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Thread: Knurling Specification and Tool Size Chart

  1. #1
    Associate Engineer
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    Knurling Specification and Tool Size Chart

    Hello All.

    I am currently working on designing a copper component which I intend to press-fit into a plastic component. I would like to design-in a straight knurl feature so as to help prevent rotation of the copper component.

    I have just recently started to become familiar with the knurling process and would like to inquire about the Knurl Specification and Tool Size Chart:

    http://www.engineersedge.com/manufac...nurl_chart.htm

    My first attempt of the knurl design is to use a 128 Diametral pitch with 36 teeth.

    Am I reading the chart correctly? Will the starting diameter of the copper shaft need to be .281 inches and then have a final knurled diameter (I'm assuming major diameter) become .293in?

    Would this mean that the minor diameter will become approximately .269in? In other words will the tool need to "roll" into the copper part at a depth of approx 0.006in in order to displace or raise a tooth 0.006in above the blank diameter?

    I apologize if I have confused anyone due to the way I am wording this... if someone could possible explain the definitions of the chart a bit... I think that would greatly help me.


    Thanks for your time.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Yes... Your approximate Major (Knurled) Diameter is .293 with the tooth depth being approx. ~ .012

    .293 - .024 = .269

  3. #3
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    Hi, you have pretty much understood the knurling process and without actually checking your work with the chart, it sounds about right. The knurling will dig into the metal and displace it outwards. It is not a cutting action but more of a roll-forming process.

    Suggestion I have: Copper is not a good choice of material to embed as you intend, as it has a high expansion rate (coefficient). If the end product must work in a range of temperatures the "grip" to the plastic will vary as the two materials will expand and contract at vastly different rates.

    The other thing is, before deciding on a knurl geometry, look in the tooling catalogs for knurling rollers. You will find a significant range in prices, I guess based on popularity. So look at the lower costs ones (unless money is no object) and then see which combination on the chart matches those rollers. Then calculate the diameter etc.

    Often those knurled brass component one sees in plastic items are molded in. The brass insert is located on a post within the mold and the plastic then flows around and into the knurling for a good grip. Pushing the knurled item into the plastic after molding will tend to increase the hole size slightly and only provide "grip" on the tops of the teeth. That may not be sufficient grip for your project unless you are using very hard materials like Bakelite or Melamine.

  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by PinkertonD View Post
    Suggestion I have: Copper is not a good choice of material to embed as you intend, as it has a high expansion rate (coefficient). If the end product must work in a range of temperatures the "grip" to the plastic will vary as the two materials will expand and contract at vastly different rates.
    Good point - Brass (C360) knurls good and vibro-installs as well.

  5. #5
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    Thank you very much for your input.

    I agree that the CTE for copper is quite high. If I had the option, I would consider alternate materials. However, this copper component is a current carrier and I also have cost restrictions... otherwise I would consider brass. I also am not able to mold-in the components (due to cost). Therefore, I am kind of stuck between a rock and a hard place. The press-in knurl design seemed the most feasible for production and cost efficiency. I would like to point out that I am not solely relying on the knurl for anti rotation; the component will have a feature that is "trapped" inside the plastic component and will use a washer on the outside which will then be ring staked. My thoughts are that the compression from the staking operation will also provide anti rotation capabilities. Another aspect that I have kept in mind is that this copper component will not see very much repeated torquing operations. Basically, a mechanical connection will be made to it and typically left alone.

    I will need to perform some thermal shock/cycle tests to determine if the the copper component does become very loose, however. My other thoughts are that I could design it so the feature "trapped" in the plastic component could also serve as a anti-rotation feature (i.e. hex pocket design). I am skeptical about trying to work in both anti rotation features at the same time; I kind of have it in my head that it will be either one or the other (knurl vs. hex pocket)

    Any additional thoughts?

    Thanks again!

  6. #6
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    Given the latest information update, I would not consider knurling at all, but instead make the copper piece such that it cannot turn in the molded hole. Maybe a cross-pin that locates in molded slots?

    A drawing or sketch will give us a better idea of what you are trying to do.

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