The tables below are a planning guide, not a guarantee. Achievable tolerance depends on the feature, the material, the geometry and the setup - the exact figure for a part is confirmed when it is quoted.
Material grades and what they are for
| Material | Common grades | Typical general tolerance | Notes |
|---|---|---|---|
| Aluminium | 6061-T6, 7075 | +/- 0.05 mm | fast to machine, anodises well, good for most parts |
| Stainless steel | 304, 316L, 17-4PH | +/- 0.05 mm | work-hardens; 316L for corrosion, 17-4PH for strength |
| Carbon steel | 1018, 4140 | +/- 0.05 mm | 4140 often machined then heat treated |
| Brass / copper | C360, C110 | +/- 0.05 mm | free-machining brass; copper for conductivity |
| Titanium | Grade 5 (Ti-6Al-4V) | +/- 0.05 mm | slow to cut, excellent strength-to-weight |
| Engineering plastics | POM, PEEK, Nylon, PC, ABS | +/- 0.10 mm | thermal expansion is significant; design for it |
Tolerances: how to set them
A single tight block tolerance forces every feature to the same standard and drives cost across the whole part. The efficient approach is a loose general tolerance on the drawing plus explicit tight tolerances only on functional features - bearing bores, sealing faces, locating holes.
Tolerances also accumulate. Where three dimensions sit in a chain, the worst-case sum is what decides whether the assembly works, not the individual values. Sizing that stack before release is cheaper than fixing it after.
Surface finish values
| Finish | Typical Ra | Where it is used |
|---|---|---|
| As machined | ~Ra 3.2 | non-critical surfaces, internal features |
| Fine machined | ~Ra 1.6 - 0.8 | bearing and sealing surfaces |
| Bead blast | matte | cosmetic uniformity, pre-paint prep |
| Anodised | matte or dyed | aluminium corrosion and wear resistance |
| Plated / passivated | per spec | corrosion resistance, conductivity, hardness |
Material certificates and traceability
Mill certificates traceable to the heat number are available for every lot and are supplied as standard where a part carries load or goes into a regulated product. Where the material is a substitute for a specified grade, the substitution is stated rather than assumed.
Questions we get asked about materials
Why is plastic given a looser tolerance than metal?
Engineering plastics expand and contract far more with temperature than metals, and they move after machining as internal stress releases. A tolerance of +/- 0.10 mm is realistic for plastic where +/- 0.05 mm is normal for aluminium.
Does 17-4PH need heat treatment after machining?
It is usually supplied in a condition that machines well and then aged to reach its strength. Because aging shrinks the part slightly, the sequence has to be decided before the tolerance is fixed.
Can you machine titanium?
Yes, but it is slow - titanium conducts heat poorly, so the cutting edge runs hot and tool life is short. That is reflected in the price rather than hidden in the lead time.
Which stainless grade should I specify?
304 for general corrosion resistance, 316L where chlorides or marine exposure are involved, and 17-4PH where strength or hardness matters more than corrosion resistance. Machinability differs substantially between them.
Send a part and its tolerances
We will confirm what the process can hold and flag anything that will be difficult or expensive.