Most quotation delays are not caused by the machining itself — they are caused by missing information. A supplier that has to guess a tolerance, a thread or a surface finish will either quote conservatively or come back with questions. Sending a complete package removes that round trip.
1. Send a solid 3D model
STEP is the safest format: it keeps solid geometry, units and does not depend on which CAD system you use. STL is fine for 3D printing but poor for machining because it is a mesh — dimensions have to be re-measured. If your part has critical features, always add a 2D drawing as well.
- Preferred: STEP (.stp / .step)
- Accepted: IGES, SLDPRT, Parasolid (x_t), DWG / DXF for 2D
- 3D printing only: STL, OBJ, 3MF
- Always state the unit (mm or inch) if the file does not make it obvious
2. Mark the dimensions that actually matter
A drawing with every dimension toleranced tightly is expensive to make and slow to quote. A drawing with no tolerances at all leaves the supplier guessing. The practical approach is to tolerance the features that affect fit, sealing, alignment or movement, and leave the rest to a general shop tolerance.
- Bore diameters that receive a shaft or bearing
- Hole positions that must align with a mating part
- Faces that seal, mount or set a height
- Thread size, pitch and class
- Geometric controls (flatness, perpendicularity, concentricity) only where they matter
3. Specify material by grade, not by family
"Aluminium" is not a specification. 6061-T6 and 7075-T6 machine differently, cost differently and are stocked differently. The same applies to stainless steel: 303, 304 and 316 behave very differently in a machine. If a certificate or a specific standard is required, say so in the enquiry — sourcing certified material takes longer.
4. State quantity — including the repeat quantity
Machining one part and machining fifty are different jobs: workholding, tooling and inspection are amortised differently. Telling a supplier that a first order of five will be followed by fifty a year often changes the process route and the unit price.
5. Define the surface finish
As-machined is usually the cheapest and fastest. Anodising, plating, painting and polishing each add handling time and risk. If a finish is cosmetic, say which faces are visible — finishing only the visible faces is cheaper than finishing the whole part.
6. Say how the part will be inspected
If you need a dimensional report, first-article inspection or a specific gauge, ask before the job starts rather than after it ships. Inspection takes time and must be planned into the schedule.
7. Give the real deadline and destination
Lead time means production time plus finishing plus packing plus shipping. If you need parts in three weeks and shipping takes one, say so at the start — it changes what is feasible.
Summary: A complete package is: 3D model + dimensioned 2D drawing + material grade + quantity (first order and repeats) + finish + inspection expectation + destination and deadline. Everything else is a bonus.
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