A robot is a sourcing problem disguised as an engineering problem. A single arm or mobile platform can contain dozens of custom machined parts, sheet metal covers, printed brackets, cable harnesses and several circuit boards — each in low volume, each from a different process, all of which must fit together on the first build. The pain points are predictable: tolerance issues at the joints, finishing that changes dimensions, revision chaos, and suppliers who are not interested in 20-piece orders.
Typical parts and the right process
| Part | Common process | Watch out for |
|---|---|---|
| Joint housings, gearbox housings | CNC machining (aluminum 6061/7075) | Bearing bore tolerances, coaxiality between bores |
| Shafts, pins, adapters | CNC turning (steel, stainless) | Fits with bearings and couplings, surface hardness |
| Arm links, frames, base plates | CNC milling or sheet metal + welding | Flatness, weld distortion, mounting hole positions |
| Covers, guards, chassis panels | Sheet metal; later injection molding | Cosmetic surfaces, hardware inserts, powder-coat thickness |
| Grippers, sensor mounts, cable guides | 3D printing (SLS/MJF nylon) | Strength in the load direction, dimensional accuracy |
| Motor drivers, controllers, sensor boards | PCB assembly | Component availability, thermal design, connector choice |
| Cable harnesses | Harness assembly | Flex life in moving joints, strain relief |
Tolerances where they matter: joints and bearing fits
- Specify bearing seats and shafts with standard ISO fit classes (for example a housing bore tolerance class with the matching shaft class recommended by the bearing manufacturer) rather than ad-hoc ± values.
- Control coaxiality / runout between bores that carry the same axis — this is often what causes binding, noise and backlash.
- Do a tolerance stack-up across the joint assembly, not just each part individually.
- Mark these features as critical-to-quality on the drawing and require them in the inspection report; loosen everything else to a general tolerance to save cost.
Finishing changes dimensions
Anodizing, especially hard anodizing, grows the surface and partly penetrates it, so coated bores get smaller and shafts get larger. Powder coating adds considerably more thickness. Either mask precision features (bearing seats, threads, mating faces) or machine them with an allowance for the coating — and say which on the drawing.
Managing many low-volume part numbers
- One BOM with revision control: every part has a number, a revision letter and a single current drawing.
- Never change a part by message — issue a new drawing revision and confirm which revision is in production.
- Group parts by process and supplier to reduce setups and shipping.
- Kit parts per robot build (bagged and labeled by assembly) so your assembly team does not sort hundreds of loose parts.
- Keep a small stock of wear parts and long-lead items.
Electronics: plan for supply, not just function
Motor drivers and controllers rely on MCUs, power stages and connectors that can go on allocation. Review the BOM for lifecycle status and single-source parts early, and agree approved alternates. See the PCB assembly files checklist and embedded hardware.
From prototype robots to a production series
- Prototypes: machined and printed parts, fast iterations.
- Pilot series (tens of units): stabilise drawings, add fixtures, first articles for every part.
- Production: move high-count parts to more economical processes (casting, molding, stamping) where volume justifies tooling.
More on stages in EVT, DVT and PVT and our robotics industry page. Have a set of drawings? Send the BOM and files — mixed-process robot BOMs are exactly what we coordinate.