Why Aren’t Tighter Tolerances Always Better for Aviation Parts? — Understanding Precision Requirements Through “Design Intent”

Having been in aviation parts machining for over 25 years, the most regrettable thing we have seen is a designer clearly needing ±0.03 mm, but the drawing specifies ±0.005 mm.
The result: machining time doubles, the scrap rate jumps from 2% to 15%, and delivery is delayed. And that tight tolerance? It wasn’t utilized at all.

Article Outline
A Real-Life Case of “Over-Engineering”

Tighter Tolerances Don’t Lead to Linear Cost Increases—They Skyrocket

Why Are Tolerances for Aviation Parts Often “Excessively Tight”?

How Is “Design Intent” Reflected on Drawings?

Three Practical Tips for Designers

A Few Final Straightforward Truths

I. A Real-Life Case of “Over-Engineering”
Two years ago, we took on a project for an aviation connector bracket. It was made of 7075 aluminum, measuring 100×80×30mm. The drawings specified a positional tolerance of 0.005mm for the three mounting holes.

I asked the engineer, “Are you sure we need 0.005mm?”
He replied, “It’s an aviation part—the tighter the better.”

We went ahead with it. To meet that 0.005mm tolerance, we scrapped four parts before producing a single合格 piece. We quoted a unit price of 3,000 yuan, but the client found it too expensive. Later, another engineer dug up the original design documents and discovered that this bracket was merely for securing a wiring harness—a 0.03mm tolerance was perfectly sufficient.

After changing it back to 0.03mm, the unit cost dropped to 800 yuan, and the scrap rate was nearly zero. This isn’t an isolated case. The workshop is full of drawings with tolerances “set by feel,” and in the end, they all turn into real, hard-earned money wasted.

II. As Tolerances Tighten, Costs Don’t Rise Linearly—They Spike
Many people assume that doubling the tolerance doubles the cost. That’s not actually the case. When tolerances enter the precision machining range, costs rise exponentially.

Take 7075 aluminum milling as an example:

±0.1mm: Standard milling, caliper measurement, cost: 1x

±0.05mm: Precision milling + careful tool setting, cost: 1.5x

±0.01mm: Precision machining + temperature-controlled workshop + compensation, cost: 3–5x

±0.005mm: Ultra-precision + 100% CMM inspection + high scrap rate, cost: 8–12x

Why the sudden cost surge?

Cutting parameters are reduced to 1/3–1/5 of the original, while labor costs double

Tool wear of just 0.01mm causes out-of-tolerance, shortening tool life by 5–10 times

CMM inspection takes 20 minutes per part; for a batch of 500 parts, that’s 160 hours

A temperature fluctuation of just 2°C can cause a 0.005mm drift, requiring a temperature-controlled workshop

Scrap rates jump from 2% to over 15%

In aerospace parts, bearing seats and gear meshing surfaces do indeed require micron-level tolerances. But for housings, brackets, covers, and wiring harness clamps—they really don’t.

III. Why Are Aerospace Parts Prone to “Excessively Tight Tolerances”?
First, the inertia of “safety first.” Engineers instinctively believe that “the tighter the tolerance, the safer it is.” In reality, overly tight tolerances do not make parts safer; they only make them harder to manufacture. True safety comes from strictly controlling critical features while relaxing non-critical ones.

Second, a lack of understanding of manufacturing capabilities. Many designers have never spent time on the shop floor and do not understand what ±0.1 mm versus ±0.01 mm means for machining. They assume “just writing it down is enough,” unaware that the tighter the tolerance, the more time the shop floor must spend.

Third, the “contagion” of template copying. If the previous drawing specifies ±0.005 mm, the next one is copied directly without anyone questioning it. Over time, tolerances become “inflated.”

Fourth, how should “design intent” be reflected on drawings?
“Design intent” means: What exactly is this dimension for?

Bearing bore diameter → Interference fit → Specify fit grade (e.g., H7/p6)

Center-to-center distance between mounting holes → To allow bolts to pass through → For M5 bolts, a 5.5mm through-hole with a center-to-center distance tolerance of ±0.2mm is sufficient

Housing outline → To ensure it fits into the cabinet without jamming → Leave a 0.5mm clearance; a tolerance of ±0.2mm is sufficient

Seal groove → Ensures the seal is compressed without being squeezed out → Requires tight tolerances, but compression must also be considered

Specific practices:

Mark non-critical dimensions as “reference dimensions” or “non-critical features.”

Mark critical mating surfaces with fit standards (ISO 286)

Add an “undimensioned tolerances” box in the corner of the drawing (e.g., ISO 2768-m)

Include a “List of Critical Features” for complex parts

When receiving drawings, a responsible supplier will ask: “Is this 0.005 mm a functional requirement or a customary notation?” If they do not ask, you should be cautious.

V. Three Practical Tips for Designers
If you’re drafting drawings for aerospace parts, these three tips could save you tens of thousands of dollars in machining costs:

1. Specify tight tolerances only where “truly necessary.”
Ask yourself three questions: If this dimension deviates by 0.1 mm, will the part fail? Will it affect the assembly? Will it cause a safety issue?
If the answer to all three is “no,” then specify ±0.1mm or even wider. Don’t automatically apply tight tolerances just because the part is labeled “aerospace.”

2. Learn to Use “Geometric Tolerances” Instead of “Dimensional Tolerances.”
Many designers habitually use positive and negative dimensional tolerances to control position, such as “30±0.01.” However, a better approach is to use Geometric Dimensioning and Tolerancing (GD&T), which allows for some compensation and is easier to machine. For the same functional requirement, a positional tolerance of 0.03 mm is much easier to achieve than a dimensional tolerance of ±0.01 mm, because positional tolerance provides a circular tolerance zone, whereas dimensional tolerance provides a rectangular zone.

3. Communicate with the machining shop in advance
Before finalizing the drawing, send the sketch to a few reliable CNC shops and ask them: “Do you think this tolerance is reasonable? Where can we relax the tolerances? Have you manufactured similar parts before?”
An hour-long phone call could save you weeks of rework and tens of thousands of yuan in machining costs.

Bonus Tip: Build a Tolerance Database
If your department frequently designs similar parts, compile the tolerance requirements from successful projects into an internal database. When starting a new project, refer to historical data instead of guessing from scratch every time.

VI. A Few Hard Truths to Share
Aviation parts do indeed require high precision, but not every feature does.
An excellent drawing isn’t one where every tolerance is set in stone; rather, it imposes strict requirements where it matters most and allows flexibility where it doesn’t.
I’ve seen too many tragedies in the workshop caused by overly strict tolerances:
Parts are manufactured but fail inspection, even though they actually assemble perfectly fine.

An entire batch is scrapped, and materials are reordered just for a 0.005mm tolerance.
Suppliers refuse to quote when they see tight tolerances, leaving projects unable to find manufacturers.

Next time you’re specifying tolerances, pause and ask yourself: Is this 0.005mm really for the sake of aircraft safety, or just to ease my own mind?
If it’s the latter, change it. Your suppliers and your project budget will thank you.