DFM checklist
Tolerancing Basics: How to Set Dimensions Suppliers Won't Push Back On
A drawing with no stated tolerances isn't more flexible for a supplier, it's more risky, because they have to guess what matters and price in that uncertainty. Getting tolerance right before quoting avoids both fit failures and paying for precision you don't need.
What tolerance actually controls
A tolerance is the acceptable range of variation on a dimension, and every manufacturing process has inherent variation, from mold shrinkage to tool wear to sheet metal spring-back. Under-tolerancing risks parts that don't fit or seal; over-tolerancing pays a 20-50% premium for precision the product doesn't need and can push a process past what it can reliably hold.
Tolerancing vs. GD&T
Plus/minus tolerancing applies a simple range to a linear dimension and is fine for most non-critical features. GD&T (per ASME Y14.5) controls form, orientation, location and runout, and is worth using selectively on the handful of features, like mating parts, hole patterns and sealing surfaces, where fit or alignment depends on geometric relationships rather than size alone.
The cost of getting it wrong
Under-specifying leaves the supplier to default to their standard shop tolerance, which may be looser than the assembly can tolerate, and the mismatch often surfaces only at first article inspection, after tooling is cut. Over-specifying gets every dimension flagged for clarification or quietly ignored, and it signals to an experienced supplier that the drawing wasn't reviewed by someone who understands the process.
Practical checklist
| Stage | Practical guidance | Why it matters |
|---|---|---|
| Critical dimensions | List the 5-15 dimensions where fit, function or sealing depend on precision; leave everything else at a looser, process-standard tolerance. | Focuses supplier attention on what actually matters instead of every dimension. |
| Process match | Confirm what the target process can realistically hold: CNC is typically ±0.05-0.1 mm, injection molding often ±0.1-0.3 mm, sheet metal looser still. | Prevents specifying below what the process can hold without costly secondary operations. |
| Stack-up and default block | Add up worst-case variation across mating parts, and state a default tolerance (e.g. "±0.2 mm unless noted") for uncontrolled dimensions. | Catches assembly-level misalignment and removes guesswork on the rest of the drawing. |
Common mistakes to avoid
Approving visuals too early
A polished image can still hide structure, cost, tolerance, material or supplier risks.
Skipping useful outputs
Each stage should produce files, notes, samples or checklists that support the next decision.
Sharing sensitive files too soon
Start with safe context and move to detailed files after fit and NDA terms are clear.
Related questions
Do I need full GD&T on every dimension?
No. Most consumer hardware products only need GD&T on the handful of features where fit, alignment or assembly depend on geometric relationships, such as mating features, hole patterns or sealing surfaces. Plain plus/minus tolerancing is fine everywhere else.
What tolerance should I use if I'm not sure yet?
State a general default tolerance block on the drawing, such as "±0.2 mm unless otherwise noted," and reserve tighter, explicitly called-out tolerances for the dimensions you've identified as critical to fit or function.
When should tolerancing be finalized?
After material and process are confirmed, since achievable tolerance depends on both, and before drawings go out for quoting, since it directly affects the price and the number of clarification rounds a supplier needs.
Need help applying this to your product?
Send product type, stage and target market. We will reply with the next practical development path.