True Position - Arc Length

[[Ri...]]

As the title says, has anyone ever heard of True Position of an Arc Length? I know what an Arc Length is, and I know how to calculate it, but I'm struggling on if True Position would apply to it, and how.

Any thoughts?

[[Br...]]

Position tolerances are only for locating features of size. Features of size must have directly opposed elements. With partial arcs, It rarely makes sense to have a position tolerance applied to it (e.g. Majority of the diameter is present with an interruption in the surface) . Think about this, a position tolerance of a cylindrical regular feature of size at RFS has a tolerance zone that the feature axis must not violate. The axis of a cylindrical feature of size is derived by the actual mating envelope.

Y14.5 defines an actual mating envelope as a similar perfect feature counterpart of the smallest size that can be contracted about an external feature or largest size that can be expanded with an internal feature so that it coincides with the surface at the highest points.

So here we have a cylinder of perfect form expanding or contracting (depending on if its an OD or ID), until the opposing sides stop it from being able to expand or contract.

The point here is, an actual mating envelope can not really be produced in something like a partial arc unless it is more than 180°.

Radii and other arc shaped surfaces should be located with a profile tolerance as they are not features of size. Their axis cannot be reliably reproduced

https://youtu.be/wNTmeSkgYh4

[[Ri...]]

Hi Brett,

Yeah - Arc Length is only a feature of size it is derived from the central angle and the radius. The formula is as follows: arc length = 2πR(C/360)
a. C is the central angle of the arc in degrees
b. R is the radius of the arc

I explained to the customer that Arc Length only tells you its feature of size - it does not tell you where it is in space.

[[Br...]]

My apologizes. I misunderstood that your concern was a tolerance applied to the formula itself. Its just a mathematical definition. That's basically what a CAD model is. So yeah in that regard, it just needs to be fully defined on the drawing and its true position must be established by basic dimensions. If you have all that, then I don't see why you couldn't treat it like any other cylindrical feature. Is the formula listed in a table or something on the drawing? Sometimes they are listed with their X and Y values with a note saying all tabulated dimension are basic. Also, if the "C" value is less than 180°, I don't believe it can be considered a feature of size--Which is what I thought you were getting at with your initial question.

[[Aa...]]

Haha! terminology confusion...I love it. In ASME, "True Position" merely means the basic position. This is frequently confused with a position control or a position result. (Is this Calypso's fault, or were they just following the generally accepted misuse of the term?)