5-Axis Machined Suspension Knuckle Prototypes in 7075-T6

The challenge

A suspension knuckle connects the wheel bearing, brake caliper, steering arm, lower ball joint and strut. Each of those interfaces has its own feature, and they all have to sit in the right place relative to one another, because the knuckle defines camber, caster and steering geometry.

Key requirements on the drawing:

• Hub bearing bore at H7 fit, with tight roundness and perpendicularity to the bearing shoulder
• Ball-joint taper that must seat the stud at the correct height
• Brake caliper mounting holes with a true position of 0.05 mm relative to the bearing bore axis
• Strut clamp bores at compound angles to the bore axis

Production knuckles are usually forged or cast and then machined. For a prototype, the customer wanted billet parts in 7075-T6 so that they could change the design between iterations without tooling cost. That means a large block is reduced to an organic, ribbed shape, and the part must be held securely through operations without distorting thin webs.

Our approach

We planned the job so that every critical feature was machined from as few datums as possible.

1. DFM review. We reviewed the 3D model with the customer and suggested adding two temporary clamping lugs to the billet outline. These lugs give a rigid hold during the main operation and are removed at the end.
2. Op 1, 3-axis roughing. The block was roughed on one side to leave the lugs and a flat datum face, with about 1.5 mm of stock on the final surfaces.
3. Op 2, 5-axis main operation. Held by the lugs, the part was turned on the 5-axis trunnion to reach the bearing bore, caliper mounts, steering arm and strut clamp in a single clamping. Machining these features together is the main reason the hole positions stay consistent: there is no re-clamping error between them.
4. Bearing bore. The bore was rough-bored, then finish-bored with an adjustable boring head after the part had cooled, so the H7 size was cut at a stable temperature.
5. Ball-joint taper. The taper was produced with a dedicated taper reamer and checked against a gauge stud before the part left the machine.
6. Op 3, lug removal and blending. The lugs were cut away and the area blended to the surface model.
7. Finishing. Edges were deburred, the part was bead blasted, and bores and the taper were masked before Type II clear anodize.

Toolpaths for the organic surfaces used constant-scallop finishing so the blend radii carried no step marks that could become stress risers.

Quality plan

Because the knuckle sets wheel geometry, inspection focused on relationships between features, not only individual sizes.

• Full CMM layout of the first article against the 3D model, with the bearing bore axis as primary datum
• Bearing bore size and roundness measured with a bore gauge, perpendicularity to the shoulder on CMM
• True position of caliper and strut mounting holes reported relative to the bore axis
• Ball-joint taper checked for seating height using the customer's gauge stud
• Threads checked with go/no-go gauges
• Visual inspection of blend radii for tool marks and sharp edges

Every pilot-run knuckle received 100% inspection of the bore, taper and hole positions, with a serial number laser marked on a non-functional face.

What was delivered

We delivered the prototype knuckles and a follow-up pilot run made on the same fixture and program. The delivery package included:

• CMM first-article report against the 3D model
• Per-part report for bearing bore, taper seating height and hole true position
• Material certificate for the 7075-T6 bar stock
• Laser-marked serial numbers that trace each part to its report

The customer used the parts for fit and vehicle testing. Design changes between iterations were handled by updating the model and toolpaths, with the same datum scheme kept so reports remained comparable from one revision to the next.

Questions about this project