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Thread: How to control the Size and Position of 2 Very Odd Openings

  1. #1
    Associate Engineer
    Join Date
    Sep 2013

    How to control the Size and Position of 2 Very Odd Openings

    Hi all,

    I am trying to find a way to control the size and position of 2 very odd openings of a molded plastic part. The opening looks like a perfect circle on one end, and a slot/ellipse on the other due to the draft and odd angles. This part mates with another plastic part which has 2 cylinderical holes. 2 self tapping screws are used to asseble both parts, so it is a fixed fastener assembly. The openings on my part function to clear the screws. Ideally, I would like to be able to use MMC.

    Can I use a Surface Profile (ALL AROUND) for the size and a Position (BOUNDARY) for the position of these openings?
    Or do I have to use Surface Profile callouts as in the attachment?

    Thank you all for your feedback.

    Odd Openings.jpg
    Last edited by Tarator; 09-20-2013 at 10:49 AM.

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Well, you could use the profile of surface tolerance for location and size as you have shown within your attached illustration. You do need a basic angle dimension (TED if you’re ISO) somewhere to specify the draft angle. You don’t need the “All Around Opening” note however it does not hurt the specifications clarity.


    Because this is plastic injection molding and in that world limit tolerances are normally adequate..

    Simply indicate the nominal diameter with tolerance and draft angle (clearly define from which surface) and locate using either simple limit tolerances or geometric position (if you feel inclined to do so) relative to your datum’s’. Since this is a simple clearance hole –all you need to do is control the inner boundary cumulative of size, form and location at the smallest diameter of the drafted hole feature. Well, do the math at the large end as well.

    In the plastic injection molding world, the process is normally more capable than the fit requirements in addition to the fact that plastic are often semi-rigid in nature. Be aware that most plastic molders are quite accustomed to seeing and building from the 3D CAD models only.

    End Note: Be prepared to have a conversation with the molders to explain the Profile of Surface Specification..

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