Description of BS 1790:1961 1961

This British Standard relates to length bars of the cylindrical type, in inch sizes up to 48 in, having parallel end faces finished by lapping.

If longer bars are required, they shall be uniform in diameter and straight overall to the limits laid down in Clauses 7 a) and b); tolerances for flatness and parallelism of faces shall be proportionate to those specified in the appropriate Table 1, Table 3, Table 5 and Table 7, and their lengths shall conform to the requirements of Clause 7 d) ii) .

Provision is made for two designs of length bar, one with complete plane faces and the other with annular faces surrounding an internally threaded hole at one end or both ends of the bar.

Section 1 provides for four grades of accuracy, namely:

1. Reference
2. Calibration
3. Inspection
4. Workshop.

Reference grades bars, as their name implies, are intended for use as reference standards. They have completely plane faces and embody the highest order of accuracy as regards both their end faces and the finishing of their actual lengths to their nominal sizes.

It is recommended that they should be supplied with a certificate by the National Physical Laboratory which confirms their full accordance with this British Standard and gives the deviations from their nominal lengths to an appropriately high order of accuracy.

To derive the maximum benefit from these very high grade reference bars they should be used only in standards rooms which are temperature controlled at 68 ± 1 °F (20 ± ½ °C) and with comparators of suitably high sensitivity.

Calibration grade bars possess the same degree of perfection in their completely plane end faces as the reference grade bars . They differ only in the somewhat wider tolerances allowed in the adjustment of their lengths to the nominal sizes, which thus permits them to be produced more economically than reference grade bars. It is recommended, however, that calibration grade bars should, like those of reference grade, be supplied and used with a N.P.L. certificate giving the actual deviation in the length of each bar from its nominal size.

NOTE The wider tolerances permitted on the lengths of calibration grade bars render them no less service able than reference grade bars when used as standards for the accurate measurement of other bars.

As already mentioned for reference grade bars, calibration grade bars require to be used under suitable conditions of temperature and with appropriate comparators if the fullest advantage is to be gained of the very high accuracy associated with their certified sizes.

Inspection grade bars have internally threaded ends and can thus be used in combination with each other. They are intended for use in inspection rooms and tool rooms.

Workshop grade bars also have internally threaded ends and can be used with slip gauges, comparators and their own accessories for measuring gauges, jigs, work pieces, etc . As their name implies, they are intended for use in workshops.

General dimensions, tolerances on length and standards of accuracy for flatness, parallelism and squareness of faces are specified for all grades of bars.

Section 2 relates to the essential features of design and accuracy of accessories for use with inspection and workshop grade bars.

Recommended sets of inspection and workshop grade bars and the sources of papers on the refined type of screwed connection are given in Appendices.

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