Material removal processes-Drilling

Material removal processes include all those where, by the nature of the process, the material is cut in order to arrive at a predetermined size. There are five basic metal-cutting processes:

1. Drilling
2. Turning
3. Planing
4. Milling
5. Grinding

All of the other metal-removing processes are closely related to or are modifications of these five basic processes. For example, the process of boring is internal turning； reaming, tapping, and counterboring modify drilled holes and, therefore, are related to drilling； hobbing is a milling operation； honing, lapping, superfinishing, polishing, and buffing are refined grinding operations； sawing can be either milling (if it takes a circular saw) or planing (if it is done by hacksawing or bandsawing)； broaching is a form of planing.

The amount of material removal of the various cutting processes may be quite small，as in polishing and buffing operations, or it may be relatively large, as in milling and turning processes. It is the purpose of this chapter to present the various material removal processes that are available to the production design engineer so that he will be able to specify the most favorable manufacturing procedure.

DRILLING

Drilling is probably the widest used machining operation. There are only a few processes, such as punching, boring, and burning, which can be substituted economically for drilling operations. All of these, however, have decided limitations； and in many cases, although another process can be used, it is still more economical to drill. Good drilling practice will result in little variation in location of holes, and size and shape can be depended upon at a minor tool cost.

The principles of cutting metal apply to drills as well as to single-point tools. The surface of the drill flutes must be smooth so that friction will not retard the movement of the chips up and out of the drilled hole. The cutting angles must be ground to suit different materials, and adequate lubrication must be provided.

The most important factor to be controlled to assure satisfactory drill performance is the accurate grinding of the drill. If one lip is ground at a different angle from the other, the tool will feed off-center and will drill an oversized hole. Also if the angles are the same, but one side is longer than the other, a thicker chip will be cut on one side, causing the drill to cut oversize. In addition, improper grinding results in an unequal distribution of forces acting on the drill, which may cause drill breakage. Drills should be ground in a drill grinder rather than by hand so that an unskilled operator can provide drills that are ground properly and save the time of the skilled operator.

Drills for different kinds of material, such as plastics, nonferrous metals, cast iron, steel, and alloy steels should be stocked in the tool crib, available for use with the particular material. The use of proper feeds, speeds, jigs, and equipment with true-running spindles will increase the life of tools, result in the drilling of more holes per hour, and give greater accuracy. Drilling is a major operation, and a small percentage saved can amount to a considerable amount of money.

Improvement in drill performance has recently been made by the introduction of better tool material, polished flutes chromium-plated to reduce wear, and improvement of the shape of the cutting edge.

DRILL TERMINOLOGY

There are 20 different types of twist drills, as well as flat drills, straight-flute drills, core drills with three or four flutes, multicut drills, step drills, multiland drills, and combination drills and reamers. In order that the possibilities of the various types of available drills might be understood, the suppliers of drills have developed data which suggest the best shape of drills for each of the large variety of materials.

DRIU EQUIPMENT

One design may require the use of several types of drilling equipment, jigs, and fixtures. A good design reduces the number of different machines and the sizes of drills, reamers, and taps used. Good designs endeavor to place the holes on a single plane and maintain constant depth of all holes drilled. Effective design and production will be made possible by a knowledge of drilling equipment, cutting tools, and auxiliary equipment.

When drilling is done, the work is brought to the fixed spindle of sensitive and upright drilling machines (both single spindle and gang drills) or the spindle of a radial machine is brought into position as the part is held stationary.

Parts are held in vises or jigs and moved under the drill spindle along the table. Often two, three, four, six, or eight spindles are mounted over one table so that the part can be drilled, reamed, tapped, or counterbored without removing the part from the jig. Quick-change chucks also enable additional drill sizes to be used on the same machine and spindles.

When radial drills are used, the part is stationary and usually in a jig. Often the jig is suspended in a trunion and can be drilled from any side parallel to the axis of the trunion. Quick-change collets enable the operator to change from one drill size to another, or from drill size to reamer, boring bar, or tap. Radial drills are designed so that feeds and speeds can be changed quickly to accommodate, use of the large variety of cutting tools.

Multiplc-spindle machines and single-spindle machines equipped with multiple-spindle drill heads may be fed simultaneously into the work. The length and time of feed are determined by the longest hole to be drilled and the drill having the lowest feed requirement. Rpm can be varied in some cases with special gearing，but rpm，s, in general, are the same for all drills mounted in the head. The drills are guided by bushings mounted on the drill press head or in the jig. Drilling, reaming，and tapping can be performed in multiple-spindle heads. Usually each operation is done in a separate machine. Wherever a number of holes can be drilled at the same time in a part of moderate activity, the multiple-spindle drill is economical. Multiple drilling is applied widely in mass production and can be economically adapted to job shop work. Bolt holes for cover plates, bearings, glands, and housings, can be standardized to use similar jigs and setups and promote the use of multiple-spindle drill presses.

The part, jig, and tool must be designed to withstand the pressure of the tool as it is cutting. The new cutting tool materials have raised speeds, feeds, and pressures until only the most modern equipment can take full advantage of the new features. The engineer should use the data furnished by equipment and tool suppliers as a guide, and experiment with feeds and speeds in order to remove the greatest amount of material consistent with an economical tool life. The cost of the operation and machine must be balanced against the cost of tool wear, and against sharpening and setup time.

A drill, on entering material, has a tendency to wobble until the entire drill is cutting. Thus the accuracy of the location of a drilled hole is increased by using a punch mark, a smaller diameter drill (which wobbles less), a stub or short drill rigidly held in position by a good spindle, and a guide bushing. If accurately located holes are required, it is necessary to make the part on a precision machine like a jig borer or a horizontal boring machine, or to use guide bushings mounted in a jig .

Numerically controlled turret-type drill presses with the table automatically positioned can drill, ream, tap, chamfer, and counterbore any quantity of parts without a drill jig. The table holding the work is accurately positioned, the turrets are rotated, proper speeds selected, proper advance and cutting feeds selected by a master tape or punched card. The operator merely places the tape in the control, installs the cutting tools in the turret, and locks the part on the table with vee jaws or clamps.

DESIGN CONSIDERATIONS

The most important considerations for a designer to observe are：

1. Avoid deep holes. Any hole longer than five times its diameter is considered a deep hole and requires special procedures in the shop, such as withdrawing the tool at intervals to clear the chips, and forced lubrication.
2. Start and finish holes on surfaces at right angles to the direction of the hole.
3. Provide room for a bushing and its support in the jig or a fixture to guide the drill.
4. Use standard-size drills for tapped holes and clearance holes for bolts, screws, rivets, and bushings, so that minimum stock of drills can be maintained.
5. Use drilled holes instead of reamed holes wherever possible, provided the shop can produce good quality holes through proper grinding and tooling.
6. Use the same size hole, or tap wherever possible, so that the minimum number of spindles and drills will be required.