5-Axis Machined Drone Motor Mounts and Brackets in 7075-T6

The challenge

On a multirotor, every gram on the arm tip counts, and the motor mount sits at the end of the arm. The customer's design used deep pockets and ribs down to 1.2 mm thick to remove weight, which brings the usual thin-wall problems: chatter, deflection under the cutter and distortion after unclamping.

At the same time, the mount has functional features that cannot move:

• Motor face flatness of 0.03 mm, so the motor sits square and the propeller runs true
• Motor bolt pattern positioned relative to the arm axis, with clean, accurate tapped holes
• Carbon fiber tube clamp bore held to ±0.02 mm, sized so the clamp grips the tube evenly without point-loading the laminate
• Compound-angle features for motor tilt, which is common on multirotors to improve yaw authority

The bracket at the frame end had similar clamp bore requirements plus a mounting pattern that had to match the frame plates. Each part also had a target mass on the drawing, so extra material left for safety was not an option.

Our approach

We sequenced operations so the thin ribs were cut last, when the stiff features were already finished.

1. DFM review. We checked minimum rib thickness against cutter reach and suggested small corner radii in deep pockets to match available end mills. Larger radii let a stiffer, shorter tool be used, which reduces chatter on thin walls.
2. Op 1, datum and clamp bore. From a 7075-T6 blank, we machined the datum face and rough-bored the tube clamp.
3. Op 2, 5-axis main operation. On a dedicated fixture, the motor face, bolt pattern and tilted features were finished in one clamping, so the motor axis relates directly to the clamp bore axis.
4. Pocketing and ribs. Pockets were cleared with trochoidal (adaptive) toolpaths that keep tool load constant, then ribs were finished in alternating light passes from each side so neither side was left unsupported.
5. Clamp bore finishing and slotting. The tube bore was finish-bored, then the clamp slot was cut.
6. Deburring. Thin rib edges were deburred by hand to avoid bending them.
7. Anodizing. Parts were Type II black anodized; the clamp bore size allowed for coating growth, and tapped holes were chased after coating where needed.

The bracket used the same sequence, with the frame mounting pattern and clamp bore finished together.

Quality plan

Inspection covered fit, flatness and mass.

• First-article inspection of all dimensions on CMM, including motor face flatness and bolt pattern position relative to the clamp bore axis
• Clamp bore size checked with pin gauges and bore gauges after anodizing
• Rib thickness sampled on CMM at several points
• Threads checked with go/no-go thread gauges
• Mass of every part weighed on a calibrated scale and compared with the drawing target and tolerance
• Visual inspection for chatter marks, bent edges and anodize uniformity

Clamp bore, motor face flatness and mass were 100% inspected on the pilot run. Material certificates for the 7075-T6 stock were included.

What was delivered

We delivered prototype motor mounts and brackets for the first airframe build, followed by a pilot run on the same fixtures and programs. The delivery included:

• CMM first-article reports for both parts
• Per-part records of clamp bore size, motor face flatness and measured mass
• Material certificates for the 7075-T6 stock
• Notes from the DFM review on corner radii and minimum rib thickness for future revisions

Each part was serial marked so its mass and dimensional record could be traced during the build. Flight testing and vibration evaluation were carried out by the customer on their airframe.

Questions about this project