On-Site Disc Machining

This article was first published in 2004.

Brake disc machining is one of the few car maintenance jobs that isn’t within the realm of a Do-It-Yourselfer – not many of us have a lathe and machining tools. Sure, you can remove the discs at home but by the time you drop them off and pick them up from the machinist, it becomes a real pain in the backside.

Well, here’s the solution – professional on-site disc machining. In this article we’ll follow Adelaide’s Mobile Brake Specialists during an on-site disc machining job.

Step-by-Step

The first step is to jack up the appropriate end of the car and place it securely on chassis stands. In this case, the front discs are to be machined.

Caution - jacking up a vehicle on gravel should be avoided wherever possible. 

Next, the wheels’ centre caps/hub caps are removed and both wheels are taken off the vehicle.

To remove the brake discs on our demo Nissan 180SX, the calipers must be separated from the hub assembly. This requires the removal of two bolts.

Before any machining can take place, the thickness of the disc must be carefully checked. In the case of our 180SX, the standard disc thickness is 18mm but they had already been worn down to 17.4mm at their thinnest. Fortunately, the minimum allowable disc thickness for this vehicle is 16mm – this meant we could remove a considerable amount of material from the disc without major concern. Each vehicle has a minimum allowable disc thickness.

The discs are now removed from the vehicle. In the case of our 180SX, the discs simply slide off – only the wheel and caliper assembly hold each disc in place (the discs are ‘fully floating’).

Each disc can now be individually fitted to a mobile brake lathe. This lathe incorporates an electric motor that spins the disc at a relatively low speed which avoids heat build-up. There are two manually adjusted cutting tips that sweep across both faces of the disc.

The thickest areas of the discs are machined off manually before the lathe is set to an auto-run mode. In auto-run, the cutting tips move from the inner edge of the disc face to the outer edge. A constant rate of movement is maintained and the lathe automatically shuts down when it reaches the outside of the disc.

When the discs are finished being machined they are cleaned up with abrasive paper. This is a very important step that removes the fine grooves left by the cutting tips. A smooth surface is essential to avoid brake noise.

The discs are now refitted to the vehicle. Note that the disc mating surfaces are first cleaned to ensure a proper fit.

The calipers are now reinstalled, the wheels are fitted and the vehicle is lowered to the ground.

Job finished.

Integrated Disc/Hub Assemblies On some vehicles it may not necessary to remove the brake discs. Where the brake disc is incorporated as part of the hub, the brake lathe can be installed on-car – so long as there’s no brake dust shield to foul access to the inboard face of the disc. Where possible, the lathe is bolted to the wheel studs and its electric motor spins the disc while on the vehicle. The body of the lathe can be balanced inside the wheel arch or propped against part of the under-car bodywork to prevent it moving due to the torque of the electric motor. Note that this approach eliminates the need to repack the wheel bearing with grease – this is often unavoidable when the disc/hub assembly is removed from the vehicle.

Simultaneous Pad Swap

Ideally, a vehicle’s brake pads should be changed when the discs are machined. Retaining the old pads can cause uneven disc wear and reduced braking performance.

In the case of our Nissan 180SX, the used brake pads are easily removed when the calipers are separated from the hub assembly.

The caliper piston must then be pushed into its bore to accommodate the greater thickness of the new brake pads. In this photo, the piston is being pushed back using a special tool.

Note that the rubber brake lines are first clamped near each caliper. The caliper bleed nipples are then opened to provide an escape for the brake fluid that’s displaced when pushing the caliper piston back. Once this piston is pushed back, the bleed nipples are retightened.

The thin metal backing plates must be refitted along with the new brake pads. These backing plates prevent brake rattles and squeaks.

The brakes are now reassembled, the brake pedal is pumped and the necessary amount of brake fluid is added to the under-bonnet reservoir.

The vehicle should now be driven to bed in the new pads.

We’re told that modern brake pads don’t require the frantic bedding in efforts of ‘the old days’ – all they need are a few firm brake applications from about 80 to 20 km/h. If you overwork the pads at this stage they will glaze and you’ll never achieve their potential braking performance.

Where, When and How Much?

South Australia’s Mobile Brake Specialists Pty Ltd operates three vans within the Adelaide metropolitan area. On-site work is generally performed at private residences but professional workshops are regular customers as well. The company operate 5½ days a week and you can arrange a call-out typically within a day.

An on-site brake disc machine job costs between AUD$80 – AUD$120 – the jobs at the higher end of this range include the labour for a brake pad swap. The total cost to have our 180SX’s front discs machined and new pads installed was just AUD$165 (including purchase of the pads).

An on-site disc machining certainly makes a lot of sense when you weigh up the time involved removing/fitting a car’s discs at home and then running around to machine shops. Why would you bother doing it yourself?

Contact: Mobile Brake Specialists Pty Ltd (Adelaide, South Australia)         +61 8 8211 8442

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