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GD&T: True Position Question
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Posted by: rubinora

10/11/2006, 11:40:27

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I'm confused as to the factor of 2 used in the calculation that relates actual x,y position of a feature and its desired true position. All the GD&T books glibly cite an equation that looks pythagorean but includes a factor of 2?

Z=2*sqrt(x^2 + Y^2)

The above equation has the net effect of making the actual positions of my part features measured by a CMM look twice as bad as they really are.

Can anyone give me a solid explanation of this factor of two other than cite "that's how its always been done":(








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Re: GD&T: True Position
Re: GD&T: True Position -- rubinora Post Reply Top of thread Forum
Posted by: Kelly Bramble

10/11/2006, 21:25:29

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Look at the following web page: http://www.engineersedge.com/true_position.htm. The illustration at the bottom of the web page shows shows location of the tolerance zone by "BASIC DIMENSION (DRAWING)" in the x and y directions. The "ACTUAL MEASURMENT (PART)" shows where the feature was actualy manufactured in the x and y directions.

The "ALLOWABLE TRUE POSITION (DRAWING)" shows that the tolerance zone is diamteric. The offsett calculated by the Pythagorean therom actually calculates the radial offset of the as-built feature. To determine the actual true position of the hole feature, one has to convert the radius into a diameter.

Hence, Position (Diametric)= 2*sqrt(x^2 + Y^2)







Modified by Kelly Bramble at Wed, Oct 11, 2006, 21:28:49


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Re: Re: GD&T: True Position
Re: Re: GD&T: True Position -- Kelly Bramble Post Reply Top of thread Forum
Posted by: mattsmith07 mattsmith98

05/30/2007, 13:41:12

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I hate the stupid x2... The diametric introduction into the positional GD&T callouts have only a usefull effect when allowing for Bonus Tolerancing. For example: 1. The actual measured minus 2. The low diameter limit = 3. The bonus tolerance. This method in my opinion is junk compared to establishing position (1.) and then diameter (2.).. See;(radial-pythagorim)+(radius of meas. hole)= Radius of absolute maximum deviation.. Then we can go ahead and project all the tolerance zones etc.. To me this is the only logical way - at this point.







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Re: Re: Re: GD&T: True Position
Re: Re: Re: GD&T: True Position -- mattsmith07 Post Reply Top of thread Forum
Posted by: Kelly Bramble

05/30/2007, 14:37:08

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I'm not sure why you think the 2X is not optimal. When you calculate the true position from a vector offset you are effectiveley calculating a radius. So, 2x gives you the location tolerance as it is expressed, diameteric. When a simple limit tolerance is expressed +/- .005, you still have to muliple the "true position" by two to reflect the entire tolerance zone (.010). Additionally, you may still need to calculate the offsett using pythagoron theorem to figure out actual tolerance - nothing gained.

You said that "positional GD&T callouts have only a usefull effect when allowing for Bonus Tolerancing" is quite honestly bunk. Did you know that a +/- .005 square tolerance zone can actually allow up to +/- .007? So is you redesign using a round zone at .014 (no MMC onlt RFS), which is equivalent to a +/- .007 square zone you actually get the same function and approximately 57% MORE available tolerance?

I could list about thirty more good reasons round zones referenced to a common origin is better than square...

You stated "radial-pythagorim)+(radius of meas. hole)= Radius of absolute maximum deviation" Can you clarify?








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