ASME B89.1.2M:1991 pdf free download Calibration of Gage Blocks by Contact Comparison Methods (Through 20 in. and 500 mm)
It follows that, in order to achieve the level of accu-racy required for the calibration of gage blocks, the de-viation of the environmental ambient tenmperature fromthe standard must be held to a minimum.A suggestedmaximum is +0.5°F(0.25 C) or less. For a completeanalysis of the subject，refer to ANSI B89,6.2-1973,Termperature and Humidity Environment for Dimen-sional Measurement.
4.3 Humidity
It is recommended that the relative humidity in themeasuring environment shall not exceed 45%. Humid-ity significantly beyond that value may cause problemswith rusting of iron and steel surfaces,and cause per-sonnel discomfort.
4.4 Vibration
Excessive mechanical vibration in the measurementlaboratory may seriously affect the accuracy required forgage block measurement by causing instability at thepoint of mcasurement and at the readout. Vibrationshould be minimized by locating the laboratory awayfrom vibration sources and by using insulating mount-ings for measuring equipment. An ideal solution, usedby some laboratories, is to support all measuring equip-ment on a massive floating subfloor isolated from thesurrounding structure，the building foundation and thework floor.
5 GAGE BLOCK GEOMETRY5.1 Shape
Gage blocks are made in three cross sectional shapes:rectangular, square and round, with and without holes(see Fig.1).
5.2 Reference Points
The reference point is a point located in the plane ofeach gaging surface.The top reference point is locatedfor each shape in relation to the size marking as shownin Fig. 1. The bottom reference point is the mirror im-age of the top reference point.
5.3 Gage Block Length
The mechanical length of a gage block, for the tol -erance grades 1, 2,and 3 is the length perpendicular toone gaging surface and between the two referencepoints. The length is obtained by comparison to a reference gage block at 68RF (20°c).When the test blockand reference are of different materials, corrections mustbe made for differential deformation, and for differentialthermal expansion when the temperature of the blocksdiffer from standard. (Refer to Bi and B2.)
5.4 Parallelism
The parallelism of the gaging surfaces is the greatestdifference in length obtained from scanning the gagingsurfaces with mechanical contacts (see para. 8.6.)
5.5 Flatness
The flatness of a gaging surface is the distance be-tween two theoretical parallel planes with minimumseparation which envelop the gaging surface.
6MEASURING INSTRUMENT
6.1 Design
The measuring instrument shall consist of a compar-ator and readout specifically designed to measure thelength of a gage block between the top and bottom ref-erence points by comparison with a reference gageblock,and shall meet the requirements stated in paras.6.2 and 6.3,over its full range.
The instruiment may consist of one of, but not limitedto, the following:
(aType A. Acomparator with dual opposedmatched upper and lower gage heads， platen and read-out.The readout indicates the differential of the move-ments of the uppcr and lower gage head contacts.Theplaten is used to support the gage blocks being measuredand is not a reference.
(b) Type B.A comparator with a single lower gagehead,adjustable upper reference contact,platen and .readout.The measuring system floats with respect to afixed platen which supports the gage blocks and is nota reference.
(c Type c. A comparator with a single upper gagehead and a lower platen having a flat, serrated, or dualtrack configuration. The platen is the lower reference.(Refer to para.A5.)
6.2 Functional Requirements
6.2.1 Alignments
It is not the intent of this Standard to specify the nec-essary alignments and geometry of the components in-volved in the measuring instrument, since these factorsare inherent in the accuracy attainable. The manufacturer or a certified laboratory must warrant that the in-strument will meet the accuracy, linearity , and precisionrequired in para. 6.3, over its full range.
6.2.2 Contact Tip Material
The gaging contact tips and referehce contact tips，where applicable, shall be made of diamond.
6.2.3 Contact Tip Geometry
The contact shall have a spherical shape.The spher-ical radius of the tip may vary from one instrument toanother and is not specified in this Standard since itsvalue is not critical in the calibration procedure whenreference and test gage blocks are made of the same ma-terial.
However, when the reference and test blocks are madeof different materials，the contact tip radii must beknown in order to compute the correction factor to beapplied to the test block reading to compensate for thedifferential penetration of the contact tips in the differentmaterials.(Refer to para.8.1 and B1.)
6.2.4 Measuring Force
The upper gage head contact, or upper reference con-sact,(Type A or B, para. 6.1), as applicable, shall exerta force of 3 to 4 oz.(0.8 to 1.1 ncwtons).
The lower gage head contact shall exert a force of 1to 2 oz.(0.28 to 0.56 newtons).
The upper gage head contact or upper reference con-tact force must always exceed the lower gage head con-tact force to assure that thin lightweight blocks will seaton the platen.
A single head (Type C, para.6.1) suggested force is2 oz.(0.56 newtons).
ASME B89.1.2M:1991 pdf free download
