Turning a brake rotor -- not so hard after all!

[awake]

I just came in from doing a "quick" brake job on our minivan. Well, it WAS going quickly, until I discovered that the sound I was hearing was NOT the wear indicator, as I had hoped, but rather was one of the shoes that had worn down to the metal and was gouging lovely ridges in the rotor. Problem is, I discovered this at 10:00 pm, and my wife needs the van in the morning.

I thought I had read (here or elsewhere) that turning your own rotors is a waste of time, hard to get right, etc., etc., but I was desperate, and the rotor had way more than enough thickness, so I chucked it up on my 12-1/2" Cincinnati TrayTop. Had to turn the compound to 90º and do a little maneuvering to get to where the tool could sweep the face. No carbide tooling on hand, so I'm hoping HSS will do.

And ... it did. Very well. In fact, the job was so easy, I wondered what all the fuss was about. I kept a shop vac on it and that helped keep most of the mess contained, though I still wiped the ways down carefully afterwards. Took two passes at .010" DOC to clean it up -- probably way too cautious, but I didn't want to take off any more than I had to -- then a "finish" pass with slower feed and .005" DOC. It didn't come out with a mirror finish per se, but it cleaned up very nicely indeed. Remounted it on the van, put in the new pads, put the wheels back on. Tested it, and wow! it stops much smoother than it's been in a good while.

So, here's my question (gotta have a question, right?): Did I just get lucky, or is it just not that hard after all?

 

[Finegrain]

I think that the prevailing belief is that the time it takes to do a 1-off is more costly (figuring shop rates) than it would be to have the NAPA guy do it. Certainly any machinist can do a brake rotor, just a matter of what is your time worth, or in your case, is it even possible to job it out and meet your time constraints.

Regards.

Finegrain

 

[Perry Harrington]

I really don't understand why more people don't turn rotors or use a flywheel grinder to clean them up. I've machined flywheels on the lathe many times, it's a piece of cake. A brake rotor is just more prone to chatter due to lack of rigidity. I've had problems turning large rotors (13in plus) on the brake lathe, but thought the flywheel grinder would be a cinch.

 

[Bruce Griffing]

I am one step further back. As long as the rotors are not distorted, what is the problem with a few grooves?? Why bother turning them at all? (Assuming you replace the pads) I have never turned them at all.

 

[DieselPower]

Nobody ever told me it was hard to do, so I think that was one of the first things I ever did on the lathe. Been doing them ever since.

 

[HuFlungDung]

I'd turn an old set of rotors if new pads are going on. I figure they'd wear in pretty rapidly on a gouged surface.....and not produce good braking effect for quite a while. There is never a whole lot of excess material left on a worn set of rotors, and one should be cautious about turning them below minimum thickness (usually stamped on them somewhere). So for the sake of not turning them under thickness, I would not worry about machining all the wear grooves out, but I'd like it to look like it was 80% flat.

The bad thing about turning rotors, if anything acts up about the braking system, like noisy, squealing pads, vibration, or whatever else a paying customer may complain about, you'll have to put up with the flak, even if they took the car out and warped the rotors themselves through some unusual driving condition.

 

[DocsMachine]

The main issue with doing a rotor is typically parallelism- getting both sides even and square.

There's no real 'trick' to it, just a careful setup, no real difference between that and any other workpiece. But, it's too easy, if you're not paying attention, or have to cobble an iffy setup on a too-small lathe, for example, to turn one side, flip the rotor and turn the other side, and wind up with an eccentric cut- where one side of the rotor is thicker than the other.

That kind of error is all to easy unless your lathe is big enough to be able to turn both sides in the same setup. A brake lathe turns both sides at the same time, virtually guaranteeing parallelism.

And it doesn't take much error to produce a "pulsating" pedal- I'm told that even just .005 to .008" can make a very noticible vibration, and anything over .010 supposedly makes it almost intolerably so. That's an old hotrodders' "rule of thumb" and I haven't tried it myself, so take it as you will.

And as noted, today, most parts stores will turn a rotor for maybe $15- for anyone but a hobbyist with the machines and the time, it's almost not worth turning the lathe on for that.

'Course, I have a real live brake lathe so the point is moot for me.

Doc.

 

[Marcibb]

I do not think one would want to turn rotors to a minor finish to smooth of a finish and the pads will not grab properly (grab may not be the proper term here) and cause the rotor to glaze

A brake mechanic once explain to me that the problem with grooved rotors comes in play when the pads retract the pads float between the calipers and the rotor the groves cause the pads to rub unevenly against the rotor causing hot spots/areas and uneven ware and possibly rotor surface glazing all leading to reduce of breaking action from that wheel after a time uneven breaking between the wheels will result causing the vehicle to pull to one side or the other and require longer breaking distances.

Marci

This subject has been disused here before see http://www.practicalmachinist.com/vb/showthread.php/machining-disc-brake-rotors-151383.html

 

[ToolPost]

As said above, the main thing is to keep both sides of the rotor surface parallel to each other, so the pads ride straight in thier grooves. This also comes down to how true the rotor is chucked up (rotor surface relative to the hub), but as most are cast surfaces, indicating the old rotor surface with a 0.001" indicator more than suffices.

The main premise behind a brake lathe is that there are usually two tools that cut each side of the rotor at the same time, advancing on the same cross slide, ensuring the rotor is cut square. In terms of surface, anything you can get on a lathe is fine, just don't go cylindrical grinding them.

As for the reason why no one really does this, cost would be the issue here. Why bother when you can purchase a new set of rotors for $40? I can see where it makes sense to as in the OP's case, as he needed them urgently, but otherwise, there's no reason to.

I've never read a service manual that advised turning brake rotors; most state to replace the rotors.

 

[Greg White]

Yep I turned em too.
My bud snuck his corvette rotors in and milled them on the Hass whilst I kept the forman busy with silly questions, I was pleased with the finish he produced,we thought it would stop good,it does.
Gw

 

[sandiapaul]

This brings back good memories...when I was about 17 my Austin Healy Sprite's front rotors needed turning. All my Dad had at home was a 6" Atlas lathe, so we carted them over to his cousin's house. He was a classic old time machinist, long since retired. He had a few old converted flat belt machines in his dark basement. We chucked them up in an ancient lathe. He guided me through the whole process and never once did any of the work himself, he made me do it all. One of my fondest memories.

 

[Pierce Butler]

I have done rotors on a lathe before. It didn't turn out well. The problem with some braking systems is that they require some pretty tight tolerances in terms of runout. Unless you are willing to hold sub .001 runout and have a readily available reference point, the time spent turning the rotor will be wasted. Then there is the nasty mess left behind.

If it were my car I would have thrown pads on it and let it go for a day. Later I would have changed or turned the rotors at the auto parts store.

 

[Question Boy]

As long as the rotors are not distorted, what is the problem with a few grooves?? Why bother turning them at all? (Assuming you replace the pads) I have never turned them at all.

Ditto on that. Just go easy on the brakes for a few hunnert miles when the pads are replaced. The only difference between a smooth rotor and a grooved (from normal wear) rotor is a few thousand miles anyhoo.

I suspect much of it comes down to liability in a shop/customer setting.

QB

 

[CalG]

I just take the side grinder with a metal sanding disc too 'em to knock off the ridge high lines.

A few moments on each side with a coarse flat stone to smooth them off, and wash down with soap and water. Slap 'em back on!

Works once or twice. Then new rotors ;-)

less than 15 min. each, Way better than the drive to town ;-)

 

[Garwood]

I just take the side grinder with a metal sanding disc too 'em to knock off the ridge high lines.

A few moments on each side with a coarse flat stone to smooth them off, and wash down with soap and water. Slap 'em back on!

Works once or twice. Then new rotors ;-)

less than 15 min. each, Way better than the drive to town ;-)

Exactly. A coarse grinding disc on a handheld grinder works excellent.

For the cost of new rotors these days (even USA raybestos) it's hardly worthwhile to turn them unless you have an on the car lathe. If a rotor is warped I toss it, there's too much hassle to turn it on strong odds the driver's just going to warp it again especially now that the rotor is thinner.

One aspect of brake repair that gets me is many people tend to cheap out on the rotors by reusing their old crap and then buy rebuilt or new calipers. If I have the time I tend to buy new rotors and quality rebuild kits for the calipers. Often the kits come with new pistons, all seals, boots and bleeders for less than $5 a piece. Doing it yourself you know it's done right.

 

[Newman109]

I recently replaced the front rotors on my 1998 Dodge Dakota. They began to lurch when I stopped due to having become warped. They also had hot spots on them when I examined them.

NAPA wanted $20 apiece to turn them or $29.00 apiece for new ones, presumably made in China. I bought the new ones and some new pads, which I needed anyway. Prices on rotors have come down tremendously in the past 10 years.

Since that's the case, I don't think it's worth one's time to bother setting them up and turning them. Not only that, they are then thinner and much more prone to warp again.

I've had them go bad on motorcycles and they are much more expensive. Conversely, they are much thinner and generally, it's not recommended to try to turn bike rotors. I would just replace bike rotors if they went bad.

 

[strokersix]

12" rotors but only 11" lathe

I stood an extra spindle up on my mill table, hung a motor off the side, and wrapped a vee belt around the rotor. Then a simple matter of a fixed cutter in the locked spindle and I was good to go. Arguably even better than an off-car brake lathe since the rotors were turning on their own bearings.

The time versus $$ argument may be different for vintage rotors not easily replaced if your friendly parts counter person ruins them.

 

[chuckey]

I remember that I used to get a recurring problem with my Volvo 240 series discs with the encroachment of rust especially from the hub area into the active area which glazed up and I removed on a lathe. I suspect that the big thing with proper brake turning lathes is that you only have to fix and line up the disc ONCE. Even if they have two carriages, the disc still has to be centered some how between them to ensure equal depth of cut. Which with swinging calipers I don't think is a big deal, at least not until the error gets huge (.05" difference in thickness?).
I had a friend who was having a bit of vibration when he braked hard. Turned out that the ventilated discs were more like like two discs over half their diameter, due to the separaters(?) vanes(?) spacers(?) having rusted and fallen out. (Ford Scorpio).
Frank

 

[gary350]

My time is more valuable than money saved by turning my own rotors. I just put new brakes and rotors on my GMC for $76.

OK.........so I could have saved $60 to turn them myself but as it is it took me 1 hour to do the brake job. If I had turned the rotors myself it would have taken all day. First I would have to make a holder for the lathe then turn each rotor. If I value my time at $20 per hour it would cost me $160 to do the brake job thats $100 more than I paid.

 

[Newman109]

My time is more valuable than money saved by turning my own rotors. I just put new brakes and rotors on my GMC for $76.

OK.........so I could have saved $60 to turn them myself but as it is it took me 1 hour to do the brake job. If I had turned the rotors myself it would have taken all day. First I would have to make a holder for the lathe then turn each rotor. If I value my time at $20 per hour it would cost me $160 to do the brake job thats $100 more than I paid.

That's the way I see it too. Furthermore, on some of the larger GMC's and Fords, turning them is not all that effective since they have a tendency to warp when re-installed. There are turning machines used at the dealerships that can do them in place when torqued down with a plate, not unlike motorcycle cylinders.

New rotors are so inexpensive now that I don't bother.