Slip Gauges (Johnnson Gauges ) or Gauge Blocks
Slip gauges are rectangular blocks of steel having a cross section of 30 mm face length and 10 mm face width, and are most commonly used end standards in engineering practice. The size of slip gauge is defined as the distance ‘l’ between two plane measuring faces. They are mad up of high grade steels with a range of sizes in a set enabling dimensions to build up to 0.005 mm, 0.001 mm or 0.0005 mm according to the set chosen. Slip gauges are also manufactured from tungsten carbide, which is an extremely hard and wear resistant material.
The slip gauges are first hardened to resist wear and carefully stabilized so that they are independent of any subsequent variation in size or shape. The longer gauges in the set, and length bars are hardened only locally at their measuring ends. After being hardened, blocks are carefully finished on the measuring faces to such a fine degree of finish, flatness and accuracy that any two such faces when perfectly clean may be “wrung” together. This is accomplished by pressing the faces into contact and then imparting a small twisting motion while maintaining the contact pressure. The phenomenon of wringing occurs due to molecular adhesion between a liquid film and the mating surfaces. by wringing suitable combination of two or more gauges together any dimensions may be build up.
To reduce wear on slip gauges a pair of protector gauge blocks are used and they are wrung to the ends of slip gauge combinations. The protector gauge blocks are made of tungsten carbide or similar wear resisting material which do not wear out and protect the slip gauges from wear. They are marked with letter ‘p’ on the measuring face.
Basic forms of Slip Gauges
Following are the three basic forms of slip gauges
- Rectangular Slip gauges
- Square Slip gauges
- Square with Center hole slip gauges
- Rectangular Slip gauges : This form is used because they are less expensive to manufacture. They are used where space is limited or where excess weight is undesirable.
- Square Slip gauges : These slip gauges are preferred in certain applications even though they are expensive. Due to their large surface area they adhere better to each other when wrung to build long lengths. Further they will not wear out easily
- Square with Center hole slip gauges : these Slip gauges are inserted on to the tie rod to ensure that the wrung stocks do not fall apart while handling.
Major Requirements for slip gauges
Slip gauges are used to provide end standards of specific length by combining several individual slip gauges into a single gauge bar. The success of combination of slip gauges depends on formation of a bar of reasonable cohesion between individual slip gauges and its actual dimension truly representing within specific limits, and the desired nominal dimension. For achieving this, the individual slip gauges must be available in dimensions need to achieve any combination with minimum number of gauges. The accuracy with which the individual slip gauges must be attached so closely to each other, such that the length of built up combination is equal to the added size of the individual slip gauges of the assembly. This is achieved by wringing the slip gauges. Further the attachment of individual gauges must be firm enough to permit a reasonable amount of handling as a single unit. And also it should be possible to detach all individual slip gauges so that they are re-usable without any damage to their original size or other properties.
The phenomenon of wringing takes place when two flat lapped surfaces are placed in contact with a sliding movement. If the surface are cleaned they adhere strongly when slid carefully together. Generally speaking a minute amount of grease or moisture must be present between the surfaces for them to wring satisfactorily. The phenomenon of wringing is partly due to molecular adhesion between a liquid film and the mating flat surfaces and partly by atmospheric pressure. It has been found that the gap between the two wrung flat pieces is approximately 6 nano meters, which has no effect on the total length. The technique of wringing together two slip gauges is quite simple, provided the surfaces are clean and free from burrs. The surfaces should be washed in petrol, benzene, carbontetrachrloride or other de-greasing agents and wipped dry on a clean cloth. The one gauge is placed at right angles to the other and slid one over the other, while pressing them together, a twisting motion is applied until the gauge blocks are lined up. In this way the air is expelled from, between the gauge faces causing the two slip gauges to adhere.
Similarly, for separating the two wrung slip gauges, combined sliding and twisting motion should be used and no attempt should be made to separate them by direct pull which may damage the slip gauges.
Manufacture of Slip Gauges
The method of manufacturing slip gauges developed by National Physical Laboratory (NPL) is as follows
- Section of high grade steel gauge blocks by preliminary operations.
- Initial hardening heat treatment to increase hardness and wear resistance.
- Stabilizing treatment is performed by heating and cooling the gauge blocks successively, after rough grinding all over to remove hardening stresses. The successive temperatures used in the four stages are 40 degree, 70 degree, 130 degree and 200 degree centigrade, the gauges are heated in sand cooled slowly at each stage.
- Eight gauges of one size are then mounted on a special type of magnetic chuck and spot ground on each face. A preliminary lapping operation is also carried out which makes all the gauges parallel to about 0.0002 mm and within about 0.002 mm of size.
- The final lapping process is carried out on a solid steel chuck, on which the gauges are wrung. When the gauges have been lapped in this position, all the faces will lie in one plane, which will not necessarily be exactly parallel to the place of the chuck. In order to eliminate the wedging effect, one gauge in each corner is removed and re-arranged, each gauge being turned end for end in the process. A little consideration of this re-arrangement will show that, in whichever direction the wedge may lie, there is an almost equally high spot at each corner.
Further lapping produces very high degree of parallelism and equality of size between the eight gauges. Then all the eight gauges are compared with a standard equal to their nominal aggregate size.
Question and Answers
- The accuracy of micrometers calipers dial indicators can be checked by a
Answer : Slip Gauges