March 2011 / Volume 63 / Issue 3|
By Michael Deren
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.
When preparing to design a fixture, get input from the part designer, in-house or outside fixture designer, manufacturing engineer and operator(s). This ensures everyone is on board with the design intent and part manufacturability. Design reviews should be held until the design suits everyone’s satisfaction.
Once the design is approved and built, a manufacturer should conduct tests to verify fixture performance. These tests should cover fixture load and flexing as well as cycling of the hydraulic or pneumatic system. This ensures the part does not move when clamped and the fixture does not flex. Too often, there is some form of movement after a part gets clamped. Unless this is detected early and corrected, it will cause part problems.
Repeatability and capability studies should be performed when testing the fixture to ensure it performs as designed. These studies tell how well the fixture was built and allow any changes to be made to machine high-quality parts.
Once the fixture has been placed in service, there is one more vital element to consider: preventive maintenance. A fixture can last a lifetime if taken care of properly. Frequently, companies continue to use a fixture year after year without giving it a second thought, unless the fixture crashes.
When problems occur with the finished part, many are eager to blame the operator or the casting. But it could be that a fixture component is worn, such as a locating pin. It’s usually a simple fix, but it’s not fixed until a lot of time has been wasted compensating for the fixture problem.
When considering a fixture, determine which type—modular or dedicated—best serves your needs. Then have it designed, built and tested. Most of all, take the investment as seriously as when buying a machine tool. CTEAbout the Author: Mike Deren is a manufacturing engineer/project manager and a regular CTE contributor. He can be e-mailed at firstname.lastname@example.org.
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