Precision-ground bars sold in matched pairs of equal height, set under a part in a mill vise so it sits above the jaws and parallel to the table. After clamping, the part is tapped down with a dead blow hammer until both parallels are trapped and will not slide. Thin spring-steel versions fit under thin parts, and adjustable parallels cover odd heights.
Tap it down till you can't move the parallels, if one still slides, the part's rocking and the face won't be flat.
If a parallel will still slide after tapping the part down, the part is not seated, so the finished face will not be parallel either.
Also heard as
- vise parallels
- precision parallels
Related terms
1-2-3 blocks
Hardened, ground steel blocks measuring 1 x 2 x 3 inches, used for setups, squaring and height stacks.
Fixturing
The design and use of fixtures that locate and clamp parts the same way every time.
Soft jaws
Vise or chuck jaws of aluminum or mild steel, machined in place to cradle a specific part.
Surface plate
A thick slab of granite or cast iron, lapped flat, that every height check and layout line is measured from.