Henry Schein Orthodontics:
Pin Welding Fixture
Pin location in the orthodontic appliance being laser welded together
During my internship at Henry Schein I was in charge of designing a way to determine the correct clearance for a two part metal injection molded (MIM) part to finalize the design of a new product. Since the molds are expensive to make they wanted to verify their designed clearance between the pin diameter and hole diameter to ensure both assembly was easy and welding was successful. The offset height of the pin from the surface of the hole was also considered to allow for excess material to fill the gap when being welded.
A clearance and offset height of .001", .002", and .003" with a machined accuracy of ±.0005" was decided upon for a total of 9 configurations. Each configuration was tested 3 times for a total of 27 tests for better statistical significance. It was not tested more than 3 times due to the complexity of the CNC operation required to create the fixture plates.
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This was designed with intent to locate the pins central to the holes of the plate by placing the pins on a square extrusion with high tolerance. The square was machined with a difference of .0005" and a tolerance of ±.0005" between the extrusion and cavity to make the tolerance stackup near between .0000" and .001".
Finished text fixture after being welded and pulled apart to failure
CAD model of the test design, base held in vice and top pulled with Instron
Real test setup
The plate and block were then welded together with a Laser Star welding machine. This is a pulse type machine that fuses the metal pin and material around the hole together. The block was placed in a vise and the plate was pulled using threaded hangers attached to high strength wire.
It was found that too much clearance resulted in weaker welds and too little clearance was difficult to assemble. The test was worth the effort to determine the correct design parameters for the final design.