High speed machining is a hot topic. But, what is "high speed" machining really? is it simply running at maximum feed rates and taking multiple shallow passes? this strategy is often less efficient than taking few passes at slightly greater depths! achieving the shortest cutting time is related to feed rate, but the relationship is not necessarily "fastest feed rates = most efficient."
The term High Speed Machining (HSM) commonly refers to end milling at high rotational speeds and high surface feeds. For instance, the routing of pockets in aluminium airframe sections with a very high material removal rate. Over the past 60 years, HSM has been applied to a wide range of metallic and non-metallic workpiece materials, including the production of components with specific surface topography requirements and machining of materials with a hardness of 50 HRC and above. With most steel components hardened to approximately 32-42 HRC, machining options currently include:
· Rough machining and semi-finishing of the material in its soft (annealed) condition
· Heat treatment to achieve the final required hardness (= 63 HRC)
· Machining of electrodes and electrical discharge machining (EDM) of specific parts of the dies or moulds (specifically small radii and deep cavities with limited accessibility for metal cutting tools)
- Finishing and super-finishing of cylindrical/flat/cavity surfaces with appropriate cemented carbide, cerment, solid carbide, mixed ceramic or polycrystal - line cubic boron nitride (PCBN).
With many components, the production process involves a combination of these options and in the case of dies and moulds it also includes time consuming hand finishing. Consequently, production costs can be high and lead times excessive. Typical for the die and mould industry is to produce one or a few tools of the same design. The process includes constant changes of the design. And because of the need of design changes there is also a corresponding need of measuring and reverse engineering. The main criteria are the quality of the die or mould regarding dimensional, geometrical and surface accuracy. If the quality level after machining is poor and if it cannot meet the requirements there will be a varying need of manual finishing work. This work gives a satisfying surface accuracy, but it always has a negative impact on the dimensional and geometrical accuracy. One of the main targets for the die and mould industry has been, and is, to reduce or eliminate the need of manual polishing and thus improve the quality, shorten the production costs and lead times.
The main cost-effective and technical factors for the development of HSM
Survival - The ever increasing competition on the marketplace is setting new standards all the time. The demands on time and cost efficiency is getting higher and higher. This has forced the development of new processes and production techniques to take place. HSM provides hope and solutions.
Materials - The development of new, more difficult to machine materials has underlined the necessity to find new machining solutions. The aerospace industry has its heat resistant and stainless steel alloys. The automotive industry has different bimetal compositions, Compact graphite iron and an ever increasing volume of aluminium. The die and mould industry mainly has to face the problem to machine high hardened tool steels, from roughing to finishing.
Quality - The demand on higher component or product quality is a result of the hard competition. HSM offers, if applied correctly, solutions in these areas. Substitution of manual finishing is one example. This is especially important to dies or moulds or components with a complex 3D geometry.
Process - The demand on shorter through-put times via fewer setups and simplified flows (logistics) can be solved to a big extent via HSM. A typical target within the die and mould industry is to make a complete machining of fully hardened small sized tools in one set-up. Costly and time consuming EDM-processes can also be reduced or eliminated via HSM.
Design & development - One of the main tools in todays competition is to sell products on the value of novelty. The average product life cycle on cars is today four years, computers and accessories 1 to 5 years, handphones 3 months… One of the prerequisites of this development of fast design changes and rapid product development time is the HSM technique.
Complex products - There is an increase of multifunctional surfaces on components, such as new design of turbine blades giving new and optimised functions and features. Earlier design allowed polishing by hand or with robots (manipulators). The turbine blades with the new, more sophisticated design has to be finished via machining and preferably by HSM. There are also more and more examples of thin walled workpieces that has to be machined (medical equipment, electronics, defence products, computer parts).
Production equipment - The strong development of cutting materials, holding tools, machine tools, controls and especially CAD/CAM features and equipment has opened possibilities that must be met with new production methods and techniques.
HSM in todays scenario
The discussion about high speed machining is to some extent characterised by confusion. There are many opinions, many myths and many different ways to define HSM. Looking upon a few of these definitions HSM is said to be:
· High Cutting Speed ( ) Machining.
· High Spindle Speed (n) Machining.
· High Feed ( ) Machining.
· High Speed and Feed Machining.
· High Productive Machining.
In the following paragraphs, the parameters that influence the machining process and having connections with HSM will be discussed. It is important to describe HSM from a practical point of view and also give as many practical guidelines for the application of HSM as possible.
The Practical definition of HSM
· HSM is not simply high cutting speed. It should be regarded as a process where the operations are performed with very specific methods and production equipment.
· HSM is not necessarily high spindle speed machining. Many HSM applications are performed with moderate spindle speeds and large sized cutters.
· HSM is performed in finishing in hardened steel with high speeds and feeds, often with 4-6 times conventional cutting data.
· HSM is High Productive Machining in small-sized components in roughing to finishing and in finishing and super-finishing in components of all sizes.
· HSM will grow in importance the more net shape the components get.
· HSM is today mainly performed in taper 40 machines.
|Posted : 10/26/2005|