Automotive assembly technicians rely on fixture gages to verify part accuracy and analyze fit and quality problems if they occur. The industry standard gage material is aluminum, which produces fixtures with accuracy but takes weeks to machine. To decrease the time required to introduce new vehicle designs, fixture makers have started investigating methods and materials for rapid production of master gauges.
The Detroit-area tooling facility of MSX International (Madison Heights, Michigan) developed a time-saving process based on its experience in the design, building and delivery of fixtures and gages to the automotive OEM community. In the past, the tooling group fabricated separate outer and inner checking fixtures for a truck manufacturer using a multi-step procedure . . . beginning with masters machined from Ren Shape boards, followed by lay up of outside material (OSM) outer and inner laminated skins on support structures that were bolted to steel bases.
Since then, the company has realized significant time savings by developing fixtures using a combination of fiberglass and epoxy with the Ren XD 4569-1 R/H seamless epoxy modeling paste from Vantico (formerly Ciba Specialty Chemicals, Performance Polymers) of East Lansing, Michigan. By using the epoxy paste to produce OSM outer and inner dimensions on a single fixture, MSX has reduced fixture production time by 25 percent, compared with using conventional fiberglass/epoxy materials, and maintained the precision and strength needed for a long service life.
Dan Crowton, MSX International’s fixtures and plastics foreman, is pleased with this. “Fabricating fixtures from fiberglass and epoxy was a labor-intensive process that required extensive spotting during assembly of the inner and outer sections of the tool,” he says. “With the Ren seamless epoxy, we can produce both fixture sides on the same support structure to save time and hold tolerances to plus or minus 0.010 inch.”
Now, MSX starts by machining a tooling aid for the outer side of the fixture and then lays up a fiberglass/epoxy laminate for the OSM outer. A support structure is installed, and then the surface for the OSM inner is built up by applying 0.8- to 1.5-inch thick layers of Ren XD 4569-1 R/H epoxy paste using an automated mixing and dispensing machine. The equipment features a special nozzle designed to extrude the sag-resistant paste directly onto the fixture base.
The epoxy paste is allowed to cure for 24 hours at room temperature and is then cut to the desired dimensions on a Tarus 15-horsepower CNC machine using high speed steel, ball-nosed, four-flute end mills run at 3,000 rpm and 150 ipm. The resulting seamless epoxy surface requires virtually no secondary finishing. Moreover, the paste’s linear shrinkage of 0.0012 inch/inch, shore hardness of 80D and compressive strength of 9,000 psi generates fixtures with the required accuracy and durability. Added benefits of the epoxy paste are that it is strong enough to hold threaded inserts, and the epoxy paste surfaces are easy to re-machine to reflect design changes.
Mr. Crowton says, “Use of Vantico’s new epoxy paste supports our ability to produce nominal fixtures faster and more cost effectively than ever before.”