To reduce unsprung weight and improve gas mileage, disk rotors are not as thick as they used to be, so once they reach a minimum thickness, you replace them.
It costs money to turn the rotors and unless it's done properly, they won't have a uniform thickness and you get pulsing, and if they are too thin they warp. It was a money maker for repair shops and an unnecessary expense for the consumer. There was a myth about needing a smooth rotor surface for good stopping. In fact, drilled and grooved rotors work better, so minor rotor scoring actually enhances the stopping power.
Bob
Yep...
I went through this several times over... I do most all my own mechanic work...
I'm kinda hard on brakes on the farm trucks, pulling trailers in the 6,000-8,000 pound range with a half ton truck tends to do that... The rotors on my 96 F-150 were pretty well shot, warped and pulsing stops, so I pulled them and did a brake job-- took them to Auto Zone and had them turned. Went and picked them up, and they had uncut "crescent moons" on them (turned improperly). I told the guy that they were worthless like that-- he had to turn them down to get the surface even and flat all the way across-- no "smooth shiny" half-moons... shoulda figured I got Forrest Gump's retarded cousin turning the rotors at the Auto Zone... I come back again and he's re-turned them, but they still have some small uncut areas on one side of the disk on the inside... I dubiously take them home and put them on, install the pads and stuff and put it all back together, take her out for a test drive, and they're FIVE TIMES WORSE than when I started! I pull the rotors off again and start mic'ing everything, and sure enough, the dumb-bunny has milled them CROOKED... he obviously didn't have them centered in the milling machine! What a moron...
(Almost as bad as the idiot pressing bearings into a housing in the shop one time that was too stupid to realize you STOP when the bearing goes crooked, and pull it out and start again... that and you never press a bearing in a bore by the INSIDE RACE... instead the idiot simply keeps pumping until he's putting his whole body weight on the handle and BOOM! the bearing explodes like shattered glass and razor-sharp shards fly across the shop in all directions...
) I call around and find new rotors are like 35 bucks for the pair for the 96 pickup so I just replace them and call it a day... I spent $20 for the botched turning job...
Pulled the rotors off my 2002 F-150 and went to have them turned... "Sorry, but they're 12.9 mm and the cutoff is 13mm... we can't do it... liability..." What a load of crap! Like 0.1 mm is going to make a difference...
Oh, they'd gladly sell me NEW rotors... at $87 bucks EACH!!! :eyepop: No thanks... did a little digging and got my brother onto the job since he was partsman at a tractor salvage and did a lot of internet searches for parts, and he got me crossdrilled and slotted racing rotors for the pickup for $125 for the PAIR... SOLD! Ran those for a few years, but I noticed my wheel bearings seemed loose... got worse and worse. Finally did a brake job and when I took the wheel bearing out, the cup was LOOSE in the rotor (should be pressed in and TIGHT... this on fell out, and it had evidently spun in the rotor enough that it was loose and floppy in there... THAT explains the loose wheel bearings I could never seem to get tightened up! No fix for that but new rotors... SO, I check around, and now the stock rotors from the auto supply are like $45 bucks each, while the crossdrilled/slotted rotors are up to
$160 bucks for the pair... so she gets stock rotors from O'Reilly's... I measure the new ones out of the box-- get this-- THEY'RE 13.5 MM THICK! What a crock... no wonder you can't get them turned... basically by the time the first set of pads on them is worn out, the rotor is "too worn out" to be turned because 0.6 mm of steel has worn off... what a crock...
SO, unless they're worn completely through or warped beyond all recognition, I just slap new pads in there and run them as-is... My Dad thought I was daft for pulling them off and having them turned years ago... he drove his old '77 Ford LTD 52 miles each way to work, and he'd do a "30 minute $9 brake job" on it on a Sunday afternoon-- pop the wheels off, pop the pads off, slap new ones in, and pop the caliper back on and put the wheels back, BAM yer done! I used to laugh because the car had RIVETED pad material to the pad backing plates, and would pick up gravel on our rural roads that would wedge in the rivet holes and wear deep grooves into the rotor-- "you need to turn that!" I'd say... he's just grin and say "oh, no... that increases the surface area and reduces the stopping distance... Heck I've seen rotors on cars in the junkyard before that were so worn the flat surface was COMPLETELY GONE and the pads were wearing on the COOLING FINS... :y: BUT, it was still working when the car was junked!!!
They don't want to turn them, fine... screw 'em... I'll run 'em til they drop... LOL
Later! OL JR