In the world of precision machining, it’s easy to fall into the trap of thinking: “The tighter the tolerance, the better the quality.” While tight tolerances are essential for certain high-performance applications, they’re not always the smartest—or most cost-effective—choice in custom metal part manufacturing.
In fact, over-specifying tolerances can lead to increased costs, longer lead times, and unnecessary manufacturing complexity, without any real benefit to functionality.
Let’s explore why “as tight as necessary, as loose as possible” is often the golden rule in tolerance design—and how thoughtful tolerance planning can add value to both suppliers and customers.
What Is a Tolerance?
A tolerance is the allowable deviation from a specified dimension. In manufacturing, it defines the acceptable range within which a part can vary and still function correctly in its intended assembly or system.
For example, a hole diameter of Ø20.00 mm ±0.05 mm means the actual hole can range from 19.95 mm to 20.05 mm and still be within tolerance.
Why Tighter Tolerances Cost More
While advanced CNC machines can hold tolerances within microns, every reduction in tolerance range increases machining time, tool wear, inspection requirements, and overall cost. Here's why:
l Increased Machining Time: More passes and slower feed rates are needed.
lTool Life Reduction: Tools wear faster when working under strict tolerances.
lHigher Rejection Rates: Even minor variations result in scrapped parts.
lAdditional Quality Control: More frequent inspections, more specialized gauges.
For example, moving from a tolerance of ±0.10 mm to ±0.01 mm may double or triple the machining cost, depending on the complexity of the part.
Function-Driven Tolerance Design
Instead of defaulting to tight tolerances, good engineering practice involves evaluating:
l Fit Requirements: Does the part need to slide, press-fit, or rotate?
l Mating Components: What are the tolerances of the parts it connects to?
l Operating Environment: Will temperature or stress affect the material?
l Material Properties: Different materials respond differently to machining and stress.
l Production Volume: Is this a prototype or a high-volume run?
A shaft that rotates inside a housing doesn’t need the same tolerance as a fuel injection nozzle—function dictates tolerance.
Real-World Tip: Collaborate with Your Manufacturer
At Densen Group, we often help our clients optimize their tolerance requirements based on real-world production experience.
By working together during the quoting or DFM (Design for Manufacturing) phase, we can:
l Recommend achievable tolerances without compromising function
l Suggest design adjustments that improve manufacturability
l Reduce lead time and cost while maintaining part performance
In many cases, relaxing non-critical tolerances can result in faster delivery, better consistency, and significant cost savings.
Conclusion: Precision with Purpose
Precision machining is all about delivering parts that perform as needed, when needed, at the right cost. And tolerance design plays a critical role in balancing these goals.
So next time you design a part, remember:
Tighter isn’t always better—smarter is better.
By aligning design intent with manufacturing reality, we can achieve the perfect balance of quality, functionality, and efficiency.