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GD&T Course: True Position (A Better Way to Tolerance a Position)

Jul 8, 2016   //   by Bob Warfield   //   Beginner, Blog, NewFeatures, Products, Software, Techniques  //  2 Comments

The latest chapter in our Free GD&T Tutorial is now available, and it is all about True Position.  This is the first of a number of chapters that will be focused on the individual symbols used for Geometric Dimensioning and Tolerancing.  The symbol for True Position is this:

GD&T True Position Symbol

The GD&T True Position Symbol…

 

This chapter explains the in-and-outs of True Position in some detail, but the best thing about it is the pictures.  These concepts are Geometric, and a picture is worth a thousand words, so having good pictures is critical for GD&T learning.

Let me give you a quick elevator pitch on True Position versus conventional Old School plus/minus tolerances.  Let’s say we’re going to tolerance the location of the center of a hole on a part.  With the Old School, you’d indicate a plus/minus allowance on each of the X and Y coordinates of the hole center.  With True Position, you might only indicate one number–the True Position tolerance for that center.

Let’s say you get handed a print that calls for a True Position of 0.0015″ on your part.  Hey, that’s one and a half thousands, shouldn’t be too hard at all, right?  Not so fast, buddy–it doesn’t work quite like that.  Here’s one of those pictures so we can see what we’re talking about:

TruePos2

True Position is the red circle, plus/minus tolerancing is the yellow box…

I’m using a True Position in this diagram of 1.0 so it is easy to scale things to any example.  I’m also using fewer digits just so we can keep it easy to do some math in our head.  Now 1.0 is the diameter of the red circle, and that circle is the Tolerance Zone that describes where the actual center of the hole could be and still be within tolerances.  For our example, that red circle would have a diameter of 0.0015″.

Now the yellow square is how we want to think about Old School plus/minus tolerancing relative to True Position.  It gives a square (or rectangular depending on the actual plus/minus values for each dimension) box for the tolerance zone.  The first thing we can see from the diagram is that if the square is entirely within the circle, it can only be 0.71 (actually 0.707) of the True Position tolerance.  In other words, our plus/minus tolerance has to be 0.707 times the True Position tolerance.  For our example, that would be 0.001061″.  But it gets worse, because we’re talking diameter in the case of True Position and the total length of a square side for plus/minus tolerance.  So to put it into apples to apples (sort of) terms, we actually need a plus/minus tolerance of half that.

In other words, a True Position tolerance of 0.0015″ corresponds roughly to a plus/minus tolerance of 0.00053″.

Crikeys!  Suddenly we’ve gone from thousandths to tenths–our job is much harder than we thought at first.  But, there’s some hope in that the actual area of the circle is larger than the area of the square.  In that sense, GD&T is actually allowing us looser tolerances than the old plus/minus system.  The article explains all of this in full.

Read through this new chapter on True Position.  It’s a key concept for any CNC’er that has to read a print with GD&T on it to understand.  In fact, it may be the single most important place to start.

 

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GD&T Course: True Position (A Better Way to Tolerance a Position)
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2 Comments

  • I don’t know if you mention in your Geometrical Tolerancing course about using ANSI B4.1-1967 for limits and fits, you should; all Europe uses the ISO standard they don’t give the tolerance as a number but by say a hole H5 and a shaft g4 and use the shaft size to determine the final tolerance.

    Many years ago I worked as a tool designer for Parker Hannifin. One day they decided to hire an engineer to go through all their drawings and apply geometrical tolerancing to all their drawings. This made my job as a tool designer easier I would now know what to locate the part from and other engineers working on the same part would also locate from the same feature.
    I remember seeing previous to the GT implementation a cylinder foot mounted cap where the 4 tie rod holes where not parallel to the feet. GT made a dramatic increase to the quality of all the parts made. Parts would now assemble with no problems.

    We always had to consider a parts MMC (maximum metal conditions) when designing say cutting tools so that they can be sharpened several times and still stay within tolerance or that a pin locator would fit in a parts hole.

    I also worked for the war department and GT enabled us to design the right gauges for checking finished parts.

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