A tolerance is an agreement about how much a dimension may vary and still be acceptable. It is not a statement of how accurate a workshop is; it is a decision about what the part needs. A dimension that sits against a machined face and a dimension on the outer skin of a bracket rarely need the same latitude, and treating every dimension as equally critical makes a part expensive without making it better.
The useful way to think about it is by function. Every feature exists to do something: locate, seal, clear, align or carry load. Features that locate or seal need tight control. Features that only have to look right or stay out of the way can be looser. Sorting them into those two groups is the first step in any sensible tolerance discussion.
Variation enters a fabricated part at several points. Cutting and forming introduce their own deviation. Welding adds heat, and heat moves metal, so a part can be correct on the bench and out of shape once the weld has cooled. Assembly and fixturing add whatever the fixture allows. Each step contributes, and the total is what the finished part shows.
Because the sources stack up, controlling the total means controlling the sequence as much as the individual steps. A part that is checked and corrected between operations finishes closer to the drawing than one that is welded completely and inspected at the end, when the only remaining options are expensive.
Flat faces, straight cuts, circular features produced on a machine and hole patterns drilled from a fixture are relatively easy to control, because the tooling itself defines them. If the setup is correct and the fixture is rigid, the feature repeats. Hole positions in a flat plate are a good example: once the pattern is set in the tooling, every part follows it.
Round features on a turned or machined part behave the same way. Diameter, concentricity and the relationship between a bore and its mating face are set by the machine and the setup rather than by hand skill, so they repeat from part to part and are straightforward to verify after the fact.
Large thin-wall assemblies are the classic difficulty. Thin tube and sheet move when they are heated, so a long welded structure accumulates distortion that cannot simply be measured away afterwards. This is managed with sequence, restraint and sometimes correction, not by demanding a tighter number and hoping for the best.
Freely formed and hand-shaped surfaces are also harder to hold to a tight figure, because the result depends on the tooling and on the operator's control of the process. Where a part combines a formed shape with a dimension that has to mate precisely, the sensible approach is to make the critical interface a machined or jigged feature rather than part of the formed shape.
In the custom work process, a drawing containing dimensions, materials and special requirements is the document the mould is made from. That is the place to record which sizes matter and which do not. Marking critical features, stating datums and noting where a feature mates with something else gives a workshop the information it needs to spend effort where it counts.
Materials and surface finish belong on the same document, along with colour, logo and any revision notes. The services offered include customised 3D file, materials, colour, logo, drawings and surface finish, and every one of those is easier to hold to once it is stated rather than assumed.
The stage that follows mould making is deliberately measurement-based. The mould is tested, samples are made, and an engineer measures their sizes and tests hardness to confirm quality. Size tells you whether the geometry matches the drawing; hardness tells you whether the material and the way it was processed are what they should be. Both matter, and neither substitutes for the other.
Samples then go to the customer for testing, and mass production starts after the customer confirms them. For titanium fab components, that sequence is also a check on the material and the process, carried out before the tooling is committed to volume.
