Measuring the True Position of a Cylinder's Pierce Point


Mr. Friend

Hi Friends,
I'm pretty good with GD&T (1994 & 2009), but wish to verify something before acting.

I'm interested in measuring the actual position of a pierce point (i.e., "Point G") of a cylinder axis intersecting a plane. This point is given three locating Basic dimensions: one in Z, one in X, and one in Y. I typically would discover the deviations from the print's true position and input them into the formula 2SQRT(x^2+y^2) to get the actual. But here, we have a third Basic. How would I measure a pierce point's true position?

I would be so grateful for any help on this.

Mr. Friend

Ok. I have sketched a drawing of my own (see attached). Please take a look.
The sectional view is drawn at the wrong angle, but this shouldn't disrupt our inquiry.


  • true position drawing.PDF
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This will be measured the same as normal true position. The pierce point is telling 'where' to specifically measure the cylinder at. The GD&T frame is calling A B C.

Think of it this way: you have a cylinder, we will project the cylinder to be 20 inches long (or miles for extreme effect). Lets say the pierce point is called out to be at the exact center, the end points of the cylinder usually are required to fit within the GD&T but at 20 inches (miles) long any angle deviation from perfection will probably fail because of the high length, In these cases, I usually see another call out for cylindricity call out, or another GD&T call out, independent of the measurement at the pierce point.

David DeLong

You do not measure a point but you do measure the feature which in this case is two holes which share the same center. The basic dimensions to point G develop the axis in which the two holes lie. So, there should be a positional tolerance with a diametrical tolerance zone for the two hole having a location shown with point G with a basic angle.

Mr. Friend

Thanks guys. Your input is much appreciated.

Unless new insights arise, I will proceed with interpreting this positioning as being a position requirement for the cylinder at Point G's location only (the point G itself not having anything to do with being toleranced with a sperical tolerance zone, halting translation of the cylinder along the 18 degree axis, etc.). Point G merely designates a point where the Basic axis exists, and at 18 degrees. This means the cylinder may translate for infinity up and down the axis, as long as the cylinder's axis does not violate the .010" (plus bonus tolerance) cylindrical tolerance zone existing around the center of the 18 degree Basic Axis.

Using a cmm, people had been constructing a pierce point out of the intersection of the cylinder's axis with the Flange's face, and testing its distance from Point G's location using a spherical tolerance zone.

Instead, we'll merely translate our origin to Point G, and take several hits around the inside of the Flange's hole to see if the hole's center violates the diametrical tolerance zone at that point. I should also do the same with the back of the cylinder (2nd flange) in order to prove that this cylinder as a whole does not violate the tolerance zone.

Thanks again, guys!
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