Why are Tolerances Important in Manufacturing?

There’s a common saying in manufacturing, “Measure twice and cut once.” But how do you accurately measure parts for a complex product like an autonomous oceanographic vehicle or a telecommunications device? With so many moving parts, it can be hard to wrap your head around all of the individual component dimensions and how they’re meant to fit together in the end product.

This is why tolerances are important in manufacturing. Tolerances determine exactly how much room for error you have when you manufacture each part. When you appreciate the vital role that tolerances play in the manufacturing process, you’ll design higher quality products and make fewer costly manufacturing mistakes.

What Is Tolerance? 

Tolerance is a defined range of measurements or other physical traits that enable the product to function properly and/or look aesthetically pleasing. In most cases, the tolerance refers to the minimum or maximum range of measurements of a product in X, Y, and Z dimensions. However, you can also express tolerance in terms of color, texture, shape, or profile.

For example, if you’re designing a product that has geometric features like holes in a part, the manufacturer needs to know certain tolerance details before they can create the part, including:

• The nominal diameter of the hole—can it be smaller or larger in diameter in the end product, and if so, how much larger or smaller can it be before it affects core functionality?
• Exactly where the hole has to be placed in X, Y, and Z dimensions—how much variation in placement is allowed for the part to still function properly?

The more detailed these estimates are, the easier the product will be to manufacture. This is why tolerances are important in manufacturing. Defining them gives you greater control over the consistency, accuracy, precision, and quality of your products.

Why Tolerances Are Important in Manufacturing 

Just about every product has some feature that needs to be controlled through tolerance measurements. There are six main reasons why tolerances are important in manufacturing:

1. They improve the fit and functionality of parts: If you’re making parts that are compatible with other components, you absolutely have to define your tolerances. Even a small mismeasurement or variation in size will make your product functionally useless or incompatible.

2. They improve the look of the end product: If aesthetics are important to you, tolerances should be too. For instance, if you want a part to sit flush up against another part so there’s no obvious gap between them, you need to carefully control both parts’ dimensions and positions.

3. You can account for a reasonable amount of error: Tolerances assume a certain amount of error, but only to the extent that the part remains functional. When you define your tolerances from the start, you’re less likely to have to remake parts later.

4. Manufacturing is more cost-effective: When you define your tolerances, a product is only as precise as it needs to be. This means you’ll only pay for the materials, manufacturing tools, and labor that you actually need to get the desired end result.

5. You’ll get products to market faster: Although it takes a little longer to work out the tolerances up-front, this extra step saves you time in the long run. Mismeasurements and inconsistent products can delay your time to market by weeks or even months while you wait for new parts to be made.

6. Tolerances reduce manufacturing ambiguity and complexity: Ambiguity is the enemy of manufacturing. If you don’t specify what you want, you may end up with a product that you can’t use. Tolerances remove all ambiguity from the equation. Tolerances also make manufacturers’ lives easier. For instance, if your product doesn’t need to be accurate within ±0.002mm, then there’s no reason for manufacturers to break their backs trying to achieve this high level of precision.

Is there any reason not to define tolerances when you design a product? The answer is no. Defining tolerances is a no-brainer no matter what type of product you’re designing. The hard part is deciding which tolerances matter most and how tight or loose those tolerances should be.

Weighing the Costs and Benefits of Tolerances

There are many reasons why tolerances are important in manufacturing, but tolerances can also drive up the cost of production depending on how “tight” or “loose” they are.

A tight tolerance allows for relatively minimal variation in size, position, or other physical properties, while a looser tolerance permits a little more leeway. However, this precision comes at a steep price. In general, the tighter the tolerance, the more the product will cost to manufacture. That’s because it takes more time, skilled labor, and expensive tools to create products with a high degree of precision and accuracy.

So if loose tolerances cost less, why don’t all product designers use them by default? That’s because some products require tighter tolerances to function properly. For example, if you need to manufacture a very precise moving part that will fit snugly inside another part without causing friction, then you probably need to set a very tight dimensional and radial tolerance. But if a part doesn’t move and doesn’t have to fit inside another component, then the tolerance can probably be a little looser without negatively affecting its core function.

A skilled contract manufacturer will help you estimate the most cost-effective tolerances for your products. However, before you manufacture your product, you have to consider more than just how tight or loose you’d like your tolerances to be. There are certain challenges that come with estimating tolerances, which is why it’s important to think carefully about what you need.

How to Avoid Common Tolerance Challenges

Although it’s easy to see why tolerances are important in manufacturing, that doesn’t mean the process is simple. For instance, it is possible to underestimate or overestimate tolerances. These miscalculations usually come from:

• Inexperience with materials or manufacturing tools: If you try to define tolerances yourself or you work with an inexperienced product designer, your tolerances may be way off the mark. Accurate tolerance calculations depend on a number of factors that are specific to your product and the manufacturing tools you use. For example, tolerances are very different when you weld metal vs. using a CNC machining tool to cut it. Your tolerances can generally be tighter when you use a CNC machine because the process is inherently more precise.
• A shallow understanding of the manufacturing process: Another common mistake is forgetting to adjust tolerances at each step of the manufacturing process. For instance, if you coat a part in primer or metal plating, then you have to make a new tolerance that takes this extra layer of material into account. Defining tolerances requires a deep and detailed understanding of every single step in the manufacturing process and what order they are performed in.
• Missing tolerances: If you want to fit one part inside of another, you need to define tolerances for both of these parts. Failure to define even just one important tolerance measurement can lead to product failure down the line.
• Lack of clear direction: Defining all of the necessary tolerances requires you to know which ones are important. If you don’t tell your product designer or manufacturer exactly what you need the product to do or how you want it to look, then they can’t accurately estimate tolerances.

This is why it’s important to work with an experienced contract manufacturer you can trust. Skilled contract manufacturers have a deep understanding of materials, manufacturing tools and methods, product functionality, and aesthetics.

The Number One Thing You Should Tell Manufacturers

If you want to see firsthand why tolerances are important in manufacturing, you need to make sure you and your contract manufacturer are on the same page. This means expressing your expectations in as much detail as possible. When you get in touch with a contract manufacturer, start by having an in-depth conversation about what you want to accomplish with your product.

Ask yourself:

• Are there any moving parts?
• Do you need to make multiple parts and assemble them later?
• Is precision important in your industry, and if so, how important is it?
• Is the weight of the product important, and if so, how much would you like it to weigh?
• What materials are you planning on using?
• What’s your manufacturing deadline?
• Is there a specific aesthetic you’re going for? Are you particular about the color, texture, shape, or profile of your product?

The bottom line is if a design detail matters to you, then you should talk to your manufacturer about defining a tolerance for it. One of the biggest reasons tolerances are important in manufacturing is all the valuable information they provide manufacturers to develop a top-quality product. It’s the manufacturer’s responsibility to ensure that each part falls within tolerances. Even if you know next to nothing about defining tolerances, when you work with a manufacturer that understands and values this important step in the process, you’ll create consistent products that drive your industry forward.