A machine tool is no more efficient than its cutting tool. There is nothing in shop work that should be given more thoughtful consideration than cutting tools. Time is always wasted if an improperly shaped tool is used. The cutting action of the tool depends on its shape and its adjustment in the holding device. Lathe cutter bits may be considered as wedges which are forced into the material to cause compression, with a resulting rupture or plastic flow of the material. The rupture or plastic flow is called cutting. To machine metal efficiently and accurately, it is necessary that the cutter bits have keen, well-supported cutting edges, and that they be ground for the particular metal being machined and the type of cut desired. Cutter bits are made from several types of steel, the most common of which are described below.
- Carbon Steel. Carbon steel, or tool steel is high in carbon content, hardens to a high degree of hardness when properly heated and The carbon-steel tool will give good results as long as constant care is taken to avoid overheating or “bluing,” since the steel will lose its temper or hardness at a relatively low heat becoming ineffective as a cutting tool. For low-speed turning, high carbon steels give satisfactory results and are more economical than other materials.
- High-Speed Steel. High-speed steel is alloyed with tungsten and sometimes with chromium, vanadium, or Although not as hard as properly tempered carbon steel, the majority of lathe cutting tools are made of high-speed steel because it retains its hardness at extremely high temperatures. Cutter hits made of this material can be used without damage at speeds and feeds which heat the cutting edges to a dull red.
- Stellite. These cutter bits will withstand higher cutting speeds than high-speed steel cutter Stellite is a nonmagnetic alloy which is harder than common high-speed steel. The tool will not lose its temper, even though heated red hot from the friction that is generated by taking a cut. Stellite is more brittle than high-speed steel. To prevent breaking or chipping, it requires just enough clearance to permit the tool to cut freely. Stellite is also used for machining hardened steel, cast iron, bronze, etc.
- Tungsten Carbide. Tungsten carbide is used to tip cutter bits when maximum speed and efficiency is required for materials which are difficult to Although expensive, these cutter bits are highly efficient for machining cast iron, alloyed cast iron, copper, brass, bronze, aluminum, Babbitt metal, and such abrasive nonmetallic materials as fiber, hard rubber, and bakelite. Cutter bits of this type require very rigid support and are usually held in open-side toolposts. They require special grinding wheels for sharpening, since tungsten carbide is too hard to be redressed on ordinary grinding abrasive wheels.
Tantalum Carbide and Titanium Carbide. These cutting tools are similar to tungsten carbide tools but are used mostly for machining steel where extreme heavy cuts are taken and heat and pressure tend to deform the cutting edge of the other types of cutting tools.
Terms And Definitions Applied To Single-Pointed Cutter Bits
The terms and definitions in described below are applied to tools used for turning, planing, boring, , which have a cutting edge at one end. This cutting edge may be formed on one end of a solid piece of steel, or the cutting part of the tool may consist of an insert or tip which is held to the body of the tool either by brazing, welding, or by mechanical means.
- Shank. The shank is the main body of the tool.
- Nose. The nose is the part of the cutter bit which is shaped to produce the cutting edges.
- Face. The face of the cutter bit is the surface at the upper side of the cutting edge on which the chip strikes as it is separated from the workpiece.
- Side. The side of the cutter bit is the near-vertical surface which, with the end of the bit, forms the profile of the The side is the leading surface of the cutter bit used when cutting stock.
- Base. The base is the bottom surface of the shank of the cutter bit.
- End. The end of the cutter bit is the near-vertical surface which, with the side of the bit, forms the profile of the bit. The end is the trailing surface of the cutter bit when cutting.
- Heel. The heel is the portion of the cutter bit base immediately below and supporting the face.
Angles of Cutter Bits
The successful operation of the lathe and the quality of work that may be achieved depend largely on the angles that form the cutting edge of the cutter bit. The profiles of the bit may be of any shape so long as the cutting edge is properly The five angles are used to define the cutting edge, to prevent supporting surfaces of the bit from rubbing against the workpiece, and to establish a path for the chips being removed. Improperly ground angles will result in weakening and breaking the cutting edge and overheating the bit.
Recommended Clearance and Rake Angles
When grinding cutter bits, the lip angle should he considered in selecting the proper angles from the table.
Common Types Of Cutter Bits
- General. Cutter bits are made from standard sizes of bar stock to fit into cutting toolholders which in turn are fastened to the toolpost of the lathe. If the cutter bit is to be used for heavy roughing, where a finished surface is not expected, the nose should be ground with a very small radius (approximately 1/64 inch). If the cutter bit is to be used for general shaping and finishing, the nose should be more rounded (approximately 1/32-to 1/16 inch radius). The following cutter bits are identified by their function.
- Right-Hand Turning Cutter Bit. The right-hand turning cutter bit is shaped to be fed from right to left. The cutting edge is on the left side of the bit and the face slopes down away from the cutting edge. The left side and the end of the tool are ground with sufficient clearance to permit the cutting edge to bear upon the workpiece without the heel of the bit rubbing against the workpiece. The right-hand turning cutter bit is ideal for taking light roughing cuts as well as general all-around machine work.
- Left-Hand Turning Cutter Bit. The left-hand turning cutter bit is just the opposite of the right-hand turning cutter bit, being designed to cut the metal when fed from left to right. It is used for all around machine work when right-to-left turning is impractical.
- Round-Nose Turning Cutter Bit. The round-nose turning cutter bit is used for all around machine work and may be used for taking light roughing or finishing cuts. Usually the face is ground with a right sloping side rake so that the bit may be fed from right to left, although it is often ground without any side rake so that the feed may be in either direction.
- Right-Hand Facing Cutter Bit. The right-hand facing cutter bit is intending for facing on right-hand shoulders and the right end of the workpiece. The cutting edge is on the left-hand side of the bit, and the nose is sharp to permit machining a square corner. The direction of feed for the facing bit should be away from the axis of the workpiece.
- Left-Hand Facing Cutter Bit. The left-hand facing cutter bit is just the opposite of the right-hand facing cutter bit; it is intended for facing the left side of the shoulders.
- Parting Cutter Bit. The parting cutter bit has its principal cutting edge at the end. Both sides must have sufficient clearance to prevent binding and should be ground slightly narrower at the back than at the cutting edge. The bit is convenient for machining necks and grooves, square corners, etc., as well as for cutting-off operations.
- Thread Cutter Bit. The thread cutter bit has its cutting edge ground to a 60° angle. This form will cut sharp V-threads. Usually the face of this bit is ground flat and has clearance ground on both sides so that it will cut on both sides. For American (National) Standard screw threads, the bit is ground with a flat at the nose to cut the flat root of the thread. The width of the flat at the nose is determined by the pitch of the screw thread that is to be cut.
Special Types Of Lathe Cutting Tools
Besides the common cutter bits, special lathe operations and heavy production work require special types of cutting tools. Some of the more common
special tools will be described in below.
- Tipped Cutter Bits. Tungsten carbide, tantalum carbide, and oxide or ceramic tipped cutter bits are commonly used in production work where high speeds and heavy cuts are necessary, and where exceptional hard and tough materials are encountered. The tipped cutter bit generally has a shank size larger than the common cutter bit and is mounted in an openside cutting toolholder, a turret tool block, or directly in the toolpost of the lathe. Tipped cutter bits come in shapes for use in left-hand and righthand turning, general purpose work and cutting threads.
- The Threading Tool With Toolholder. The threading tool with the toolholder is used where considerable thread cutting is to be accomplished. It is used in a specially designed toolholder which in turn mounts to the lathe toolpost. The threading tool has a formed cutter which needs to be ground on the top surface only for sharpening, the thread form being accurately shaped over a large arc of the tool. As the surface is worn away by grinding, the cutter can be rotated to the correct cutting position and secured there by the setscrew.
- Knurling Tool. The knurling tool consists of two cylindrical wheels, called knurls, which rotate in a specially designed toolholder. The knurls contain teeth which are rolled against the surface of the workpiece to form depressed patterns on the workpiece. The knurling tool accepts different pairs of knurls, each pair having a different pitch. The diamond pattern is most widely used, and is generally supplied in three pitches: 14-pitch, 21- pitch, and 33-pitch to produce course, medium, and fine diamond patterns.
- Boring Tools. Boring tools are ground similar to left-hand turning cutter bits and thread cutter bits, but with more attention given to the end clearance angle to prevent the heel of the bit from rubbing against the surface of the bore. The boring cutter bit is clamped to a boring tool which in turn is supported in a boring toolholder that mounts to the lathe toolpost.