# 5-Axis Machined Robot Joint Housings in 6061-T6

Industry: Robotics & Automation

A robotics OEM needed joint housings and linkage arms for a collaborative robot arm, machined from 6061-T6. The housings carry a strain-wave reducer and a crossed-roller bearing, so bore coaxiality and mounting-face flatness drive the accuracy and smoothness of the finished joint.

- **Part:** Robot joint housing and connecting linkage arm
- **Material:** 6061-T6 aluminum
- **Process:** 5-axis CNC milling and boring, 4-axis drilling
- **Critical tolerance:** Bearing bores down to ±0.01 mm; coaxiality 0.02 mm
- **Finish:** Type II black anodize, bearing seats masked
- **Batch type:** Prototype → pilot run → low-volume production
- **Inspection:** CMM, bore gauges, flatness on surface plate

## The challenge
In a robot joint, the housing is the reference that the motor, reducer, bearing and encoder all align to. If the bores are not coaxial, the reducer runs with a misalignment load, which shows up as extra friction, noise and lost positioning accuracy at the tool tip.

The drawing set these targets:

- **Reducer and bearing bores held down to ±0.01 mm**, with **coaxiality of 0.02 mm** between bores on opposite ends of the housing
- **Flatness of 0.02 mm** on the flange faces that bolt to the next arm segment
- **Bolt circles** for the reducer and bearing positioned relative to the bore axis
- **Wall thickness of 3 mm** in places, to keep arm weight and inertia low
- **Internal cable passages** through the hollow center of the joint, with smooth edges so cable insulation is not cut

The combination of thin walls and tight bores is the main risk. A housing that is stiff while clamped can relax when released and pull the bores out of round.

## Our approach
We designed the route so that the bores were finished last, from a stable part, in one clamping.

1. **DFM review.** We confirmed with the customer's engineer which bores were functional pairs and agreed on one datum scheme for machining and inspection, so both shops measure the same way.
2. **Roughing.** Housings were roughed from 6061-T6 bar on a 3-axis machine with even stock (about 1 mm) on every surface, including internal cavities.
3. **Stress release.** Roughed parts rested before finishing so that any movement from released stress happened before the finished surfaces were cut.
4. **5-axis finishing in one clamping.** On a zero-point fixture, the outer surfaces, flange faces, both end bores and both bolt circles were finished without re-clamping. Machining opposite bores in one setup is how coaxiality is held: the 5-axis table indexes 180 degrees rather than the operator flipping the part.
5. **Bore finishing.** The bearing and reducer bores were finished with a fine boring head on a final spring pass, with low clamping force to avoid distorting the thin walls.
6. **Cable passages and edges.** Internal passages were milled and every edge was deburred and radiused.
7. **Linkage arms.** The connecting arms followed a similar route, with both flange interfaces finished in one 5-axis setup.
8. **Anodizing.** Bearing seats and grounding points were masked, and the parts were Type II black anodized.

## Quality plan
The inspection plan matched the datum scheme agreed during DFM.

- **First-article inspection** of all dimensions on CMM, reporting bore size, roundness and coaxiality against the primary bore axis
- **Bolt circle position** reported relative to the bore axis
- **Flange flatness** checked on CMM and confirmed on a granite surface plate
- **Bore sizes** checked with bore gauges on every part after anodizing, to confirm masking held
- **Threads** verified with go/no-go thread gauges
- **Visual check** of cable passages for burrs and sharp edges that could damage cable insulation during assembly
- **Anodize appearance** checked for even black color on visible surfaces

During pilot and production runs, bore size, coaxiality and flange flatness were 100% inspected. Material certificates were supplied with each batch.

## What was delivered
We delivered prototype housings and linkage arms for the first assembly build, then a pilot run and low-volume production orders made with the same fixtures, programs and inspection plan. Each delivery included:

- CMM first-article report using the agreed datum scheme
- Per-part records of bore size, coaxiality and flange flatness
- Material certificates for the 6061-T6 bar
- Masking drawing for the anodize step

Joint assembly, run-in and accuracy testing were carried out by the customer. Keeping one datum scheme for machining and inspection meant our reports and the customer's incoming checks could be compared directly.