Every drawing that lands on our floor carries tolerances. Most of them are fine. A few of them add cost that the buyer never intended to pay for.
The default is not free
A general tolerance block of ±0.1 mm is comfortable for most machined features. Move to ±0.05 mm and the machinist starts thinking about fixturing and tool wear. Move to ±0.01 mm and you have changed the job: it now needs a finishing pass, in-process measurement, and often a temperature-stable environment. The part is the same shape. The process behind it is not.
A rough guide from our own quoting:
- ±0.1 mm — baseline, no cost impact on most features
- ±0.05 mm — usually absorbed, may add a finishing pass on some geometries
- ±0.02 mm — adds setup time and inspection; expect a visible line item
- ±0.01 mm and tighter — separate finishing operation, dedicated inspection, higher scrap allowance
Tolerance is not a wish, it is a budget
The most expensive drawings we see are the ones where a tight tolerance was applied globally rather than to the feature that needs it. A bore that locates a bearing genuinely needs to be held. The clearance hole 20 mm away almost never does. When the block tolerance is tightened to cover the one critical feature, every feature inherits the cost.
The fix takes five minutes: leave the general block loose, and call out the two or three features that actually control fit or function.
Ask what the tolerance is protecting
Before tightening a number, it is worth asking what fails if it drifts. Three answers come up repeatedly:
It has to fit something. Then the tolerance belongs on the mating dimension, and often a slip fit tolerance band is cheaper than a symmetric ±.
It has to seal. Surface finish usually matters more than dimension here. A perfectly sized bore with a rough finish still leaks.
It has to be interchangeable across a batch. This is a process capability question, not a single-part question — and it is the one worth discussing with your supplier before the first order, not after.
Geometric callouts often cost less than tight limits
Position, concentricity and flatness callouts frequently solve the real problem at lower cost than shrinking a linear tolerance. A hole pattern that must line up with a mating plate is better controlled by a position tolerance with a datum reference than by tightening every X and Y dimension. It also tells the machinist what you actually care about, which changes how the part is fixtured.
What to send with the drawing
Quotes come back faster and more accurately when the drawing arrives with three things:
1. Which features are critical, and why
2. The mating part or assembly context, even as a sketch
3. Expected annual volume, so the supplier can decide between a soft-jaw setup and a dedicated fixture
The short version
Tolerances are the single easiest place to remove cost from a machined part without changing the design. Loosen the block, tighten the two features that matter, and say what each one is protecting. Most buyers who do this see the same part come back cheaper — not because the supplier discounted it, but because it genuinely became easier to make.
