Tolerances
What each process can hold, and what it costs to hold it tighter
Tolerance is the single biggest driver of price in a metal part, and the easiest thing to over-specify. The tables below are the ranges we quote against. Your drawing always wins—but if a callout is tighter than the assembly needs, we will say so.
Mill · Turn · Laser · Bend · Stamp
01 Dimensions
General tolerances by process
Values in millimetres unless noted. “General” is what we hold without special measures; “tight” needs slower feeds, extra setups, or a second operation—and it is priced accordingly.
| Process | Feature | General | Tight | Notes |
|---|---|---|---|---|
| CNC milling | Linear dimension | ± 0.05 | ± 0.02 | Below ± 0.02 usually needs grinding or temperature control. |
| CNC milling | Hole diameter | ± 0.03 | ± 0.01 | Reaming or boring instead of helical milling. |
| CNC milling | Flatness (per 100 mm) | 0.05 | 0.02 | Thin walls and long parts move after clamping is released. |
| CNC turning | Diameter | ± 0.05 | ± 0.015 | Slender parts need a steady rest or a second pass. |
| CNC turning | Concentricity / runout | 0.05 | 0.02 | Depends on how the part is held between operations. |
| Laser cutting | Profile position | ± 0.15 | ± 0.08 | Kerf and heat input vary with thickness and material. |
| Press brake | Bend angle | ± 1° | ± 0.5° | Springback and grain direction matter more than the machine. |
| Press brake | Bend-to-edge distance | ± 0.2 | ± 0.1 | Very short flanges need special tooling or a different process. |
| Stamping | Pierced hole / blank | ± 0.05 | ± 0.02 | Set by die build quality and press condition. |
| Stamping | Form height | ± 0.10 | ± 0.05 | Material thickness tolerance often dominates the result. |
Indicative ranges for RFQ discussion. Final acceptance follows the tolerances stated on your drawing and the inspection plan agreed in the quote.
02 Surface
Surface roughness we quote
Ra is usually specified on sealing faces, sliding interfaces, and optical or cosmetic surfaces. Elsewhere it is a cost with no function—so we ask before chasing it.
If the drawing says “Ra 0.8 all over”, expect a question. A note like “Ra 1.6 on sealing faces, Ra 3.2 elsewhere” is both cheaper and easier to inspect.
| Process | Typical Ra | Reached by |
|---|---|---|
| CNC milling | 1.6 – 3.2 µm | Standard finishing passes with a sharp tool. |
| CNC milling (fine) | 0.8 µm | Smaller stepover, slower feed, or a finishing-only setup. |
| CNC turning | 0.8 – 1.6 µm | Finish pass with correct nose radius and feed rate. |
| Surface grinding | 0.4 – 0.8 µm | Post-machining operation on flat faces. |
| Laser cut edge | 3.2 – 6.3 µm | As-cut; improves with nitrogen assist on stainless. |
| Bead blast / tumble | Cosmetic | Uniform matte; hides tool marks rather than reducing Ra. |
03 Cost
What actually drives the price up
Most of the cost in a machined part is not material. It is the time spent getting the part located, measured, and re-located. These are the six things that move a quote.
Tolerances tighter than the assembly needs
Every dimension below ± 0.02 mm adds inspection time and often an extra operation. If only two features matter, mark those two.
Thin walls and deep pockets
The part deflects while it is being cut, so the job needs lighter passes, more steps, and sometimes a stress-relief step.
Multiple setups
Each re-fixturing adds handling and a new source of positional error. A design that can be made in two setups is usually cheaper than one that needs four.
Cosmetic requirements
“No visible tool marks” on a large face means a different finishing strategy, and every part has to be handled like a finished surface.
Heat treat or coating between operations
Sending parts out mid-route adds calendar time and can move dimensions. It has to be planned, not discovered.
Volume mismatch with the process
A die quoted for 300 pieces will look expensive. Machining 50,000 pieces will look worse. We route to the volume you actually have.
04 GD&T
What we need on the drawing
Position and profile callouts are only meaningful with a datum scheme. If the drawing leaves datums implied, two suppliers will measure two different parts and both will believe they are right.
- Datum features identified (A/B/C), not left to interpretation.
- True position with the material condition modifier you intend (MMC or RFS).
- Profile callouts with the tolerance zone direction stated.
- Thread callouts with class, depth, and whether they are measured or gauged.
- Cosmetic vs functional surfaces separated—by note or by view.
If your drawing is incomplete
We list the missing callouts in the quote instead of assuming. It adds a round of questions, but it removes a much more expensive argument later.
Standards we work to
Drawings are read as ISO/ASME-style geometric tolerancing. If your program requires a specific standard edition or a customer-specific supplement, state it on the PO.
Inspection to match
Tighter callouts only mean something if they are measured. Pair this page with the inspection levels so the report matches the drawing.
Quality & inspection05 Finishes
Common finishes and what they change
Finishing runs through partners we use repeatedly. Cost and lead time are quoted with the part, not discovered at the end.
| Finish | What it does | Watch for |
|---|---|---|
| Anodize (Type II / III) | Controlled oxide layer on aluminum; clear or dyed. | Build-up on threads and bores—mask or allow for it. |
| Passivation | Removes free iron from stainless; improves corrosion resistance. | Does not hide scratches or weld discoloration. |
| Zinc / nickel plating | Corrosion protection for steel parts. | Hydrogen embrittlement risk on high-strength parts. |
| Powder coat | Durable color layer over formed and welded parts. | Thickness bridges small gaps; mask mating and grounded surfaces. |
| Electroless nickel | Uniform wear and mild corrosion protection. | Uniformity depends on racking and part geometry. |
| Bead blast / tumble | Cosmetic matte, deburring, edge softening. | Changes surface texture—re-check Ra if it is specified. |