# True Position of a Cylinder

True Position (Cylinder perpendicular to the axis system)

This is how most people think of True Position.  It is simply derived from the “offset” of the actual to the nominal.

In our example here, we have a tolerance band of +/- 0.025 (TP tolerance of 0.050).  The cylinder is shifted 0.010 to the right.  The difference between the nominal and actual is 0.010 mm.  This is doubled for TP output, so our TP actual would be 0.020.  Well within limits.

True Position (Cylinder tilted to the axis system)

True Position also considers the tilt (and form) of a 3d feature such as a cylinder.

This is the same cylinder.  It still has the same basic 0.010 “offset” in the center, but it is also tilted somewhat (exaggerated here for clarity).  The True Position is still calculated based on the original nominal which is still perpendicular to the axis system!  The largest distance from nominal is used for the calculation.  In our example here, the top end of the cylinder is at about 0.025 from nominal.  Doubled, that would be 0.050 which would be at the TP tolerance limit.

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