DESIGN FOR MANUFACTURING (DFM) GUIDE

A Practical Guide to Tolerances

Specifying tolerances is one of the most cost-sensitive aspects of part design. This guide explains how to apply tolerances wisely to create functional parts without unnecessary expense.

What is a Tolerance?

No manufacturing process is perfectly exact; there will always be some small, acceptable variation in the final dimensions of a part. A **tolerance** is the total permissible amount a dimension can vary from its nominal (ideal) value.

For example, a hole dimension specified as 10.00 ±0.05 mm means the final hole diameter is acceptable as long as it measures between 9.95 mm and 10.05 mm. Tolerances are essential for ensuring that parts in an assembly will fit together and function correctly.

A CMM machine inspecting the precise dimensions and tolerances of a manufactured part

The Cost of Precision: Standard vs. Tight Tolerances

The tighter the tolerance, the more expensive the part. This is the most important rule of thumb in machining.

Our Standard Tolerance

HyProto's standard machining tolerance is **±0.125 mm (±0.005")** for metal parts and ±0.25mm for plastics. This is achievable with standard tooling, setups, and inspection methods. For over 90% of features on a typical part, this tolerance is more than sufficient. You should always default to the standard tolerance unless a feature is truly critical.

Tight Tolerances

Tolerances tighter than our standard require special manufacturing steps that increase cost, such as: slower cutting speeds, multiple finishing passes, specialized tooling, and advanced inspection methods (like CMM). Only apply tight tolerances to the specific features that absolutely require them, such as bearing bores, press-fit pin holes, or critical mating surfaces.

Tolerancing FAQ

How do I specify tolerances on my drawing?

The best way is to include a title block on your 2D drawing that states the default tolerance for all dimensions (e.g., "UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE ±0.125 mm"). Then, for any critical features that need a tighter tolerance, you can specify it directly on that dimension (e.g., a bilateral tolerance like ±0.02 mm, or limit dimensions like 10.00 / 9.98).

What is GD&T?

GD&T (Geometric Dimensioning and Tolerancing) is a symbolic language used on engineering drawings to define the allowable deviation of feature geometry. It goes beyond simple size tolerances to control form (flatness, straightness), orientation (perpendicularity, parallelism), and location (position, concentricity). It is a more precise way to define a part's functional requirements.

If I don't specify any tolerances, what will I get?

If no tolerances are specified on a drawing, we will manufacture your parts to our standard tolerance of ±0.125 mm for metals. This ensures a consistent and high-quality result for every order.