Quality Magazine

Control Tolerances Before They Control You

May 19, 2003
Tolerancing will not necessarily determine if a good part or a bad part will be produced, but it can help determine if the final assembly is going to be in tolerance as opposed to the individual parts themselves. It is possible to produce parts that conform to individual tolerances, but when put together the parts don't work because the overall effect on assembly has not been analyzed.

"A good tolerancing scheme is needed all the way through to final assembly," said Steve Scigliano, business unit manager of quality at the San Jose, CA-facility of Tecnomatix Technologies (Herzeliya, Israel). "Companies should manage tolerances not only in the design phase, but also in the manufacturing phase.

"Typically when design engineers tolerance parts, they really don't know what tolerances to put on parts so they look at previous drawings and assign values from the past. If asked why a particular tolerance is being used, often there isn't an answer because no one really knows," said Scigliano. "That's probably the most fundamental problem--the fact that the designers are just adopting tolerance values and tolerance schemes from previous designs without really understanding what the impact of individual part tolerances have on final assembly."

Using past values isn't the only reason today's tolerances are not met, said Dave Meadors, operations manager for Varatech Inc. (Holland, MI). "Quite often we find that an engineer will design something that a manufacturing facility is not capable of producing," he said. "They're not capable of producing it because the process that they've been dictated to use is not capable."

Tolerances also aren't achieved because they are put on drawings too tightly. As engineers design tolerances for the end application, they someArial lose sight that some of those tolerances might not be realistic and achievable in the manufacturing process, said Jack Gaughan, manager of custom gaging at Edmunds Gages (Farmington, CT).

Gaughan added that companies often use different engineering departments at different stages, which can further complicate the pro-cess. For example, a product engineering department designs the product and a manufacturing engineering department makes the product, yet these two departments don't communicate with one another in the development of the product. "It's more like the product engineering department throws it over the wall and says, 'You make it.' And therein lies a bunch of trouble," he said.

Tolerances and scrap
Scigliano estimates that from 30% to 50% of scrap and rework is caused by poor tolerancing. In high-volume industries, such as automotive and consumer electronics, scrap and rework are caused not only by poor tolerancing, but by process variation. In production lines, dies and fixtures wear, and other changes occur in the process, eventually leading to a poor product. "In those industries it's important to monitor the process closely with statistical process control techniques to ensure the process is in control. Once the process starts to go out of control, then there are tools that can be used to actually measure the product and analyze the as-built product compared to the designed product and determine what is going wrong in the process," said Scigliano.

The aerospace industry is different because it's not a high-volume industry. "In aerospace, you measure each part to make sure it conforms to its tolerances. It's a matter of having a good analysis tool that analyzes each part and allows you to determine what's going wrong at that point," said Scigliano.

Analysis tools will help control warranty costs. Meadors said companies budget millions of dollars on warranty issues. "Companies always have the time and money to spend on warranty issues, but they don't have the time and money to spend upfront in the design process, to make sure they don't have to spend the time and money on warranties," said Meadors. "Don't spend the money in the tail end when there are warranty and quality issues, and unhappy customers that will buy from someone else next time."

Varatech has conducted surveys that show that about half of the time a typical engineer spends at work is fire fighting and debugging existing products that are in production, instead of spending time developing new concepts. "We propose spending a little bit of that fire fighting time to solidify the designs ahead of time via analysis tools, virtual simulation and other tools the industry offers," said Meadors.

Many software products are graphical in nature, which allows operators to emulate varying parts on the screen, cut sections into them and see all of the mechanisms on a computer screen. Meadors considers the visual feedback an advantage because a trained expert is not needed to decipher the code.

Scigliano said, "A good analysis tool is a software package that can take the specified design tolerances and measurements of the actual part and confirm that the part conforms to the design tolerance." He added that an analysis program for individual parts should have the ability to analyze different types of tolerances, whether it's linear dimensioning or geometric dimensioning and tolerancing (GD&T). Scigliano said that consumers should carefully review software packages. Some software claims to analyze tolerances but doesn't analyze all the possible tolerances or analyzes the tolerance only under certain restrictions.

A systematic approach
To avoid future problems, companies must realize that problems may exist and determine where those problems lie. Often companies will continue to do things the way it's always been done, and are not aware of problems because of poor tolerancing practices, said Scigliano. First and foremost, a company needs to take a serious look at what type of production problems are occurring.

Scigliano suggests using a tool that analyzes assemblies and tolerancing schemes. In the manufacturing phase, follow the assembly scheme that was defined in the design stage. If a change is deemed necessary, go back to the design and reanalyze it to try and fix the potential out-of- tolerance manufacturing results. Once a history of good parts is collected, use this information to understand the manufacturing capability and feed this back to the designers so that they know what the manufacturing process is actually doing, said Scigliano.

A systematic approach to tolerancing begins by understanding the capabilities of the manufacturing process, said Meadors. He said that in design situations a particular vendor or manufacturing facility will use "get the job tolerances." "They'll say, 'Yep, we can meet all of your requirements,' when in fact, they don't know if they can."

A systematic approach includes:

  • Defining the build objectives. What is going to be produced?
  • Looking at the sensitivity of the geometry. Is the geometry capable of producing the objective?
  • Defining the datum structures and GD&T. How do the parts functionally relate to each other in the manufacturing process?
  • Looking at the process capability. Is the company capable of building this product? Can it assemble the product?
  • Running simulations representing virtual factories on a computer by emulating the design and running it through a virtual plant allows the designer to compare the design to the build objective. "Often, industrial design or concept guys are passing things to a designer and that gets passed to manufacturing and then passed to quality. The final product doesn't look anything like they originally intended. It's a constant correlation and checking of what the up-front objectives were and making sure you go back to that," said Meadors.
  • After running computer simulations, look at building fixtures and gages. After a product has been built and designed, and objectives are defined, the product can be functionally measured. Specify the critical dimensions to be measured.
  • Producing quality documents. A living audit trail of what was done in the design process is necessary in the event that problems occur later on down the road. With the documents, the quality people are able to go back through the audit trail and determine where in the process the problem lies.
  • Continuous maintenance helps continually review and monitor particular manufacturing processes to help ensure the build objective is met.

"We encounter companies that don't understand that they have to be systematic about the application of tolerances and managing their tolerances all the way from the predesign and design phases all the way through the manufacturing phase," said Scigliano.

"We're such a strong believer in managing tolerances through the whole process because if you look at the cost of making an engineering change at the design phase vs. prototype phase vs. manufacturing phase vs. field phase, the cost of making an engineering change grows exponentially," said Scigliano. "The cost is low in the design phase and high in the field phase, so try and catch problems as early as possible.