The cutting tool is that part of a cutting machine which serves for removing material from revolving work. If either incorrect or faulty cutting tools had been used for metal-cutting operations, the quality of work would have become poor and cost would have been higher. That is why careful attention should be given to the cutting tools in any metal-cutting operation.
Cutting tools are made of hardened and tempered steel or alloy metals. All the cutting tools are adapted to perform certain work in the most efficient manner and, accordingly, they may be subdivided into turning tools, boring tools, milling cutters, planing tools, shaper tools, etc. These tools having one effective cutting edge along which excess material from the workpiece is removed are known as single-point cutting tools.
Other tools removing excess material on two or more cutting edges simultaneously are known as multiple-point cutting tools. Each cutting tool consists of a sliank for holding the tool in the machine and a tip or cutting edge for removing chips from the work.
The single-point cutting tools fall into several types, such as: (I) solid, forged tools having the same material throughout; (2) solid tools having a tough steel shank and a tip made of high alloy steel which is welded on to the shank; (3) solid tools with a tip brazed (Fig. 19) on to the shank; and (4) inserted tools having a small piece of the cutting edge made of carbide steels. Inserted tools held in a tool holder owing to a screw or wedge are used for machines of a complicated nature when it is necessary to prolong tool life as long a time as possible.
Fig.20. Cutting Tip:
A - top rake; В - side rake; С - front clearance; D - side clearance
The various types of cutting tools differ in shapes and in the angles to which the surfaces of the tools are ground. The cutting tipshould be ground by hand or by machine with correct angles (Fig. 20) on the top face (rake angles) and sides (clearance angles) to a desired shape. The shape of the tool, an ell as the proper rake and clearance angles depend upon a large number of factors, such as the specific operation, the material to be cut and the material from which the tool is made. The top rake is usually provided for the tool holder by the tool being set at an angle, which is correct for machining steel and cast-iron. On solid tools it is necessary to grind the top rake in the tool.
By adjusting the tool in the tool post through a wedge, this top rake can be varied somewhat to suit the material being turned. The softer the material the less the top rake should be as there is a tendency for the tool to dig in if the rake is too great. The side rake also varies with the material being machined. The proper angle is from 6° for soft material to 15° for steel. The front clearance depends on the diameter of the work to be turned. To turn cast-iron it is advisable to set the tool above center. If the tool were ground square1 without any front clearance, it would not cut, but rub on the material to be turned below the cutting edge of the tool. The front clearance should be less for small diameters than for large diameters, ranging from 8° to 15°. The tool is ground with the side clearance to prevent the dragging of the tool on the shoulder formed by the cut. This angle is usually about 6 from the vertical and is constant.
For efficient operation of the machine, the proper surface speed of the work being machined must be maintained. If speed is too slow, the job takes more time than necessary and often the work produced is unsatisfactory. On the other hand, if the speed is too great, the cutting edge will be worn down too rapidly. Frequent grinding will be necessary, which is also wasteful. For ordinary
production work the speed should be as great as the tool will stand without requiring sharpening more often than every two or three hours when cutting continuously.
Cutting tools used for longitudinal turning are subdivided into roughing tools and finishing tools.
Roughing tools are applied for roughing or removing the excessive metal from the work. Such tools are usually carbide-tipped and they have a long cutting edge. Angular roughing tools are very convenient for turning surfaces of the parts which are at the chuck cams, as well as for facing.
Finishing tools are used after the work has been turned with a roughing tool to give accurate size and clean surface to the work being machined.
Before starting the cutting operation tools should be clamped in the tool-holder (Fig. 21) by means of two or more bolts.
Fig. 21. Tool Holder
1 - cutting tool; 2 - bolt; 3 - shank; 4 – bolt
Side tools are used for cutting faces. A side tool has a long cutting edge set in an angle of about 5° with respect to the surface of the work to be cut, and a short cutting edge. This cutting edge is largely beveled to facilitate the approach of the tool tip to the center of the part fastened between the two lathe centers.
Necking tools are used for grooving, since the width of grooves is usually small. The cutting edge of a necking tool is narrow, which increases the danger of its breakage. To prevent this breakage the height of the head is made several times larger than the width of the cutting edge.
Material is cut off by means of tools known as cutting-off tools, which are similar to necking tools. The difference is that they have a longer head which should be a little larger than one-half of the diameter of the blank to be cut.
1. If the tool were ground square - если бы резец затачивался под прямым углом
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