So the True Position in this example would be 0.7071, which is within the 1.00 called out for the True Position–we’re okay! The measured position of our holw differs from True Position by 0.25 in both X and Y. The True Position is 1.00 (keeping it in round numbers to make it easier to scale your real numbers and see what all this means). In this example, the Red Circle is the tolerance zone for the hole center. The Red Circle is the tolerance zone for the hole center… Since a picture is worth a thousand words, geometrically, what we’ve been talking about looks like this: True Position can be tougher than it looks on first glance! We’re off from the 0.0015 True Position tolerance by almost 2x!Įven with half the differences, so X and Y are within 0.00075″ of the true center, the True Position still works out to be 0.002121″. Let’s say we’re off by 0.0015 in both X and Y. So, we take the difference in X (difference between actual and measured X), square it, add that to the difference in Y squared, take the square root of that sum and multiply by 2. True Position = 2 x SQRT(XVAR^2 + YVAR^2) Here’s the usual formula for True Position in X and Y:
#Position definition how to#
But is that really true? To understand the answer, we must understand how to calculate true position: How to Calculate True Position Many who are not familiar with GD&T True Position may jump to the conclusion that they just need to locate within a thou and a half (0.0015″) on X and Y and all will be well. So how far off can the center actually be? Let’s say the callout gives a true position tolerance of 0.0015″.
Consider the true position of the center of a hole, a very common application. Depending on how it is called out, true position can be used in a lot of different ways. True Position is the total permissable variation that a feature can have from its “true” position–that is, the total variation from the position if there was no error on an ideal part. True Positions are relative to Datums, so you will want to spell out which datums in the Feature Control Block associated with a True Position. The GD&T Symbol for True Position is a little crosshairs: There are two forms of GD&T True Position–one for a feature size under a material condition (Maximum Material Condition or Least Material Condition), and one for True Position Regardless of Feature Size (RFS). GD&T uses a notion called True Position when tolerancing locations. Now let’s put all of that together by taking our first look at the GD&T concepts around tolerancing positions or locations and let’s also see how to calculate true position by formula or using a true position calculator. We just finished going over plus/minus tolerancing–the way most drawings that don’t use GD&T are toleranced. You know how Datums and Feature Control Blocks work, for example. We’ve picked up a lot of fundamentals in prior chapters.
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