The Three Symbols That Cost You Money
You do not need to read every GD&T symbol on a drawing to evaluate a quote, but you do need to know that three symbol groups — true position, profile, and tight form tolerances — drive cost more than any other callout. Flatness and perpendicularity are the most common and usually affordable; it is the composite position and profile frames that consume inspection time.
Quick Reference: What Each Symbol Means for Price
| Symbol | One-Sentence Meaning | Price Impact |
|---|---|---|
| ⌀ Position | A feature (hole, boss) must lie within a cylindrical tolerance zone relative to datums | Medium to high — requires CMM inspection; tighter zones add setup time |
| ⏥ Flatness | A surface must sit between two parallel planes regardless of other datums | Low to medium — achievable on machined faces but requires extra finish passes below 0.02 mm |
| ⟂ Perpendicularity | A surface or axis must be square to a datum within a stated tolerance | Low — standard squaring operations cover this for most applications |
| ⌓ Profile | A curved or irregular surface must match its CAD model within a zone | High — requires 3D scanning or CMM point clouds; often the most expensive callout |
Three Actions When Reviewing a Drawing
- Quickly count the feature control frames (boxed symbols): more than 8–10 on a simple part usually means an over-dimensioned drawing that will cost more
- Before accepting a quote, ask the supplier which single callout adds the most cost — their answer tells you whether they read the drawing carefully
- If you relay GD&T requirements verbally or in email, always attach the actual drawing PDF. Text descriptions of tolerance zones are ambiguous and cause disputes.
Matching GD&T to Supplier Capability
A drawing with ⌀0.05 mm true position or ⌓0.05 mm profile requires CMM inspection. Ask what CMM model the shop uses and when it was last calibrated. A shop without a CMM can still machine parts to tight tolerances, but they cannot prove it — which matters if your customer requires inspection data.
Common Buyer Mistakes
- Assuming tighter GD&T is always better: over-toleranced parts cost 20–50% more with zero functional gain
- Treating GD&T as a negotiating lever: refusing to loosen tight callouts while demanding lower quotes guarantees corners will be cut
- Accepting 'we do not need GD&T to make it' from a supplier: this usually means they do not do formal inspection, not that the tolerance is unnecessary
TruPart Precision is a Dongguan-based ISO 9001 CNC shop that reads every GD&T frame before quoting and flags over-tight callouts. Our glossary explains more engineering terms that come up in CNC sourcing.
Frequently Asked Questions
Does a drawing with more GD&T always cost more?
Not always — many GD&T callouts (perpendicularity, concentricity at generous values) are achievable in standard machining. It is tight position (below ⌀0.1 mm) and profile tolerances that add real cost.
I do not understand GD&T — how do I confirm requirements with a supplier?
Send the drawing unchanged and ask one question: 'Which callouts on this drawing require CMM inspection, and does that change the quoted price?' Their answer tells you what matters.
Is it normal for a drawing to have no GD&T — only ± tolerances?
Yes, it is common and acceptable for simple parts. GD&T becomes important when feature relationships (hole patterns, sealing faces, bearing bores) need tighter control than ± dimensioning can express.
Can a supplier refuse GD&T requirements?
They can and should say if a tolerance is beyond their equipment capability. If they say they can meet it, ask to see a sample CMM report for a similar feature.