Lathe cutting tools-tool angles and their functions
. functions of angles and clearance given on the cutting tools
. the angles of a lathe cutting tool
. the characteristics of a rake angle
. the characteristics of a clearance angle
The tool acts like a wedge during turning. The wedge shaped cutting edge penetrates into the work and removes the metal. This necessitates the grinding of a tool cutting edge to a wedge shape. When we sharpen a pencil with a pen knife by trial and error, we find that the knife must be presented to the wood at a definite angle, if success is to be achieved.
If, in the place of a wooden pencil, a piece of soft metal such as brass is cut, it will be found that the cutting edge gets crumbled. For tha blade to cut the brass successfully, the cutting edge must be ground to a less acute angle.
Angles ground on a lathe cutting tool
All the angles may not be located or found in every tool. As an example a roughing tool is chosen. The angles and clearances ground on this tool are
-approach angle(1)
-trail angle(2)
-top rake angle(3)
-side rake angle(4)
-front clearance angle(5)
-side clearance angle(6)
Approach angle
This is also known as side cutting edge angle. This is ground on the side of the cutting tool. The cutting wil be oblique while cutting. The angle ground may range from 25° to 40° but as a standard a 30° angle is normally provided. The oblique cutting the advantages over the orthogonal cutting, in which the cutting edge is straight. More depth of cut is given in the case of oblique cutting, since, when the tool is fed to the work, the contact surface of the tool increases gradually as the tool advances, whereas in the case of the orthogonal cutting, the length of the cutting edge for the given depth fully contacts the work from the begining itself which gives a sudden maximum load on the tool face. The area over which heat is distributed is greater in oblique cutting.
Trail angle
It is also known as end cutting edge angle, and is ground at 30° to a line perpendicular to the aixs of the tool, as illustrated.
The approach angle and trail angle ground form the wedge angle of 90° for the tool.
Top or back rake angle
The rake angle ground on a tool controls the geometry of chip formation. Thereby, it controls the cutting action of the tool. The top or back rake angle of the tool is ground on the top of the tool, and it is a slope formed between the front of the cutting edge and the top of the face. If the slope is from the the front towards the back of the tool, it is known as a positive top rake angle, and if the slope is from the back of the tool towards the front of the cutting edge, it is known as a negative back rake angle.
The top rake angle may be ground positive, negative or zero according to the material to be machined. When turning soft, ductile materials, which form curly chips, the positive top rake angle ground will be comparatively more than for turning hard brittle metals
When turning hard metals with carbide tools, it is the usual practice to give a negative to rake. Negative top rake tools have more strength than tools with positive top rake angles.
Side rake angle
A side rake angle is the slope between the side of the cutting edge to the top face of the tool width wise. The slope is from the cutting edge to the rear side of the tool. It varies from 0° to 20°,according to the material to be machined.
The top and side rake, ground on a tool control the chip flow, and this results in a true rake angle which is the direction in which the chip that shears away from the work passes.
Front clearance angle
It is the slope between the front of the cutting edges to a line perpendicular to the axis of the tool drawn downwards which is known as the front clearance angle. The slope is from the top to the bottom of the tool, and permits only the cutting edge to contact the work, and avoids any rubbing action. If the clearance ground is more, it will weaken the cutting edge.
Side clearance angle
The clearance angle is the slope formed between the side cutting edges of the tool with a line perpendicular to the tool axis drawn downwards at the side cutting edge of the tool. The slope is from the top of the side cutting edge to the bottom face. This is also ground to prevent the tool from rubbing with the work, and allows only the cutting edge to contact the work during turning. The side clearance angle needs to be increased when the Fred rate is increased. When grinding rake and clearance angles, it is better to refer to the standard chart provided with the recommended values and grind. However, then actual operation will indicate the performance of the tool, and will indicate to us, if any modifications are needed for the angles ground on the tool.
Side relief angle
This angle is ground on parting and undercut tools on both sides. This will provide for the width of the cutting edge to be slightly broader than the back of the cutting edge.
A side relief angle permits clearance between the sides of the tool and the groove walls, formed by the plunging action of the tool, thereby, preventing the tool from getting jammed in the groove, causing breakage. The relief is kept as minimum as possible. Too much of relief will weaken the tool cutting -edge tail, and also permit the chips to clog in the gap, in both cases causing the tool to break. Side relief is also provided sometimes to the main cutting edge of the facing tool, permitting the cutting point only to perform the operation, whenn the tool axis is set perpendicular to the lathe axis. The side relief angle normally does not exceed 2°.
Relationship between rake angle and wedge angles
The rake angle, clearance angle, and the wedge angle have close relationship for efficiency in cutting. Excessive rake angle reduces the wedge angle, which helps in good peneteration, and it is particularly useful for cutting soft metals. A decreased wedge angle weakens the tool strength. Therefore, for cutting hard metals, the rake angle is zero or negative. The clearance angle is generally fixed depending on the geometry of the surface being cut.
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