![]() 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 maximum allowed positional deviation 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″. That gives us the following True Position Formula: 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 True Position Formula for X and Y: 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 permissible position deviation that a feature can have from its “true” or theoretical position–that is, the total variation from the actual 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 are 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 take a look at the true position formula and 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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