Posted by: totoro ® 07/04/2006, 23:58:45 Author Profile eMail author Edit

i've a GD&T question. I need to align 2xM3 holes with the 2X 0-80 holes as shown in the drawing. With the GD&T in the drawing, am I able to tell the need of alignment of these holes?

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Re: positioning tolerance
Re: positioning tolerance -- totoro	Post Reply	Top of thread	Forum

Posted by: Kelly_Bramble ® 07/05/2006, 09:06:43 Author Profile eMail author Edit

First things first - what do you mean "am I able to tell the need of alignment of these holes?"? I do not understand the question. Second, the datum specified "C" resolves into the middle or an axis located at the medium location of the two hole features. Datum "A" appears to be at the same location. Personnaly, I would never define a datum feature the way is has been done for C. I would select one of the threaded holes or the other, since the declaration given is not clear. Additionally, if you define one of the threaded hole features as datum C, the tolerance accumulation is almost identical anyway. Additionally, I'm not a fan for declaring a V-thread as a datum. These types of features are not very stable, reapeatable, or easy to discern. The OD, datum "A" would do a great job in this application. In fact I would locate all of the threaded features relative to primary A, and secondary datum B. This should work great since all of the hole features will be probably be manufactured in the same setup anyway. Third, within the right view (and top left), you have located the M3 (0-80) thread with a position tolerance and used a MMC modifier relative to datum C. The modifer is applied to the axis derived from the pitch cylinder. Do you know what the size tolerance range is for the "axis derived from the pitch cylinder" ? I don't either, in fact I don't think it is defined anywhere within the applicable thread tolerances. Major and minor diameter tolerance of the thread will be defined within the applicable thread specification, but not the tolerance range of the pitch axis. I would remove the MMC modifier.

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Re: Re: positioning tolerance
Re: Re: positioning tolerance -- Kelly_Bramble	Post Reply	Top of thread	Forum

Posted by: totoro ® 07/05/2006, 20:54:36 Author Profile eMail author Edit

I'd like to have the 2X M3 that at the bottom surface always align with the 2x 0-80 holes that at the top surface of the part, how should I define the GD&T? should I define one of the threaded hole as the datum? can it define as shown in the example attached?

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Re: Re: Re: positioning tolerance
Re: Re: Re: positioning tolerance -- totoro	Post Reply	Top of thread	Forum

Posted by: Kelly_Bramble ® 07/06/2006, 08:49:50 Author Profile eMail author Edit

Yes, you will have to establish a feature on one side of the part as a datum to orient the features on the other side. In this case, since you don't have a slot or other better orienting feature, define one of the threaded hole features as a datum. For the 2X M3 hole I would use the datum order of precedence - primary "A", Secondary "B", and Tertiary "C". The "C" datum will orient the threaded hole patterns together. BTW - loose the MMC modify on the M3 holes.

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Re: Re: Re: Re: positioning tolerance
Re: Re: Re: Re: positioning tolerance -- Kelly_Bramble	Post Reply	Top of thread	Forum

Posted by: totoro ® 07/06/2006, 20:02:16 Author Profile eMail author Edit

thanks for the advice!

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