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Q: turning eccentrics

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  • Q: turning eccentrics

    when turning shafts with eccentric bits.. dialing in the 'eccentricity' in a 4jaw.. do i dial in the actual eccentric value, or double what i need?

    i ask because i 'mussed' up today. needed a 3mm eccentric in the middle of a shaft.. and set my 4jaw 3mm offcenter..

    when i finished, while checking, i noticed my eccentric was only 1.5mm -- half of what i needed! (doh)

    i suppose tomorrow i'll redo it and dial in 6mm instead of 3.. but this just isn't making sense to me. if i dialed in 3mm off center, shouldn't my eccentric now be 3mm off center?
    why's it half?

    someone please shed some light.

    thanks,
    -tony

  • #2
    Kinda hard for me to explain, but remember that your lathe is taking depth of cut for the radius, not the diameter. While your brain says that it should be an equal value to offset, your lathe cut the depth from both sides of center. Clear as mud?
    Arbo & Thor (The Junkyard Dog)

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    • #3
      Drives you nuts, eh?

      When you say you want a 3mm eccentric, you mean that the distance between the axis of the main shaft and the axis of the eccentric should be 3mm.

      If you could measure the offset from the lathe's axis to the main shaft's axis, you'd need to measure 3mm.

      However, you're probably clocking the diameter of the main shaft with a dial gauge to determine offset - in which case you'll need to adjust until you have a measured movement of 6mm.

      You sort of gain 3mm on the front, and also lose 3mm on the back; measuring on the diameter is double dipping.

      hth,

      Ian
      All of the gear, no idea...

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      • #4
        but.. but .. but
        (i know you're right because i have my bad shaft to prove it)..

        devils advocate: using a DTI and 4jaw chuck.. if i measure 3mm on the DTI then that means i moved the axis of my workpiece 3mm away (radially) from my lathe's axis. i'm measuring it *directly* aren't i? no radius/diameter conversions. my DTI measures how much i really moved off-axis.

        now i start cutting..
        no matter how you figure it (radius or diameter) the axis of my spinning work is *still* 3mm offset from my lathe's axis.

        i'm having trouble visualizing..
        black magic?

        -tony

        [This message has been edited by knucklehead (edited 01-30-2006).]

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        • #5
          I'm with knuckle on this, if the you moved the axis over 3mm, the eccentric should have a 3mm offset. bet your mic is busted

          on the other hand, if you moved it over by indicating a 3mm delta between the surfaces on the first axis, then i'm with Arbo, you are picking 1.5 from each side
          .

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          • #6
            here's what i did:
            chucked 20mm round in 4jaw..
            instead of indicating to center, i indicated 3mm out using a DTI.

            turned the eccentric bit.

            took it outta the late, measured the eccentricity: 1.5mm.

            -tony

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            • #7
              im baffled too!, dont take much though to do that!!

              bill

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              • #8
                I'm curious how you measured the eccentricity.
                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  being a slow afternoon (or rather this is more fun than what i should be doing) I did this pic on where the disappearing 1.5 mm went. make sense?



                  [This message has been edited by Mcgyver (edited 01-30-2006).]
                  .

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                  • #10
                    ah - did you do the following:

                    - Chucked the round shaft in the 4 jaw, set it to run dead true

                    - Set a DTI horizontally, touching the shaft

                    - Set the 4 jaw so that two jaws were horizontal, 2 vertical

                    - Slack off one horizontal jaw and tighten the opposite jaw, and keep doing this until the reading on the DTI had changed by 3mm

                    If this is what you did, then you moved the axis of the shaft by 3mm. Turning the lathe spindle through 180 degrees would then cause the DTI needle to swing 6mm.

                    Machine an eccentric at this setting, and it'll have a 3mm throw (unless you forgot to tighten the other 2 chuck jaws, in which case it'll have a throw of several feet...)

                    (from what you say, I think the DTI only swung 3mmm when turning the lathe spindle 180 degrees - this'll give you a 1.5mm throw eccentric)

                    Ian
                    All of the gear, no idea...

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                    • #11
                      McGyver, slick drawing.
                      IanB & Evan, i set the 4jaw so the DTI measured 3mm in 180deg turn.. ie.. swinging the chuck through one complete revolution made the DTI go from 0 to 3 and back to 0 again. seemed obvious enough for a 3mm offset.

                      using a step-mic i measured from the highspot of the eccentric down to the shaft.. it read 1.5mm .. it should've read 3mm.

                      -tony

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                      • #12
                        Like Mcgyver showed if yu want 3mm actual motion you set over 1.5mm. The rotating shaft will make a 3mm offset.

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                        • #13
                          You need to set it so that one side is offset by 3 mm. Depth of cut to make it round will be 6 mm.

                          [This message has been edited by Evan (edited 01-30-2006).]
                          Free software for calculating bolt circles and similar: Click Here

                          Comment


                          • #14
                            i guess the drawing wasn't that good

                            it was supposed to show as Evan pointed out that moving the axis 1.5mm, for a 1.5mm offset, would show up as a 3mm delta in the indicators reading between 180 degrees.
                            .

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                            • #15
                              Mcgyver, yes the drawing was perfect.
                              I've been shown the error of my ways, just wanted to answer Ian/Evan re: how i set-up and measured.

                              the drawing i'm working with shows a center offset (between Mcgyver's two circles) as 3mm. so i dialed in 3. guess i didnt think it through so was surprised to find the 1.5 offset.

                              i'll have another go at it tomorrow.. and i'll set it for 6mm.

                              thanks everyone. this is for a little steam engine, by the way. my first. Elmer's Chapter 24. "Open Beam"

                              -tony

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