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Thread: Straightness of rod

  1. #1
    Join Date
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    Default Straightness of rod

    I'm working on a model engine here that needs a pretty straight 1/8" dia. piston rod. I have several choices here but the best one runs out .003-.004 over about 2". I tested it in a 3c collet, and proofed the setup with a gage pin which shows less than .001" @ 1.5" stickout. If I could thread the gage pin, it would be perfect.


    So, you guys think drill rod would be the best choice? Something else?


    Thanks in advance TC

  2. #2
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    Default

    It comes out of the factory neatly straight and if handled with care for the folks you buy it from it'll be nice and straight.

    Best if you can to check for straight with the material sitting on two "V" blocks and a dial gauge sitting on the middle of the length of rod. And if you do this right in the drill press (doesn't need much force) with a dial gauge reading off a "follower" beam you can shift the beam over and press any high spots down to straight.

    Would using it in the annealed state be OK? If you harden and temper it back to a nice blue spring temper it's likely to distort depending on the manner you use for hardening and tempering. But once back to a blue temper you can again measure in blocks and with that follower idea on the drill press and tweak it back to straight.

    By a follower I mean a bar or narrow lathe parallel sitting on some other block so the dial gauge can be out from under the drill press chuck. Due to the leverage ratio the reading will only be half or whatever ratio of the actual bend but once you get it down to a fine degree where the needle barely moves I'm thinking "close enough" as it would only be out at most a half thou or so. And that way you have it sitting right under a pusher rod in the drill chuck for any straightening.

  3. #3
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    Default

    I've seen a spec of .005 per foot. Same as stated here: https://www.diesupplies.com/drill-ro...rod-tolerance/ and on the Starrett website.

  4. #4
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    Aug 2009
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    How about a dowel pin?? They are centerless ground and very straight.
    I've seen 1/8" pins up to 2 1/2" long.
    Ground drill blanks would also be a good choice but are probably softer.

    JL.............

  5. #5
    Join Date
    Jun 2008
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    Default

    Quote Originally Posted by JoeLee View Post
    How about a dowel pin?? They are centerless ground and very straight.
    I've seen 1/8" pins up to 2 1/2" long.
    Ground drill blanks would also be a good choice but are probably softer.

    JL.............
    If it is a steam engine, and bigger than about 1/2 " bore then provided the rod is reasonably straight, ie as you are measuring, then it will not be a problem.
    Give a reasonable allowance for oil to lubricate glands etc( Ie 0,125 rod in 0.126 or 7 reamed holes)
    The folks who have trouble actually running their models are those who build them with very tight clearances.
    I hope these hints help, regards David Powell.

  6. #6
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    This is for a steam engine- will this part need to be rust-proof?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  7. #7
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    Shafts are straighter than drill rods. At least that is what I have been told.

    Plain shafts:

    https://www.mcmaster.com/precision-shafts

    Straightness = 0.0036" per foot. That should be well under a thousandth in two inches. 12L14 carbon steel so you should be able to thread it. And there are other alloys you can choose from.
    Paul A.

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

  8. #8
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    Spot heating may work as well if not better than pressing on the high spot.

  9. #9
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    Default Steam is an interesting world.

    IF the model is to be properly cared for then a mild steel rod will be satisfactory. That is, if it will always be put away dry, with oil lightly applied and kept in a dry place.
    If it is likely that the model will be used hard and maybe put away wet and dirty in a damp place, even for just a day or two then I would advise you use stainless steel piston and valve rods.
    IF the rods get rusty, even with just " overnight" rust the gland packings pick up the rust and can rapidly wear the rods. and the glands will leak.
    As one full size steam engine fellow said, " any oil is better than no oil"
    Regards David Powell.

  10. #10
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    Quote Originally Posted by elf View Post
    Spot heating may work as well if not better than pressing on the high spot.
    I attended a short lecture and demo by Prof. Richard Holt about flame straightening, spot heats along with other patterns. This was at U.W. late 90's IIFC. I still have the handout from that. His dad was one of the pioneers of the method way back when. Seems like he was in his mid-late 70's at the time. I've never tried it on anything very small.

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