Machining Standards and Reference Tables
The numbers a drawing is quoted against, in one page. ISO 2768 is the published standard. Everything else is a typical range, not a promise, because what a shop holds depends on the part, the material and the machine.
Last reviewed 2026-09-23. Print it, or keep it open next to the drawing.
General tolerances, ISO 2768-1
Every dimension without its own tolerance falls under the general class. If your drawing names no class, the shop assumes medium, and that assumption is inside your price.
| Nominal size | f fine | m medium | c coarse | v very coarse |
|---|---|---|---|---|
| 0.5 to 3 | ±0.05 | ±0.1 | ±0.2 | - |
| over 3 to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1 |
| over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| over 400 to 1000 | ±0.3 | ±0.8 | ±2 | ±4 |
| over 1000 to 2000 | ±0.5 | ±1.2 | ±3 | ±6 |
| over 2000 to 4000 | - | ±2 | ±4 | ±8 |
ISO 2768-1:1989, permissible deviations for linear dimensions in millimetres. For nominal sizes below 0.5 mm the deviation is stated next to the dimension.
Which class to put on the drawing
- f, finePrecision parts and mating features. Costs more and needs a shop that measures properly. Do not apply it to a whole drawing.
- m, mediumThe usual choice for a machined part. Most shops hold it without special effort, and it is what a quote assumes if you say nothing.
- c, coarseCovers, brackets and features that do not mate. Using it deliberately on the non critical majority is the cheapest line on a drawing.
- v, very coarseFlame cut, cast or rough stock. Rarely relevant to a machined part and not defined below 3 mm.
The cheapest drawing is usually a coarse or medium general class with three or four features called out tightly. The expensive one is a fine general class applied to everything. How to decide which features need it.
Surface finish by process
Typical achievable roughness. A finer value is not better, it is slower, and it has to be measured to mean anything.
| Process | Typical Ra | Notes |
|---|---|---|
| Rough milling | 6.3 to 12.5 | Stock removal. Not a finish you would specify on a visible face |
| Finish milling | 1.6 to 3.2 | The default as machined finish on most parts |
| Fine finish milling | 0.8 to 1.6 | Extra passes at slow feed. Specify only where it matters |
| Turning | 1.6 to 3.2 | Comparable to finish milling on a cylindrical face |
| Fine turning | 0.4 to 0.8 | Sharp tooling and light cuts |
| Reaming | 0.8 to 1.6 | The normal way to hold a bore at ±0.02 mm |
| Boring | 0.8 to 3.2 | Used where a reamer of that size is not practical |
| Grinding | 0.2 to 0.8 | A separate operation and a separate cost line |
| Bead blast | 1.6 to 3.2 | Cosmetic and uniform. Changes appearance more than measured roughness |
Tolerance and finish for each process, and which part features need which: CNC machining processes.
Coatings and how much they grow
Coating thickness is a dimensional change. A bore toleranced before anodising is a bore that will not pass after it.
| Finish | Typical thickness | What it changes |
|---|---|---|
| Anodise type II | 5 to 25 µm | Roughly half grows outward, so tight bores need masking or post machining |
| Hard anodise type III | 25 to 50 µm | Significant dimensional growth. Account for it on the drawing |
| Passivation | No measurable build | Chemical treatment for stainless. Removes free iron, does not coat |
| Zinc plating | 5 to 15 µm | Corrosion protection on steel. Masking needed on threads with a tight fit |
| Powder coat | 60 to 120 µm | Thick. Never apply it to a mating face without masking |
| Black oxide | 1 to 2 µm | Minimal build. Appearance and mild corrosion resistance |
Materials and what to ask about them
Local stock availability is the largest hidden variable in an overseas quote. These are the questions, not claims about the market.
| Material group | What to expect | Ask the shop |
|---|---|---|
| 6061 and 6063 aluminium | The most widely stocked machining material in the region | Which temper, and is it bar or plate |
| 7075 aluminium | Available but less widely held than 6061 | Is it stocked or ordered in, and what lead time |
| 303, 304 and 316 stainless | Common. 316 for marine and food contact | Bar diameter held, and whether a mill certificate comes with it |
| Mild steel, S235 and S355 equivalents | Common in plate and bar | Which local grade is being substituted and whether that is acceptable |
| Tool steel, H13 and D2 | Ask first. Often ordered in | Lead time, and whether heat treatment is in house or subcontracted |
| C360 brass and copper | Available, smaller range of sizes | Stock sizes held, since a near size saves machining |
| POM, nylon and PTFE | Common engineering plastics, widely available | Whether the grade is virgin or recycled |
| PEEK and other high performance polymers | Expect an import lead | Lead time and minimum purchase quantity for the stock |
| Titanium and nickel alloys | Specialist. Expect an import lead and a smaller pool of shops | Whether they have cut it before, and on which machine |
Grades with UNS, EN and JIS equivalents and typical strength: materials and equivalents.
Machine envelopes
What sits on a typical floor. Ask which specific machine your part would run on, not what the shop owns in total.
| Machine type | Typical working envelope | Notes |
|---|---|---|
| 3 axis vertical mill | Up to roughly 1000 x 500 x 500 mm | The most common machine on any floor. Larger beds exist but are less common |
| CNC lathe | Up to roughly 300 mm diameter | Bar capacity through the spindle is the limit that matters for turned parts |
| Lathe with live tooling | Similar diameter, fewer machines | Removes a second setup on parts with cross holes or flats |
| 4 axis mill | Similar to 3 axis | A rotary table added to a 3 axis machine. Common |
| 5 axis mill | Usually smaller than the 3 axis machines | Fewer of them. Ask which specific machine your part would run on |
Inspection levels
Each level is work and each has a cost. Ask for the ones you will read.
| Level | What it is | When to ask for it |
|---|---|---|
| In process check | The operator measures key dimensions during the run | Always. It should be inside the price |
| First article report | Every drawing characteristic measured on the first piece, with actual values | First order, and after every drawing revision |
| Dimensional report on a sample | Named characteristics measured across an agreed sample | Production orders, on the features you care about |
| 100 percent inspection | Every piece measured on the named characteristics | Safety critical features only. It is expensive |
| Material certificate | Mill document with chemistry and mechanical properties | Whenever the grade is functional |
| Certificate of conformity | A supplier declaration that the goods meet the order | Always. It costs nothing |
What a quotable drawing carries
Ten things. A drawing with all of them is priced on the first pass instead of after three rounds of questions.
- Title block with part number, revision and units
- A general tolerance class or a general plus or minus note
- Tight tolerances only on the features that need them, each with its datum
- Surface finish as an Ra value, with the faces it applies to
- Coating or plating specification and class, or the words "as machined"
- Thread callouts with pitch, and depth for blind holes
- Material grade and temper, with "or equivalent" if a substitute is acceptable
- Edge break or deburr note
- Any feature that must not be coated, marked as masked
- Inspection requirements, if documents must travel with the parts
Have a drawing that fits these numbers?
Send it and we match it with up to three shops in Vietnam that run that process and material.