Fixturing 101: General Industry Coverage

Many machined parts require fixtures. That’s especially true for repeat and higher-volume jobs, because a fixture enables quicker and more consistent setups. Fixtures can be modular or dedicated to a particular part or family of parts.

Modular fixtures can be used numerous times and then dismantled until another design is required. They are appropriate for production runs that might have a limited life, as well as shorter production and prototype part runs.

Modular fixtures are relatively inexpensive, costing a few thousand dollars for the base plate and components. The base plate typically has a series of tapped holes for clamping parts and accessories along with precision dowel locating holes. Each tapped or dowel hole has a letter and number combination corresponding to a specific hole location.

When creating a fixture for a given part, a machinist generally labels the holes on a sheet of paper and creates a “map” for placing the fixture components. If that particular part runs again, the map recreates the required modular fixture design.

Various sizes of risers—which might include blocks or threaded rods—place clamps in the appropriate positions to hold a part. Most modular fixtures clamp parts manually, but some are equipped to clamp parts hydraulically or pneumatically.

Dedicated fixtures are most cost-effective when used in high-production runs or when parts are run frequently. These fixtures can be manual, pneumatic or hydraulic, or a combination of methods.

I have seen dedicated fixtures used primarily for cast parts, due, in part, to their irregular shapes. Dedicated fixtures typically cost tens of thousands of dollars. Some companies employ workers whose sole purpose is to design and build fixtures for in-house use. Some create a secondary profit center by accepting outside fixture design work.