What is High-Speed Machining? High-Speed Machining is a term that has become widely used, mainly because of the benefits it provides to those able to accomplish it. However, like a lot of machining terms, its clear definition has become a little clouded with use. We though little refresher on the basics might be in order.
Over the past 60 years, HSM has been applied to a wide range of metallic and non-metallic materials, including the production of components with specific surface topography requirements and machining of materials with a hardness of 50 HRC and above.
Survival--the ever-increasing competition on the marketplace is setting new standards all the time. The demands on time and cost efficiency are getting higher and higher, forcing 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 aluminum. The die and mold industry mainly has to face the problem of machining highly hardened tool steels, from roughing to finishing.
Quality--the demand for higher component or product quality is a result of the hard competition. HSM offers, if applied correctly, solutions in this area. Substitution of manual finishing is one example. This is especially important on dies or moulds or components with a complex 3D geometry.
Processes--the demands on shorter throughput times via fewer set-ups and simplified flows (logistics) can be solved to a big extent via HSM. A typical target within the die and mold 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 today's competition is to sell products on the value of novelty. The average product life cycle for cars today is three years, computers and accessories 1.5 years, and Mobile phones 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 designs for turbine blades giving new and optimized functions and features. Earlier design allowed polishing by hand or with robots (manipulators). The turbine blades with the new, more sophisticated designs have to be finished via machining.
Production Equipment--the strong development of cutting materials, toolholders, machine tools, controls, and especially CAD/CAM features and equipment have opened possibilities that must be met with new production methods and techniques.
Original HSM Definition
Salomon's theory, "Machining with High Cutting Speeds" on which he got a German patent 1931, assumes that "at a certain cutting speed (5-10 times higher than in conventional machining), the chip removal temperature at the cutting edge will start to decrease ..."
Giving the conclusion: "seems to give a chance to improve productivity in machining with conventional tools at high cutting speeds ..."
Modern research has unfortunately not been able to verify this theory to its full extent. There is a relative decrease of the temperature at the cutting edge that starts at certain cutting speeds for different materials. The decrease is small for steel and cast iron, and bigger for aluminum and other non-ferrous metals.
The definition of HSM must be based on other factors.
The discussion about high speed machining is to some extent characterized 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 (vc) Machining;
* High Spindle Speed (n) Machining;
* High Feed (vf) Machining;
* High Speed and Feed Machining;
* High Productive Machining.
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.
True Cutting Speed
As cutting speed is dependent on both spindle speed and the diameter of the tool, HSM should be defined as "true cutting speed" above a certain level. The linear dependence between cutting speed and feed rate results in "high feeds with high speeds."
The feed will become even higher if a smaller cutter diameter is chosen, provided that the feed per tooth and the number of teeth is unchanged. To compensate for a smaller diameter the rpm must be increased to keep the same cutting speed ... and the increased rpm gives a higher vf.
Very typical and necessary for HSM applications is that the depths of cut, ae and ap and the average chip thickness, hm, are much lower compared with conventional machining. The material removal rate, Q, is consequently and considerably smaller than in conventional machining. The exception is when machining in aluminum and other non-ferrous materials.
To perform HSM applications it is necessary to use rigid and dedicated machine tools and controls with specific design features and options. All production equipment has to be designed for the specific process of HSM.
It is also necessary to use advanced programming techniques with the most favorable tool paths. The method to ensure constant stock for each operation and tool is a prerequisite for HSM and a basic criterion for high productivity and process security. Specific cutting tools and toolholders are also a must for this type of machining.
Tool Steel HSM
Within the die & mold area the maximum economical workpiece size for roughing to finishing with HSM is approximately 400mm X 400mm X 150mm (1, w, h). The maximum size is related to the relatively low material removal rate in HSM, and of course also to the dynamics and size of the machine tool.
Most dies or moulds have a considerably smaller size, than mentioned above, in complete machining (single set-up).
Typical operations performed are, roughing, semi-finishing, finishing, and in many cases super-finishing. In many cases 3 - 4 tool types are used.
The common diameter range is from 1 - 20 mm. The cutting material is in 80% -90% of the cases solid carbide end mills or ball nose end mills. End mills with big corner radii are often used. The solid carbide tools have reinforced cutting edges and neutral or negative rakes (mainly for materials above 54 HRC). One typical and important design feature is a thick core for maximum bending stiffness.
It is also favorable to use ball nose end mills with a short cutting edge and contact length. Another design feature of importance is an undercutting capability, which is necessary when machining along steep walls with a small clearance. It is also possible to use smaller sized cutting tools with indexable inserts, especially for roughing and semi-finishing. These should have maximum shank stability and bending stiffness. A tapered shank improves the rigidity, and so also do shanks made of heavy metal.
The geometry of the die or mold could preferably be shallow and not too complex. Some geometrical shapes are also more suited for high productive HSM. The more chances there are to adapt contouring tool paths with few changes in direction, the better the result will be.
HSM, A Practical Definition
* 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. (MAN--Modern Applications News thanks Sandvik Coromant for its assistance in the preparation of this article.) Sandvik Coromant
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|Title Annotation:||CUTTING TOOLS|
|Publication:||Modern Applications News|
|Article Type:||Cover Story|
|Date:||Jul 1, 2005|
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