GD&T Position vs Concentricity - Comparison

[@孙小圣-d8u]

Please launch a detailed comparison course between concentricity and circular runout, and coaxiality and total runout!

[@GeoTolPro]

I have about 5 videos on my channel showing the difference between all those symbols. Check them out. Our GeoTol Pro fundamentals course (unit 13) has even more.

[@seanhurd4633]

I agree Concentricity is misused and misunderstood by lots of uncertified engineers but that doesn't mean that isn't incredibly useful for making sure the axial CG of a high-RPM part is well balanced. Concentricity uses opposed pairs of points which controls the unbalanced outward mass. I don't care if a rod is "lobed" or has a conical taper if it's primary duty is to spin and only takes torque loading.

[@GeoTolPro]

I've heard that its used for "high speed balance". But the part must be pretty simple as every feature on the part would need to be nominally centered (no hole patterns) and also have those concentricity controls. What about the datum feature? The datum axis is created by only the high points (unrelated AME) and wont be "balanced". Also, see the comments from @2004Timber, there is not a clear definition of the "median points" in Y14.5. Best to keep it simple and use a circular runout on a high speed rotating part. High speed rotation parts are usually balanced afterwards anyway. Circular runout in that application would be a little tighter than concentricity (runout doesn't allow the "lobes"), but its well-defined, easy to explain, and widely understood.

[@chrmeiss67]

ASME follows the principle of dependence. This is suitable when most linear sizes are functional based on assembly connections. In my opinion, ASME is consistent with the nature of mechanical parts. ISO GPS made a big mistake when it chose independence principle (oherwise nobody needs ISO GPS).

[@GeoTolPro]

I agree. ASME Y14.5 sets a few dependence defaults that you probably want with mechanical parts that fit with others (nX simultaneous requirement relationships and rule#1) . ISO may have taken the underlying rule of independency too far. It makes the system more complicated when you must always reestablish the relationships.

[@2004Timber]

Regarding concentricity and it's diametrically opposed points, what establishes the diameter or center point? Is that based on the datum axis at the cross-section, the 2-D AME of the cross-section (difficult to determine with extreme form variations like the peanut shape), the axis of the feature's AME, or something else I'm not considering?

[@GeoTolPro]

This is a good question without a clear answer. Y14.5-2009 only states "diametrically opposed elements" without a definition of how those points are found. Are they opposed about the datum axis or about the as-produced feature (like the 2D cross-sections normal to the AME as you mentioned)? Y14.5.1M-1994 (math standard) explains it as the "centroids obtained by intersecting a pattern of symmetry rays with the actual feature". This means it could be a centroid across 3 or 5 points and not just 2. Lots of confusing and conflicting definitions here.

[@sem7207]

​@@GeoTolPro Aren't the median points for concentricity simply the points at the middle of distance of each "ray" projected through and normal to the datum axis? Kind of same idea of symmetry and the relationship of it to a datum center plane, only related to a datum axis; essentially symmetry "wrapped around" the datum axis.

[@GeoTolPro]

That's an good try at filling in the missing words of the Y14.5 standard, but not everyone agrees. Unfortunately, Its up to a reader to come to their own conclusion of how to find the median points with both concentricity and symmetry in 2009. Another reason to avoid it.

[@saurabhroy7100]

I believe ASME has done a very good job. They make it easy and simple that there is a cylindrical tol zone and you need to adjust your axis in between it and that's it. This median and all are useless concepts as there are many confusions there. ISO will not remove concentricity as Engineers have big EGO issues I believe

[@GeoTolPro]

Agree, I'm a believer of "keep it simple" when you can.

[@felixwaverley6458]

very very good explanation

[@maryamgoli-x3w]

Like Like Like, for the most beneficial man in the world.

[@Kiran-cd9uj]

1:16 unrelated actual mating or related actual mating envelope?

[@GeoTolPro]

The video is correct. The unrelated AME determines the feature's axis that must lie within the tolerance zone.

[@Zach-ed4nx]

Concentricity.