Product category:
EDM wire cutting
News Release from: Agie Charmilles | Subject: Wire EDM - cutting hard metal press tool parts
Edited by the Manufacturingtalk Editorial
Team on 01 December 2006
Making hard metal tools and dies with
wire EDM
Hard metals appear to provide the optimum press tool life and advanced wire EDM technology is shown the most effective method of their manufacture.
Today's modern metal cutting, punching, embossing and metal forming manufacturing processes - place ever increasing demands on the performance and life of a tool and/or a die With the high volume manufacture of parts - it is not uncommon for a tool to be expected to have a life in excess of 1 - 10 million strokes
This article was originally published on Manufacturingtalk on 17 May 2001 at 8.00am (UK)
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(Sometimes a total operating life of 500 million strokes is required - and certain heavy duty presses can achieve over 2000 strokes/min).
Against this backdrop it is not surprising that conventional tool and die materials/metals fail to deliver the consistent quality, reliability and cost efficiencies required.
These limitations have driven tool and die manufacturers to search for more high-performance, economically-viable materials with which to manufacture their tools.
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Hard metals (HM) appear to provide the optimum solution and, say market-leading EDM specialist - Agie, advanced wire EDM technology is the most effective method of manufacture.
* Hard metal characteristics - HM, based on tungsten carbide and cobalt, have proved to be the most suitable materials for manufacturing tools as they display the following properties.
High-compressive strength.
Low thermal expansion.
Good adhesive behaviour.
Tough and durable.
Good machining capability.
With steel tools - wear increases continuously necessitating costly tool replacement or repair/modification.
However, with a HM tool - wear is significantly reduced thereby ensuring increased accuracy and repeatability over a longer period of time.
* Hard metals explained - HM is a generic name associated with a group of materials that exhibit hardness and high wear resistance.
Typically in such materials Cobalt (Co) is the alloying metal - and is found in over 95% of all HMs.
Cobalt cements/binds the ceramic components (carbides) in a material - and it is the carbide element that gives HMs their hard wearing characteristics.
The most commonly used carbide is tungsten monocarbide (WC) - and is often mixed with small amounts of other carbides - titanium, tantalum, niobium, vanadium, molybdenum, chromium etc WC is unstable under high temperatures so cannot be manufactured using conventional metallurgical processes - smelting, alloying, casting, etc Instead, powder metallurgy is used which involves grinding, mixing and compacting the materials into the desired form at temperatures from 1350-1500 deg C.
(This thermal process is called sintering).
The high temperatures used melts the metallic (binder) cobalt element and, through capillary diffusion, closes the pores that exist between the different carbide elements.
At the end of the sintering process - a solid compact block (99.8% density) is formed - which can further be pressed (1000-1500 bar) to create a material called HIP (Hot Isostatic Pressing).
* The traditional method of tool manufacture - owing to the hardness of HM - parts are sintered, rather than machined, to their final form.
However, there are difficulties associated with this - namely due to volumetric shrinkage (as much as 10-15%) that occurs during the sintering process.
This has created problems when high-precision complex tools are required.
Conventional grinding and milling can be used - but require expensive diamond tools and long machining times to achieve the desired results.
Advanced Agie wire EDM machines - with their inherent versatility, guaranteed high precision and high degree of automation potential - are much better suited to HM tool manufacture.
* EDM process - EDM is a thermal process - and, dependent on the energy exerted in the process, may affect the structure and surface integrity of the material being machined and, in extreme cases, cause micro-cracking.
To prevent this - care and attention needs to be paid to the current pulses used - especially during trim cuts (final finishing passes) where the power and size of cut are reduced.
To achieve optimum results - trim cuts should be performed using constantly diminishing power (energy) to reduce the thermal effects of the EDM process on the surface being machined.
It is critical then to remove the thermally influenced area caused by the previous cut - until there is little or no thermally affected area left.
(From this it can be seen that there is a false economy in trying to reduce the number of trim cuts on carbide - to save machining time).
* Variable quality of HM - because HMs are manufactured via a metallurgical process (sintering - see above) - the quality of the metals can vary - and this can create headaches for manufacturers.
A common problem is decarburisation of the metal (loss of carbon from the material's surface due to the effects of oxygen) - which in turn affects the mechanical properties of the HM making it hard and brittle (the eta-phase).
Decarburisation leads to the formation of free carbon on the surface of the material which, as a result of subsequent EDM machining, may lead to unwanted corrosion and micro-cracking.
(Only impeccably conducted sintering can avoid decarburisation and the problems associated with it).
To avoid corrosion - it is important that EDM machining of HM parts occurs without interruption and that once machined they are removed from the work tank and dried quickly to avoid 'pitting'.
It is also critical that the dielectric pH value is kept constant (neutral) at about 7-8 - and that deionising resins are replaced in good time.
* Agie advanced EDM machines - achieving optimum results when EDM machining HMs requires know-how and advanced EDM technology.
With Agie - manufacturers get both.
All new Agie wire EDM machines are equipped with advanced technology (i e, machine parameter settings) - that can be altered according to materials being machined; the results required (i e, surface finishes, accuracy, removal rates, etc).
In addition - Agie's special technologies such as Super Finish - can, with Agie's new generators (IPG-V) and enhanced process controls (AGIEVISION 5) - reduce the thermal effects on the material and deliver the highest (best) surface finish of 0.05 micron Ra on the most complex of tools.
An example of this is Agie's Vertex duo wire machine.
The machine delivers a surface finish of 0.05 micron Ra; 1 micron positional accuracy; 17 micron corner radii machining capability and uses wire diameters down to 0.02mm - which all combine to make it the most accurate, and versatile, wire EDM machine on the market - ideally suited to the machining of HMs.
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