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Straightening 1/4" rod?

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  • Straightening 1/4" rod?

    I have a 1/4"(6mm) cold-rolled mild steel shaft 36"(900mm) long. It is held by bearings on the ends and in the middle. It runs 10,000 rpm plus. The shaft has a bend in it that produces a lot of vibration, even though it is constrained by the bearings (the bearings are held by a thin wall aluminum tube).

    I would like to get the shaft to be much straighter to reduce the vibration. The bend was such that the center of the shaft was >0.050 out. By trial and error un-bending in the vise I got it down to 0.020, but I'd like to get it much less. I doubt that the trial and error will let me do that. Anybody have any good ideas (especially experience-based ones)? Let me inject here that ground rod is not in the budget.

    Thanks, Bob

  • #2
    When I worked in a marine machine shop propeller shaft straightening was done by putting the shaft between V blocks, pressing with a hydraulic press, while checking with a dial indicator. However, the shafts were more likely to be 4 or 6 inches in diameter, and they guy who did it had experience, but there was still some trial and error involved because of spring back. So none of this may apply to straightening a 1/4 in shaft..

    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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    • #3
      My friend, a pro machinist, wrote this years ago and I have saved it. It was probably written about a larger shaft, so scale it back mentally as needed. - metalmagpie

      Here's the method:

      Saw a couple vee blocks out of aluminum.
      Support the shaft on these blocks in your press.
      Mount a 1 inch travel indicator beneath the shaft under the bend.
      Rotate the shaft such that the high spot is up.
      Press against the shaft with a piece of aluminum between the ram and the shaft.
      Deflect the shaft about .100".
      Rotate the shaft and see if it has gotten any straighter.
      Once the shaft starts to become straighter lessen the deflection increments. It may take a .250 deflection to make the shaft move .001 but only .260 to make the shaft move .002.

      Use the indicator to monitor the deflection and be careful once the shaft starts to straighten because it will be easy to over bend it. The aluminum vee blocks will indent to fit the shaft and will work better as the pressure rises. This indenting also protects the shaft. Steel vee blocks will damage the shaft. It takes a long time to tell you this but straightening the shaft will be quick.

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      • #4
        McMaster Carr


        Inch—1566 Carbon Steel
        Lg. Lg. Tolerance Straightness
        Tolerance
        Surface Smoothness,
        microinch
        Edge Type Hardness
        Rating
        Hardness Heat Treatment Yield
        Strength, psi
        Surface Yield
        Strength, psi
        Each
        36" -0.03" to 0.03" 0.002" per ft. 8 Chamfered Very Hard Rockwell C60 Case Hardened 75,000 250,000 6061K61 $18.59











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        • #5
          On the opposite extreme is arrow straightening. Might be something you can use if you look up a video or two.
          *** 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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          • #6
            If you have many parts a bar straightener is an excellent tool for such work, most centerless grinding shops have them.
            Run the bars through it before machining.

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            • #7
              You can also use a torch to straighten a shaft.

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              • #8
                I like reggie_obe's suggestion, replace it for $20.00. Now the question does come up for me, a HSM not the sharpest tool in the shed kind of guy that often thinks outside of the box for good or bad. Does it have to stay at 1/4"? Could you make a new shaft that is larger in the center between bearings and either machine it down in size, or drill, ream, and press 1/4" shaft into the ends of the larger shaft to get the full length.
                We don't know the full application, but to turn a shaft that size at that RPM this might be the best fix.

                TX
                Mr fixit for the family
                Chris

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                • #9
                  Is there any way you could add a couple of extra bearings, a 1/4" shaft supported at 18" spacing and at that speed is a poor design.

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                  • #10
                    Originally posted by old mart View Post
                    Is there any way you could add a couple of extra bearings, a 1/4" shaft supported at 18" spacing and at that speed is a poor design.
                    ........at 36" spacing and.......
                    What is the device? Very long high speed mixer or agitator?

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                    • #11
                      This is a technique that I have used often for small rods. Works well and should appeal to your cheap side.
                      This youtube is a decent explanation.
                      Youtube titled: Saxophone Repair Topic: Straightening a Rod
                      https://www.youtube.com/watch?v=RvPTAIAF5EQ

                      You would need to remove the bearings from your shaft.

                      Chuck the shaft in your lathe by the end and flex the shaft with a 2x4 with an appropriate hole in it. As you slide the 2x4 along the shaft, you can feel where the bends are and flex the shaft until they are gone. Go easy and feel your progress. You will feel when you are getting the bends out. If you get too aggressive, you can make it worse.

                      Don't spin the shaft very fast, something less than 100 rpm. You need to control this, or the shaft can bend and flail around and do a lot of damage!
                      You have to control the shaft as it rotates and is flexed by the 2x4. If you are not comfortable working around spinning metal, then this technique isn't for you.

                      1/4" inch shaft is about as large as I have ever tried, but it works.

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                      • #12
                        I’d have a guess the shaft is twisted myself, don’t know what kind of torque it dealing with, I’ve found twist causes the wiggly in the past, perhaps replace it ?
                        if not I’d be tempted to pull the thing if you can rig it, ie stretch straighten, just my instinct ( I don’t have a good track record straightening shafts that thin)
                        mark

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                        • #13
                          I've been biting my tongue, waiting for Doozer to chime in with some comment about his "Shaft Straightener of the Month Calendar". (I'll just leave now...)
                          "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

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                          • #14
                            Curious what it is as it buzzes to 10K.

                            If it's a weed eater, toss it and get a new one.

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                            • #15
                              Originally posted by I make chips View Post
                              Curious what it is as it buzzes to 10K.

                              If it's a weed eater, toss it and get a new one.
                              Straight shaft weed eater would have a shaft longer than 36".

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