Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations pierreick on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

What is a difference between Center Point and Median Point? 2

Status
Not open for further replies.

pmarc

Mechanical
Sep 2, 2008
3,214
These two terms show up in two different definitions throughout Y14.5. Center Point is used in Derived Median Line definition (1.3.31), while Median Point is something that Concentricity deals with (7.6.4). How would you describe a difference between them? Is there any? Thanks a lot for any input.
 
Replies continue below

Recommended for you

pmarc,

Here is my understanding of the distinction between center points and median points:

-Median points are derived from pairs of surface points. In the definition of Concentricity, the points are intersections of piercing lines passing through and normal to the datum axis. In the defintion of Derived Median Plane, the surface points are intersections of piercing lines that are normal to the actual mating envelope of the feature.

Center points are derived from surface or cross-sectional geometry that is more than just pairs of surface points. For a spherical feature of size, the center point is the center of the actual mating envelope (e.g. minimum circumscribed or maximum inscribed sphere). The definition of the Derived Median Line mentions center points of each cross section of the feature. However, the method of deriving these center points is not clearly defined. Y14.5 points to the B89.3 standard, which does not clearly define the center points either. It's a mess.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
Center is defined by the true geometric counterpart (tgc) of the feature (e.g. a collet engaging a boss); the actual surface irregularities don't come into play when establishing the center because you find the center of the tgc.

Derived median is the averaging of directly opposed points. If you picture a boss, at one cross-section, you take opposing points and average those two opposed points out. You then repeat that process at every rotational position around the perimeter at that same cross-section. At each cross-section, you end up with a point cloud. You do the same thing at every cross-section along the length of the feature. You end up with a point cloud that "wobbles" (I tell my class it looks like a garden hose or a snake).

In mechanical design, we anticipate a center mating condition regardless of actual feature surface, so the TGC center makes sense. I haven't heard of any valid arguments for the derived median, or proposed functionality that can't as effectively or more effectively be serviced by using the tgc center. Concentricity and Symmetry, per Y14.5, use derived medians. Thus the debate over their value as geometric controls in Y14.5.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
Thank you Guys,
Your understanding is in line with mine. I basically wanted a confirmation that someone besides me thinks that these two are results of readings/calculations done in different cross-sectional planes. The planes could be identical only if unrelated actual mating envelope of a feature was perfectly parallel to datum axis (assuming that print required both: concentricity and straightness of median line check).

As for the process of getting to those entities, I agree with Evan that the wording could be better, especially if we are talking about finding the center points of derived median line.

Thanks again.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor