machining brake rotors

Machining Brake Rotors

Machining rotors is a little controversial. There are a lot of reasons for this. One is that there are two styles of brake pads and manufacturer recommendations tend vary depending on which style of pad is used.

Brake pads can “work” by abrasion or adhesion. I like to think of it like this —

Adhesion analogy
Imagine a slippery polished hardwood floor. If you tried to run in socks, you’d be slipping and sliding. If you put on some shoes with a soft rubber sole, you’d get good traction. The shoes stick (or adhere) to the floor. If you walk around for years, the soles would wear down more than the floor.

Abrasion analogy
Imagine that same hardwood floor, only this time you put on some shoes with sharp metal spikes on the soles. The spikes dig into the floor and you get traction. The floor doesn’t look so good after you go running around though.

European cars seem to use pads that work on abrasion more than adhesion. They make a ton of nasty brake and rotor dust that sticks to the rims, and the rotors are chewed up by the pads. European cars generally recommend replacing pads alone until the rotors are undersized, and then they say to replace the rotors. They prohibit machining the rotors.

Japanese cars seem to use pads that work on adhesion more than abrasion. Way less dust is generated and the rotors barely wear during the life of the pads. Almost all of the cars we repair allow for machining rotors. However, some companies like Honda discourage the practice unless it can be proven it’s necessary. Their rational is that mechanics are typically incompetent and are more likely to introduce a problem that correct a problem. Thanks for your confidence Honda.

Honda does have a point. If a rotor is machined “crooked”, it will eventually cause problems. The hub face must be scraped to remove rust, uneven paint, and any other debris that might sit between the chuck on the brake lathe and the rotor hub face. Any debris in this area can cause the rotor to wobble on the lathe, and if it’s not corrected before cutting, the mechanic will make the rotor worse than when it arrived.

Another issue is surface finish. The rotor is machined with a single point cutting tool. (Well two single point cutting tools since there’s one for each side of the rotor). The rotor spins on a spindle and the cutting tools slowly move from the inside of the rotor to the outside of the rotor, cutting off metal as they go. This can leave a surface that looks like an old vinyl record, with peaks and valleys. If this surface is coarse, the pads will sit on the peaks and have less surface contact.

However, just because something bad can happen doesn’t mean you shouldn’t do it. You can crash your car and die, but you still drive, right? You exercise discretion and do your best to drive properly and safely. The same is true of machining rotors. If a mechanic is knowledgeable and does the job properly, the rotor will be true and the surface finish will be just as good as what’s produced in a factory setting.

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