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Finishing piston o.d. with toolpost grinder

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  • J Tiers
    replied
    Centers IS best, but you may need to allow for the volume of the center hole in such a small piston.

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  • Mcgyver
    replied
    Originally posted by brian Rupnow View Post
    The only issue I have with grinding the outside diameter of the piston, is that once the piston stock has been removed from my lathes 3 jaw chuck, it will never go back in the same way.
    before taking something out of the 3 jaw, put a witness part on one jaw and the part - a simple magic marker line is good enough. Try it and marvel at how the turned section is very close to concentric on remounting the part. Not saying it would be good enough for you in this instance, but its a good trick when the situation permits (non critical stuff).

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  • brian Rupnow
    replied
    I haven't made another piston. I think the only way I could use a toolpost grinder to finish the final outer diameter is to do it without ever removing the piston from the original set-up in the 3 jaw where it was machined. Once the piston is fully machined, there would be nothing left to hang onto if I used the four jaw chuck after originally turning the piston in my 3 jaw chuck. So far, the only real work I have done with the toolpost grinder is to reduce the pilot diameter on some of my counterbore tools..
    Last edited by brian Rupnow; 01-24-2022, 04:26 PM.

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  • Doozer
    replied
    Running the piston on centers is the ONLY reliable way of taking it in and out
    of the lathe and having it perfectly centered and coaxial again.

    -Doozer

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  • J Tiers
    replied
    A) Use a 4 jaw.

    B) use a fixture. I'd think the piston needs that anyway unless you do it all in one go (I think you did).

    c) Both.

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  • brian Rupnow
    replied
    The only issue I have with grinding the outside diameter of the piston, is that once the piston stock has been removed from my lathes 3 jaw chuck, it will never go back in the same way. My chuck has about 0.003"total indicated runout. What that means is that getting the piston back into the chuck exactly as it was before depends solely on good luck. I'm not sure that would really affect the way the piston works or not. If I use an external lap to bring the piston down to size, it doesn't really matter if the piston is exactly centered in the chuck or not, because the external lap is free floating and is held in my hand.

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  • eKretz
    replied
    If you are going to run it that close you might want to lap both. Better chances of getting as near to dead straight as possible.

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  • brian Rupnow
    replied
    I asked this question because I want to bring a cast iron piston down to just a couple of tenths under the bore of a cylinder. I created a split external lap to use as well. So far, I haven't used either. I'm gathering information, thank you to those who gave an answer.---Brian

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  • J Tiers
    replied
    Bottom line...... While I am not a fan of the grinder you have (based totally on looking at pics), it ought to be able to do a good job of sizing the piston. I think it would work better on a CI piston than aluminum.

    With my Dumore, I find it useful to; 1) use a wheel that is finer than the usual ones, and 2) run the lathe quite slowly in the SAME direction as the wheel, so they are moving parallel where it contacts the work, not opposite as is suggested by most. No idea what that does, other than give better results for me. I have an impression that it led to less "chatter".

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  • Sparky_NY
    replied
    Originally posted by brian Rupnow View Post
    Luthor--I really do read the comments. I posted this question on three forums and have a very wide range of answers. I post a lot on this forum, and really do read the answers. Sometimes I just don't have anything intelligent to say.---Brian
    Reminds me of the annular cutter thread..... tons of suggestions for you to get/use them and not a peep of a response from you. THEN...... a bit later you showed the new annular cutters you bought.

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  • brian Rupnow
    replied
    Luthor--I really do read the comments. I posted this question on three forums and have a very wide range of answers. I post a lot on this forum, and really do read the answers. Sometimes I just don't have anything intelligent to say.---Brian

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  • Sparky_NY
    replied
    Originally posted by luthor View Post
    Is it just me or has anyone else ever wondered why someone would put up a post asking for comment and then, even after many replies, doesn't bother to check back in to the thread?
    Pretty standard procedure for Brian. I am pretty sure he reads the replies just does not comment very often at all. On the surface it would appear he ignores the suggestions/comments but I don't think that is the case. So, he probably checks back in but just remains silent.

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  • luthor
    replied
    Is it just me or has anyone else ever wondered why someone would put up a post asking for comment and then, even after many replies, doesn't bother to check back in to the thread?

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  • luthor
    replied
    Getting back to the question asked by the OP, a TPG will do the job nicely although I am not familiar with the Little Machine Shop product, it looks a bit lightly built. For cast iron I would use a Norton 39C60JVK grinding wheel or finer grit, 80 to 100 maybe if available.

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  • eKretz
    replied
    There's no question that a machine built solely for grinding is better, and easier to use. As I mentioned, to get a TPG to do its best, you'll need to take care to get everything right, and that may include trying a few different approaches with rotation direction, RPM, FPM, dressing, wheel type, DOC, feed, sparkout etc. But a TPG absolutely can do almost as well as a "real" grinder - I've done it plenty of times. I've also run plenty of "real" built for purpose grinders and am not denying that they are better -mainly in ease of use, and rigidity. Exhibit 1, have a look at luthor's photo. Doesn't get much better than that.

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