Technical and Training Technical Articles Disc Machining, cleaning & bedding-in new brake pads

Disc Machining, cleaning & bedding-in new brake pads

Machine discs using the slowest feed rate with minimal depth to prevent grooving. For final clean-up use brake part cleaner. Do not use an oil based solvent. Please note that on vehicles fitted with ABS systems the wheel speed sensor needs to be covered during on car machining processes to avoid the sensor being contaminated with metal filings.

Machining may leave a fine film of graphite on the surface that could affect braking. Clean with brake part cleaner. Never use petrol for cleaning!

After fitting, once again wipe down the disc with a clean rag and brake part cleaner.

Bedding-in is always advisable to condition material surfaces correctly. At 60 kph apply brakes using light to moderate pedal effort reducing to 10 kph. Do this at least 10 times.

Rotors should be checked for run-out using a dial indicator and thickness variation, preferably using a micrometer for accuracy. In the absence of these, a reasonably accurate check can be made by rotating the disc by hand with the caliper and pad still in place. If the rotor drags in some places as it rotates, it can be assumed that some amount of run-out or thickness variation exists.

In addition to this, a smooth rotor surface is essential for correct bedding in of new friction material and the prevention of noise. It should be noted however, that smooth does not mean glazed. A highly polished surface that readily reflects light will not allow modern friction materials to bed in correctly. Should any of these conditions exist, the rotor, providing it is not below its minimum thickness, will require machining. Rotors which have been machined to or past their maximum limit will have to be replaced.

A point worth mentioning here is that after the caliper is removed, it should be suspended from a piece of wire attached to some part of the suspension or body. This prevents the brake hose having to take the weight of the caliper. Rotors should always be machined in two stages. The first stage is used to remove all surface irregularities and is done in even cuts from both sides of the rotor to a depth of between 0.125mm and 0.25 mm. The final cut is done to a depth of approximately 0.1mm on both sides of the rotor. A slow feed speed is necessary to give an acceptable surface finish to the rotor.

To finish off, 120 grit emery paper is used to remove any traces of graphite brought to the surface by the machining process which could contaminate the surface of a new pad.

The last important point about disc rotor machining that is often overlooked is final disc thickness. Disc rotors should be the same thickness on both wheels of an axle. This ensure that both rotors heat up the same rate, maintaining stable braking performance as the temperatures rises. Also remember, whether the disc has been removed for machining or not, the wheel bearings must be adjusted in accordance with manufaturers' recommendations.

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