4140 Machining and Possible Distortion
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  1. #1
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    Default 4140 Machining and Possible Distortion

    I have a large (for me) compression mold to machine. Material is
    4140 prehard (~Rockwell C30). Main mold plate is ~9 x 24 x 0.625".
    Almost all of the machined features are on one side of the plate-
    see attached image. Not having used 4140 prehard I have questions
    about how much this may distort when I take material out of one
    side and not the other. How much do I have to worry about this
    given the material and condition?


    mold-top.jpg

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    You are not going to cut the back side at all?

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    Just a couple of counterbores.

    Starting with ground plate and hoping not to do anything on the flip side (besides counterbores). Not sure if this is possible.

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    Better take out warp insurance you are going to need it...Phil

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    maybe start with 1" and face it off on both sides, whole lotta extra, and it still might warp, but you gotta chance.
    how about asymmetrically so the c of mass is on the centerline of original plate?

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    Ok this sounds dicey to say the least.

    I assume if I start with 4140 annealed material it would be more likely to stay flat?

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    Rolled plate is always hit or miss.

    5/8" is very thin.

    The slot in the middle of the plate doesn't help things.

    Is this your own design or someone else's? The first thing I would do, if I could, is thicken it up substantially. Based on the longest dimension, 24" in this case, I'd want to start with either 1.5" or 2" thick plate.

    If cost is no object, blanchard grind -> machine -> surface grind -> finish machine.

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    Another thought- stress relieve plate, rough machine, heat treat, grind, finish machine.

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    Snee, you'll need to plan for it moving, and include some cuts on the flat side. Use sharp tooling and low-force cutting paths to minimize adding stress due to cutting forces, so factor additional endmill costs into the project. And I'd use endmills over insert cutters, they tend to cut more freely.

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    Quote Originally Posted by sneebot View Post
    I have questions
    about how much this may distort when I take material out of one
    side and not the other.
    It won't be flat if that's what you're asking.
    Is there a reason you can't machine both sides? Mill flip mill flip mill

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    Thanks for the replies so far, very helpful.

    Not my design-- it's an existing design from my customer's customer.

    I can machine the flip side I was originally hoping not to-- but this does not sound possible.
    I originally quoted this with the understanding that the tolerances were quite loose. Get
    farther into it and a document appears that has some rather tight tolerances on some of the
    features as far as flatness. Trying to re-assess feasibility.

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    Quote Originally Posted by sneebot View Post
    Not my design-- it's an existing design from my customer's customer.


    I originally quoted this with the understanding that the tolerances were quite loose. Get
    farther into it and a document appears that has some rather tight tolerances on some of the
    features as far as flatness.

    LOL...gotta love customers like that. Quote Item A with loose tolerances, no problem.

    Then the loose tolerances get reeled in
    ...holes become bores

    ---through holes become blind tapped holes.

    ***now we start talking about surface finish and...


    Just recently ran a job like that...

    It was originally quoted as make this piece we already made for them in 316SS, but in Aluminum.
    No problem, I ran the numbers, $10.00 was the quote.

    Then I had to supply the certified material...no problem, $12.00

    Then they needed it longer... Sure, but understand it will reflect in the finished cost. Wasn't an issue.

    Then change a couple angles.

    Then I'm asked if it will work...beats me, I just make stuff...
    So a sample was needed for approval.

    Sample worked perfect...but they didn't like the "look". So it went from square to rectangle and new samples.

    Can we engrave...

    Can we have all surfaces machined.


    New sample made and approved for Rush production.


    We run job.
    When final bill was handed over they actually asked why the pricing was not what I originally quoted.


    ...and were serious.

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    Circling back to this...

    I ended up machining these from material I bought from McMaster:
    McMaster-Carr

    There was virtually no warpage-- easily less than 0.005 over the 24" length.

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