vdoc

92 Part II: Metrology
The unit standard of angular measurement is the degree. The measurement and
inspection of angular dimensions is somewhat more difficult than linear measurement
and may require instruments of some complexity if a great deal of angular
precision is required.
The Sine Bar
Part II.D.2
The sine bar is a hardened, precision-ground and lapped tool for measuring
angles in different settings. It is a relatively simple device for precision
measuring and checking of angles. It consists of an accurately ground, flat steel
straightedge with precisely affixed round buttons a defined distance apart and of
identical diameters.
Sine Bar Application. Figure 10.4 illustrates one method of applying a sine bar
in the determination of the angle a on the conical surface of a part located on the
surface plate.
For precise results, a sine bar must be used on true surfaces. In Figure 10.4 the
center-to-center distance of the sine bar buttons = 127 mm (5 in.), and the distances
A and B are determined by means of gage blocks, or a vernier height gage:
Thus,
A = 25.400 mm (1.0000 in.)
B = 89.794 mm (3.5352 in.)
sine a = (89.794 – 25.400)/127.00
= 0.50704.
From trigonometric tables, the angle a is 30°28’.
Sine Tables. Sine tables are a further development of the sine bar. The sine table
has a larger working surface and is much more robust than the sine bar. It is more
suitable for larger, heavier parts.
A = 25.400 mm
(1.0000 in.)
127.00 mm
(5.000 in.)
Sine bar
B = 89.794 mm
(3.5352 in.)
Surface plate
Figure 10.4 Application of a sine bar.
Reprinted with permission of the Society of Manufacturing Engineers, Manufacturing Processes and
Materials, 4th edition, copyright 2000.