Automobile & Light Truck Certification Test A5 BrakesMachiningHard
A technician is machining a brake rotor on an on-car brake lathe. After several passes, the technician observes a slight taper across the rotor surface, with the outer edge being thicker than the inner edge. Which of the following is the MOST likely cause of this condition?
- AThe vehicle's wheel bearing has excessive play.
- BThe rotor was not properly cleaned before mounting on the lathe.
- CThe cutting bits are dull or improperly installed.
- DExcessive feed rate was used during the machining process.
Show answer & explanationAnswer & explanation
Correct answer: A. The vehicle's wheel bearing has excessive play.
Excessive wheel bearing play allows the rotor to wobble slightly as it rotates during machining, leading to an uneven cut and creating a taper. The lathe's cutting head is fixed, but the rotor's movement causes the discrepancy.
Why the other options are wrong
- B. Improper cleaning usually leads to runout or an uneven initial cut, but not necessarily a consistent taper during subsequent passes.
- C. Dull or improperly installed bits typically cause poor surface finish or chatter, not a consistent taper.
- D. Excessive feed rate can cause a rough finish or chatter, but not usually a consistent taper across the rotor.
On-Car Lathe Taper Cause
A common issue where an on-car brake lathe produces a rotor surface that is thicker on one side (inner or outer) than the other, often caused by underlying vehicle component issues.
- Indicates uneven material removal.
- Can lead to brake pulsation after machining.
- Often points to issues with wheel bearings or suspension components.
Memory trick: If the wheel wiggles, the rotor won't be even!