Consequences of the elimination of lead from alloys in machining with machine tool

Mikron Machining

 

Consequences of the elimination of lead from alloys in machining with machine tool
The recent RoHS environmental standards require all connector manufacturers to eliminate lead from metal alloys. This creates many difficulties in machining by chip removal.

 

A few years ago, the need to remove lead from alloys came from the food sector (e.g. water faucets, manifolds, ..). Now environmental protection is moving into all sectors and creating many technological changes, just think of the explosion of electric mobility that the car market has undergone in a few years. This generates new developments in many industrial branches: mechanics, electromechanics, electronics and software. As far as electrical connectors are concerned, they have become increasingly complex, small and must conduct a lot of current; this requires high quality components. New environmental laws have imposed the elimination of hazardous materials, including lead from metal alloys, from all production processes.

The directives RoHS 2002/95 / EC required the elimination of lead from metal alloys. The latest directive made the deadline of June 12, 2019 imperative. Meanwhile, other countries adopted similar laws in the US and China.

Problems and solutions related to lead-free brass machining:

Electrical connectors are, in most cases, made of brass. Removing lead from brass has the following consequences in the processing of pieces in large volumes:

Roughness: with traditional tools with cutting geometry for leaded brass, machining on lead-free brass is problematic: it is very difficult to obtain the roughness required by modern electrical connectors which must be low to ensure high specific amperages. So, this aspect is of fundamental importance. Obviously, if there are roughness problems, the tolerances are automatically affected as well.

An example of a lead-free brass electrical connector; on the left machined with dedicated technology (cutting tools and cutting parameters), on the right with traditional technology

With an appropriate cutting-tool geometry it is possible to obtain the previous roughness again and reach the values required by the market which are approximately Ra 0.40 µm.

Tolerances: Conventional cutting-tools make it unreliable to achieve form tolerances because of the so-called “material rejection” phenomenon: sometimes it cuts well and sometimes it rejects; a small temperature or lubricant variation is enough to trigger the phenomenon. Therefore, there are variations on the diameter, ovalities or errors in length; it is therefore impossible to maintain the tolerances of ± 0.0004” on the diameters required by the current market of high-tech connectors.

Roundness measurement of an electrical connector

A more aggressive cutting-tool geometry (like that of aluminum) makes the process more reliable

Cutting-tool life: to overcome the above-mentioned quality phenomena, the user often tends to use tools with more aggressive cutting angles. Unfortunately, in the machining of unleaded brass, these tools quickly lose the “edge” of the cutting edge, drastically reducing the tool life to a few tens of minutes, which tragically affects the efficiency of the entire system, raising the cost of the piece to the point of making production unprofitable

An example of cutting-tool with evident signs of wear

 

Therefore the cutting geometry must be “special”, the oversize between roughing and finishing and the cutting parameters must be harmonized to each other like the elements of an orchestra.

Overheating of the material: where the walls are thin (and in connectors this often occurs), if the tool cutting edge is not perfect, there is overheating of the material such that it thermally affects the material to the point of making the produced part inoperable (the annealing of the material makes it lose its elasticity). Tests have also highlighted new lubrication parameters. For some machining operations it was necessary to adopt lubrication through the tool, which can be easily retrofitted on modern & flexible transfer machines.

Long chips: long chips create skeins and dams that prevent their evacuation and tend to pile up to the point of making automatic production impossible.

The tool must be shaped in such a way as to avoid long chips, and chip breakers must be used.

Mikron Machining has carried out numerous tests in collaboration with Mikron Tool, determining the best parameters in order to obtain the same quality as before, without losing productivity.
Mikron Tool now has such know-how in lead-free tools with specific cutting geometries and spirals for this material that it can offer excellent solutions to existing and future customers.
Tests have shown that the oversize between roughing and finishing operations is important in order to guarantee the respect of the connector tolerances, with a good tool life without overheating the thin connector walls; of course all this maintaining the high productivity typical of Mikron transfers.

Practical example of machining a connector; different techniques compared:
Piece dimension: Ø=0.12” L=1”

 

 

A B C D
Output rate 180 parts/min 180 parts/min 155 parts/min 180 parts/min
Roughness 0.38 µm Ra 0.62 µm Ra 0.43 µm Ra 0.38 µm Ra
Ø Tolerance ± 0.01 mm
± 0.0004”
± 0.025 mm

± 0.001”

± 0.015 mm
± 0.0006”
± 0.01 mm
± 0.0004”
Length tolerance ± 0.02 mm
± 0.0008”
± 0.028 mm
± 0.0011”
± 0.020 mm
± 0.0008”
± 0.02 mm
± 0.0008”
Material Integrity OK OK Overheated OK
Tool life 144 hours 80 hours 0.3 hours 24 hours

 

A.  Previous situation. Leaded brass material, cutting tools (carbide) with traditional geometry for leaded brass
B.  Lead free brass material, traditional tools for leaded brass. The deterioration of the quality of the piece produced is unacceptable
C.  Lead-free brass material, tools with aggressive geometry, adapted machining parameters. The overheating of the piece and the loss of productivity due to the very short tool life and reduced cutting speed are unacceptable. The quality of the component is slightly out of tolerance
D.  Lead-free brass material, tools and cutting parameters resulting from Mikron Machining’s experience

The new technology for lead-free brass machining (dedicated tools + parameters + machinery) has made it possible to keep the good quality of the piece produced without losing cutting speed. The only drawback, which is negligible, concerns the reduction of the tool life that, however, is at a level of easy management and does not substantially reduce the efficiency of the system.

This technology can of course be applied not only to electrical contacts, but to any brass piece, such as parts for faucets, locks, decorative elements, etc … …

A very similar technology is also applied in the machining of steel, which no longer has to contain lead or sulfur. These two elements had the benevolent effect of reducing chip length; the short chip is particularly appreciated in machining with automatic machines.

Examples of lead- & sulfur-free steel chips. Left with dedicated technology, right with conventional tools

 

Modern tool monitoring ensures reliable production:

Machining lead-free copper base alloys is and remains challenging! Dedicated tools for lead-free brass machining and optimized parameters allow economic production despite the elimination of lead from electrical connector alloys. However, premature wear of the tool cutting edge is always lurking and could have negative consequences on the quality of the produced part.

Noticing it even after half an hour, at the production rates of a modern transfer machine, means having produced thousands and thousands of defective pieces; going to try to recover them in the containers of finished parts is a long and risky undertaking: even one defective piece could compromise the reputation of the manufacturer. For this reason, the entire production batch should be thrown away.

High-tech companies have recently launched tool monitoring systems with extraordinary features, which, thanks to increased sensitivity and extremely fast sampling, allow tool wear to be detected immediately. This creates an immediate stop of the machine, allowing the operator to replace the worn tool and eventually to eliminate the few pieces produced of non-compliant quality. These modern monitoring systems are also easy to use thanks to automatic learning of load limits.

Thanks to Mikron’s modern miTool tool-monitoring system, surface finish and tolerances can be ensured over time by detecting tool wear immediately; damaged tools can be replaced before thousands of defective parts are produced

The modern Mikron Multistar CX-24 transfer machine is particularly suitable for reliable mass production of complex and accurate electrical contacts in lead-free copper base alloys.

Examples of connectors made of lead-free copper base alloys. Customers struggling with complex geometry and difficult material find a profitable solution with Mikron Machining.

 

Contact
Mikron SA Agno, Via Ginnasio 17, 6982 Agno, Switzerland
Mr. Boris Sciaroni
Marketing
boris.sciaroni@mikron.com
Phone +41 91 610 65 11
www.mikron.com

 

 

 

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