Gage Block Tolerances - Grade B Gage Blocks

[Bubba]

Our company purchased a set of Grade B Gage Blocks years ago that only receive moderate use. We send them out to our calibration vendor every two years for recalibration. When we purchased the blocks, we understood the tolerance to be ±50 millionths of an inch. Now our vendor says that they should be calibrated to Class 3 tolerances of +8/-4 millionths of an inch. I briefly searched the web and could not find information on Class B specifications or why they would need to be calibrated to Class 3 tolerances. Does anyone here know the history of these classes and can explain it to me? If we calibrate to Class 3, we will have 3 of our blocks fall outside the tolerance by 2 or 3 millionths of an inch.

 

[Ryan Wilde]

Wow, I guess you bought these awhile ago, since there hasn't been a Grade B since 1975. Okay, here is the skinny on your blocks...

Grade B went away in 1975 when GGG-G-15B was superceded. It was replaced by GGG-G-15C, which stated that Grade 3 is the lowest grade used in the U.S. (for gov't service, actually, which is why these fed specs are written). Grade 3 is what they called 'a compromise between Grade A and B. Off the top of my head, I can't remember the tolerance for Grade B (it was somewhere in the 12-16 µin range as I remember), but what your vendor is correct, Grade 3 is (soon to be was) the replacement for Grade B, and the tolerance up to an inch is +8/-4 µin. There is no American specification for ±50 µin, although it is a common "Economy Import" set tolerance, common enough that we used to cal to it and called it a "Workshop" Grade.

Here is the where the REAL fun begins. GGG-G-15C was cancelled a few years back, and was replaced by ANSI B89.1.9, which thankfully kept the same tolerances. But, the final touches on the NEW B89.1.9 are almost done, and the NEW version uses the tolerances used in Europe (to facilitate global trade, yada yada yada). The NEW tolerances look NOTHING like the old tolerances.

So what should you do? Is ±50 µin good enough for your process? If it is, then specify on your P.O. that ±50 µin is the tolerance you want used. Every quality system allows for you to set tolerances on gauges to meet your needs, and in this case, you probably should do just that. Your vendor should have no problem with a custom tolerance, and if they do, then let them know that there are a lot of very good quality calibration vendors that do gage blocks.

Good luck,
Ryan

 

[Bubba]

Thanks for the information Ryan! I don't know the exact date the blocks were purchased, but I know that it was somewhere in the early 90's. That's something I don't quite understand. While doing my search on the web, I found several manufacturers advertising Class B sets without specifying what class B means. If Class B went away in 1975, why is it still around as an alternative to Class 2 or 3? I guess there are enough companies like mine that are willing to buy it without fully understanding the implications.

 

[metrologyguy]

The set of gage blocks must be the import sets mentioned above (since they were purchased in the 90s). I see these gage blocks everyday, as our customers send them in for calibration. In the absence of purchase order directions, we apply accept/reject criteria of +/- .000050".

ASME/ANSI B89.2, Calibration of Gage Blocks is also undergoing a rewrite. There are significant improvements over the existing version. Once released, it will take years to educate the end users of gage blocks of all the changes. We still have grades of gage blocks in service that were made in the 1940s.

These are the grades of gage blocks (I am going by memory since I am at home now) in use at present in the United States: A,A+,B, .5, 1,2,3.

The new grades will be: K, 0,00, AS1, AS2.

The import sets that are not made to these specifications, will not be calibrated to a grade. We will simply calibrate them to a tolerance.

It is going to get a lot more interesting for us in the calibration business when the new grades become common.

 

[Ryan Wilde]

Keith,

You are right, the import sets tend to vary on their expected tolerance, and most are made to hold ±50 µin. They do not meet any US specification, and we too defaulted to the ±50 µin when the only writing on the case or block is "Made in China". The strange thing is, ±50 µin is often all that a user needs. By the way, these blocks also do not meet flatness, parallelism, or surface finish specifications for the US either.

As far as the grades that are used in the US (without resorting to obsolete standards) are:

Grade 0.5: ±1 µin
Grade 1: ±2 µin
Grade 2: +4 / -2 µin
Grade 3: +8 / -4 µin

The grades of AAA, AA, A, A+, and B went away in 1975, as those were the ones specified in GGG-G-15B, which was replaced by GGG-G-15C. I didn't know anyone still used them.

Ryan

 

[Bubba]

Well, I finally got in touch with the manufacturer of our gage blocks and found what they consider to be the tolerances in microinches for Grade B.

1" or less +10/-6
2" +20/-12
3" +30/-18
4" +40/-24

I also found it interesting that they are still manufacturing to this "Grade B" and they do it all in the U.S.A. They make four different grades of gage blocks: Grade 2, 3, B, and Economy. So much for an "American" standard.

 

[Ryan Wilde]

I know why they offer these specs, because there is a market for them. As you know, a manufacturer does not HAVE to meet any published specification, so they could make a set ±0.001" and sell it if you'd buy it from them.

But, the bottom line is, their "Grade B" and "Economy" are "Manufacturer's Specifications", and don't meet Federal Specifications BY DESIGN. The work involved to meet tight gage block specs grows exponentially with the tightness of the spec, therefore the price goes up exponentially as well. Loosened tolerance saves them time in manufacturing, it saves you money, and most often, it meets the needs of your processes.

Ryan