Magic Made With Fixtures

May 5, 2003
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Traditional flexible fixtures used on coordinate measuring machines (CMMs) can hold most parts, but some complex, geometric parts with multiple levels and steps, cannot be held by standard CMM fixtures. That is why many manufacturers maintain inventories of dedicated fixtures.

R&R Sales (Grand Haven, MI) thinks it can eliminate these inventories. The distributor of the patented Rayco CMM fixture that is used to hold parts on CMMs and vision measuring systems, now offers R&R Magic Fixtures software. A Belgium-based company, Materialise, developed the software for R&R Sales to hold complex parts that are difficult to hold or that require a dedicated fixture.

Rayco's standard line of flexible CMM fixtures can hold a variety of part types through a system that uses components such as vacuum cups, clamps and magnets that work with holders and standoffs. These are deployed on a universal base plate that contains a grid of pre-drilled and threaded mounting holes. Special stand-offs, which come in a variety of sizes, can be configured and attached to the appropriate holes in the base for mounting the part being fixtured. This allows the fixture to hold most parts regardless of size or material. The holding component conforms to the geometry of the part being fixtured and keeps the part from shifting during measurement.

This system works well, but not well enough, says Larry Ray, the inventor of the Rayco CMM fixture. "Flexible fixtures can hold about 70% to 80% of the parts out there, but people still need dedicated fixtures for the rest," he says. "It is that 30% of the parts that require dedicated fixtures that we are trying to eliminate."

Ray says that a company could literally save hundreds of thousands of dollars, considering that the cost of a dedicated fixture can run from $10,000 on up to $50,000 and higher. The cost of the Fixture Magic software is expected to be less than $10,000, including a standard Rayco CMM or vision fixture system. Those companies that already have a Rayco fixture system only need to purchase the software.

The Fixture Magic Software takes a computer-aided design (CAD) program of a complex part, and creates a stereolithography (STL) computer file that mirrors the part's shape. These details can then be used to create a rigid, yet inexpensive CMM fixture.

The STL file can be used to program a computer numeric controlled milling machine to create aluminum or steel fixtures. Also, the program can be downloaded into a stereolithographic machine for producing polymer fixtures. A stereolithographic machine creates a 3-D object in a layer-by-layer process using an ultraviolet laser and a vat of UV-curable polymer liquid. As the laser light is beamed onto the surface of the liquid in the appropriate CAD pattern, the polymer hardens where contacted by the light, forming a layer. As each layer is cured, a platform below the surface of the liquid is lowered by the thickness of one layer and the laser makes another pass, so that when the object is completely built, it is entirely submersed in the vat.

For long term gage repeatability and reproducibility (R&R) and continuous process control, the end user will want the details to be made out of aluminum or steel, says Ray. This can be done on R&R's milling machines or by the customer. Polymer fixtures are useful when the customer wants a quick way to hold a part for first-time checks or short-term gage R&R, says Ray.

"If the customer wants the details to have a long shelf life they will want the detail beams made out of aluminum or steel," says Ray. "Stereolithography works great when you want a quick temporary detail to perform first piece checks, or short term gage R&R and capability studies."

Fixture programs can be created in a few steps beginning with loading a 3-D CAD file of the part that can be created using most popular CAD programs. Then, the user chooses the appropriate Rayco base plate from a library of plates stored in the software. The base plates range in size from 10 to 50 inches square and can be linked together when large parts are involved, according to Ray.

"The software allows you to select the size of the Magic detail you need to hold the part," says Ray. "Once your part is loaded into the software, you simply use the Rayco Fixture base plate library to select the base plate that corresponds to the part."

The next step is to point and click on any area of the part drawing to generate the first detail in the Fixture Magic Software program. Alternatively, detail locations can be specified as an exact datum point to emulate a dedicated fixture or part-holding nest. The software automatically creates fixture beams, also known as detail beams. When the fixture is later built, it is on these beams that the part details will be carved.

"The fixture beams can be created vertically or horizontally and the user can place one or more beams at an exact location on the part to create a precision fixture nest with an unobstructed probing path for measurement," says Ray.

The beams have two standard 0.5-inch diameter through holes for attachment to the Rayco base plate using standard Rayco components. The beam size and holes can be modified in the software to fit any part regardless of size or shape.

The details are automatically assigned a hole location and detailed part description, and this information will be engraved onto the fixture beam. The fixture beam will be created with identification and details of where it should be placed on the base plate. The identification on the beams allows for easy set-up and removal from the base plate when multiple parts need to be checked.

Additional capabilities
In addition, the software has other features, including the ability to repair and use corrupt STL files, which are a common problem, according to Ray. The software also comes with a communication package that can be used by manufacturers and suppliers. They can communicate in real time via the Internet about part, fixture or quality problems, and the system will automatically document the meeting in text and graphics. Currently, the communicator only allows two participants to be on line at the same time, but future versions will allow multiple users to simultaneously participate in design part and fixture meetings on line, according to Ray.

Ironically, the beam also has the ability to measure parts, says Ray, judging by a recent example at a Beta site. "In a sense we have almost created a go gage," he says. "At the Beta site, we created the fixture beam from the CAD drawing and when we placed the part on it we found out it didn't fit. It wasn't the beam that was off, it was the part that had been manufactured out of tolerance."

TECHNOLOGY CONTACT
For more information on the R&R Magic Fixtures, contact:
R&R Sales LLC
1810-F Industrial Park Dr.
P.O. Box 161
Grand Haven, MI 49417
(616) 847-6045
Fax: (616) 847-6082
E-mail: rrsales@cmmfixture.com
URL: www.cmmfixture.com

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