Slip gauges
Slip gauges are also known as precision gauge blocks. These are used for taking accurate and precise measurement of parts and for verifying measuring tools such as micrometers, snap gauges etc. These are rectangular blocks of steel having a cross section of about 30x10 mm. These are first hardened to resist wear and carefully established so that they are independent of any subsequent variation in size and shape. Measuring faces of the gauges are ground and lapped to such a degree of finish, flatness, accuracy that any two such faces when perfectly clean may be wrung together.
Wringing is accomplished by pressing the faces into contact (Keeping them perpendicular) and then imparting a small twisting motion whilst maintained the contact 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 x 10-6 mm) and the mating surfaces. When two gauges are wrung together they adhere so that considerable force is necessary to separate them, and the overall dimension of a pile made of two or more blocks so joined is exactly the sum of constituent gauges.
Factors affecting the desirable wringing effect are,
1) Flatness of the block.
2) Surface finish
3) Freedom of foreign elements such as dirt, grease, scratches and burrs.
1 Grades of slip gauges
Indian standard specification for slip gauges specifies three grades of slip gauges: grade 0, grade 1, and grade2.
Grade 2 intended for use in workshops during the actual production of components. Grade 1 is in inspection departments; and grade 0 is for use in laboratories and standards rooms .
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 widths.
Grade1 Used for more precise work, such as that c 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 has 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.
Grade00 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 the 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.
Maximum Permissible Errors in the Mean Length of Gauges at 20°c(BIS Specification)
Nominal Size
|
Grade 0
|
Grade 1
|
Grade 2
|
10mm
|
0.02 µm
|
0.10 µm
|
0.35 µm
|
20mm
|
0.04 µm
|
0.15 µm
|
0.45 µm
|
50mm
|
0.10 µm
|
0.30 µm
|
0.75 µm
|
100mm
|
0.20 µm
|
0.50 µm
|
1.20 µm
|
2 Manufacturing of slip gauges :
The following sequence of operation is followed in order that slip gauge posses certain desirable qualities.
1) .Making the approximate size by preliminary operations.
2) A special form of heat treatment to make the block hard and wear resistant.
3) An artificial and natural seasoning process to ensure stability for the whole life of the blocks. Stabilizing is generally carried out by heating and cooling the gauges successively, after rough grinding.
4) A final grinding process to reduce the block to an approximate required dimension
5) A final lapping operation to reduce the blocks to exact size and impart a beautiful finish to the surface. A special lapping machine is used for lapping.Lapping is done in a room maintained at 20°C and controlled humidity of 50%. The blocks are held in a moving spider between upper and lower cast iron laps. The spider is imparted both rotating and reciprocating motion to produce a complicated path of travel for each block and wear is distributed all over uniformly.
6) Comparison of finished gauges with grand master sets.
Sets Of Gauges
The recommended slip gauge sets in the metric units are:
M112, M105, M87, M50, and M27
3 Length Bar
These are used for the measurement of larger sizes of work. These are made in sizes varying
from 10 to 1200mm.These are used in standardising laboratories. Lengths bars are available in
four grades of accuracy :
1, Reference 2, Calibration 3, Inspection and 4, Workshop
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