# Thread: center of a irregular circle

1. ## center of a irregular circle

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Hello.

Could you tell me how to find a center of irregular shape (for example circle).

2. ? Using a CAD program, on the kitchen table, mathematically?

3. Mathematically.

Let's say we have a circle that is toleranced with position tolerance|DIA.005|A|B|

So the center of our circle should be inside circular tolerance zone that has a diameter of .005 and is positioned from datums A and B.
If our measured circle is perfect it won't be hard to find its center, but what if it's irregular, for example in a shape of an egg.

Sorry for my english, it's not my native language.

4. Originally Posted by kocykowaty
Mathematically.

Let's say we have a circle that is toleranced with position tolerance|DIA.005|A|B|

So the center of our circle should be inside circular tolerance zone that has a diameter of .005 and is positioned from datums A and B.
If our measured circle is perfect it won't be hard to find its center, but what if it's irregular, for example in a shape of an egg.

Sorry for my english, it's not my native language.
Ok then, so you actually want to measure the positional tolerance (true position) of a cylinder relative to datums "A" and "B"...

What kind of measurement equipment do you have? CMM, height gages, dial indicators?

What kind of features are datums A and B? Surfaces, cylinders, complex surfaces?

5. Let's say I've got CMM, datums A, B and C (if we want to talk about cylinder) are flat surfaces.

CMM establishes datums A, B, C then measures cylinder let's say at 3 diffrent depths so it has a cloud of points at each cross section that represent location of the surface (if we connect points) of a cylinder. Cylinder is in tolerance if center of each cross section lies within given geometric tolerance zone.

I assume that by some formulas CMM software calculate a location of center. The question is what kind of formula is used to find a center of each cross section.

6. As with many forum questions determining what the OP is actually looking for can be a challenge.

I ~ think what your fundamental question really is - “how to establish an axis on an irregular shaped cylinder feature so I can measure its positional tolerance properly”.

For example, the as-manufactured cylinder feature could be any thing including egg shaped and bowed at the same time.

I assume that by some formulas CMM software calculate a location of center.
Most correctly there should NOT be any funny math going on when measuring the positional tolerance of a cylinder however CMM’s do come with the capability to mathematically massage data.

So what is meant by “Most correctly”? The axis of an as-built cylinder for positional measurements at RFS, MMC or LMC should be established from the Actual Mating Envelope.

Definition: Actual Mating Envelope.

A similar perfect feature counterpart of smallest size that can be contracted about an external feature(s) or largest size that can be expanded within an internal feature(s) so that it coincides with the surface(s) at the highest points.

There are two kinds of "Actual Mating Envelopes" - unrelated and related.

I assume you have an ASME, ANSI or ISO GD&T or G&T book, standard available?

7. Right -- as Kelly says, you really want to find the position tolerance based on the Actual Mating Envelope. On a CMM, this might also be called the Maximum Inscribed Circle (for a hole). In this way, the irregular shape of the hole won't "pull" the axis in any particular direction, so there's no funny math.

As an aside, if a hole is irregular by design, meaning intentionally, then you wouldn't even care about the axis. You would be measuring the "boundary" which gets back to the same idea as the actual mating envelople. Just thought I'd mention it

8. So it's all about actual mating envelope.

Ok, thank you both for replies to my question

See you soon!

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