Inspection of hole diameter

[ALOBrazil]

I know that due to Rule #1 the maximum inscribed cylinder measured in a hole must be bigger (at least equal) than the MMC diameter. I know also that each measured distance between two opposite points must be smaller (at the most equal) than the LMC diameter.

The question is: has anyone measured holes in a coordinate measuring machine exactly as established in the ASME Y14.5M? How do you usually measure a hole diameter?

Ademir

 

[dingy2]

One could measure holes on a CMM machine using cylindricity. It will give you the minimum cylinder diameter and that value must not be smaller than the smallest allowable size of the hole which is MMC.

One would take a number of spyralling points around the ID of the hole from the top to the bottom.

Dave D.

http://www.qmsi.ca

 

[ALOBrazil]

Thanks your reply dingy2! I agree with you that the cylindricity must be inside the diameter tolerance (if the Rule 1 is specified), but the cylindricity alone controls only the form deviation, not the diameter. Do you mean a cylindricity control plus an average diameter control?

I see that people in general measure a hole with only few points (e.g. 8 points) and they adjust a "Least Square Cylinder" to the points cloud. In my opinion, sometimes it is necessary a higher number of points and the use of the mathematical adjust "Least Square" is not conform the ASME Y14.5M. The measured diameter in general depends of the measurement strategy, resulting in different values just as the used strategy.

Ademir

 

[dingy2]

Sorry for the delay in answering this question. I really had to think about it.

Cylindricity controls the diameter with the inner cylindrical boundary not allowed to be smaller than the smallest size of the hole. This controls size and shape of the feature per rule #1. It will give one an inner and outer boundary and also the average cylindrical size. It does not control location or position of the hole.

Now as far a positonal tolerances at MMC, that would be another question.





Dave D.

http://www.qmsi.ca

 

[ALOBrazil]

Due to rule #1 a size tolerance controls also the cylindricity. But a cylindricity tolerance alone does not control the size. A hole could have a very low cylindricity error and a very big diameter error (imagine a perfect cylinder with another diameter). The tolerance zone for the cylindricity is bounded for two concentric cylinders, whose diameters could vary to better adjust to the measured hole. The distance between the two cylinders is fixed and equal to the cylindricity tolerance.

If the specification is a profile tolerance to the cylindrical surface of the hole, then in this case the diameter and the cylindricity are been controlled!

Back to the first question: Someone could control a hole diameter by a profile specification, but I think this measurement strategy is more rigorous...

 

[dingy2]

ALOBrazil:

Rather than performing cylindiricy, here is the easiest way to control size and shape. Get a pin that is the smallest allowable size of the hole. If it goes in the hole, you have met rule #1. The confirm the lower specification limit and also the shape.

Now measure the hole with a hole gauge and micrometer, depending upon the accuracy you need. If the hole size tolerance is rather loose, use the male end of vernier calipers. This confirms that the hole is not LARGER than the outer specification.

Do not use a profile specification. This is use for surfaces rather than features of size.

Dave D.

http://www.qmsi.ca

 

[ALOBrazil]

Hello dingy2

Thanks for the discussion! Yes, this is a very good way to control a hole diameter. But with this procedure it is not possible to calculate a Cpk index. An alternative is measure the hole in a CMM, but I see several problems in these traditional measurement procedures...

About the profile specification: Yes, normally it is not used for holes, but if we are interested in diameter and location control of a hole, a profile specification can be used - with some advantages! I found a very good link about it:

http://www.tec-ease.com/tips/february-99.htm

Ademir

 

[dingy2]

Ademir:

I went over the link provided and the profile of a surface is correct although the thought of positional at MMC giving a bonus of 0.7 is not quite correct.

Now, do you want life difficult or easy. Myself, I like easy so if you want to calculate a Pp/Ppk of a hole size, just get a hole gauge making sure you orient the 3 expandables in the same direction each time one measures a hole. Use a digital micrometer to measure the hole gauge.

When I was a Quality Manager so many years ago, I had my people do such a thing on an extruded hole in a stamping since the Customer demanded it. This method worked out well.

You COULD go profile of a surface but it is too time consuming and is it practical? I would never do it.

Dave D.

http://www.qmsi.ca

 

[underspec]

Depending on what type of diameter it is. If it's a stud then use a minimum circumscribed best fit. If it's a hole, then use max inscribed best fit. For any other I use least squares (RMS) best fit.

 

[ALOBrazil]

Underspec,

Do you use some data filtering or outlier elimination resource in the CMM software to avoid the influence of bad points? I ask that, because the minimum circunscribed and the maximum inscribed features are very sensitive to individual points - the diameter is directlly affected by probe repeatability and particles on parts surface. A least square feature is more stable, but it is not functional...

Ademir

 

[underspec]

I'm using touch points so I don't use any filtering. Now if i'm scanning then yes I would filter the data depending on the surface finish.

 

[ALOBrazil]

I see that it is very difficult for the touched points hits the highest peaks that define the functional diameter. I think the measurement of a hole diameter with few points and maximum inscribed cylinder is ok for not precise measurements or parts with low form deviations. But, in these cases, I would use a least square best fit instead of, because its better stability.

For precise measurements of parts with significant form deviation, I think it is necessary a scanning, outlier elimination, filtering and maximum inscribed cylinder (for holes)...

Ademir