Most cost overruns on machined parts are locked in before the first chip is cut. The drawing either gives the supplier enough to plan a stable process, or it doesn't — and the difference shows up as a higher quote, a longer lead time, or parts that fit the gauge but not the machine. Here is the checklist we work through with every new enquiry.
1. Give geometry as drawing + 3D model
- 2D drawing carries the tolerances, surface finish, thread and keyway standards — the 3D model alone cannot.
- STEP / IGES model lets programming start immediately and eliminates re-drawing errors.
- If you only have a sample, say so: reverse engineering with a full dimensional report is routine, but expect one extra round for approval.
2. State the material with an accepted equivalent
"Steel" is not a specification. Give the grade (45#, 40Cr, 42CrMo, 6061-T6, 304 …) and which standard you accept — GB, DIN, ASTM or JIS. Where the part is safety-related, request a material certificate EN 10204 3.1 with the heat number traceable to the melt.
3. Tolerance: specify function, not everything
| Feature | Practical class | Comment |
|---|---|---|
| Bearing / press-fit seats | h6 – h7 | Fit class must be given, not just ± |
| General mating dimensions | h7 – h9 | Tightening beyond this adds cost without function |
| Non-critical surfaces | ±0.2 mm or general tolerance | Let the shop use its standard capability |
| Positional / profile | GD&T callout | Clearer than coordinate tolerancing for hole patterns |
4. What actually drives the price
- Set-ups: a part needing 4 orientations costs far more than one finished in 2. Design features accessible from fewer directions.
- Tight tolerances across the whole part rather than on functional features only.
- Deep holes, thin walls, long slender features — slower cutting, more scrap risk.
- Batch size: 5 pcs and 500 pcs use different process routes (and different fixtures). Always quote the annual volume, not just the first order.
- Material form: bar stock is cheapest; forgings or castings cost more upfront but save machining on large volumes.
5. Surface treatment and inspection
| Treatment | Typical use | Note |
|---|---|---|
| Black oxide | Internal parts, mild corrosion resistance | Dimensional change negligible |
| Zinc plating | General outdoor / humid | Risk of hydrogen embrittlement above HRC 40 |
| Nickel / chrome | Wear surface, appearance | Adds thickness — allow on threads |
| Anodising | Aluminium parts | Colour options, hard anodising for wear |
Ask for a first article inspection report on the first batch and a dimensional report per lot. For repeat orders, a simple agreed inspection sheet prevents most disputes.
6. Packing and export
Machined surfaces need separation, not just a box: individual wrapping, anti-rust paper or VCI, foam separation in cartons, and fumigated wooden cases for heavy or long parts. Labelling by part number on every package saves hours at goods-in.