Reported position of cylinders

1) Yes, if you are able to physically measure the holes as a cylinder then you should.

2) With the Geometric Tolerance command (and previously XactMeasure), the wort end of the cylinder axis is reported for position.  Legacy dimensions give you the option of start, end, axis average or worst end.  However, since position applies to the entire length of the feature, you should always report the worst end.

Neil, I read somewhere that when using cylinders, the distance between the start and end offsets should be at least the same as the diameter (or more).  Is this true?  If so, why?

Also, I know you mention below in the thread about doing a study for different math types.  I'm unclear what measurement uncertainty is and how to determine it/see if it's less than the form error.

Neil, I read somewhere that when using cylinders, the distance between the start and end offsets should be at least the same as the diameter (or more).  Is this true?  If so, why?

Also, I know you mention below in the thread about doing a study for different math types.  I'm unclear what measurement uncertainty is and how to determine it/see if it's less than the form error.

There are many 'things' to be aware of when doing cylinders, and the calculations of Pcdmis are the reason.  IF you have a 10mm cylinder and you check it using 2 levels with 4 hits each level, with the levels 10mm apart, there are a total of 6 cylinders that can be created using those points.  There are many ways around this, MY method is to manually learn the cylinder, using either 2 levels of points with a different number of hits per level (short cylinders) or 3 levels with different number of hits per level (long cylinders), then type in the nominal XYZIJK's after.  This will prevent Pcdmis of coming up with the wrong cylinder.  How much of the cylinder you can measure will also effect the 'work arounds' to prevent the wrong cylinder from being created.

For the cylinder question, see Matthew D. Hoedeman 's reply.

For

Smashworth87 said:

Also, I know you mention below in the thread about doing a study for different math types.  I'm unclear what measurement uncertainty is and how to determine it/see if it's less than the form error.

As a general rule, you can look at your CMM's accuracy specification (shown on the calibration certificate).  For example, a typical Global CMM might have an accuracy statement of 2+L/1000 which means that you have ±2µm (0.002mm) uncertainty + an additional amount which varies depending on the length of the measurement.  So if, for example, you had to measure a Ø100mm feature, you would have at least ± 2.1µm uncertainty (2+100/1000).  Therefor, it might not be possible to get repeatable results if your size or form tolerance was less than or equal to 4.2µm (0.0042mm).

Okay, thanks guys.  Much appreciated.