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machining of brake drums with polycrystalline cbn inserts

A brake drum is an automotive component that helps the vehicle to stop moving when the driver applies the brakes. It stops the tires from rotating through the friction produced when two pads are pressed against the inner surface of the drum. Turning is the main form of machining performed on brake drums. It can be done on a brake lathe or any other lathe that is big enough to mount the drum for efficient turning. Brake drums are usually made of grey cast iron. So production shops that need to machine a fairly large number of units per day should use polycrystalline cbn inserts because they are designed for high speed machining of cast iron and they have very good wear resistance.

First Stage Turning of Brake Drum (Roughing)
During the casting of brake drums, some distortion and warping occurs when the molten iron is poured into the mold and during cooling. This results in some degree of ovality in the shape of the brake drums. For this reason, the first stage of machining of brake drums is a roughing cut to make the internal and external surfaces round. This first roughing stage also removes any impurities that may be on the surface of the drum. On large brake drums used for pick-up trucks and buses, the ovality could be up to 4 mm. In this case, there will be variable load on the insert because on one end the insert will cut 4 mm while at the other end it will cut nothing.

Therefore, cbn turning insert that provides adequate toughness will be required for such roughing operations. Good examples of polycrystalline cbn inserts that can be used include square-shaped SNMN, SNGN series, the diamond shaped CNMN, CNUN and CNGN series and the CNGA, SNGA, TPGA and DNGA series. Suitable cutting conditions for roughing of brake drums include: a cutting speed of 1000 m/min, a feed rate of 0.5 mm/rev, and a cutting depth of 3 mm. For grey cast iron, the cutting may be done without any coolant.

Second Stage Turning of Brake Drum (Semi-finishing or Finishing)
The purpose of the second stage of brake drum machining is to ensure that the internal and external surfaces have a clean cut that meets the customer or manufacturer's specification. The cutting speed at this stage may be increased to about 1,300 m/min while the feed rate may be increased to 1 mm/rev. The depth of cut could be reduced to 0.5 mm. The type of insert used for the roughing operation may also be used for the finishing operation.

Inner Re-surfacing of Brake Drum
The brake drum's inner surface usually becomes very rough and dented due to impact from the brake pads. Most drums can be refurbished by turning the inner surface to a diameter within the manufacturer's specification. This inner re-surfacing is usually done on a brake lathe machine at a relatively low speed. Great care must be taken to prevent excessive removal of material while turning the brake drum. It is advisable to use a cbn turning insert with the smallest cutting edge such as the hexagonal shaped WNMN or WNMG inserts.

Conclusion
Tool inserts for machining of brake drums should be chosen carefully to suit the operation and the qualities of the cast iron used for the drum. The ferrite and silicon content should also be checked before machining. Cast iron with higher silicon content will usually require slower cutting speeds.

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