Beginner’s Guide to Picking the Right Fit in Engineering

Intro

When you’re designing a part, there are a lot of things to consider: what material will you use, how will you design the features, and what fit will you specify? If you didn’t consider the final question, then you came to the right place.

In this beginner’s guide, we’ll teach you how to pick the right fit in engineering. This will help your parts perform better, and it can save time and money in the machine shop.

What Is a Fit?

In engineering, a fit refers to how a shaft or part fits into a hole or another part. If you want a cylinder to perfectly slide into a drilled hole, then you need to pick the right fit. It’s an engineering concept that’s a little tougher to understand, but we’ll talk about specific examples to help explain this.

With an incorrectly designed fit, your parts won’t go together. Think about threading a bolt into a hole — if the hole is too small, then the bolt won’t fit. The same story is true for putting a bolt through a drilled clearance hole.

When you put together fits and tolerances, you’ll better understand how the assembly will go together. Everything can either assemble with plenty of extra space, slide together perfectly, or require a mallet to get the job done: all three of these examples are different types of fits. 

Different Types of Fits

Let’s review the three fits that we just mentioned. To add correct terminology to the three scenarios we mentioned, these fits are called clearance, transition, and interference. 

Clearance Fit

A clearance fit is something that most engineers know about. This is when the hole is larger than the shaft. When you assemble the shaft, you can jiggle it around and you’ll notice extra space.

As an engineer, you can specify how much extra space you want in the clearance fit. It can be very “sloppy”, meaning there’s plenty of extra room.

A clearance fit is good if you’re making low-precision parts or want to save money on manufacturing. It also makes assembly easier.

Transition Fit

A transition fit means that the shaft and hole are the same dimension. With high-precision engineering builds, like in the military or aerospace industries, you’ll see a lot of transition fits. 

To assemble, you’ll need to add some force. A rubber mallet typically gets the job done if you can’t simply squeeze both parts together.

It results in a high-precision assembly. You could use a transition fit on your alignment pins that ensure both parts are lined up perfectly. 

A transition fit is great if you need accuracy and don’t mind spending more on manufacturing.

Interference Fit

The tightest fit is an interference fit. It’s so tight that you can’t put the shaft into the machined hole without a lot of force. In fact, you typically need an arbor press to put these parts together.

If you look at both parts under a microscope, you’ll see that the shaft is cutting through the walls of the hole, and lodging itself in place. 

Once two parts are assembled in an interference fit, it’s really hard to separate them. For that reason, you should only use this fit for parts that assemble once and stay together for life.

In some cases, you’ll call out heating the hole and freezing the shaft to force the parts together. Any time you disassemble parts in an interference fit, you run the risk of breaking one or both parts.

How to Pick the Right Fit

Picking the right fit is a critical step in your product design. So, how do you do it? Consider the following:

Budget

Each of these fits comes with its own price tag. Clearance fits are the least expensive to make and transition fits are typically the most expensive.

The rule of thumb is whenever tolerances get small, prices get big. When you spec both parts of your fit, consider how tight you need the tolerance to be. 

Accuracy

If accuracy matters a lot, then you’ll have to disregard the budget. In this case, the accuracy would be the driving factor. 

A sloppy clearance hole has the lowest accuracy, and an interference or transition fit typically has the highest accuracy. 

You won’t typically use an interference hole for alignment pins or high-accuracy parts, but the holes can still require a lot of accuracy to machine correctly. 

Application

Finally, consider the application of your parts. If you’re designing a standard desk for the office, then there’s no need to use interference or transition fits. You can use loose clearance fits and everything will work well.

It’s also worth mentioning that the tolerances across your parts should be considered. If all of your tolerances are loose, but then your hole tolerance needs to be an interference fit, you’re in trouble. With loose tolerances, it’s hard to perfectly place the hole on your part — it could be a few hundredths of an inch in any direction. Sloppily-toleranced parts and tightly-toleranced holes do not go well together. 

When your part is precise, then the fit can also be precise. 

Conclusion

Understanding which fit to use can save you time and money during the manufacturing phase. Remember the different types of fits and when to use them in order to make the best parts. If you want a high-quality machine shop in your corner, you can trust Rapid Axis. We’re here to help create high-precision parts with whatever fits you call out. Reach out today for a free quote.