slip gauge conclusion
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Answer :
Slip gauges are often called Johannsen gauges also, as Johannsen 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 stabilised 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 perpen-
dicular) 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 no 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 x 10-6 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.
Johannsen AA slip gauges are accurate to plus or minus two microns per metre. Type A is
guaranteed accurate up to plus or minus four microns per metre, 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 um for flatness and
parallelism. The corresponding figures for types A and AA are 125 and 75 u.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 widths.
Grade 1. Used for more precise work, such as that carried out in a good-class toolroom. 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
toolroom 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 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.
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