Gear cutting – milling – or planing – Milling – Process
Reexamination Certificate
2000-07-11
2001-09-25
Briggs, William (Department: 3722)
Gear cutting, milling, or planing
Milling
Process
C451S028000
Reexamination Certificate
active
06293741
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for machining of work pieces made of MMC material (Metal Matrix Composite) for shaping components such as piston rods, pistons, brake discs and other mechanical components.
BACKGROUND OF INVENTION
Construction materials referred to as MMC materials have become known during the latest decade. The MMC materials are composites formed from a binder material such as aluminum, titanium or alloys from these with a reinforcement by admixture of fibers or particles from substances such as silicon carbide, boron carbide or aluminum oxide. Typically, the reinforcement content is about 15% by weight to about 70% by weight of the MMC material.
MMC materials have very interesting properties, which may be tailored depending upon the field of use, whereby achieving advantages such as making a component lighter, stronger, more rigid and providing the component with better endurance properties than what is possible to achieve by using conventional materials within the specific field of use.
Vehicle technology where fast moving parts, such as piston rods, suitably could be performed by means of these metal composites is an example of a field of use for MMC materials. Vehicle manufacturers constantly try to attain increasing performance with respect to fuel consumption, emissions, vibrations, noise, comfort and the like. Essential with respect to all of these parameters is decreased weight, especially in non-resilient mass and in fast moving engine parts. Especially within competition activities where motor vehicles are used, the above mentioned properties for engine parts are very desirable. Piston rods represent, as mentioned, an example of such components where decreased weight is very favorable.
Within racing activities for vehicles light materials such as aluminum, titanium or coal fiber composites are generally used instead of steal for the mentioned types of components.
Another interesting field of use for MMC products is brake discs for cars, trucks and trains.
One major drawback when using MMC material is that the material has been very hard to machine. When shaping a component employing a MMC material, methods such as casting the component in a mold which closely corresponds to the finally completed shape of the component are applied. Another method is to use a forged work piece or a portion of an extruded rod, whereby spark-machining of the surface of the component and conventional machining may be used to arrive at the final shape of the component. Attempts have been made to produce, by example, piston rods for motorbikes by using conventional manufacturing machining methods. Hereby, the purpose of arriving at the desired component with its desired properties, such as lower weight, has been achieved. The use of such a piston rod in an engine has given as a result an engine which more willingly moves into a higher gear and further induces lower vibrations to the engine. The problem is, however, that the costs for manufacturing the engine part are very high, which imply that the use is restricted or limited to fields where the costs are of minor importance.
A number of patent documents disclose different methods for a final stage shaping of components made by MMC materials. U.S. Pat. No. 5,765,667 is here mentioned as one example. This patent describes a method for manufacturing a component, in this example a disc brake, by means of casting the component to a shape which very closely corresponds to the final shape of the component, in order to, and this is distinctly expressed, as far as possible avoid the need for machining by cutting tools. It is obvious for the person skilled in the art to avoid the need for machining by cutting tools, as the MMC material, when it is composed of an aluminum base and reinforcement in the form of silicon carbide particles contains exactly the composition which is generally used for grinding cutting tools.
The silicon particles embedded in the MMC material have a devastating effect on the cutting tools when machining by the use of conventional machining technique, as the edges of the cutting tools rapidly are worn out by the grinding particles within the composite material.
The present invention discloses an unexpected solution to the above-described problem.
SUMMARY OF INVENTION
One aspect of the invention is based on a method of shaping a work piece of a MMC material by means of what is referred to herein as High Speed Machining (abbreviated HSM) whereby a component can be given its final shape directly from the work piece by means of this machining method. The work piece may be forged, cast, being a piece of an extruded rod or a raw material produced in some other way.
High Speed Machining involves operating the cutting tool at a very high speed in relation to the work piece being machined as compared to the case of conventional machining technique. The cutting tools of current interest are preferably milling tools and drills.
As used herein, the term “high speed machining” (HSM) represents a process which differs from conventional machining methods. It happens that the term is sometimes used to denote also conventional machining, where new methods appear to push the limits for conventional machining to higher levels. However, this is not what is meant here as will be shown below.
HSM machining is characterized by:
very high cutting speeds,
high shear strain speed (the ability to separate a chip from the work piece)
a very high effect density is generated in front of the cutting edge (typical value: MW/mm
3
),
at the chip forming process, a very high temperature prevails locally at the cutting place,
the chip is flowing without being in contact with the cutting edge,
the shearing forces asymptotically approaches zero.
The following are some examples of the high cutting speeds when high speed machining some materials according to the present invention:
aluminum about 3000 m/min (conventionally about 100-400 m/min),
titanium about 15000 m/min (conventionally about 15-100 m/min).
To find the correct cutting speed depends on the kind of material being machined to obtain the above-discussed states held to characterize high speed machining. Such can be determined by persons skilled in the art without undue experimentation once aware of this disclosure.
When testing to determine an optimal cutting speed for an HSM machining of a new material, the shearing forces can be studied. These forces asymptotically approach zero when the criteria for the HSM machining state are attained. Thus, HSM state may be said to prevail when the shearing forces are decreasing with increasing cutting speeds. At said HSM state, the objective is to determine an optimal cutting speed for the machined material. At conventional machining, the shearing forces are increasing with increasing cutting speeds. This means that, as it is now understood, the shearing forces as a function of the cutting speed may be represented by a curve having a global maximum (local maximums or minima may occur). If machining data is such that machining is performed at the increasing side of the curve, conventional machining prevails. On the other hand, HSM state prevails when machining is performed under such conditions that machining is performed at the decreasing side of the curve, or in other words: HSM machining prevails when the global maximum point is past.
Another advantage of using HSM machining is that the chip absorbs the major portion of the heat generated at the cutting point, typically 80%, whereby a work piece is left essentially unaffected by the heat generated at the machining.
It has been discovered that high speed machining gives unexpectedly good results when used for MMC materials. Despite the high portion of grinding substances within the material it appears that the cutting tools maintain their sharpness for a long time, as if they were unaffected by the presence of the grinding substances in the MMC material. The reasons behind this are not quite understood, as the courses inside the material, that is, it is not quite known w
Ekblad Stefan
Karlsson Sven-Åke
Strand Kent
Briggs William
SAAB AB
Swidler Berlin Shereff & Friedman, LLP
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