BS-887:2008 pdf free download – Precision vernier callipers – Requirements and test methods.
B.1 Deviation of reading
Support the caliper along the side of the beam with the jaws horizontal. Measure the separation of the external measuring faces by combinations of calibrated gauge blocks and other end gauges or by other means of similar accuracy such as a length measuring machine. Check the deviation of reading at a minimum of at least five approximately equally spaced positions which cover the measuring range of the main scale and the vernier scale.
Ensure that the method of measurement of the separation of the measuring faces does not distort the calliper. Ensure that the calliper and the measuring equipment have stabilized at 20 °C (particularly on the longer separations) and that differences in temperature are not introduced by handling while carrying out the measurements.
B.2 Measuring faces
B.2.1 Flatness of the faces for external measurement
The flatness of the faces for external measurement shall be checked either by applying an optical flat or a dial test indicator (see BS 2795 for lever types, BS EN ISO 463 for plunger types) working from a grade 1 surface plate (see BS 817), or by another suitable method giving the same degree of accuracy.
B.2.2 Parallelism of the faces for external measurement
The parallelism of the faces for external measurement shall be checked either by inserting gauge blocks between them at different points on the jaws and at different measured lengths by varying the sizes of the gauge blocks, or by using a measuring machine or an indicating comparator The parallelism shall be checked in at least two positions, the 0 to 20 mm or 0 to 1 in the range of reading and in the 20 mm or 1 in length close to full range.
Parallelism shall not be affected by clamping the slider.
B.2.3 Squareness of the fixed faces for external measurement
The squareness of the fixed faces for external measurement with the guiding edge of the beam shall be checked either by sighting against a knife-edge square of stock length comparable with the length of the slider, or by another suitable method giving the same degree of accuracy.
B.2.4 Parallelism of the faces for internal measurement
The parallelism of the faces for internal measurement shall be checked either by a micrometer with reduced faces or by another suitable method giving the same degree of accuracy.
B.2.5 Combined width of the internal measuring jaws
The combined width of the internal measuring jaws shall be checked either by means of a micrometer used in conjunction with a GO ring gauge, the diameter of which exceeds the nominal combined width of the internal measuring jaws by 0.013 mm or 0.0005 in, or by another suitable method giving the same degree of accuracy.
B.3 Scale lines
The thickness of the main scale lines and vernier scale lines shall be checked either by direct measurement with a microscope fitted with a micrometric device, or by using a measuring machine.