How to Solve Multi-Hole Tolerancing Problems

Have you struggled to optimize tolerances for assemblies that depend on pins floating in multiple holes to ensure correct fit of their stacked components?

Electric busbars are critical elements for conducting large levels of current. Ensuring correct fit during assembly depends on specifying optimal tolerances.

These kinds of assemblies are sometimes referred to as pin-hole, pin-in-hole, or pin-through-hole (PTC) assemblies.

This video shows you how to solve this kind of multi-hole tolerancing problem the easy and reliable way, using Enventive’s Concept software for tolerance analysis.

The mathematical calculations for analyzing the impact of multiple floating pin in holes on alignment are very complicated — as anyone who has attempted to make this analysis with a spreadsheet can confirm. Because of the highly complex spreadsheet calculations, you aren’t confident in the output and basing design decisions on that output is risky.

This video shows you what you need to know to take advantage of Enventive Concept for rapidly predicting assembly failure rates and then iterating to quickly make optimal GD&T design decisions.

The video demonstrates how to do this using an example of an electrical busbar assembly. However, the approach is valid for other multi-hole design applications too, such as for: Printed Circuit Board (PCB) assemblies, housing assemblies, and other assembly systems that combine a centering diameter to limit dispersions and a secondary positioning pin to lock the rotation.

Busbars are critical for conducting large levels of electric currents within a wide of variety of products, including increasingly electric vehicles.

Let’s get started.

modeling the busbar assembly and pin in hole

The busbar assembly is made of a case with two mounted plastic mounts upon which a metal bar is assembled with a centering hole, using a pin in hole, and a slotted hole to ensure correct alignment.

This is what the assembly looks like in Concept. The assembly is made out of a case and on the case two plastic blocks are mounted. They use a centering pin that is assembled with a regular floating “pin in hole” and a slotted hole to fix the orientation of the plastic block.

Now we will load the metal busbar component instances. We first want to apply a rotation on these instances. And then we will assemble them in the same way, which means using the centering hole with a regular “pin in hole” and for the slotted hole a construction circle defined to be tangent to the slot. Finally, we apply a “floating pin in hole” between this construction circle and the bolt.

Analyzing for assembly collision problems

Now let’s proceed with the analysis.

A “tolerance-in-motion” study identifies any potential collision problems during assembly between the pin and its alignment hole.

The next step consists of running a “tolerance-in-motion” study on the gap between the bolt and the hole of the busbar to detect potential collision problems. So we will study the -3 sigma value around the 360° of the hole. From there, Enventive Concept will tell us if there is a collision problem anywhere within the entire circumference.

Adding a chart, we can visualize all the negative values that we have and identify the worst position for a possible collision. In this case that seems to be at around 16° for an interference of -.84. From that, we can position the analysis at 16°. and see the list of contributors that we have for the variation.

Optimizing contributors to meet Failure Rate objective

Now, let’s proceed with the optimization.

By taking advantage of Concept’s reporting of failure rates, as measured by the statistical Cpk value, and a ranked list of contributing parameters to the Cpk values, mechanical designers can rapidly iterate to make optimal tolerancing decisions.

A first solution to fix the problem would be to simply enlarge the hole on the busbar. The “tolerance-in-motion” study shows that 7.9 would avoid any interference. So if we use this value and update the report we see that we have an assembly failure rate, as measured by Cpk, a statistical calculation parameter, of 1.

But this design cannot afford having such a big hole — it can only afford 7.6 and still meet functional requirements for electric current. So if we use this value and update the Cpk, we see that we are down to 0.85, which would be an unacceptable failure rate.

So we need to change something else. Looking at the contributor list, it can be seen that the “pin in hole” is positioning the plastic block along with the concentricity on the second plastic block are responsible for these dispersions. So directly from the report, we can enter a smaller value on the diameters and also tighten the value of the concentricity and now the Cpk is back to a value very close to 1, which in this example is considered optimal.

The result is that with only a few intuitive iterations we achieve the targeted assembly failure rates while maintaining the functional performance of the busbar for carrying electrical current loads.

Interested in knowing if Enventive Concept could help you tackle your variational challenges ? Chat with us to figure out.

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