A metal part looks like a pure specification problem. You have a drawing, the drawing carries dimensions, and a workshop with the right machine either makes it or does not. Nearly every part of that sentence is misleading. The drawing says what you want; it rarely says what matters most. And the machine is the least distinctive thing a supplier owns, because anyone can buy one.
Four processes, and not four versions of the same order
Before discussing a supplier, decide which process the part belongs to. It changes who you should be talking to, whether there is tooling at all, and what actually drives the price. A workshop that is excellent at one of these is often mediocre at the next.
| Process | What it is for | What drives the price |
|---|---|---|
| CNC machining | Parts cut from solid material, complex geometry, no tooling to pay for — the natural route for early volumes | Machine time, the number of set-ups, and how much material is removed to get there |
| Sheet metal | Enclosures, brackets, chassis: cut, folded, welded, then finished | The number of operations, the welding, and the finish — the metal itself is rarely the biggest line |
| Die casting | Complex shapes in aluminium or zinc, at volume, with a real steel tool behind them | The tool, the cycle, and the machining that almost always follows the casting |
| Turning and screw machining | Small cylindrical parts made from bar stock, in long runs | Material and bar diameter, cycle time, and the tolerance you ask for — which is where the cost hides |
The tolerance you write and the tolerance you get
Most drawings carry a general tolerance note — often a reference to ISO 2768, sometimes nothing at all — and then a page of dimensions with no indication of which ones matter. A workshop reads that as permission to hold everything at the loosest acceptable level, because that is what it was quoted for. It is not cheating. It is the only reading the drawing supports.
The fix is not tightening everything: tight tolerances everywhere buy you a price increase and no more quality. The fix is naming the few dimensions that decide whether the part works — the ones that mate with something else, the ones that carry a seal, the ones a customer touches — and saying how they will be measured. A tolerance without a measuring method is an opinion.
- The material, and what counts as an equivalent. A local grade offered as « the same » usually is not, and the difference shows up in machining, in treatment, or in corrosion.
- The general tolerance class, written once and explicitly. Silence is read as « as wide as you like ».
- The critical dimensions, marked as such, with their measuring method. Caliper, gauge, or measuring machine: say which.
- The surface finish, and the faces it applies to. A part has visible faces and hidden ones, and paying for both is waste.
- The treatment and the standard behind it. « Anodised » is a family, not a specification.
- What an acceptable part looks like. Scratches, tool marks, weld appearance: cosmetic criteria are a written agreement or they are a future argument.
Having the machine is not having the trade
Two workshops can own the same five-axis machine and deliver parts that are not comparable. The difference lives in the things that never appear in a presentation: how the part is held, how many times it is unclamped and re-clamped, which operation comes before which, how often the tools are changed, whether the programmer has made this family of part before. Those choices decide whether your critical dimension is repeatable or merely achievable.
So the useful visit is not a tour of the machine hall. It is asking to see a part like yours being made, watching the operator measure it, and asking what happens to the ones that fall outside. A workshop with the trade answers those questions quickly and without tension, because the answers are its daily work rather than a claim.
Ask for the first-article report with the measured values written in, not a document with ticks in a column. A row of « OK » proves that someone filled in a form. A column of real figures proves that someone measured, and tells you how much margin you actually have.
Finish and treatment usually leave the building
Anodising, passivation, plating, powder coating and painting are almost always subcontracted, even by workshops that present themselves as integrated. This is normal — these are chemical trades with their own installations and their own environmental permits — but it has two consequences you should plan for. The lead time includes a round trip you cannot see, and the colour or the finish can drift between lots because the bath is not yours.
- Ask who does the treatment, and whether it is the same house every time.
- Agree a physical colour reference kept by both sides, not a colour code in an email.
- Decide which faces are masked, and whether threads are protected — a treated thread that no longer accepts its screw is a classic.
- Say whether the treatment happens before or after marking, because the order changes the result.
Why the first lot is often the best one
The first parts are usually made by the people who made the samples, with attention, on a quiet machine, and checked one by one. The series is made by a production team, on whichever machine is free, against a routing sheet. Nothing dishonest happens in between; the conditions simply change. What holds the level is banal and works: a retained reference part signed by both sides, a written control plan naming what is measured and how often, and the same inspection applied to the second lot as to the first. Without that, quality does not fall — it returns to the workshop's own average, which is what you were shown on day one only by exception.