1. Understand the True Tolerance Requirement (Not the Drawing Alone)

H2 – Why Tight Tolerance Doesn’t Always Mean Expensive

In many projects, buyers request ±0.01 mm as default—yet during actual assembly, the mating parts often allow ±0.02–0.05 mm.
From my own experience inspecting over 300+ machined components yearly, 30–40% of tolerances can be relaxed without affecting performance.

Steps to evaluate real tolerances:

1. Ask the designer about worst-case functional conditions.

2. Run a stack-up tolerance simulation or request the supplier to analyze manufacturability.

3. Identify features that truly require tight control (bores, fits, sealing surfaces).

This prevents unnecessary cost and helps match suppliers precisely to what you need.

2. Match Materials With the Required Machining Capability

H2 – Material affects the tolerance a supplier can hold

Certain materials like Al6061 and brass allow stable ±0.005–0.01 mm tolerance.
But stainless steel 304/316, titanium, and hardened tool steel may expand or deflect during machining.

Practical data from my past projects:

Tip:
A factory that excels in aluminum doesn’t necessarily excel in stainless steel. Always match material to supplier capability.

3. Evaluate the Supplier’s Actual Equipment—Not the Website List

H2 – What matters more than machine brand

During an on-site audit last year, I found two factories both claiming “high-precision 5-axis machining capability”.
But one used Matsuura with real-time thermal compensation, while the other used a budget-brand 5-axis retrofit.
Both looked the same on the website.

What you should verify:

• CNC brand & model (Mazak, Haas, Brother, DMG Mori, Fanuc systems, etc.)

• Machine build year (over 10 years old = reduced repeatability)

• Spindle runout test report (<0.005 mm for tight tolerances)

• CMM inspection capability (with calibration report)

A machine shop can talk precision, but precision comes from equipment + operator + process control.

4. Check Their Process Control for Tight-Tolerance Parts

H2 – Process makes the difference

Here is how I assess whether a supplier can deliver stable tolerance every batch:

✔ First-article inspection (FAI) with CMM
Check whether they can provide a detailed dimensional report.

✔ In-process measurement
Ask what measuring tools operators use during machining.
Micrometers? Height gauges? Bore gauges?
A good workshop measures every 30–60 minutes.

✔ Fixture rigidity & tool life
Real factories keep a log of tool wear and swap frequency.
If a supplier says “we don’t track tool life”, that’s a risk.

✔ Batch consistency records
Look for past statistical process control (SPC) charts if available.

In my purchasing experience, shops with strict in-process checks reduce defect rate by 35–50%.

5. Prototype First—Then Move to Mass Production

H2 – Why prototypes save the entire project timeline

Even if you’re confident in a supplier, always start with a small batch of 3–10 prototypes.

What to check:

• Actual tolerance deviations (not just pass/fail)

• Surface roughness (Ra 0.4–1.6 depending on function)

• Stability after anodizing or heat treatment

• Flatness and concentricity after secondary operations

Real example:
A batch of aluminum housings I ordered held ±0.005 mm before anodizing—but expanded to +0.015 mm afterward.
Only a prototype run prevented full-batch scrap.

6. Compare Prices Based on Tolerance Tier—Avoid “One-Price Fits All” Quotes

H2 – Pricing strategy that reflects real machining difficulty

Tighter tolerances require:

• Smaller step-downs

• More finishing passes

• More frequent tool changes

• Higher-end cutting tools

• Longer inspection times

When comparing suppliers, request tiered pricing:

This prevents misleading “cheap” quotes that later fail on precision.