A recent trend in the precision machining industry is that customers’ tolerance requirements are becoming increasingly tighter. 

While a tolerance of +/- 0.0005” was once considered tight, we now regularly see prints with microscopic tolerances down to +/- 0.0001”. For visual context, a +/- 0.0003” tolerance is like taking a piece of hair, which is about 0.003”, slicing it into ten pieces, and having one of those pieces represent the permitted variance.

Various factors are fueling this trend, but our goal here isn’t to get lost in speculation or finger-pointing. As a solution-focused aerospace machine shop, we’d rather take this opportunity to educate customers about this issue. Because at the end of the day, good communication between shops and customers is key to success.

The Problem with Inspecting Microscopic Tolerances 

Achieving microscopic tolerances isn’t a precision machining challenge so much as it is an inspection challenge. 

When it comes to quality control, shops have rules of 4, 7, or 10 dictating that the accuracy level of the inspection equipment should be 4, 7, or 10 times greater than the tolerance. So if there is a tolerance of +/- 0.0004”, and a shop is looking for 10 times greater accuracy during inspection, we’re talking about 0.00004”—or 1.3% the width of a human hair.

Only the most advanced inspection equipment in the world is that accurate. Many shops’ CMMs are only accurate to 0.0007” - 0.002”.

Are you getting the part you think you’re getting?

If a shop is tasked with creating holes with microscopic tolerances, they can buy the right tool, place it in the best tool holder, and create those holes perfectly. But when they go to check the holes, they could check them five times and get five different measurements. It’s impossible for them to say with certainty that the holes meet the customer’s specifications. 

Maybe the shop’s inspection equipment can verify tolerances of +/- 0.0004” instead of the customer’s requested +/- 0.0001”. The shop will assume the part is acceptable and send it to the customer, but they can’t be 100% certain. And chances are the customer won’t have the ability to inspect the parts better than a CMM at a machine shop. 

This common scenario isn’t doing anyone any good—not shops and not customers. Engineers will continue requesting unrealistic tolerances based on the (incorrect) assumption that the shop is meeting their requirements. Shops will be stuck taking on incredible risk to machine parts with rigid specifications without even being able to confidently verify that they got those parts right during inspection.

At Focused on Machining, we consider ourselves up for most challenges. (Case in point: our aerospace machine shop has mastered some of the most difficult materials.) But we end up no-quoting many parts with microscopic tolerances. Why? Because at the end of the day, we only want to take on projects when we’re 100% confident we can deliver what the customer asked for.

A Smarter Way to Approach Tolerances 

Keep in mind that nine times out of ten, tolerancing is what drives up the cost of a part. So before calling out a microscopic tolerance on your print, ask yourself if it’s truly necessary for your part’s functionality. 

The industry standard tolerances for specific features defined by the American National Standards Institute and the American Society of Mechanical Engineers are sufficient for many parts. The machinists at our aerospace machine shop follow those standards when consulting with customers about their parts, and we advise customers to follow them from the beginning.

For instance, these resources will have standard tolerances for something like an interference fit. You can feel confident that if you adhere to that standard, your part will function well and your shop will be able to meet your specifications quickly and cost-effectively.

At Focused on Machining, we’re always happy to collaborate with customers about the best approach to tolerances. We aim to set every project up for success, which means never promising something we can’t deliver and working with you to ensure you get what you genuinely need. Request a quote to work with us.