The Three Questions That Separate Real Quotes From Guesswork
Before placing a CNC order, ask your supplier three questions: what is your standard tolerance, how do you price tighter tolerances, and how do you prove parts are within spec. Most sourcing problems trace back to one of these three points being left vague on the purchase order. Not all tolerances are created equal; the table below shows what each tier costs and what inspection backs it up.
| Tolerance | Cost Impact | Typical Inspection | When You Need It |
|---|---|---|---|
| ±0.1 mm | Standard (baseline) | Calipers / visual | Non-functional surfaces, brackets, covers |
| ±0.05 mm | +10–20% | Micrometer / height gauge | General machined parts, most fit surfaces |
| ±0.02 mm | +30–60% | CMM spot checks | Bearing bores, locating pins, slide ways |
| ±0.005 mm | +100–200% | Full CMM report | Precision shafts, seal diameters, gauge components |
| ±0.002 mm | Quote dependent | CMM + temperature control | Hydraulic spools, optical mounts, medical implants |
Question 1: What Is Your Standard Tolerance?
Shops do not all work to the same default. Some quote ±0.1 mm on all linear dimensions unless the drawing says otherwise; others hold ±0.05 mm routinely. If your drawing has no general tolerance block, find out what the shop will apply by default before you approve the quote, because a shop that defaults to ±0.05 mm may charge more upfront while a shop defaulting to ±0.1 mm may produce parts that do not fit.
Questions 2 and 3: Pricing Tighter Tolerances and Proving Results
Tight tolerances require slower feeds, additional finishing passes, more frequent tool changes, and extra inspection. A reputable shop breaks this out or flags which dimensions drive cost; be wary of a supplier who says any tolerance is no problem without a price adjustment. For proof, ask what inspection data comes with the parts. A CMM report with measured values for each critical dimension is standard below ±0.05 mm; a shipped stamp or certificate of conformity without numbers does not verify anything.
Common Buyer Mistakes
- Leaving no general tolerance on the drawing, which leaves the shop to guess — some default to loose, others assume you want tight and overcharge
- Telling the supplier verbally to make it accurate without putting numbers on the PO
- Receiving parts without checking the inspection report against the drawing, missing non-conforming dimensions until assembly
- Tightening every dimension uniformly when only two or three features actually need precision
TruPart Precision, a Dongguan-based ISO 9001 CNC shop, quotes standard and tight tolerances separately and provides CMM data on request. Clear tolerance communication is the fastest way to align expectations between buyer and manufacturer.
Frequently Asked Questions
A supplier says ±0.01 mm is no problem. Should I believe them?
Treat it as a starting point for negotiation, not a guarantee. Ask what inspection method they use for that tolerance and whether the price reflects it; ±0.01 mm on a multi-axis part usually requires CMM verification and a price premium.
Should I write tolerance requirements into the contract?
Yes. Put critical dimensions with their tolerances on the drawing referenced by the PO, and specify what inspection report is required. Verbal agreements on precision are not enforceable.
How can I verify a supplier actually held tolerance?
Request a CMM or micrometer inspection report with actual measured values for each critical dimension, and cross-check a random sample with your own calibrated tools or a third-party lab if the batch is high-value.
Is it normal to pay extra for tighter tolerances?
Yes. Tighter tolerances require additional machining passes, slower cutting, specialized tooling, and inspection time, all of which add cost beyond the standard price.