A Comprehensive Guide to Tolerancing Terms Relating to CNC Machining Services

Tolerancing Terms

A Comprehensive Guide to Tolerancing Terms Relating to CNC Machining Services

Unusable parts. Wasted materials. Delayed deliveries. Increased costs.

These issues can—and do—occur when one of two things happens: the manufacturer makes a machining error, or the engineer or designer neglects to accurately define the part’s critical tolerances.

During precision machining, a tolerance deviation of merely a few thousandths can upend a job and render the parts useless. 

Thankfully, tolerancing errors are completely avoidable! How? By ensuring that your engineer or designer has adequately defined the allowable deviations from the part’s nominal values. 

Here, we’ll discuss tolerances for CNC machining services and the terms you should know before finalizing your part’s design for manufacturing.

An Overview of Machining Tolerances

For the purposes of this primer, we’ll focus on tolerancing as it relates to a feature of size. A feature of size is a geometric shape defined by size dimensions, such as length and angle.    

The machining tolerance is the permitted variance from the part’s nominal dimension. We trust our customers to define the maximum and minimum dimensional limits for each feature within their part. (If you need help fine-tuning your design for manufacturing, let us know!)  

This variable dimension is called the tolerance band. If your specifications permit a significant difference between the feature’s upper and lower dimensional limits, you’ve defined a loose tolerance band. A tight tolerance band means there’s a small difference. 

More than a measurement

Your tolerance bands help us determine which manufacturing services are best for your part’s various features.

For example, we can drill a hole with a loose band tolerance. A tight band tolerance, however, would require boring or reaming to achieve the desired results.

Accurate tolerance ranges help ensure a perfectly fitted part. And when you clearly specify your required tolerances, you allow your manufacturer to quote an accurate price for CNC machining services. 

Under-specifying tolerances can result in parts that aren’t fit for use. However, on the other end of the spectrum, over-specifying tolerances can drive up costs and lead times unnecessarily. In most cases, you don’t need to specify tolerances for every part feature.

At Alchemy Industrial, we advise our customers to prioritize design time for defining a part’s critical tolerances. A bit of extra work on the front end will ensure a top-quality product that fits, functions, and makes everyone proud—without being prohibitively expensive. 

3 Tolerancing Terms Every Designer and Engineer Needs to Know

Designers and engineers commonly include three basic tolerances on their working drawings. Adopt these terms to enable clear communication across teams, departments, and manufacturing services vendors:

1. Bilateral toleranceRequest a quote

A bilateral tolerance permits variation above and below the feature’s nominal size, with variance in both directions. The variances don’t necessarily have to be equal, but most drawings show bilateral tolerances with equal variation in both directions.

Express the upper limit of the variance with the + symbol and the lower variance with the – symbol. For example, a bilateral tolerance of ten-thousandths of an inch should be written as +/- 0.0001”.

2. Unilateral tolerance

A unilateral tolerance permits variation above or below the feature’s nominal size. It does not permit variation in both directions, meaning the size may only deviate toward a looser or smaller tolerance band. 

Express unilateral variances with only one symbol: either the + or the -, not both. For example, a unilateral tolerance allowing ten-thousandths of an inch in one direction might be written as +0.0001”/-0.000”.

3. Limit tolerance

With a limit tolerance, the allowable upper and lower dimensional values are specific and non-variable, identifying a nominal dimension rather than an allowable deviation. 

A limit tolerance would appear as: 1.0001”/0.999”, with no + or – symbols.

Quick Guide to Common Tolerancing Vocabulary

When drafting or discussing a design, everyone involved should be familiar with these standard tolerancing terms: 

Basic size: The diameter of the bolt, shaft, or hole

Upper deviation: The difference between the part’s maximum possible size and its basic size

Lower deviation: The difference between the part’s minimum possible size and its basic size

Total tolerance: The value that describes the maximum amount of variation

International tolerance grade: The maximum size difference between the component and its basic size

Fundamental deviation: The minimum size difference between the component and its basic size

Maximum material condition (MMC): The maximum amount of material that can be contained within tolerance; the part is at its heaviest at MMC

Least material condition (LMC): The least amount of material that can maintain the part’s tolerance; the part is at its lightest at LMC 

Allowance: In reference to mating parts, the allowance is the minimum required amount of clearance and the maximum allowed amount of interference

Datum: Tolerances for critical assembly features may reference specific datum or datums, or an exact plane, line, axis, or point location using the GD&T system

Aligning Terms to Ensure Proper Fit and Function

When designers, engineers, and manufacturers speak the same CNC machining language, the result is a completed assembly that functions as it should, with critical features that fit thanks to accurate tolerances.

Our manufacturing services team at Alchemy Industrial stays on top of the latest technology and terminology to ensure seamless communication and impeccable design for manufacturing every time.

Stick around for future tolerancing-themed articles that will help you get your next part from prototype to production without delays, errors, or unanticipated expenses.

Come create with us! Request a quote today.