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CNC blends surfaces better than CAD.

Empak in Colorado Springs, CO, a designer and manufacturer of molded plastic containers, found that its basic CAD systems couldn't create the complex surface blends needed to form the corners of its molds. Efforts at creating 3D models in a parametric system were unsuccessful. The solution to its problem was to enlist the services of CNC Products New York Inc, Highland, NY, which used Mill from CNC Software Inc, Tolland, CA, to model precisely the complex construction of Empak's model, and then quickly create toolpaths for the prototype.

Empak's specialty is manufacturing molded-plastic containers used by the semiconductor industry to store and protect silicon wafers. Single-crystal silicon wafers are 8" in diameter and approximately 1/32" thick. The wafers go through approximately 300 steps on the way to becoming semiconductors. As they are moved from process to process, Empak's containers are designed to provide an extremely clean shipping and storage environment for them.

Empak needed to build a fully detailed solid model of the 1 ft x 1 ft x 1 ft box called a "shipper" that holds 25 silicon wafers as they are transported from the manufacturer to a semiconductor fabrication company. The shipper consists of six parts, but its corners made the mold difficult to design. Fillets with different radii led into each. For example, a typical corner might involve the convergence of a fillet with a 3/8" radius with two that have 1" radii. The challenge was to get the relatively large radii and the smaller radius to converge smoothly, with geometry accurate enough to drive a machine tool.

Because the cost of a mold can be $50,000 to $500,000, Empak often has a prototype made by CNC Products. At CNC Products, the 2D drawings were entered into MasterCam. While running MasterCam, the prototype designer put the 2D drawings off to the side of the screen in a window and used information from them to construct 3D wireframe cross sections in another window. Turning 2D information into a 3D model--first as a wireframe and then complete with surfaces and corner blends--was the real challenge.

"The first step, creating 3D wireframes from what our designer saw in the 2D drawings, was followed by orienting the cross sections to form the rough shape of the container," says John K Boone, president, CNC Products New York Inc. "Then the designer joined the sides of the container with fillets and went to work blending the fillets, which had different radii, to create the corners."

Creating the wireframe geometry was the most difficult part of designing the Empak shipper. After that, surface definition was relatively simple, and creating the toolpath for the milling machine was nearly automatic. The design team typically spends three or four days designing a certain part of the container followed by one day of machining.

"Use of the software's Coon's-patch function for surfaces gave a great deal of flexibility to tool selection. MasterCam's ability to display the toolpath on screen was a significant time saver," said Mr Boone. "If the simulated toolpath sequence showed that a 1/8" end mill wasn't getting in everywhere, the designer could try a 1/16" end mill and view the new toolpath before actually milling a piece."

It took CNC Products 1 1/2 months to deliver the shipper prototype to Empak. That's longer than most jobs take, but the fact that it got done at all was impressive to everyone involved with the project. It would not have been possible to machine the shipper using hand coding, and it was far too difficult to model on a high-end CAD system. PC-based MasterCam provided the combination of high-end functionality and ease of use needed to create a molded plastic container that is as complicated as they come.
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Software Solutions & NC Machine Controls; computer numerical control; computer-aided design
Publication:Tooling & Production
Date:Nov 1, 1993
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