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When replacing brake pads, a problem with the brake discs may arise. Should you leave them as they are, replace them with new ones, or machine (turn) them? Unfortunately, there is no single clear answer.
As usual in such cases, the decision should depend on the condition of the given component.
Deciding when to replace brake pads is very simple – even a not very experienced driver can tell a good pad from a worn one. With brake discs, however, it is 
a bit more complicated.
Disc thickness varies greatly (in passenger cars) from 10 mm to 28 mm, so assessing the disc condition correctly can be difficult. Thicker discs are not necessarily more durable, because regardless of the original thickness, the allowable wear for further use must not exceed 1 mm per side. For example, if a new disc is 19 mm thick, the minimum thickness is 17 mm. Using a disc below the permitted minimum thickness is forbidden and very dangerous.
A worn disc heats up faster (even up to 500°C) and cannot dissipate large amounts of heat. As a result, brake overheating occurs much sooner and braking effectiveness is lost. This often happens at the worst possible time (e.g. when driving downhill). A thin disc is also more prone to cracking.
If the disc thickness is above the minimum, it can still be used. In such a case, when replacing pads, it is recommended to machine the disc surface to remove unevenness created during operation with the old pads.
Installing new pads on an old, unevenly worn disc may cause strong brake heating during the initial period of use. This results from constant rubbing of the pad against the disc.
Disc machining is also recommended if the disc is corroded. Remember that after machining, the thickness must still be greater than the minimum, and the surface must be free of pitting. The amount of material that can be removed is small, so in practice this operation is quite rarely possible.
Discs with, for example, 50,000 km of mileage often have irregularities and wear so large that machining will not allow the minimum dimension to be maintained.
A common disc defect is warping. It shows up as unpleasant steering wheel vibrations after lightly pressing the brake pedal, already at around 70–120 km/h. This can happen even with new discs if there is a sudden temperature change (e.g. driving into a puddle with very hot discs) or during intensive (e.g. sporty) use. Continuing to drive with warped discs is very troublesome because, in addition to reduced comfort, strong vibrations cause accelerated wear of the entire suspension.
How to machine them?
WHY MACHINING (TURNING) BRAKE DISCS ON THE VEHICLE WITHOUT REMOVING THEM FIRST IS NOT RECOMMENDED, UNACCEPTABLE, OR MAYBE SHOULD EVEN BE PROHIBITED. Here are the objections:
1. With an on-car brake disc lathe, it is not possible to correctly mount the cutting head in a plane perpendicular to the rotation axis of the hub (= the brake disc) using two corroded points with a very small contact area. A plane is defined by three points not lying on one straight line.
2. The operator of an on-car brake disc lathe cannot reliably estimate the amount of play in the hub bearings, and even worse – most often cannot eliminate this play, which is a NECESSARY condition for the hub (i.e. the disc) to rotate around one CONSTANT axis. Even installing new bearings does not guarantee zero play, because new bearings often have a permissible factory (and sometimes required) bearing clearance.
A rotating/“wandering” hub/disc axis means the feed direction of the cutting head is not perpendicular to the changing (during machining) rotation axis of the disc.
JUSTIFICATION
Regarding objection No. 1
Most manufacturers of on-car lathes use two chassis/suspension points for mounting the cutting head – the same points used to mount the brake caliper bracket. These are two small, often corroded surfaces with bolt holes. In theory, these surfaces lie in one plane within manufacturing tolerance, but only when freshly machined at the factory. Also, these surfaces are not machined in one setup together with the hub/bearing housing, because they are on a separate bracket/carrier which is bolted to the hub housing, and then the caliper bracket is mounted to it. Each part has manufacturing tolerances, plus fit tolerances. As a result, those two caliper-mount surfaces do not have to be – and often are not even from the factory – perfectly perpendicular to the theoretical hub rotation axis. This may be harmless for caliper operation, but it can be unacceptable for mounting a cutting head and machining a damaged disc. To treat them as reference surfaces, they must be cleaned of corrosion first – and then the question arises: how to clean them while restoring the original “factory position” relative to the hub axis?
In summary: a mechanic machining discs on the car with this mounting method has virtually no chance to mount the tool in a way that guarantees the cutting head feeds perpendicular to the disc’s symmetry/rotation axis. As a result, the machined surface is not perpendicular to the disc axis because the PRIMARY requirement for correct machining was not met.
Regarding objection No. 2
Manufacturers mention the need to determine and eliminate bearing clearance, but they usually omit the fact that meeting this second PRIMARY condition is practically impossible. Most modern hubs use double-row angular-contact ball bearings, which often have an allowable or required factory clearance between the outer and inner rings. Such bearings have no clearance adjustment. This means the hub nearly always has some play, even in a new car, and certainly after 50,000–100,000 km or more. If the disc is mounted on such a hub and the cutting head is mounted indirectly to the hub housing, the disc “wobbles” between the cutting tools. It can deflect in multiple directions, including axial movement, depending on which tool generates higher forces. These forces also depend on the depth of cut, which changes as the tools move toward or away from the disc axis.
Theoretically – and very likely in practice – the machined disc surface will not be truly flat (in a single plane) and its shape will be somewhat random. While the thickness across sections may end up consistent (because the final spacing between tools is constant), the surface may resemble a very shallow “bowl.” Steering wheel vibration may be reduced, but the brake pad friction material may not contact the disc across the full area initially. Because contact area is reduced, surface pressure increases, leading to overheating at the contact zones and corrosion beginning in areas where the pad does not touch. Therefore, long service life of this part of the brake system is questionable.
ADDITIONAL NOTES
1. Manufacturers and sellers often use the key marketing claim that the disc is not removed from the car. If a workshop machines discs this way, it should be avoided. The first action of a mechanic who suggests machining should be to remove the disc and check whether:
a. something got trapped between the disc and hub during previous installation, causing runout, vibration, or faster/asymmetric wear
b. the mating surfaces (disc and hub) are corroded – they usually are, and must be cleaned, preferably with a rotating steel wire brush. These surfaces are critical for proper disc-to-pad operation and are also the base for machining. The disc should be machined so that it could be installed on the other side as well. If, for example, the owner later goes to another mechanic to replace hub bearings, the discs must be removed and refitted – nobody will track which disc was on which wheel. That is obvious.
c. the inner side of the disc is often covered by a formed metal dust shield bolted together with the bracket to the hub bearing housing. It may need to be removed to gain access to the inner surface for machining and to create space for the cutting tool – and removing it often requires disc removal.
2. To machine a disc properly on such an on-car lathe may take 2–3 hours even for one disc. Therefore, the service would have to be very expensive – often 100–150 PLN per disc. The worst part is that the quality of this service is questionable, if not unacceptable.
SUMMARY
After reading the above, the question arises: can and should brake discs be machined?
Answer: yes, you can and you should machine every disc when replacing pads, as long as the disc does not need replacement because its thickness is still above the minimum. This means: when replacing brake pads, you should also replace the disc/drum or machine it without exception. Typically a disc has a wear reserve that allows machining of about 0.8–1.0 mm per side, i.e. 1.6–2.0 mm total. The minimum disc thickness is specified by the vehicle manufacturer and must not be exceeded. However, you should AVOID machining on the on-car lathes described above.
The best approach is machining on specialized stationary lathes after cleaning the centering/mating surface.
This guarantees the highest service quality.
source:
http://www.motofakty.pl/artykul/tarcze_wymieniac_czy_toczyc.html
http://wasze.motofakty.pl/artykul/nie_przetaczanie_tarcz_hamulcowych_na_samochodzie.html
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