Process troubleshooting: FEA moves out on the plant floor.
You may have seen an example of finite-element analysis (FEA) in your living room on Sunday afternoon. It's a TV sports filler on high-tech Olympic-training methods.
The announcer points out that a high-speed camera can now capture reference points on an athlete's body and feed them into a computer to develop an animated model. A trainer then plays with the model to determine how changes in technique affect the athlete's performance. The Europeans were first to apply FEA methods to sports training, which partly accounts for their dramatic performance improvement in worldwide athletic competition.
For years FEA has been a standard CAD tool used by US product-design engineers. Now, however, it is emerging as a way for manufacturing engineers to get metalworking processes into world-class shape as well.
A major automobile manufacturer recently faced a problem when machining a newly designed cast-iron water-pump housing, Figure 1. The part was machined on a pallet-transfer line and a one-pass face-million operation required by the process generated severe deflection in one region of the housing. Machining-support pads didn't alleviate the condition.
Structural/Kinematics, Warren MI, was contracted to work with the OEM's manufacturing engineers and perform an FEA study to determine what was causing the problem. SK used a proprietary computer-analysis package to mathematically represent the water-pump housing, Figure 2. The necessary data was taken from part drawings.
The engineers applied loads (such as those experienced during the machining operation) to the model, then observed how the structure behaved. The technique permitted a "What if we change ----?' approach that is cost prohibitive when dealing with real workpieces.
Modification possibilities were narrowed down to increasing the thickness of certain wall sections, adding reinforcing ribs, or both. The OEM elected to add the ribs, Figure 3.
During the FEA study it was also determined that there was an opportunity to redistribute the part's mass and consequently reduce the weight of the housing. The analysis delineated the distribution of stress on the structure, Figure 4, which permitted reducing the weight of the cast-iron part by 9.7 percent. Then it was decided to analyze the ramifications of changing to an aluminum work material while retaining the structure's integrity. The analysis produced a phenomental 64.4 percent weight saving.
In another example, a major machine-tool company had trouble with a milling machine producing out-of-tolerance parts. The problem was maintaining the proper locations for rocker arm pivot slots on a V-8 cylinder head. Because of this condition, the machine tool could not be shipped to the customer.
For four weeks the OEM's manufacturing engineering staff tried unsuccessfully to solve the problem by making minor adjustments. They finally elected to use FEA. The study revealed that the part was deflecting 0.0045 during machining. Several design iterations of the miling fixture were then evaluated using the computer model to maximize the fixture's stiffness.
A solution requiring minimal modifications was chosen and resulted in a 56 percent reduction in part deflection. The time required to analyze and find a solution was 10 days.
In a final example, another auto manufacturer couldn't hold a flatness specification on the joint face of V-6 cylinder heads when using a single-pass surface-grinding operation. The problem had existed for six months. As a temporary solution, the amount of metal being removed during the cut was reduced by 50 percent, thus requiring two passes for each part.
A FEA model of the ginder, grinding wheel, part, and fixture was developed. It revealed that the problem was a structural weakness in the grinding wheel/cylinder head interface. A stiffer machine-tool spindle was selected, which solved the problem.
Bridging the gap
Unfortunately, there is still a big gap between product design and making the product. Many designers just don't want to hear (or care) that their elegant solutions are raising havoc on the shop floor. Aggravating the situation is that manufacturing engineers usually rely on trial and error to get parts to run smoothly through production operations. FEA cuts the guesswork out of what causes problems in discrete-parts manufacturability and quickly identifies reasonable solutions.
Importantly, FEA doesn't require prototype or even production parts for a study. This means the technique can head-off manufacturing problems before they begin. For more more information about FEA and the service offered by Structural/Kinematics, circle E17.
Photo: 1. This water-pump housing was the object of a finite-element analysis study. The workpiece deflected during a face-milling operation.
Photo: 2. A wire-mesh computer model simulates the pump housing. The model permits a manufacturing engineer to look at what happens on the inside of a part during the manufacturing process--impossible with the actual workpiece.
Photo: 3. One result of the FEA study is the addition of reinforcing webs (shown by the solid-color sections) so that workpiece can withstand the machining stresses.
Photo: 4. An energy distribution map created during the FEA study shows the part's critical areas and sections that are relatively free of stress. The information identifies where the structure must be beefed up and where weight-reduction opportunities exist.
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|Title Annotation:||computer-aided engineering; finite-element analysis|
|Author:||Coleman, John R.|
|Publication:||Tooling & Production|
|Date:||Jan 1, 1984|
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