Major automakers are a highly demanding customer base with tight deadlines and complex needs. In design terms, the “new normal” for OEM’s shifts a lot of the engineering risk to suppliers for engine-critical parts like valve train components.
That need for speed with quality in the design phase placed product design engineer Tom Rome Sr. in a quandary: how could he compress the design timeline and still deliver producible parts?
GT Technologies is a Michigan-based Tier One supplier to major automotive OEM’s. “We make the rocker arms, followers, lash adjusters, roller lifters, everything that actuates the valves. We ship to OEMs worldwide”, says Rome, adding, “Problems can come up just about anywhere. Some designs come to us, and some are done in-house.” A frequent problem with customer supplied drawings are tolerances that make it difficult or impossible to manufacture the part.
In these cases, Rome and the GT Technologies team often have to discuss the issue with the customer’s design team or take matters into their own hands: “We try to redesign it to make it fit our manufacturing capability through a product engineer that liaises directly with the customers. I get involved if we find a specific problem. The main issue is the ability to meet the tolerances the customer expects. Often it is a case of ‘if you really want that, you will have to pay for it. Here is why it drives up the cost.’ Often they’ll bring the tolerance back into line.”
Tolerances drive a lot of the manufacturing cost
The design process begins by understanding the OEM’s engine configuration, usually the cylinder head. “Sometimes we receive a model, and sometimes drawings. It is ideal to have both… I use those in my 3-D tolerance analysis. If we disagree on tolerances, we have to prove why they do not really need it (tighter tolerances). We will go through the exercise of modelling and then put it through design steps. Often the customer gives a little to allow us to manufacture the part cost effectively.”
GT Technologies designs parts in SolidWorks or PTC Creo [formerly called ProENGINEER) with tolerances and dimensioning schemes, then defines the tolerance analyses in Sigmetrix CETOL 6σ by selecting objects within the CAD environment to include in the studies. That’s when tolerances really come under the microscope.
When GT Technologies added Sigmetrix CETOL software, new possibilities in designing for production opened up. “It drastically helped improve our GDT process”, says Rome. “You can see the tolerance stacks and rethink how you dimension your drawings. It helps you make a more functional drawing, rather than worrying about whether a given tolerance is reasonable. It lets us play ‘what if’ scenarios where we move tolerances from one place to the other to make a part manufacturable.”
Understanding the “Tolerance Stack” is Key to Cost Effective Manufacturing
Making a part manufacturable means applying manufacturing tolerances strategically to where they will improve the performance and the service life of the component. In a typical tolerance stack situation, the relaxation of a tolerance in a noncritical area allows a tighter tolerance in the area with higher failure probability, resulting in an overall better part at reasonable cost. Without the ability to visualize those stacks through Sigmetrix CETOL 6σ, the only way to minimize failures is to build with expensive, tight tolerances throughout the part. GT Technologies uses Sigmetrix CETOL 6σ both during and after the part design process.
Rome would like to use CETOL concurrently with the design process but, he relates, “as a smaller company, it is often a matter of using the software after the design. The software flags us to dangerous tolerances, things that end up on the DFMEA. Certain key dimensions will show up with a high likelihood of failure, like the weakest link in the chain. We naturally try to reduce those and that is what we use CETOL for. It lets us spread tolerances across the part, reducing the cost of manufacturing.”
First and second order tolerance analysis
Sigmetrix CETOL 6σ tolerance software allows engineers to visualize their tolerance stack by using statistical algorithms offering both first and second order tolerance analysis. First-order analysis is fine for uniform production processes. In the real world however, parts are machined through numerous operations, on different equipment run by different operators, meaning that variations in each attribute will not necessarily fit a normal distribution.
Without advanced second-order analysis, an unusually high contribution to component failure caused by a single processing step can disappear in the data, forcing engineers to address the entire process rather than the real source of trouble. For complex assemblies typical of the automotive industry, there may be dozens of SPC-monitored attributes. A “shotgun” approach can easily drive costs out of control. The power of the software also significantly reduces time in the design process as well as simplifying troubleshooting post-design when problems arise.
Tolerance analysis can also help optimize existing parts
Sigmetrix CETOL 6σ allow users to quickly identify the top contributors to quality and make tolerance adjustments to understand the impact on assembly quality.
Rome also notes that this enhanced ability to visualize part tolerances more clearly can also help optimize existing products, as he discovered when tracking down a difficult internal binding issue on a well proven design: “It also points out design problems. We found that out on a lash adjuster design that we have used for years. We used it in a new application that adds new, different loads on the part. While I was redesigning the lash adjuster, I discovered a component inside that was causing a problem. The software pointed out the issue. I could see clearly how the tolerances allowed an internal part to move, causing unwanted side load. “
Reengineering a pre-existing design for a new application is useful, but tolerance analysis can also be used to pull costs out of an otherwise over-engineered design. Rome added, “There are many advantages in doing that. It helps that we’re designing an entire system, rather than a single part. With some of the engine technology today, we joke that we are making watches. It’s amazing what we can accomplish today compared to the 80’s in terms of loads and torque.”
For Tom Rome and GT Technologies, Sigmetrix CETOL 6σ results in lower production costs, higher quality and faster time-to-market, without additional costs in the design phase. Tom Rome puts it simply: “We love it”.
Note: Sigmetrix has paid a fee to ENGINEERING.com for the promotion of their tolerancing solutions. They have had no editorial input to this article. All opinions are mine. Jim Anderton