Rough Turning of Grey Cast Iron Brake Components 02:08

Most brake drums and brake discs are today made of grey cast iron. This material has been successfully machined with Seco Tools’ PCBN inserts for many years, and we have today a thorough understanding of the requirements. Vital to the success is the properties of the work piece material, and there are three parameters that need to be checked before any machining test with CBN. For these parameters, we recommend the following;

• Free ferrite content    <   5%
• Ageing time               > 10 days
• Sulphur content         =   0.1 %

If these recommendations are followed, CBN is by far the most cost effective tooling material available in the market for machining brake discs and brake drums. Cutting speeds are much higher than with any other insert material. CBN also has the wear resistance to withstand the abrasiveness of the grey cast iron.

CBN300 is Seco Tools’ recommended grade for roughing of grey cast iron. It is available in solid format, with our without cylindrical center hole, to fit any type of tool holder. Typical components where CBN300 is used are brake discs and brake drums.

With CBN300, it is possible to run at cutting speeds of 1,000 m/min or more, and reach tool lives of several thousand components (depending on operation and component size) per cutting edge. As shown in the examples below, the tool lives with CBN300 is 10 to 30 times that of ceramic inserts.

But, CBN300 is also a problem solver, and can cope with loads not possible to withstand with any other cutting tool material.

Casting of brake drums and brake discs usually result in a product that has some ovality and/or warping. Therefore, machining of brake discs and brake drum starts with a roughing cut to make the components round and/or flat, and also to remove any surface impurities.

On larger components, like brake drums for pick-up trucks, ovality can be as much as 5 mm. This means the insert on each revolution will “feel” almost zero load on one side of the component, and on the other side a very heavy load, where the depth of cut can be up to 5 mm. This requires an insert with an extreme wear resistance as well as superior toughness.

Due to its relatively high toughness, carbides can be used on these components, but tool life is short, and productivity is low. Ceramic inserts does not have the toughness needed, and will commonly break when the load variation is too high. That leaves CBN as the optimal solution. CBN can machine very cost efficiently components with “normal” ovality, but CBN can also cope with the components with higher degree of ovality and/or warping. The production can go on un-interrupted.

The option would be to sort out the “worst” components, and machine them separately, or send them out to a sub-supplier, and this will add cost.

In the example below, carbide inserts were used, but tool life was low. Ceramics was tried, but they could not withstand the large variation in depth of cut. CBN300 was the solution. Tool life is 20 times longer than with ceramics, and it was also possible to reach this tool life consistently.

After roughing, a finishing cut is made to reach the required dimensions, tolerances and surface finish criteria. Additional operations may include grinding to generate a certain surface appearance.

The finishing cut is preferably done with Seco Tools grade CBN400C, a grade developed for superior performance and productivity in finishing of grey cast iron components, which will be presented in a coming article.

For more information, please contact your local Seco representative, or our Advanced material (PCBN/PCD/CERAMIC) team.

Author: Henrik Sandqvist Product Specialist SECOMAX PCBN/PCD/CERAMIC SECO TOOLS Shanghai CO., Ltd. Level 2, Building 8, 456 HongCao Rd., Shanghai 200233, China Office: +86 21 5426 5849 Mobile: +86 186 167 90063 e-mail: henrik.sandqvist@secotools.com