Most robot part problems are decided before the first chip is cut. A drawing arrives with a dozen tolerances, quoting starts on price, and the datum strategy is never discussed. Six weeks later the parts arrive, the joint binds on assembly, and everyone blames the machine shop. This guide is the checklist we wish every buyer had — written from the shop floor of a factory that has been machining cnc machining parts for robots since 2008.
1. Which tolerances actually matter on a robot part
A typical joint flange carries thirty or forty dimensions, but only a handful decide whether the robot works. In our experience the critical six are:
| Feature | Typical callout | Why it decides the assembly |
|---|---|---|
| Bearing / pilot bore | H7, ±0.005 mm | Sets radial centring; error becomes backlash at the end of the arm |
| Datum face flatness | 0.01 mm | Any wedge gap preloads the reducer and shows up as torque ripple |
| Bolt pattern position | ±0.02 mm true position | Off-centre bolts add bending to the joint under load |
| Coaxiality of two bores | 0.01 mm | Decides whether the shaft turns freely or binds after thermal growth |
| Wall thickness | ±0.05 mm on thin ribs | Drives stiffness-to-weight; chatter marks are a symptom, not the cause |
| Edge break on wire paths | R2-R3, polished | Sharp edges cut cable jackets and cause unplanned downtime |
Everything else — cosmetic surfaces, non-critical pockets, engraving depth — can usually be loosened to cut cost. A supplier who asks which of these six you actually measure is a supplier worth keeping.
2. Why "one setup" is the difference between 0.01 mm and 0.03 mm
Tolerances do not add up linearly, but datum errors certainly accumulate. Every re-clamping introduces a new positioning error of roughly 0.005–0.015 mm. Machine a joint flange in three operations and you have three chances to lose the relationship between the bore and the bolt pattern.
Our default is therefore a single setup on a 5-axis machining centre: the datum face, the bearing bore, the port faces and the bolt circle are all produced from one reference, with roughing separated from finishing by a low-temperature stress relief. On thin-wall parts we also check that the stress relief happens before the finishing pass — otherwise the part relaxes on the customer's assembly line instead of on our machine.
Examples from our current production:
- Multi-port cavity joint flange — G7 bearing seat, 0.01 mm flatness, cavity and ports machined without re-clamping.
- Joint motor mount with reducer interface — H7 pilot bore and bolt circle in one setup, Cpk ≥ 1.33 in serial production.
- Seventh axis carriage plate — twin rail faces flat and parallel to 0.01 mm, with anodising film compensation.
3. Materials and finishes: the three decisions buyers regret
Alloy choice. 6061-T6 is the workhorse: weldable, anodises well, good strength. 7075-T6 gives roughly 40 % more strength for the same geometry but anodises darker and costs more. On a weight-critical wrist flange, 7075 often pays for itself; on a bracket, it rarely does.
Finish and fit. Hard anodising builds 25–40 µm per surface. If the drawing calls for H7 and the finish is applied afterwards, the bore shrinks. Either mask the fit surfaces or specify the pre-finish dimension — and put that decision in writing on the purchase order.
Heat treatment timing. For steel sleeves and spools, finishing before quenching guarantees distortion. The sequence must be: rough machine → heat treat → semi-finish → finish/hone → inspect.
4. Five questions to ask before you issue the PO
- Which datum will you machine from, and can I see the fixture concept? A supplier who cannot answer this has not read the drawing.
- How do you handle stress relief on thin walls? The answer should mention roughing, relief and re-fixturing — not "we machine slowly".
- What is your first-article process? Expect a FAI report with CMM data, material certificate and, for critical features, a capability study.
- How do you protect our drawings? Look for NDA before file transfer, an isolated production area, coded part numbers and a deletion policy.
- What happens when the second order differs from the first? Ask for the CMM history of an existing part number, not a promise.
5. What robotics demand looks like from the shop floor
Orders tell a story before the trade press does. In the last three years we have seen three shifts:
- Humanoid and cobot prototypes — smaller, lighter parts, thinner walls, more features per part, and quantities that start at five pieces.
- More sensors per joint — torque sensors, encoders and cable routing all need mounting faces, which means more 5-axis work and tighter position tolerances on the same block.
- Faster launches — prototype in a week, then serial production on the same program. Buyers no longer accept a re-tooling step between sample and series.
The response is a factory that can move between one piece and twenty thousand without changing its process: see our robot parts machining capability.
6. Who is behind these parts
Dongguan Changxinda Precision Technology was founded in 2008 and has grown to 90–102 people serving more than 200 customers, with 50–60 % of output exported and ISO 9001:2015 certification. The plant runs 4-axis and 5-axis machining centres, turn-mill lathes, surface grinders and wire EDM, with a metrology room for CMM, roundness, roughness and hardness checks.
What customers actually measure us on is less romantic: first-article approval rate, batch-to-batch consistency, confidentiality, and whether the promised ship date holds. Those four are the only KPIs we track weekly.
