A drawing arrives with a general tolerance note in the title block, a tight-looking value repeated on every dimension, and the words “smooth finish, deburr all edges” at the bottom. Nothing on it is technically wrong. It is also a request for a more expensive part than the buyer needs, because the supplier has to price the worst interpretation of every ambiguous line.
We are a sourcing and freight team in Shenzhen, not a machine shop. Since 2019 we have sat between engineers and Chinese workshops for 4,000+ buyers shipping to 200+ countries. This is not a standards tutorial. It is a guide to the drawing decisions that change a quote, written for the person who has to approve it.
- A general tolerance covers dimensions you did not annotate individually; it is not a precision promise.
- Name the class you intend and confirm it with your own engineer rather than copying a chart from a website.
- Mark critical, functional and cosmetic features separately instead of tightening everything.
- Replace vague notes with measurable ones, per feature.
- State how each critical dimension will be inspected before production starts.
- Ask the supplier to flag cost-driving features at quotation stage.
What a general tolerance is actually for
ISO 2768 exists to save a drafter from writing a tolerance beside every unimportant dimension. Part 1 deals with linear and angular dimensions without individual indications; part 2 deals with geometrical tolerances for features that also lack individual indications. It defines classes, commonly referred to as f, m, c and v, that a drawing can invoke by name.
Three points matter commercially. First, the class applies to dimensions that are not individually toleranced, so it does not override a specific callout. Second, it is a default convention, not evidence that a shop holds a particular accuracy on every feature. Third, the numeric limits associated with each class belong in the standard and in your engineer’s specification, not in a table copied from a supplier blog. We deliberately do not print those values here; confirm the class and its meaning with your own engineer before it goes on a purchase drawing.
If your title block says only “ISO 2768” with no class, you have left the choice to whoever is quoting. That is a pricing assumption, not a specification, and it is one of the easiest things to fix.
Where over-tolerancing comes from
- Reusing an old title block. A note written for a precision assembly gets copied onto a bracket that has no mating surfaces.
- CAD defaults. Model dimensions are exported with a uniform displayed precision that was never an engineering decision.
- Applying a fit tolerance to non-fitting faces. The bore needs it; the outside of the boss does not.
- Unmeasurable notes. “Smooth”, “no burrs” and “high quality finish” are read as risk by a quoting engineer, and risk is priced.
- No distinction between critical and cosmetic. A scratch on a visible face and a bore out of tolerance are treated as the same class of defect.
- Fear of asking questions. Buyers tighten everything rather than discuss which features actually matter, because a conversation feels slower than a note.
The result is a drawing that is easy to reject and hard to quote. A good supplier will either add a risk allowance or come back with questions; a poor one will simply quote the easier interpretation and let the discrepancy surface at inspection.
What a tight tolerance costs
Tolerance is not a free attribute of a drawing. Each step tighter changes how the part is made and how it is proven.
| Consequence | Why it adds cost | What to do instead |
|---|---|---|
| Extra finishing passes | Roughing alone no longer reaches the required accuracy | Restrict the requirement to the features that need it |
| More setups or specialist processes | Grinding, reaming or a different machine may be needed | Review whether geometry can be held in one setup |
| Higher tool wear and scrap | Tight features leave less margin before a part is out of specification | Agree the acceptance criteria and who bears rework |
| Longer inspection time | CMM programs, reports and calibrated gauges take time per batch | List critical dimensions and the inspection method explicitly |
| Fewer capable suppliers | Shops without the equipment or environment decline or subcontract | Match the requirement to the process you actually need |
| A risk allowance in the quote | Ambiguity is priced as uncertainty | Replace vague notes with measurable ones |
Notice that most of these are process consequences, not material costs. That is why two quotes for the “same” drawing can differ so much: one supplier assumed a standard finishing route and the other priced in a CMM report and a second setup. Our quote breakdown shows how to make those assumptions visible.
Mark only what matters
Before a drawing goes out, split its features into three groups.
- Critical. Fits, sealing faces, safety-related dimensions and anything that stops an assembly from working. These get explicit tolerances, datums and a defined inspection method.
- Functional but not tight. Mounting hole positions, overall envelope, clearance features. These get a sensible general tolerance class, named in the title block.
- Cosmetic. Visible faces, edges and finish appearance. These get an acceptance definition agreed against a sealed sample, not a dimensional tolerance.
Use datums and geometric callouts to express how a part actually functions, rather than tightening every linear dimension. A positional relationship between two bores is often what matters, not the absolute accuracy of each bore to an arbitrary reference. Name the datum scheme, then attach the tight callouts only where the function requires them.
Finally, put the critical dimension list into the inspection plan. A first article report should measure every numbered feature, but production inspection needs to know which characteristics are checked on every piece and which are sampled. Our supplier verification guide sets out how we write those gates into a purchase order, including where AQL sampling is and is not appropriate.
Drawing notes that quietly change a quote
| Vague note | How a supplier may read it | Rewrite it as |
|---|---|---|
| “Aluminium” | Whatever grade is on the rack | Grade and condition, with an approved substitute if you allow one |
| “Smooth finish” | A risk allowance, or a finish you did not want | A roughness requirement on the specific feature, “as machined” elsewhere |
| “Deburr all edges” | Manual finishing on every edge, including internal ones | Define which edges, what is acceptable, and which are left as machined |
| “Tight thread fit” | A special gauge or a non-standard process | Thread class, depth, and whether cut or roll-formed |
| “No scratches on visible face” | Every face is cosmetic, with full protective packing | Identify the cosmetic faces and agree acceptance against a sealed sample |
| “ISO 2768” | Whichever class the quoting engineer assumes | Name the class, confirmed by your engineer |
Every rewrite has the same effect: it moves an interpretation out of the supplier’s head and onto the drawing, where it can be quoted consistently by three different workshops.
Ask the supplier to price the tolerance
Add one line to the RFQ pack: “Please flag any feature that increases cost or lead time before quoting.” A shop that replies with two or three specific observations has read the drawing and will usually save you more money than a negotiation does. Ask for a short DFM response covering material, setups, tolerance hot spots and finishing, and ask whether a small geometry change would remove an extra operation.
Then request quantity tiers with one-off setup and tooling itemised, because a tolerance that is expensive on a small prototype run may be trivial on a production run with a dedicated fixture. The full file list and the tiered-quote discipline are in our sourcing checklist.
If a drawing revision changes a critical feature later, treat it as an engineering change with a quoted cost and an approval step, not an informal message. That is the same discipline we recommend for modified tooling in the tooling ownership guide.
When tight really is necessary
None of this is an argument for loose drawings. Bearing fits, sealing surfaces, pressure-containing parts, alignment features and anything safety-related deserve explicit, tight requirements, and those features should never be value-engineered away by a supplier without written approval. The point is that a tight requirement costs less when it is confined to the five features that need it than when it is applied to all fifty.
Agree the measurement method at the same time: which gauge or CMM program, under what conditions, against which datum, and who keeps the record. A tolerance without an agreed method is a future disagreement, not a specification.
Frequently asked questions
What is ISO 2768 used for on a machining drawing?
It supplies a default tolerance class for dimensions that are not individually toleranced, so a drafter does not have to annotate every non-critical feature. Part 1 covers linear and angular dimensions, part 2 covers geometrical tolerances. It does not override an explicit callout and it is not a guarantee of a shop’s accuracy.
Which ISO 2768 class should I specify?
That depends on your part’s function, material and process, and should be decided by your own engineer against the standard text. Do not copy a class or a numeric chart from a website. Name the class in the title block so every supplier quotes the same assumption.
Does a tighter tolerance always increase a China CNC quote?
It usually increases the work required: extra finishing, more setups, more inspection, more scrap risk and fewer capable suppliers. But the increase is often small when confined to a few critical features and large when applied to every dimension. Ask the supplier to identify the cost-driving features before quoting.
Is AQL sampling enough to check tight tolerances?
Sampling is reasonable for general workmanship, appearance and quantity. Critical dimensions need their own plan, agreed before production: either checked on every piece or verified against a documented full dimensional report. Mark those dimensions on the drawing rather than leaving the choice to the inspector.
Should I use GD&T on drawings sent to Chinese suppliers?
Yes, where function depends on the relationship between features. Datums and geometric callouts describe how the part actually assembles, and usually allow wider tolerances on everything else. Give the supplier a clean datum scheme and a critical dimension list so the inspection plan matches the drawing.
Send us your drawing and the quotes you have received. We will have the shortlisted workshops flag the cost-driving features, review the tolerance notes with your engineer, and come back within 24 hours. Send your drawing and we will start with the title block.