Automobile & Light Truck Certification Test A5 BrakesMachiningMedium
A technician is inspecting a brake rotor that has been machined. The rotor surface shows a series of fine, concentric grooves, often referred to as a 'phonograph record' finish. What is the MOST likely cause of this condition?
- AInsufficient depth of cut was used during the finishing pass.
- BAn incorrect feed rate setting on the brake lathe.
- CExcessive runout in the rotor before machining.
- DThe cutting bits were too sharp and dug into the material.
Show answer & explanationAnswer & explanation
Correct answer: B. An incorrect feed rate setting on the brake lathe.
A 'phonograph record' finish, characterized by visible concentric grooves, is typically caused by using too high of a feed rate on the lathe during the finishing pass. This causes the cutting tool to move too quickly across the surface, leaving distinct tool marks.
Why the other options are wrong
- A. Insufficient depth of cut might not clean up the surface effectively, but it wouldn't inherently create a 'phonograph record' finish; it might just leave existing imperfections.
- C. Excessive runout usually causes a tapered or non-flat surface, not necessarily uniform concentric grooves.
- D. Sharp bits are desired; dull bits would cause a rougher, torn finish or chatter.
Phonograph Record Finish
A brake rotor or drum surface finish characterized by visible, fine concentric grooves, resembling an old vinyl record, often caused by an improper (usually too high) feed rate during machining.
- Indicates an uneven surface at a microscopic level.
- Can lead to premature pad wear or noise.
- Requires adjustment of lathe feed rate for correction.
Memory trick: Fast feed, fancy grooves, but not for your brakes!