to a piece of something like 1/2 x 3/4 to go into your BXA holder and you're good to go. If it gets dull after a few then loosen the center screw and turn it around to the other side. If the whole thing gets dull before you're all done then stone or grind the face back a little to freshen up the edge.
                          Chilliwack BC, Canada

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                          • #14
                            Originally posted by Bob La Londe View Post
                            My son installed Rapid Aire plastic aluminum tube in my shop for me, and he just straightened it by hand. Ours did not come on a spool It came in large coils. I think we went with 3/4 ID so maximize air flow. Its big stuff. We installed just shy of 200 feet of the stuff. We used 90s in inside corners in the machine room, but in the large shop areas we just curved it around the corner support beams and laid it on the horizontal wall purlins. Then we threw in a T and a drop line anyplace we wanted a manifold block. We bought some blocks, but towards the end of the project we just made our own.
                            I use a similar design , and I cut steel with it., I use broken center drills to make the cutters.

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                            • #15
                              I do it by using the coordinate position method mentioned above (-:
                              https://photos.smugmug.com/My-First-...5B1%5D-640.mp4
                              Last edited by Bented; 02-23-2021, 04:43 PM.

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