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Making the Bionic Wrench

A fixture made of metal. The outline of a wrench is visible on one piece of the fixture
A Bionic Wrench

The Challenge

Professor Dan Brown of the Northwestern Segal Design Institute is well-known for his Bionic Wrench™, a product he invented and manufactures. Every year, he challenges teams of manufacturing students to design their own fixtures and procedures for assembling his wrenches.

My Role

On our team of five manufacturing and design students, I focused on the design for manufacturing and construction of our fixtures themselves. I laser-cut prototype fixtures and milled the final steel plates. In testing, I also assembled a few Bionic Wrenches of my own.

Getting a Feel for the Product

The Bionic Wrench is constructed of two sets of plates and six metal jaws, riveted together to allow smooth motion. A reliable fixture is necessary to quickly and reliably make suitable riveted joints.

To design such a fixture, my team first measured the wrench plates with an optical comparator to generate accurate dimensioned drawings. These drawings were then used to begin designing prototype fixtures.

Sample fixtures were laser-cut from acrylic and tested directly on the shop riveter. Two major versions were used in testing, each one undergoing several rounds of adjustment, testing, and analysis.

Two pieces of an assembly fixture, made of acrylic and metal
A metal assembly fixture with part of a wrench positioned in the fixture
The underside of a metal assembly fixture

Improving Accuracy

The sample fixtures were able to make functional wrenches, but the low precision of the laser cut acrylic made it easy to rivet off-center, making the wrenches "stick" rather than articulating smoothly. The laser-cut perimeter holding the wrench also tended to overconstrain, making it difficult to add and remove components.

The final fixture located the wrench more precisely using tight-tolerance dowels on the top surface. The bottom surface held a pattern of reamed holes, one for each rivet. A separate, fixed plate underneath held a single pin, which fit into these reamed holes to locate the fixture.

Sheet metal handles provided an simple, reliable way to hold and move the fixture under the riveter.

Final Deliverables

  • Six wrenches produced by a single teammate in less than 30 minutes

  • A plan to scale production to 1,000,000 units per year

A wrench being riveted on an assembly fixture
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