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Radius in a Shoulder

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  • Radius in a Shoulder

    I'm still plucking up my courage to make a 2 1/4 x 8 chuck mount for this monster 10" 4 jaw I brought home. I got as far as chucking up a 6" x 2" round of 4140 tonight and then began the "think it thru" process I've found is so absolutely necessary, when I realized the 8" Skinner mount I'm modeling has a radius in the corner. Other than a file I haven't got a clue how to duplicate that ....

    I suppose I could do a square shoulder but this a 55 lb chuck. I'm as willing to experiment as much as the next guy but this doesn't seem like a good project to learn on by making mistakes! If a square corner's fine that's what I'll do. I just haven't seen enough chuck mounts to know any better.

    FWIW, the radius seats a 3/8" shaft with little or no gaps. Half of that is a 3/16 radius. Can I grind a bit to match and send that in there? Seems like that'd be a whole lotta chip load towards the end.

    All ears for any suggestions. Thanx.

    SP

  • #2
    The only way I know to produce an inside radius like that is to use a form ground tool as you already know. As for the load when turning the corner with a large radius tool, if it is too much for your lathe you can use a tool with a smaller radius to rough it out. Cut up to the radius in step fashion, leaving a few thousanths for the finishing tool with the final radius. You may have to make a sketch and do some calculations to get the numbers for the last several cuts at the radius. Then change to the radius tool and finish up.

    As for a square corner, I don't know. I would suspect that the design you are copying had a reason for it. And any inside corner is stronger with a radius as a square corner will concentrate the stress. A 10" chuck can hold some serious work and the cutting forces can be large. But there are other ways to relieve stress. For one, you could use a series of several steps, each at a progressively greater angle to approximate a radius. Perhaps 22.5*, 45*, 67.5*, then 90*. Or 15, 30, 45, 60, 75, 90.
    Last edited by Paul Alciatore; 05-24-2007, 02:34 AM.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

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    • #3
      Thanx for the response Paul. Other than a groove for an external snap ring I haven't ground a form tool yet, but it sounds like I'll get to grind a couple to get this radius in there.

      SP

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      • #4
        for reasonable radius, most designs of radius tool or ball turning tool will allow extending the tool farther past the center in order to produce an inside radius. It just depends on how bulky the tool holder part is.

        A form tool for a larger radius will often chatter like crazy due to the long edge in contact with the work, and the fact that it is usually blunt (flat-topped).

        If you just want a little radius, a form tool is fine. I was kinda assuming you wanted something like 6mm radius (1/4" or so).
        Last edited by J Tiers; 05-24-2007, 01:44 PM.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          Just leave a small "square" in the corner, between your facing and turning operations. Then take out the square with an appropriate radius tool.
          Just got my head together
          now my body's falling apart

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          • #6
            Not all that long ago I ran a manual vertical boring mill.
            One of the more common jobs was weld repair wire rope sheaves.
            We'd make a gage out of thin sheet steel or stiff paper and nibble away at it
            with a turning tool until the gage fit, and both sides were tangent to the flats. That's still how I cut a radius on my home lathe.
            That perfect radius is already in there. All you have to do it remove the
            overburden. Chuck up a piece of scrap and try it. It won't take long to develop the confidence to do it free hand.

            Rick
            Home Model Engine Machinist

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            • #7
              I like to use a form tool with a smaller radius, say 5/32 for you. Nibble out excess stock, trying to avoid having the whole tool touch at once. that will chatter, even on a heavy lathe. I got some chain saw files for cleaning up fillets. the files come in a few sizes and are not tapered, and they serve as good holders for emery cloth for that final polish. and they're semi cheap too... remember to use a FILE HANDLE !
              Last edited by Davyboy; 05-25-2007, 12:08 AM.

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              • #8
                1st Radius

                Just to let you guys know I use and greatly appreciate all the info everyone gives me, I thought I'd put up a pic of the radius I came up with. Probably not much of a pic. Maybe someone can fix it ....



                I left a square, cut that to a 45 flat, and then, using my recently acquired 7x reticule loupe, started with a 1/16 radius. I just kept grinding the same bit to increasingly larger radii and wound up somewhere near a quarter when it kind of filled in. It's not perfectly symetrical but I'm sure it'll work.

                So Thanx for all the advice guys. There's a chuck mount in that piece of metal somewhere and I'm gonna do my best to find it.

                SP

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                • #9
                  For those with failing sight??

                  That looks sweet enough, nice job.
                  Maybe I am reinventing the wheel here but it occurs to me that a homemade holder using round toolsteel or hardened rod could be utilised here.
                  I have in mind a piece of stock with a hole drilled at,say, 15* to vertical to accept the toolsteel and provide clearance then a grubscrew to secure it as per normal. Grind the top of the toolsteel for rake and you have an eliptical shape that would provide a tidy enough radius.
                  Some of us may already have a boring bar that uses rnd toolsteel that could be used by operating at 90* to normal ( or 270* if you turn it the other way )
                  Ken.

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                  • #10
                    Pic 8/10 - job 12/10

                    Originally posted by pntrbl
                    Just to let you guys know I use and greatly appreciate all the info everyone gives me, I thought I'd put up a pic of the radius I came up with. Probably not much of a pic. Maybe someone can fix it ....

                    So Thanx for all the advice guys. There's a chuck mount in that piece of metal somewhere and I'm gonna do my best to find it.

                    SP
                    My assessments:

                    pic: 8/10 - mightn't suit the purists (buy what does?) as it had done all that it needs to do.

                    job: 12/10 - for very experienced machinist: 9.8/10 (maybe) but under the circumstances - 12/10 (easy).

                    I'd be pleased to leave it on my bench or lathe for who-ever comes in to see.

                    No need to ask 'em how you've done - just take your well-earned bragging rights - and just show and don't bother to ask 'em!!

                    Gives you a real "lift" when it turns out as good as this.

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                    • #11
                      So far, looks really sweet. I'd like to see more pics when threading and boring is done.

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                      • #12
                        Is this what you wanted in an image fix?

                        Paul A.

                        Make it fit.
                        You can't win and there is a penalty for trying!

                        Comment


                        • #13
                          The radius is not that important as it does not locate onto anything. The reason there is a radius is that if any cracks appear they usually start from a sharp corner the radius just lessons the stress produced in a sharp corner. If your backplate is steel and not cast iron it matters even less, however a large radius is more aesthetically pleasing than a sharp corner.
                          MBB

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