Parallels (engineering) - Manufacturing

Manufacturing

There are two main grades of surface tolerance:

• Grade A - Used for close-tolerance work, such as inspection or precision machining
• Grade B - Lower surface tolerances, and are used for lower-precision machining

Parallels are first machined to rough dimensions, leaving a few millimeters to allow the rest to be ground. Parallels that only have 2 or 4 precision faces will often have the tool-marks from the machining on the non-ground sides. They are then paired and placed in a grinding machine, and each face is ground until the overall dimensions are correct - they are paired during this stage so that even if the dimensions are not correct, they are still parallel to each other. Then, the individual finishes are applied, from drilling to machining a chamfer along the edges to remove any burrs or sharpened edges. They may also be lapped to achieve a mirror smooth surface. Most parallels are also hardened.

Parallels are manufactured to either imperial or metric dimensions, and are often sold in a set, with several pairs of different sizes.