Slip gauges are often called Johannes gauges also, as Johannes originated them. These are rectangular blocks of steel having a cross- section of about 30 by 10 mm. these 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 (keeping them perpendicular) and then imparting a small twisting motion whilst maintaining the contact pressure.
The contact pressure is just sufficient in order to hold the two slip gauges in contact and additional intentional pressure. It is found that phenomenon of wringing occurs due to molecular adhesion between a liquid film (whose thickness may be between 6 to 7 ? 10? mm) and the mating surfaces. When two gauges are wrung together and the overall dimension of a pile made of two or more blocks so joined is exactly the sum of the constituent gauges. It is on the property of wringing units together for building up combinations that the success of system depends, since by combining gauges selected from a suitably arranged combination, almost any dimension may be built- up.
These may be used as reference standards for transferring the dimensions of the unit of length from the primary standard to gauge blocks of lower accuracy and for the verification and graduation of measuring apparatus, and length measures for the regulation and adjustment of indicating measuring apparatus and for direct measurement of linear dimensions of industrial component.
Slip gauges are classified according to their guaranteed accuracy:
AA for master slip gauges,
A for reference purpose, and
B for working slip gauges.
Johannes AA slip gauges are accurate to plus or minus two microns per metre. Type A is guaranteed accurate up to plus or minus for microns per meter, while type 'B' for plus or minus eight microns per metre. The guaranteed error is not divided for a block which is less than 25 mm; such a slip gauge has same tolerance as 25 mm gauge. The workshop type, i.e. B type gauges are finished on their measuring faces approximately to within 250 ?m for flatness and parallelism. The corresponding figures for types A and AA are 125 and 75 ?m respectively.
As regards grades or classes of slip gauges, these could also be designed in five grades as under:
Grade 2:
This is the workshop grade. Typical uses include setting up machine tools, positioning milling cutters and checking mechanical width.
Grade 1:
Used for more precise work, such as that carried out in a good-class tool room. Typical uses include setting up sine bars and sine tables, checking gap gauges and setting dial test indicators to zero.
Grade 0:
This is more commonly known as the Inspection grade, and its use is confined to tool room or machine shop inspection. This means that it is the Inspection Department only who have access to this grade of slips. In this way it is not possible for these slip gauges to be damaged or abused by the rough usage to be expected on the shop floor.
Grade 00:
This grade would be kept in the Standard Room and would be kept for work of the highest precision only. A typical example would be the determination of any errors present in the workshop or Grade 2 slips, occasioned by rough or continual usage.
Calibration grade:
This is a special grade, with the actual sizes of slips stated or calibrated on a special chart supplied with the set. This chart must be consulted when making up a dimension, and because these slips are not made to specific or set tolerances, they are not as expensive as the Grade 00. It must be remembered that a slip gauge, like any other engineering component, cannot be made to an exact size. All slip gauges must have tolerances on the length, flatness and parallelism of measuring faces.
Except for the calibration grade, all slip gauge sets are manufactured to within specified limits; the closer the limits the more expensive the slip gauges, but in the case of the calibration grade, greater tolerance on length are permissible. Because the actual lengths are known or recorded in the calibration chart, due to allowance can be made when the slips are used.
Slip gauges are available in sets both in inch units and in metric units.
The five most usual sets available in inch units contain 81, 49, 41, 35 and 25 pieces respectively.
e.g. in the 81- piece set, the slip gauges are arranged in the following order:
9 pieces from 0.1001", to 0.1009" in steps of 0.0001".
49 pieces from 0.101" to 0.149" in steps of 0.001".
19 pieces from 0.050" to 0.950" in steps of 0.050".
4 pieces of 1.000", 2.000", 3.000", 4.0000"
In metric units, sets of 103, 76, 48 and 31 pieces are available. Metric unit sets of 103 pieces are made up as follows:
49 pieces with a range of 1.01 mm to 1.49 mm in steps of 0.01 mm.
49 pieces with a range of 0.50 mm to 24.50 mm in steps of 0.50 mm.
49 pieces of 25, 50, 75 and 1000 mm respectively and 1 piece extra of 1.005 mm.
Slip gauge set of 56 slips is made up as shown below:
9 slips 1.001 to 1.009 in steps of 0.001 mm.
9 slips 1.01 to 1.09 in steps of 0.01 mm.
9 slips 1.0 to 1.9 in steps of 0.1 mm.
25 slips 1 to 25 in steps of 1.0 mm.
3 slips 25 to 75 in steps of 25 mm.
Slip of 1.0005 mm.
Many times to reduce wear on inspection workshop grades a pair of protector gauge blocks (1 to 1.5 or 2 mm length) are also supplied and these are wrung to end of slip gauge combination block.
As these are made of tungsten carbide or similar wear resisting material these do not wear out and protect the slip gauges from wear. These are marked with letter 'P' on one measuring face.
According to OIML, gauge blocks with rectangular section are length measures in the form of rectangular plates or rectangular parallelepiped blocks consisting of two opposite flat faces parallel to each other, called measuring faces, the surface quality of which is such that they have the property of wringing to the corresponding faces of other gauges blocks or the reference face of datum surfaces.
Gauge blocks are meant to be used as:
a) reference standards for transferring the dimension of the unit of length from the primary standard to gauge blocks of lower accuracy and for the verification and graduation of measuring apparatus.
b) Length measures for the regulation and adjustment of indicating measuring apparatus and for the direct measurement of linear dimensions of industrial components.
Basic Forms of Slip Gauges:
Slip gauges with three basic forms are commonly found. These are rectangular, square with centre hole, and square without centre hole.
Rectangular form is the more widely used because rectangular blocks are less expensive to manufacture, and adapt themselves better to applications where space is restricted or excess weight is to be avoided.
For certain applications, square slip gauges, though expensive, are preferred. Due to their large surface area, they wear longer and adhere better to each other when wrung to high stacks.
Square blocks with centre holes are used to permit the use of tie rods as an added assurance against the wrung stocks falling apart while handling.
The greatest emphasis in the slip gauge is laid on the two gauging surfaces on opposite sides of the blocks. The length between measuring surfaces, flatness and surface conditions of measuring surfaces are the most important features of slip gauges which have to be controlled within desired limits.
Major Requirements for Slip Gauges:
The essential purpose of slip gauges is to make available end standards of specific lengths by temporarily combining several individual elements- each representing a standard dimension- into a single gauge bar. The combination of single blocks must result in a bar of reasonable cohesion, whose actual dimension truly represents, within specific limits, the nominal dimension sought for a particular application.
This objective is accomplished by making slip gauges in dimensions (arithmetically determined sizes) needed to achieve any combination of sizes within the designed range and graduation of the set; ensuring the accuracy of individual blocks to be within accepted tolerance limits; attaching the individual blocks so closely to each other that the resulting bar will have a length equal to the added sizes of the individual blocks; further the attachment of the elements to each other should be firm enough to permit a reasonable amount of handling as a unit, and yet when taken apart, the individual pieces should be reusable without any harm to their original size or other essential properties.
Slip gauges are used to provide end standards of specific length by temporarily combining several individual elements- each representing a standard dimension- into a single gauge bar. The success of system depends on formation of a bar of reasonable cohesion between individual elements and its actual dimension truly representing within specific limits, the desired nominal dimension. For achieving this, the individual gauges must be available in dimensions needed to achieve any combination with minimum number of gauges. The accuracy of individual blocks must be within accepted tolerance limits. The individual blocks must be attached so closely to each other that length of built- up combination is equal to the added size of the individual blocks 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 unit. It should be possible to detach all individual elements so that these are re-usable without any harm to their original size or other essential properties.
For this purpose, control of geometry of form, i.e. flatness and parallelism of faces, squareness of the gauging surfaces; surface condition, appearance, homogeneity of gauging surfaces, etc. is very essential.
The accuracy of gauges can be affected by the dimensional instability of material, or by wear in use, or damage during storage and handling. These can be taken care of periodic calibration. The health of slip gauges can be very easily checked by checking its wringing quality by wringing the gauge to be tested with an optical flat and inspecting the wrung surface for colouration (which indicates an air gap and its area should not be more than 5 per cent of the entire wrung surface).
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