Gauge Block Grades

Gauge blocks are a set of precision-cut stable metal blocks of different thicknesses used as a length standard. They usually come in a set, but you could also find them individually.

Measuring instruments such as micrometers, digital calipers, vernier calipers, dial calipers, height gauge, dial indicators, and test indicators use gauge blocks for inspection and calibration. Those blocks can also be workshop tools for several tasks such as positioning the cutting tool, angle measurement with sine bar, etc. In some cases, a gauge block will be compared to more accurate block (master gauge block) for calibration purposes.

Based on the use of gauge blocks above, there are frequently used gauge blocks and rarely used ones. The ones frequently used in the workshop whether for gauging, shop floor tools, and calibration are lower in terms of accuracy. On the other hand, the ones rarely used as the master gauge blocks are higher in terms of accuracy.

But what are the things that define accuracy on gauge blocks?

In the United States, two standards that define gauge blocks are Federal Specification GGG-G-15C and the American National Standard ANSI/ASME B89.1.9M. The ASME specifies the blocks’ geometry, standard nominal length, and tolerance grade system. This tolerance grade system further defines the classification of gauge blocks’ accuracy. [Source, Page 4 & 5]

Simply, when you use the blocks for calibration in the level of metrology lab, you can’t use the workshop grade. Vice versa, a workshop grade gauge block doesn’t meet the metrology lab’s accuracy requirement.

Although most of the countries have adopted the International Standard ISO 3650 for gauge block grades, however, some country-specific, gauge block grades are still in use besides the International Standard.

ISO Gauge Block Grades

ISO standard ISO 3650-1998 governs the grading of all Gauge Blocks of metric lengths. Following are the grades under this standard.

Grade 2

It is a working standard and is commonly used in the manufacturing area for calibration of measuring instruments and inspection of tools, fixtures, and machines.

Grade 1

It is a working standard and is mainly used for setting and calibrating plug gauges and measuring instruments in the measuring area within the manufacturing area.

Grade 0

It is a company standard and is mainly used for calibrating plug gauges and measuring instruments in a laboratory setting.

Grade K

It is a reference standard and is mainly used to calibrate gauge blocks, length standards of the same accuracy, and other measuring instruments in the accredited or unaccredited National level calibration Institutes.

US Gauge Block Grades

Two standards govern the Gauge Block Grades in the US:

1. The Federal Specification GGG-G-15C
2. ANSI/ASME B89.1.9M

ASME (in the United States) regulates the grades based on:

• the length’s limit deviation at any point
• and variation tolerance

ASME which is the prevailing standard defines the properties (the gauge blocks’ tolerance and limit deviation) of gauge blocks for gauge block length from 0 to 40 inches. It also regulates them in metric (mm) from 0 mm up to 1000 mm. There are five grades according to ASME B89-1-9 2002.

Table ASME B89-1-9 2002 Grades for Gauge Blocks (Inch)

Source: Mitutoyo Catalogue No. E12014 Page 8

Grade K

This grade has almost the same regulation of the “length’s limit deviation at any point” as Grade AS1 does. That means its deviation is greater than Grade 00 below. But, its variation tolerance has almost the same as Grade 00.

Blocks of this grade are used as a calibration master as well to calibrate, like grade 00 gauge blocks do.

Grade 00

It is a grade with the most precision flatness and accuracy requirements. The grade 00 has a tight length’s limit deviation at any point as well as its tight variation tolerance. In comparison with Grade 0, its accuracy is almost entirely twice better.

This grade is not designed for use on the shop floor. But it is designed for master gauge blocks or calibration masters. Therefore, you could find this grade in a laboratory meant to calibrate the other lower-grade blocks.

Grade 0

As said before, the gauge blocks of this grade are calibrated by Grade 00 gauge blocks. It is of lower quality grade as compared to K and 00 grades. Blocks of this grade are accurate enough to perform 95% or more applications.

If you are looking for gauge blocks in order to calibrate a 1-inch range micrometer that comes with ±.00005″ accuracy (you can find this micrometer in this list), this grade is more than enough.

Suppose that you want to apply the 4:1 rule of accuracy ratio for micrometer calibration. Then, the gauge blocks’ tolerance must have at least .0000125″; or tighter is better. Of course, the grade 0 qualifies to do it. That’s because even the lowest tolerance defining grade 0 from the range of 0-1″ is 6 µin (.000006″). That means the gauge block (.0000060″) is about twice more accurate than you require (.0000125″).

Grade AS1

It is lower than Grade 0. Blocks of this grade are used for gauging and measurement purposes on the shop floor.

When it comes to calibrate micrometer with ±.00005″ accuracy, it’s also applicable. Again, if you apply the 4:1 rule, grade AS1 is enough.

For a 6-inch digital caliper calibration that has an accuracy of ±.001″, Grade AS1 is surely more than enough in regard to 4:1 rule. The grade AS1 gauge block is at least about 8 times more accurate than the digital caliper.

Grade AS2

It is the lowest Grade. Gauge blocks of this grade are common to be used on the shop floor to set and calibrate fixtures as well as precision instruments. You could also use it for caliper calibration. It’s at least 4 times more accurate for a 6-inch digital caliper with an accuracy of ±.001″, in regard to 4:1 rule.

UK Gauge Block Grades

In the UK, British Standards govern the gauge block grades through BS 4311: Part 1: 1993. It has four grades: K, 0, 1, and 2 that regulate up to 4 inches or 100 mm of the gauge block length.

BS (in the United Kingdom) regulates the grades based on:

• Tolerance on central length’s deviation from the nominal
• Parallelism (variation in length)
• Flatness

Table BS 4311: Part 1: 1993 Gauge Block Grades (Inch)

Source: Mitutoyo Catalogue No. E12014 Page 8

Also, according to Mitutoyo Catalogue above (page 9), you can match your application based on the grade

1. Reference level. You could use Grade K as the reference gauge block. This grade has tighter flatness and parallelism compared to the lower grades. But, its tolerance on central length’s deviation is the same as Grade 0. You could use it in research. Also, it takes a role as the reference for gauge blocks for calibration. That means, it’s rare being used. It’s not used as frequently as Grade 0. It applies to verify the gauge blocks for calibration.
2. Calibration level. For calibration level, you could use the gauge blocks with Grade K or Grade o. Herewith, you could use it to verify the accuracy of workshop gauge blocks, inspection gauge blocks, and measuring instruments.
3. Inspection level. At this level, either Grade o or Grade 1 or Grade 2 is capable to handle this role. The tasks aim for instrument calibration, mechanical parts inspection, etc.
4. Workshop level. Besides the inspection level gauge blocks, Grade 1 or Grade 2 can be used also as workshop gauge blocks. Since it has lower tolerance, flatness, and parallelism, you can downgrade its hierarchy ranking in your workshop as mounting tools, precision tool positioning, etc.

Japanese Gauge Block Grades

The Standard that governs Gauge Block Grade in Japan is JIS B 7506-2004. It also has four grades K, 0, 1, and 2 for Gauge Blocks of nominal length from 0.5 mm to 1000 mm. K is the highest grade while 2 is the lowest Gauge Block grade.

Final Thought

Gauge block takes a vital role in maintaining the accuracy of dimensional measuring instruments. Micrometers, indicators, calipers, and height gauges benefit from it for inspection and calibration. If you are learning about those precision measuring tools, you need more in-depth insight into gauge block. Knowing the gauge block grades is a must.

As exposed above, gauge blocks come with different grades. This grade shows its tolerance, limit deviation, flatness, and parallelism.

Since the grades of each country are different, you should not get confused anymore. If you are based in the United States, you should look at the gauge block grades regulated by ASME. While ISO internationally regulates gauge block grades in metric.

If you are going to buy a new gauge block, then acknowledging the gauge block grade is important to help you pick the right set for your need. It is overkill to use the grade K for the machine shop. The lower grade is intended to use for less precision work that we usually do in the workshop.