Use of lead-free copper materials, especially as an antibacterial material offering high mechanical strength and increased oxidization resistance, compared to alternatives, for food, drinking water and sanitation, is increasing, due to the new EU Drinking Water Directive.
In order to improve their machineability, copper-based materials like brass have always been alloyed with lead. Dispensing with lead significantly impairs the machineability, with consequences including increased tool wear due to adhesion and material smearing, the formation of long band and snarl chips, and reductions in process reliability and productivity.
A series of lead-free or low lead copper materials has been developed in recent years to comply with the new legislation. A well-known representative is the CuZn21Si3P brass alloy, marketed under the brand name ‘Ecobrass’ or ‘Cuphin’, or other special brasses, like the wrought alloy CuZn10Si4, the cast variant CuZn10Si4-C and many others. These compounds contain silicon, which has been added to achieve similar cutting characteristics to those achieved with lead. But the higher silicon share makes the material more abrasive and so tools’ cutting edges wear out faster.
Since cutting feeds and speeds stay the same in non-leaded brass as for leaded brass, up to 1.5 m/min, depending on the cutting media, the general rule is that higher specific cutting forces and tool temperatures are expected for lead-free brass materials. On average they are double or three times those of the usual CuZn39Pb3 brass. Strong material pick-up on the chip surface and on the open surface, especially when processing CuCr1Zn, tear out TiAlN layers and expose the carbide substrate. Sharp-edged tools and those with coated edges are also subject to initial wear as a consequence of micro-chunking caused by the strong tendency toward adhesion of the lead-free Cu materials and the strong machining forces.
In comparison to TiAlN coatings, diamond coatings or diamond cutting substances offer a significantly smoother surface, a lower friction value, the lowest tendency toward adhesion and significantly higher heat conductivity. In addition, they possess wear resistance to exceptionally abrasive materials, due to their high degree of hardness, so that they are also ideally suited to dry processing.
In leaded brass, a friction-reducing lead film lowers entry of heat during processing. Since that film is no longer possible in no-lead brasses, heat entry today needs to be prevented another way. Diamond edges or coatings, with their high level of heat conductivity, reduce the entry of heat into the workpiece, which improves dimensional form and precision, especially with small and geometrically sophisticated components.
So diamond-coated edges and cutting substances like PCD, CVD-diamond (chemical vapour deposited diamond) and MCD (monocrystalline diamond) have proven to offer superior performance. The latter is unsurpassed in the production of highly precise reflective polished surfaces, although tool life is reduced when machining no-lead brass. Depending upon the Cu material or type of processing, PCD or CVD-diamond prove to be the cutting substance of choice. Each has its strengths: PCD types can be ground individually, allowing tailored inserts, while CVD-diamond edges offer the highest degree of hardness.
PCD and CVD-diamond can be used with or without coolant. The coolant type depends on the application and machine tool; high pressure coolant or minimum quantity lubricant (MQL) are possible. In contrast, MCD should never be used in dry conditions; MQL is recommended.
In roughing operations or manufacturing processes where the material is not homogeneous and the risk of inclusion is very high, PCD is the recommended cutting material. For finishing operations, the first choice is CVD-diamond for lead-free copper processing.
With these cutting materials optimally adapted to the processing task, a significant increase in tool service life, geometric precision, surface quality, chipping volume, productivity and especially process reliability is achieved. With its extensive cutting materials’ programme, Horn offers tailored, productive, safe and economical solutions for lead-free brass machining.
This article first appeared in the August 2015 issue of Machinery magazine.
