5-Axis Machined Robot Joint Housings in 6061-T6

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.

Questions about this project