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Turning eccentric work

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  • Turning eccentric work

    I need to turn a piece of CR 1018 to make a feed pump eccentric. Question: What puts the least amount of strain on the machine and tooling? Turning the hub as a facing proceedure, or, turning along the length of the stock towards the chuck? O.D. is 2", Off-set is 3/8" and hub will be 1-1/4" O.D.



  • #2

    I've just made something similar (the ecentric backgear shaft off of a Raglan lathe). It was 1 1/8" major dia. with one end 1/2" dia, and the other 5/8ths dia. The offset was 3/8ths. The 1/2" dia was 1 inch long and the 5/8ths was 2 3/4" long. The whole job about 7 1/2" long, and was in EN19 (your 4130 spec).

    The way I did it was to mark out the two sets of centres, set it up between the 'true centre' centres, turn the 1 1/8th dia. Set between the offset centres and turn the long 5/8ths eccentric diameter. I then thought about using a L.H tool to do the 1/2 dia. short eccentric, next to the headstock end, but felt that this may distort ~ so I reloaded the job into a good chuck I have with freshly bored soft jaws, picked up the centre with the tailstock, clocked it to make sure everything was OK; then turned it to the 1/2 dia. Its hard work with the intermittent cut but its the best and surest way of going about it. I did consider facing (and tried it for one cut ~ that convinced me to go back to the intermittent diameter reduction approach!)


    [This message has been edited by Ragarsed Raglan (edited 11-18-2002).]


    • #3
      Not that big a difference.
      I could have it done quicker either way, than I can write about it.


      • #4
        Not on my lathe!! She likes to work slowly if somewhat steadily.


        • #5
          I have a preference for turning on eccentrics as this does not put a "shear" type of stress on the chuck jaws or centers. The banging and intermittent stresses are more towards the chuck and the spindle bearings, which are made to absorb thrust more aptly, and keeps pressure into the chuck body rather than against the jaws. The "shear" is still there, but anyway to reduce this is always a good thing.

          With turning you also have more diameter control, and the tool is not tryign to constantly "kick back" on you throughout the operation.

          Keep your compound set at 60 1/2 degrees, like threading. There is a reason the old toolmakers do this instead of lining up longitudinally (along the Z axis). This angle is of course standard for threading, but also absorbs intermittent cuts better and with less damage due to the forces being spread along two force "vectors" or directions.
          CCBW, MAH


          • #6
            Thanks, Guys! I will turn the piece along the axis of the bed, the compound turned as in threading, with the piece in the 4 jaw chuck supported by the tailstock. After turning the hub, I will drill and ream the hole for the axle. Thanks again for your advice.