Wheel Spacers Machined from a Hand Sketch — 12 Days from Drawing to Delivery

The Brief

A UK motorsport parts supplier contacted us needing a custom set of wheel spacers for a track-day vehicle build. Their customer had a specific bolt pattern and hub bore combination that wasn't available off-the-shelf — 5×108 bolt pattern with a 73.1mm hub bore and 25mm thickness, in both silver and black anodized finishes.

They didn't have a CAD file. They sent a hand-drawn sketch with the critical dimensions written in pencil — bolt circle diameter, hub bore, overall thickness, and stud hole diameter. No tolerances specified on the drawing beyond "needs to fit properly."

Material Selection: Why 7075-T6

Wheel spacers are safety-critical components. They carry the full lateral and vertical loads of a moving vehicle. Material choice matters significantly:

Property	Al 6061-T6	Al 7075-T6	Why it matters
Tensile Strength	310 MPa	572 MPa	Higher strength under wheel loads
Yield Strength	276 MPa	503 MPa	Resists deformation under clamping torque
Density	2.70 g/cm³	2.81 g/cm³	Minimal weight difference vs. big strength gain
Machinability	Excellent	Good	Slightly harder to machine, tools wear faster
Anodizing	Excellent	Good	Takes anodize well, slightly darker natural colour
Typical use	General structural	Aerospace, motorsport	Higher safety margin for load-bearing parts

We recommended 7075-T6 over the more common 6061. The customer confirmed — the extra strength margin was appropriate for a track-day application where wheel loads are higher than road use.

CAD Redraw Process

Our engineer received the sketch photo at 09:30. By 14:00 the same day, we had a full 3D CAD model in STEP format ready for customer review. The model included:

• Hub bore diameter to H7 tolerance (interference fit with hub spigot)
• 5× stud holes on 108mm PCD — bolt circle accuracy ±0.05mm
• Chamfered stud hole entries for easier fitment
• Flat faces parallel within 0.02mm (critical for wheel seating)
• Laser engraving position for PCD/hub bore marking

The customer reviewed the STEP file in their CAD viewer, confirmed the geometry, and approved production the same afternoon.

Machining

Wheel spacers are primarily a CNC turning job with secondary milling for the stud holes. The process sequence:

• Op 1 — Turn OD and face: Billet 7075-T6 bar turned to outer diameter, face skimmed flat. Hub bore rough bored.
• Op 2 — Finish bore hub: Hub bore finish bored to H7 tolerance. Surface finish Ra 1.6μm for good hub register fit.
• Op 3 — Mill stud holes: 5× stud holes drilled and reamed on CNC mill. PCD accuracy held to ±0.05mm using rotary table.
• Op 4 — Deburr and inspect: All edges deburred. Hub bore, OD, thickness, and PCD measured and recorded.
• Op 5 — Anodize: Silver parts clear anodized (Type II, 15μm). Black parts black anodized. Both batches to same dimensional spec — anodize layer accounted for in final machined dimensions.
• Op 6 — Laser mark: PCD and hub bore specifications laser engraved on face for customer identification.

Technical Challenges

Hub bore tolerance: The H7 tolerance on the hub bore (±0.015mm) is the most critical dimension. Too loose and the spacer doesn't register accurately on the hub. Too tight and it won't fit. We bore this in two passes — rough bore leaving 0.15mm stock, then finish bore to final dimension with a dedicated boring head. CMM verification after each batch.

PCD accuracy: The stud holes must be precisely on the bolt circle diameter. A 5-stud spacer with one hole out of position won't accept all five wheel bolts. We hold PCD accuracy to ±0.05mm using a precision rotary table on the mill.

Anodize dimensional allowance: Type II anodize builds approximately 5–8μm per side. Black anodize builds 15–20μm. We machine final dimensions with this allowance factored in so the anodized part hits the target dimension — not the pre-anodize dimension.

Inspection & Quality

Every wheel spacer in this batch was inspected before shipment:

Dimension	Nominal	Tolerance	Method
Hub bore diameter	73.1mm	H7 (+0.000/+0.030mm)	Bore gauge + CMM
Overall thickness	25.0mm	±0.05mm	Outside micrometer
Stud PCD	108.0mm	±0.05mm	CMM
Stud hole diameter	14.3mm	+0.1/0mm	Plug gauge
Face flatness	—	0.02mm	CMM
Face parallelism	—	0.02mm	CMM

Project Timeline

Outcome

Twelve days from pencil sketch to parts on the car. The customer had tried two other suppliers before reaching us — one required a CAD file and the other had an 8-week lead time. We accepted the hand drawing, redrawed it in CAD within half a day, and delivered finished anodized parts faster than most suppliers can acknowledge a quote request.

The same customer has since ordered three more custom wheel spacer configurations — different PCD patterns and thicknesses — all from hand sketches or simple PDF drawings.