Software Review: Rapid Prototypes for Speedy Race Cars

Sept. 11, 2008
A NASCAR racecar shop uses direct digital manufacturing to build prototypes for car components.

Jon Rittle
Mark Bringle
Technical Sponsor Manager
Joe Gibbs Racing

Edited by Leslie Gordon
[email protected]

As a NASCAR race-car manufacturer, we call our main vision “concept to car.” The phrase sums up our push to keep a competitive edge by moving from idea to physical component as swiftly as possible. Race cars are no different from many engineering endeavors in that designs often require several changes before approval. So our shop makes lots of prototypes to test on cars for form and fit. About five years ago, to speed the prototyping phase, we started using direct-digital-manufacturing (DDM) technology called fuseddeposition modeling (FDM) from Stratasys Inc. Today, we use the FDM 400mc, one of the company’s recent machines.

Examples of parts we prototype include alternator brackets, oil-pan pieces, and steering mechanisms. In one design, a complex steering mechanism with six separate machining operations needed changes five times before it was approved. What would have taken weeks with our old method took only a few days because FDM can run 24/7. The prototype provided a part to bolt on to the car to see if the piece would fit in with the rest of the steering components.

Also, we are one of the few NASCAR shops that runs a scalemodel wind-tunnel program. Most of the parts for scale models come off the FDM machine. A recent example is a 40%-scale header, a component with a complex shape and small internal holes.

Another example comes from a fixture intended to hold an engine for testing in the dynamometer. Technicians made up the prototype piece just to check for fit before cutting such a large piece of steel. The FDM machine build envelope is 16 14 16 in., so when parts are bigger than that, we can create a tongue-and-groove design comprising two pieces that get fabricated in the same build.

Previously, the making of prototypes necessitated having a programmer write code for the CNC and an operator run parts. This process took a lot of time, tied up valuable personnel, and usually provided only about 75 to 100 prototypes annually. Today, though, we basically just dump STL files from CAD models to our file server or internal Internet. An operator downloads a file from the server to the FDM and the machine produces sample pieces in about a day. (Files can also queue to the FDM in an automated fashion.) Last year, we made 750 pieces this way.

The FDM process is straightforward. First, the machine’s Insight software takes STL files and figures out how to layer models, where boundaries reside, and where to put support material. The software lets users rotate parts, specify surfaces, and even overlap parts with the work rectangle to get more parts in one build.

The material the shop uses is PC-ABS plastic. The form it comes in looks like twine on a spool in a canister. There are canisters for the support and part material, with a backup for each. The software warns if a newly loaded job will run out of material. Also, a flashing red light on the machine indicates the need for a swap-out.

Once inside the machine, the material feeds through a nozzle, which melts the plastic and extrudes a line of material about 0.010-in. high and 0.20-in. wide with the largest tips offered, while moving back and forth to build the part. The machine extrudes the boundary layer first from support material, a soluble resin, and then fills the part interior. Parts sit on a sheet which, in turn, sits on a steel platen held down by a vacuum.

We use a polycarbonate sheet because parts are a polycarbonate blend of ABS. Fully built parts go into a tank containing water and a soaplike substance to dissolve the support material.

The shop picked FDM for several reasons. First, other DDM techniques require a controlled environment that allows only so much UV. In contrast, the FDM can go anywhere on the shop floor. Also the machine is affordable. Equipment from another developer cost $900,000. The FDM 400mc was about $225,000. Other technology used a liquid photopolymer that costs about $2,700 a gallon. Canisters for the FDM machine cost $400 to $500 each.

A handy feature: Each machine comes with a little “cheat sheet” with magnetic strips. Just place it on the machine for readily available information on everything from how to replace a canister to how to identify and replace tips. There is also an integrated Help Function in the user interface to diagnose problems on-the-fly.

In manufacturing, the absolutely best way of doing anything is rarely needed. Companies are only successful when they manufacture products as cheaply as possible. For example, to make a door wedge out of titanium is overkill. So we bypassed technologies that would be overkill for the shop. Some DDM techniques allow building parts and bolting them, say, directly to a motorcycle for running. But NASCAR likes keeping things plain and simple, so it probably will not allow the building of functional parts for race cars anytime soon. The organization doesn’t want to run the cost of manufacturing so high that smaller guys go out of business.

The FDM 400mc machine comes from Stratasys Inc., 7665 Commerce Way, Eden Prairie, MN 55344, (800) 937-3010,

A CAD model of a wind-tunnel scalemodel engine is created in Siemen’s NX Software.

The toolpaths for the same wind-tunnel scale-model engine are programmed in the FDM machine’s Insight Software.

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