Automobile & Light Truck Certification Test A5 BrakesMachiningHard
A technician is machining a brake rotor and notices that the lathe continuously produces a tapered cut, even after re-mounting the rotor multiple times and checking for runout. The outer edge of the rotor is consistently thicker than the inner edge. What is the MOST likely cause of this persistent issue on a bench-type lathe?
- AThe lathe's cross-feed mechanism has excessive backlash.
- BThe cutting bits are not installed at the correct angle.
- CThe lathe's main spindle or arbor is bent or misaligned.
- DThe rotor's mounting adapters are worn or incorrect.
Show answer & explanationAnswer & explanation
Correct answer: C. The lathe's main spindle or arbor is bent or misaligned.
If a tapered cut persists despite proper rotor mounting and runout checks, it strongly suggests a fundamental issue with the lathe itself. A bent or misaligned main spindle or arbor would cause the entire rotor to rotate off-axis relative to the cutting tools, consistently producing a tapered surface.
Why the other options are wrong
- A. Excessive backlash in the cross-feed can cause inconsistent depth of cut or difficulty in making fine adjustments, but not typically a consistent, uncorrectable taper.
- B. Incorrect bit angle usually affects surface finish or chip formation, not a consistent taper across the entire rotor face.
- D. Worn or incorrect adapters would lead to runout that could be corrected by proper mounting or new adapters. The question implies this has already been checked.
Lathe Spindle Misalignment
A condition where the main rotating component (spindle/arbor) of a brake lathe is not perfectly straight or aligned, leading to consistent errors like tapered cuts, regardless of workpiece mounting.
- Causes persistent dimensional inaccuracies.
- Requires professional repair or replacement of lathe components.
- Cannot be corrected by adjusting workpiece mounting or cutting parameters.
Memory trick: If the core is crooked, every cut is a crooked look it!