Brown, in Encyclopedia of Materials: Science and Technology, 2001. 3 Shear Angle. The shear angle is of fundamental importance in chip formation. The smaller the shear angle the larger the strain, the machining forces, and the power requirements. There is nothing in the geometry of the tool that dictates what the shear angle should be.
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Diamond MachiningDiamond machining is considered to be a finishing operation.If large volumes of material are to be removed, it should first be doneby other means. Diamond tools are generally used with a nominal rake anglenear zero degrees. For metals, a slightly negative (a degree or two) rakeangle can improve the surface finish where for plastics a slightly positiverake angle has the same effect. Primary clearance angles are generallythe same as for conventional machining, in the range of 6 to 10 degrees.Machining parameters are very different from conventionalmachining. Surface speed is generally in the range of several-hundred feetper minute. For a turning operation, the radial depth of cut for roughingis generally in the range of 50 micrometers to 15 micrometers for metalsranging from soft to hard. Roughing cuts for plastic is generally several-hundredmicrometers in the radial depth of cut.
A roughing feed is typically inthe range of 10 to 40 micrometers per revolution. Finishing depth of cutis typically in the range of 1 micrometer for hard metals and 3 micrometersfor soft metals. For plastics the finishing cut is generally about 15 micrometers.These numbers are intended to supply general information only and shouldbe adjusted based on the best results which are obtained under actual machiningapplications.Metals and crystals should be machined with the presenceof a light oil. Mineral or vegetable oil has been used but petroleum oilsare mich more difficult to remove after machining. When machining plastics,air should be used and water-based grinding fluid has also been used withsuccess. The purpose of the fluid is to help reduce friction but primarilyto help with chip removal.
During roughing, soft metals can have long andcontinuous chips which can entangle around the tool and tool holder. Diamondtools generally do not incorporate chip breakers. The fluid can help causethe coiled chip to break because of the weight of the fluid adhering tothe chip.
The fluid will also normally result in a smoother surface. Whendoing finishing diamond machining, dust is often generated as the chipare normally on the order of micrometers. Care should be taken to avoidinhaling these particles. PMMA for example has been identified as a possiblecarcinogen if inhaled, and beryllium and silicon dust are very toxic wheninhaled.Diamond machined surfaces are normally of optical quality.However, optics are normally made of glass which can withstand a relativelylarge amount of physical abuse without scratching. Diamond machined metalsthough, particularly copper and aluminum, are very soft and can be easilydamaged physically and chemically.
Metrology must be performed with greatcare to avoid damage. One qualitative way to inspect diamond machined surfacesis to use them as a reflector of overhead diffuse light. By focusing one'seyes on the diamond machined surface which is reflecting the diffuse light(a fluorescent light for example), waviness and diffraction patterns canbe easily seen. This takes some practice since the eye normally wants tofocus on the light source itself rather than on the reflecting surface.When handling diamond machined parts, handle them by theedges only. Oil from the skin is very difficult to remove and even drycotton can scratch the surface. Do not talk over the surface as salivamarks are very difficult to remove and can corrode the metals.
Never laya diamond machined surface on anything as scratching will most likely occur.Copyright Craig Friedrich1998.
Machining is one subtractive manufacturing process by which excess material is removed from pre-formed blank in the form of chips in order to improve dimensional accuracy and tolerance. A wedge shaped cutting tool (or cutter) is employed for gradual shearing of layer by layer material. Geometry, orientation and material are three paramount factors that influence machining capability and product quality.
Cutter geometry embraces crucial features of the cutting edge(s) and relevant surfaces. It includes, but is not limited to, rake angle in different directions, clearance angles in difference directions, edge radius, orientation of cutting edges, nose radius, etc. Different tool designation systems display such features in a standard manner.Rake angle of a cutter basically indicates inclination of rake surface. Since rake surface is the chip flowing surface, so rake angle also indicates chip flow direction (in orthogonal cutting). This is one crucial parameter as it directly or indirectly influences shear deformation, chip flow direction, cutting force, power consumption, machinability, etc.
By definition, rake angle is the angle between cutter’s rake surface and reference plane and measured on some other plane. Based on the plane on which this angle is measured, it may have different names, such as orthogonal rake (measured on orthogonal plane), side rake (measured on machine longitudinal plane), back rake (measured on machine transverse plane), maximum rake (measured on a plane perpendicular to master line for rake), etc.Irrespective of the plane or direction it is measured, rake angle can be either positive or negative or even zero. This concept evolved from relative orientation of rake surface with respect to fixed reference plane—in one direction it is considered as positive, in other direction it is considered as negative and when they merge together rake angle is considered as zero. A positive rake occurs when sum of wedge angle and clearance angle is below 90° on a particular plane.
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It offers a sharp cutting edge and thus can efficiently shear off material from workpiece requiring less force. Now if sum of wedge angle and clearance angle becomes equal to 90° on a particular plane, then rake angle becomes zero. Similarly, when sum of wedge angle and clearance angle is more than 90° on a particular plane, rake angle becomes negative. Negative rake offers stronger tool tip and thus enhanced tool life. Various differences between positive rake and negative rake are given below in table format.
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