Slip gauges, also called gauge blocks, can counter some of these limitations and provide a high degree of accuracy as end standards. In fact, slip gauges are a direct link between the measurer and the international length standards.
Fig.1 Slip Gauges / Gauge Blocks
The origin of gauge blocks can be traced to the 18th century Sweden, where ‘gauge sticks’ were known to have been used in machine shops. However, the modern-day slip gauges or gauge blocks owe their existence to the pioneering work done by C.E. Johansson, a Swedish armoury inspector.
Therefore, gauge blocks are also known as Johansson gauges. He devised a set of slip gauges manufactured to specific heights with a very high degree of accuracy and surface finish. He also proposed the method of ‘wringing’ slip gauges to the required height to facilitate measurements. He also emphasized that the resulting slip gauges, to be of universal value, must be calibrated to the international standard. Johansson was granted a patent for his invention in the year 1901 and formed the Swedish company CE Johansson AB in the year 1917. He started manufacturing and marketing his gauge blocks to the industry, and found major success in distant America. One of his customers was Henry Ford with whom he signed a cooperative agreement to establish a gauge making shop at his Cadillac automobile company.
Fig.2 Functional features of a slip gauge.
The development of ‘GO’ and ‘NO-GO’ gauges also took place during this time. Figure 2 illustrates the functional features of a slip gauge. It is made of hardened alloy steel having a 30 mm × 10 mm cross section. Steel is the preferred material since it is economical and has the same coefficient of thermal expansion as a majority of steel components used in production. Hardening is required to make the slip gauge resistant to wear. Hardening is followed by stabilizing at a sub-zero temperature to relieve stresses developed during heat treatment. This is followed by finishing the measuring faces to a high degree of accuracy, flatness, and surface finish. The height of a slip gauge is engraved on one of the rectangular faces, which also features a symbol to indicate the two measured planes. The length between the measuring surfaces, flatness, and surface conditions of measuring faces are the most important requirements of slip gauges.
Fig.3 Carbide slip gauges for superior wear resistance and longer life
Carbide gauge blocks are used for their superior wear resistance and longer life. They also have low coefficient of thermal expansion. However, they are quite expensive and used when rapid wear of gauges is to be avoided. Several slip gauges are combined together temporarily to provide the end standard of a specific length. A set of slip gauges should enable the user to stack them together to provide an accuracy of up to one-thousandth of a millimetre or better. In order to achieve this, individual gauges must be available in dimensions needed to achieve any combination within the available number of gauges. The surfaces of neighboring slip gauges should stick so close together that there should not be any scope for even a layer of air to be trapped between them, which can add error to the final reading. For this to happen, there should be absolute control over the form, flatness, parallelism, surface finish, dimensional stability of material, and homogeneity of gauging surfaces. While building slip gauges to the required height, the surfaces of slip gauges are pressed into contact by imparting a small twisting motion while maintaining the contact pressure. The slip gauges are held together due to molecular adhesion between a liquid film and the mating surfaces. This phenomenon is known as ‘wringing’.
Gauge Block Shapes, Grades, and Sizes
Slip gauges are available in three basic shapes: rectangular, square with a central hole, and square without a central hole. Rectangular blocks are the most commonly used since they can be used conveniently where space is restricted and excess weight is to be avoided. Square slip gauges have larger surface area and lesser wear rate because of uniform distribution of stresses during measurements. They also adhere better to each other when wrung together.
Square gauge blocks with central holes permit the use of tie rods, which ensure that the built-up slip gauges do not fall apart. Slip gauges are classified into grades depending on their guaranteed accuracy. The grade defines the type of application for which a slip gauge is suited, such as inspection, reference, or calibration.
Accordingly, slip gauges are designated into five grades, namely grade 2, grade 1, grade 0, grade 00, and inspection grade.
Grade 2 This is the workshop-grade slip gauge. Typical uses include setting up machine tools, milling cutters, etc., on the shop floor.
Grade 1 This grade is used for tool room applications for setting up sine bars, dial indicators, calibration of vernier, micrometer instruments, and so on.
Grade 0 This is an inspection-grade slip gauge. Limited people will have access to this slip gauge and extreme care is taken to guard it against rough usage.
Grade 00 This set is kept in the standards room and is used for inspection/calibration of high precision only. It is also used to check the accuracy of the workshop and grade 1 slip gauges.
This is a special grade, with the actual sizes of slip gauges stated on a special chart supplied with the set of slip gauges. This chart gives the exact dimension of the slip gauge, unlike the previous grades, which are presumed to have been manufactured to a set tolerance. They are the best-grade slip gauges because even though slip gauges are manufactured using precision manufacturing methods, it is difficult to achieve 100% dimensional accuracy.Calibration-grade slip gauges are not necessarily available in a set of preferred sizes, but their sizes are explicitly specified up to the third or fourth decimal place of a millimetre. Many other grading standards are followed for slip gauges, such as
JIS B 7506-1997 (Japan),
DIN 861-1980 (Germany),
ASME (USA), and
BS 4311: Part 1:1993 (UK).
Most of these standards assign grades such as A, AA, AAA, and B. While a grade B may conform to the workshop-grade slip gauge, grades AA and AAA are calibration and reference grades, respectively.
Slip gauges are available in standard sets in both metric and inch units. In metric units, sets of 31, 48, 56, and 103 pieces are available. For instance, the set of 103 pieces consists of the following:
1. One piece of 1.005 mm
2. 49 pieces ranging from 1.01 to 1.49 mm in steps of 0.01 mm
3. 49 pieces ranging from 0.5 to 24.5 mm in steps of 0.5 mm
4. Four pieces ranging from 25 to 100 mm in steps of 25 mm A set of 56 slip gauges consists of the following:
1. One piece of 1.0005 mm
2. Nine pieces ranging from 1.001 to 1.009 mm in steps of 0.001 mm
3. Nine pieces ranging from 1.01 to 1.09 mm in steps of 0.01 mm
4. Nine pieces ranging from 1.0 to 1.9 mm in steps of 0.1 mm
5. 25 pieces ranging from 1 to 25 mm in steps of 1.0 mm
6. Three pieces ranging from 25 to 75 mm in steps of 25 mm
Fig.4 Slip Gauge for Precise Engineering
Generally, the set of slip gauges will also include a pair of tungsten carbide protection gauges. These are marked with letter ‘P’, are 1 or 1.5 mm thick, and are wrung to the end of the slip gauge combination. They are used whenever slip gauges are used along with instruments like sine bars, which are made of metallic surfaces that may accelerate the wear of regular slip gauges. Wear blocks are also recommended when gauge block holders are used to hold a set of wrung gauges together. The purpose of using a pair of wear blocks, one at the top and the other at the bottom of the stack, is to ensure that major wear is concentrated over the two wear gauges, which can be economically replaced when worn out. This will extend the useful life of the set of slip gauges.