How to Choose Tolerances to Optimize the Manufacturing Process

Tolerances can make or break the manufacturing process. When you choose the right tolerances for your product, the manufacturing phase often runs incredibly smoothly and produces a high-quality end result. But if you don’t know how to choose tolerances, this process may not go according to plan. For example, if you manufacture 5,000 components that are meant to fit inside other components, tolerance miscalculations can lead to fit issues. You would either have to manufacture all 5,000 components again from scratch or try to salvage what you have—a process that can delay your time to market and put a serious dent in your budget.

This is why it’s essential for product designers to learn how to choose tolerances long before manufacturing begins. The earlier you begin thinking about tolerances, the easier it will be to hit your target deadlines and manufacture your product at the lowest possible cost. In this guide, you’ll learn how to talk about tolerances with contract manufacturers based on their capabilities so you can avoid manufacturing setbacks.

Why Choosing Tolerances Can Be Challenging

The biggest mistake product designers make is creating a drawing or model of the design without including tolerances. This is called a “nominal” dimension. But to optimize the product development process, you also have to include a little flexibility—tolerance—in the design. No manufacturing tool is 100 percent accurate and precise every time, so tolerances take this into account and ensure these small inaccuracies don’t cause bigger problems down the line.

However, deciding how to choose tolerances is a complex process. This isn’t a “paint by numbers” sort of task, as it requires you to think very carefully about all of the details involved in the design as well as the contract manufacturer’s capabilities. This may include, but isn’t limited to:

• The physical properties of the design (size, material, color, shape, weight, etc.)
• How the end product will ultimately be used
• How precise and accurate the execution of the design has to be
• The tools used to manufacture the design
• Whether components have to interact with other components
• The assembly process

With very rare exceptions, every product must be controlled using a tolerance measurement of some kind. Even relatively simple designs usually have at least one feature that engineers create a tolerance for, as this ensures product consistency. However, this process isn’t always easy. There are a number of common mistakes product designers make when they try to define tolerances on their own.

One Tolerance Inconsistency You Should Avoid 

Incorrect tolerances can throw a wrench in the works. For example, if you have too many defined tolerances in your design, you could produce a problem called tolerance stacking. This is especially common when you define tolerances for multiple dimensions in a row, like in the example below.

The problem is that the placement of each hole depends on the placement of the other holes around it. So if every hole is shifted to the right by just a small amount (well within the defined tolerance), this small variation adds up until the last hole is placed in entirely the wrong spot, as shown below.

Experienced engineers and product designers know how to choose tolerances that don’t lead to stacking. For example, many engineers and designers use ordinate dimensions when they make models or drawings. An ordinate dimension is one with a fixed point that you can use as a reference for each tolerance. Because the point of reference stays exactly the same, the holes will only vary in placement by a very small amount.

Starting with a solid point of reference, instead of basing tolerances on other points with tolerances of their own, is one of the main ways to avoid future manufacturing problems. But there are other methods you can use to ensure the contract manufacturer understands your vision for your product and is able to produce the most consistent finished products possible.

How to Choose Tolerances: A Step-by-Step Guide

Deciding how to choose tolerances doesn’t take a lot of time if you work with experienced engineers. You also don’t need to be an expert on tolerances to get started. Engineers can take an existing drawing or model you have and apply the correct tolerances to it based on the information you provide about the end product’s functionality. They can also select appropriate tolerances based on their tools and capabilities.

However, it’s still advisable to come prepared with at least some information and tolerance estimations before you meet with engineers. Here’s a step-by-step guide you can follow:

Step 1: Identify the tolerances that matter	Tell the engineer exactly what you want your end product to do. Be specific. Do you want one component to turn in a full circle? If so, you need to define a clearance tolerance so that part doesn’t get caught on anything. Does a tube have to fit inside another tube during assembly? The engineer needs to know how tight to make this fit so it’s easy to assemble but hard for end-users to accidentally pull apart.
Step 2: Perform a cost-benefit analysis	Tighter tolerances cost more to achieve, so weigh your options carefully. The goal is to make a tolerance that is just loose enough to reduce complexity, manufacturing time, and product development cost, but that is tight enough to ensure the part actually works.
Step 3: Consider your materials	Some raw materials expand and contract when exposed to different temperatures or moisture levels. So if you know that you’ll be storing the material in a hot warehouse after cutting it to size and that material is likely to expand under these conditions, you should define a new tolerance based on this factor. An engineer will walk you through this step.
Step 4: Consider your manufacturing tools	Different manufacturing tools and capabilities require different tolerances. For example, when you weld something, your tolerances must be slightly higher than if you use CNC machining. That’s because welding can increase the amount of material in one area of the joint and create an uneven finish, making a tighter fit. CNC machines are more precise, so you can afford a tighter tolerance.
Step 5: Consider the manufacturing process	The order that you make the parts and how you treat them also has an impact on tolerances. So, if you want to add metal plating to a part or finish it with a primer, you’ll need a new tolerance for this additional layer of material. Even seemingly minor details add up over the course of manufacturing.
Step 6: Account for tolerance slack	Tolerance slack is the total maximum or minimum tolerance of a specific dimension or measurement. Let’s say you have five components welded together side-by-side, each with a tolerance of ±0.5 mm. The tolerance of each part might be relatively small, but the tolerance of the total length is much greater at ±2.5 mm. If the total length of the product is important, then you have to tighten up the tolerances of each individual component.
Step 7: Put it all together	With all of these calculations in mind, start defining tolerances in your model or drawing. It doesn’t have to be perfect at this stage—you just need to show engineers that these tolerances are important to you. You can also check for problems like tolerance stacking during this step.

A skilled engineer and contract manufacturer should calculate all the necessary tolerances for you. However, it is helpful to have some knowledge of how to choose tolerances, as this allows you to participate more in these conversations and collaborate with engineers to find solutions during the design stage.

Discuss Tolerances with Your Contract Manufacturer   

Deciding how to choose tolerances isn’t something you can necessarily do on your own, especially if you have little to no experience manufacturing products. To define tolerances correctly, you need to work with a contract manufacturer that appreciates the value of tolerances and has experience including them in the design for manufacturing purposes.

This step-by-step guide can help facilitate the conversations you have with the contract manufacturer. When you think about what’s involved in this process and you work out some of these calculations for yourself, it gives your contract manufacturer more information to work with. Great manufacturing is a collaborative process, and by estimating some of your own tolerances, you can show manufacturers which parts of the design you consider most important and what you would like to focus on achieving with the end product. Communicating your needs early is a great way to produce a high-quality product that you can be proud to call your own.