How Tight Is Too Tight? Rethinking Unnecessary Tolerances in Your Design
In CNC machining, tolerances are often where cost and quality collide. I’ve seen countless projects where parts were designed with tolerances so unnecessarily tight that they drove up costs, slowed production, and complicated quality control, without delivering any real benefit to performance.
So, how tight is too tight? Let’s break it down.
The Cost of Over-Engineering Tolerances
Every decimal place you add to a tolerance comes with a price. A part with a ±0.010” tolerance is usually easy to machine and measure. Tighten that to ±0.001” and suddenly:
- Machining requires slower feeds, special tooling, or additional finishing processes.
- Quality inspection must use higher-precision gauges and instruments.
- Scrap rates and rework often rise because meeting the spec consistently becomes harder.
The end result? A part that costs significantly more to produce, even if the extra precision isn’t actually necessary.
The Illusion of Precision
Designers sometimes specify extremely tight tolerances simply because CAD models default to many decimal places, or because “tighter feels safer.” But a critical question should always be asked: Does the function of this part actually require it?
For example:
- A mounting bracket that simply holds a cover plate doesn’t need the same precision as a medical implant.
- A hole designed to fit a bolt may only require clearance, not a line-to-line fit.
- Features hidden inside an assembly may never interact with other components tightly enough to justify microns of precision.
Over-specifying can even backfire, forcing parts into unnecessary secondary operations, such as grinding or EDM, that extend lead times.
When Tight Tolerances Are Worth It
Of course, there are times when strict tolerances are essential:
- Components that must interchange in mass production.
- Precision sliding fits, like in valves or linear bearings.
- Critical safety or performance features where deviation could cause failure.
But these are the exception, not the rule.
Best Practices for Smarter Tolerance Design
- Ask “What’s the function?” – Tie tolerances to the actual needs of performance, safety, or interchangeability.
- Consult your machinist early – A quick review can reveal which tolerances are realistic, affordable, and which are overkill.
- Use standard fits – ISO or ANSI fit classes provide proven guidance for shafts, holes, and clearance requirements.
- Leverage GD&T wisely – Geometric Dimensioning & Tolerancing can control function-critical relationships without over-constraining everything else.
- Balance cost with precision – Don’t let “because we can” dictate tolerances—let “because we must” guide the call.
Final Thoughts
Precision is the heart of CNC machining, but over-precision is one of the fastest ways to inflate costs and slow down production. The smartest designs are those that achieve the right balance: tight enough to perform reliably, but not so tight that you’re paying for invisible accuracy.
Next time you’re finalizing a drawing, ask yourself: How tight is too tight? Chances are, your machinist will thank you!