Assembling Better Designs with Working Model

Jewett Automation uses Working Model and AutoCAD to help develop reliable and flexible assembly machines.

Assembly machines can represent a significant capital investment for manufactures. To justify this cost, assembly machines must work accurately and tirelessly - often outliving the original products they were designed to assemble. Developing durable assembly machines that can be reconfigured easily for new tasks poses considerable design challenges.

Jewett Automation, a manufacturer of assembly machines that specializes in turnkey systems for various industries including pharmaceutical, automotive, and paper products, uses Working Model 2D v.4.0 motion simulation software to meet these challenges. According to Ed Cutright, director of Machinery Development at Jewett, the software package paid for itself with the first use. The Company uses Working Model software to help develop reliable and flexible assembly machines. As a result, Jewett Automation also reduces manufacturing costs and speeds customer delivery.

Many of the assembly machines that Jewett Automation designs are built on an indexing dial machine that uses pick-and-place devices to add pieces to a customer's product. For example, Jewett Automation has produced machines that assemble fire extinguishers, deodorant sticks, fuel injectors, circuit boards and surgical scopes, among others. A typical assembly machine incorporates tooling arms attached to a column that is mounted centrally on an indexing dial.

According to Cutright, the forces placed on the machine by the tooling arms moving the objects to be assembled directly influences its final design. In developing an assembly machine, Cutright says it is critical to correctly visualize the effects of an object's mass on a tooling arm and consequently the internal components.

Jewett automation markets a standard chassis, the RAC-2 (Rotary Assembly Chassis), a completely cam-driven assembly platform. Because the RAC-2 was designed with all of the necessary automated motions required for part placement and is mechanically syncrhronized to a central indexing dial, a customer has to select feeders, escapements, grippers, and controls for building an assembly machine.

Using the RAC-2 as a standard chassis, Jewett Automation is able to achieve savings in manufacturing costs and bring systems to market more quickly. The production chassis on the RAC-2 uses a barrel cam indexer, with a tooling head mounted to a column formed by the extended center post of the indexer.

Another cam-driven component provides the lift and oscillating drive for the tooling head. This head performs the lift, extend, and retract motions of the tooling arms. Extend cams located in the tooling head of the RAC-2 convert the oscillating rotary motion into linear motion.

In developing the RAC-2, Cutright wanted to analyze the forces involved in lifting the central column and retracting and extending the tooling arms. In particular, he needed to design the proper extend cam to drive these motions.

Cutright selected Working Model as the motion simulation software tool for determining the best cam design.

Before using Working Model, Cutright manually calculated the loading by assigning an acceleration factor for the extend motion and the mass on the subassembly. When Cutright actually performed a simulation using Working Model, he found that the extend cam's loading was not symmetrical. The motion involved more torque for pulling the arms in than for pushing them out.

Using AutoCAD, he produced another cam layout and imported the drawing into Working Model to analyze the results. "With another run, I was able to easily optimize the extend cam by finding out the actual loading on the components," comments Cutright.

Cutright produced the initial model of the lift and extend motions of the RAC-2 in only a few days. He then read through the examples provided in the Working Model software package and constructed models using the motors, slots, and links features in the application. The original model used a pin on a driven disk to impart a cycloidal motion on the lifting of the tooling head and rotation of the extend cam's disk.

As Cutright mastered Working Model 2D, he was able to use the software application's formula language for constructing a more realistic model containing the appropriate dwells, the periods of time during a machine cycle when motion is not occurring. With this model, he determined the actual duration of the loads. Then, he exported the data generated by Working Model into Microsoft Excel software to create a spreadsheet.

In Microsoft Excel, Cutright found the root mean square (RMS) loading, a more accurate figure of loads, on the cam followers and predicted the operational life of the cam follower. Customers want to know this data because the system serves as a vital automation component ia manufacturer's assembly operations. Jewett Automation's RAC-2, costing an average of $35,000 to $40,000, typically will perform for 10 years at 3 shifts-per-day or more. In some cases, the RAC-2 outlives the product for which it was designed, but it can be easily re-tooled for a different product.

"By improving productivity and helping minimize prototypes, Working Model paid for itself with the first use," Cutright notes. "The software has become an essential development tool for us. I rely on its simple formula language for writing equations quickly and then running simulations." He also cites the ability to enter his own equations for simulating motors and actuators as key to successfully designing a sound RAC-2. He particularly likes how he can easily import his AutoCAD design drawings into Working Model for motion simulation.

"Working Model has proved very useful to Jewett Automation, especially as interest in our RAC-2 has grown," says Cutright, who recently received a request for a variation on this standard chassis. The customer requested more horizontal stroke than was provided with the RAC-2. Cutright used AutoCAD to modify the original design and then applied Working Model to check the extend cam design and lift loading.

"I was able to finalize the design of a 'long stroke'version of the RAC-2 and determine the top speed it would be capable of in only a few days using Working Model," he adds. "More important, the combination of using our RAC-2 as a standard chassis and using Working Model for motion simulation has enabled us to manufacture custom assembly machines in less time."

Reprinted with permission from: Computer-Aided Engineering, May 1998, pages 12-14, Penton Publishing