0028// Pin Gauge Sizing for Thru Hole

Pin gauge is one of the most common gaging method use in metrology lab. It can help us verify the hole size easily with minimal cycle time.

However, not everyone know how to select the pin gauge size correctly. So, today we will discuss how to select a pin gauge for a thru hole.

NOTE: In this post, we will not consider grade of pin gauge. We assume gauge is in perfect form and feature of size.

Example:

Consider we wanted to verify a thru hole which specified as 3mm diameter with +/-0.1mm tolerance zone size. So, what is the size of the pin gauge is needed to verify its oversize, undersize or meet specification?

Solution:

First, we needed to set the fail criteria. There are two possible condition the feature of size out of specification, undersize or oversize.

To verify oversize, we need a pin gauge with the size will pass through the hole when hole is oversize and cannot pass through the hole when hole meet at USL or below. (Note: I'm not using LMC because LMC's perfect form is not required by default... Rule#1...)

For this case, the maximum actual local size allows is diameter 3.1mm. So, pin gauge with size above diameter 3.1mm able to verify its oversize. diameter 3.10...1mm pin gauge pass through the hole implied hole oversize.

To verify undersize, we need a pin gauge with the size will not pass through the hole when hole is undersize and the pass through the hole when hole meet MMC's perfect form.

For this case, the MMC is diameter 2.9mm (remember internal feature, MMC is smallest allowable diameter). So, pin gauge with size smaller diameter 2.9mm cannot pass through the hole implied hole under size.

As a result, below is the fail criteria:

Unfortunately, criteria above may cause accept the bad parts when the hole have the surface profile as below.

This surface profile is common in chemical machining. But, why this profile will give us the risk of accept good parts?

If the example above have this kind of the hole profile and the diameter 3.10 mm pin gauge pass through the hole, you may think the part pass the criteria. However, that is not true! The gap between the pin gauge and hole surface shown in picture above implied some cross sectional diameter of the hole actually larger than specification. That mean this part fail its actual local size specification.

Therefore, we must aware this shortcoming when using pin gauge as your verification method.

return 0;

Revision for MMC perfect form:

please refer to http://gbwonggbwong.wix.com/dimensionalmetrology#!0009-MMC-perfect-forms-geometrical-characteristic/c2211/560942db0cf2f0ed7a25045e

Revision for Rule#1:

please refer to http://gbwonggbwong.wix.com/dimensionalmetrology#!0008-Rule-1/c2211/560935ef0cf25fa7fe1abab6

Revision for Actual Local Size:

please refer to http://gbwonggbwong.wix.com/dimensionalmetrology#!0004-Definitions-of-Features-Properties-Part-1/c2211/55e29efc0cf2c1d1fd6425a9