Tolerance Analysis

"Final Assembly is the moment of truth."-Charles Fine, MIT

Veteran product development professionals know the value of thorough and methodical tolerance analysis. Tolerance analysis is baked into our process and has become a core expertise at Bresslergroup. We, like many other experts took our tolerance lumps the hard way which has led to our current tolerance analysis expertise. From junior level engineers to the Director level, all staff members understand the importance of tolerance analysis. For them and many professionals, the following sequence will sound familiar:

Your parts have been carefully designed. A lot of thought and effort has gone into their form and function. Each part has been meticulously checked and refined -- they can handle the loads and there aren't any manufacturing issues.

Your suppliers received your CAD data and the first parts are coming out of the presses. Good news from quality control: all parts pass inspection.

You've received your first parts, did a visual inspection, and measured all the critical dimensions yourself…all looks good. They're laid out before you and you excitedly begin assembly.

As you assemble the first few pieces, dismay sets in: something's not right, the parts all passed inspection, but they're not fitting together -- the ones that do require an inordinate about of force -- what happened?

Surprising as it may be, when manufacturing variation isn't taken into consideration, this is probably the best scenario -- you can't assemble your parts! That’s when early investment in tolerance analysis pays off. Not that tolerance analysis ensures that your parts will fit and functional perfectly each time, but effective tolerance analysis can significantly streamline the production ramp-up process.

More often than not, when the inevitable variations of your manufacturing processes aren't taken into account beforehand, your parts may very well fit together but your assembly may function poorly (if at all!) -- and the tedious and expensive process of analysis and troubleshooting to accommodate production tolerances begins.

All manufacturing processes have their own inherent tolerances -- or, the process’ ability to meet your dimensional requirements. The variation may be quite small -- even a fraction of a thousandths of an inch -- but it’s there. Your CAD model may show a 2" feature, but when it’s actually produced, that feature will have some variation depending on the tolerances inherent in the process that was used to create it -- it could be 1.999" or 2.001.” Moreover, not all the parts will be the same…some may be larger, some smaller. They may all pass inspection.

Despite being relative, one thousandths of an inch may not seem like much, but nonetheless, it is there, and engineering must account for it. What happens when your final product is an assembly of 10 parts all with this 0.001" window of tolerance variation? If all your parts' variation is on the high-side, you now have a problem which analysis will show is ten times larger: 0.010 of an inch that was not considered by you or your CAD system. This is obviously a situation we seek to avoid in our contract engineering services.

All the parts passed inspection, but now your assembly binds, the slides are tight, the motors won't spin, etc. You could try to force your contract supplier to tighten tolerances…you may demand that all the parts be of one particular size -- but you do so by dramatically increasing the cost. Your contract supplier may make 1,000 parts, and throw out the 900 parts that don't meet your engineering requirement.

Conversely, you could take this tolerance stacking into consideration beforehand as we do in our contract engineering service -- allowing your assembly to function within a range of manufacturing variation, yielding lower cost parts that work every time.

Understanding where the variations add up and where they subtract from the overall design intent is an art and science unto itself. When done right through our contract engineering services, the resulting product is easy to assemble, at the lowest possible cost, and functions reliably each and every time.

To cite a recent example we took on in our contract engineering services practice, an airtight product proved difficult to effectively seal each and every time. This particular product’s function depended entirely on the ability to maintain an environmental seal. Although engineering performed a tolerance analysis beforehand, some units still failed their leak tests as, in some instances; the seal wasn’t seeing sufficient pressure. Tightening the manufacturing tolerances would have proven too costly.

So a second tolerance stack-up analysis was undertaken by engineering. This analysis gave primary consideration to the sensitivity of the seal’s compression. In this particular instance, the opportunity arose to increase the actual thickness of the seal by reducing the overall height of another component. Because there is a practical limit to the percent compression the gasket can see, by thickening the gasket engineering could increase the actual compression while reducing the forces seen by the case and the percent compression seen by the gasket. This allowed us to maintain a satisfactory degree of seal pressure throughout the range of part variations. All subsequent assemblies passed their leak test each and every time and our contract engineering service for tolerance analysis paid for itself many times over.