9.7 CUTTING TOOL NOMENCLATURE AND GEOMETRY
A machine tool can perform efficiently only when correct cutting tools are used. The lathe toolbit consists of a face, cutting edge, flank, nose, and base. The face is the top of a toolbit and is the surface where the chip passes as it is cut away from the workpiece. The cutting edge is that part of the toolbit that does the actual cutting of the workpiece. The flank is the surface directly below the cutting edge, the nose is the tip of the toolbit and is formed by the side and end edge. The base is the bottom surface of the tool.
9.7.1 Cutting Tool Angles
Relief or clearance angles are ground on the end and side of a lathe to keep it from rubbing on the workpiece. Relief angles are necessary to permit only the cutting edge to touch the workpiece.
Rake angles are ground on a tool to provide a smooth flow of the chip over the toolbit and away from the workpiece. A lathe tool generally uses two rake angles: side and back.
1) Side rake is ground on the tool face away from the cutting edge. Side rake influences the angle at which the chip leaves a workpiece. Most general purpose lathe toolbits use a side of 14°
2) Bake rake angles influence the angle at which the chip leaves the nose of the toolbit. Back rake angles are established in two way: by the position of toolholder, or by grinding it onto the face of the tool.
End- and side-cutting edge angles are ground on a toolbit so that the tool can be mounted in the correct position for various machining operations.
The nose radius is the rounded tip on the point of a toolbit. The nose radius has two functions: to prevent the sharp, fragile tip from breaking during use, and to provide a smoother finish on the workpiece during machining operations.
9.7.2 Types of Lathe Toolbits
They are usually classified into two categories: left-cut toolbits and righ-cut toolbits. Left-cut toolbits cut from the left to the right or toward the tailstock of a lathe. Right-cut toolbits cut from the right to the left or toward the headstock.
Finishing tools have larger nose radii to produce finer finishes on surfaces of workpiece. A light cut and a fine feed are used with these tools.
Rounghing tools are used to machine the majority of the metal from a workpiece, leaving just enough material for a finish cut.
Facing tools are used to square the end of a piece of stock with a smooth finish.
The cutoff or parting tool cuts only on the end. It is used for grooving or cutting off stock from the workpieces, which are held in the lathe.
The round nose tool is a tool that cuts in both directions. It is sometimes used to form a radius at the corner of a shoulder.
9.7.3 Grinding a General-Purpose Lathe Toolbit
1) Hold the tool firmly while resting the hands on the grinder toolrest.
2) Do not apply excessive pressure against the grinding wheel. Cool the tool in water often while grinding high-speed-steel tools to prevent overheating and burning.
3 )While grinding, move the toolbit back and forth across the face of the wheel without changing its position.
4)Hold the toolbit against the rotating wheel and tilt the bottom inward 26° for the end-cutting edge angle.
5)Tilt the bottom of the side inward approximately 6° for the relief angle and to the right 15° for the side-cutting edge angle. When ground, the cutting edge should be the same length as the side of the toolbits.
6)Hold the toolbit shank approximately 45° downward to the axis of the wheel and the bottom inward 14°.
7)Grind a small nose radius of a 1/64- to 1/32-in. point on the toolbit. When grinding the nose radius, only a slight amount of pressure is needed-too much causes a larger radius than desired.
8)Hone the cutting edge slightly with a whetstone to prolong the life and hold the toolbit firmly against the stone to avoid changing the angles.
9.7.4 Cutting Speed, Depth of Cut, and Feed
Cutting speed is the distance (in feet or meters) that the circumference of the work moves past the cutting tool in 1 min. cutting speed is expressed in surface feet per min or meters per min. Cutting speeds are established by two factors: the type of material to be machined and the cutting tool material to be used in the machining operation.
Notice that each category has a range for both roughing and finishing cuts. They are seven other factors to consider:
1) Depth of cut
2) Size and shape of the work
3) Design of the toolbit
4) Rigidity of the work setup
5) Rigidity of the lathe
6) Use of cutting fluids
7) Power available
Lathe rpm= 12*cutting speed(ft./min.) / π*D(in.)
Explanation:Multiplying the material cutting speed by 12 changes feet into inches. D equals the diameter of the work in inches and is multiplied by π (3.1416) to obtain circumference.
Depth of cut: The depth is measured at a right angle to the surface of the work and is the difference between the original surface and the surface left by the toolbit. The roughing cut removes the majority of material from the workpiece, leaving enough metal for the final or finishing cut.sometimes it is necessary to use several roughing cuts prior to the finishing cut. Finishing cuts are used to make the work surface smooth and accurate.
Feed: Feed is the distance the tool advances along the work surface for each revolution of the apindle. Generally coarser feeds are used on softer metals and rough turning, and finer feeds are used for harder metals and finishing cuts.
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