If the machine is operated by hand, with hand pressure, it is called a hand drill. The largest size drill it takes is 3/8"; therefore it is generally used only for light, small work. If operated by hand but with pressure applied by plating it at the chest and using the weight of the body for pressure, it is called a breast drill. A breast drill is generally used for heavy-duty work. It may have a two-gear arrangement for slow and fast operations.
In the push drill, the point is turned by the pressure of pushing the tool, not by turning a wheel or gear. Drills may also be operated by power other than manpower. When the danger of electric sparks must be avoided, compressed air is sometimes used for power. But most of ten power drills are run by electricity. Such power drills relieve the mechanic of effort and make it possible to do more accurate work.
The automatic chain drill is used with the common brace, but it has a chain attachment for applying pressure. The bench drill is similar to a drill press, but it is operated by hand power. Power drills that can be used only when the work is brought to them are called drill presses.
Center punch marking
The purpose of center punching is twofold: (1) to locate permanently the center of the hole that is to be drilled, and (2) to make a slight depression to provide a start for the drill point. If a punch mark is not made, the drill point will slip or "walk," and make the hole in a wrong location.
Unless great accuracy is required in locating the center of the hole (as in matching the holes with other holes or with fixed studs), approximate layouts with a chalk pencil and a steel rule are ordinarily accurate enough.
As explained elsewhere put a thin layer of topper on the surface of the metal. Scribe two or more lines crossing at the spot where the center of the intended hole is located. Place a center punch exactly at the point of crossing. Then gently tap the center punch with a hammer. Use a magnifying glass if necessary to check the location of this punch mark.
If the location is correct, scribe a circle with a pair of dividers, the diameter equal to that of the hole. With the center punch, make small indentations, called "witness marks," on the circumference of the circle. If a check is required after the hole has been drilled to see if the layout was followed, a second circle should be scribed outside the first one.
As a last step in punching, use a center punch with a heavy blow of the hammer to mark the center of the hole to be drilled. If this broad, deep punch mark were made before the circles were scribed, it would be impossible to scribe these circles accurately.
Correcting punch marks
If a punch mark is incorrectly placed, perhaps about 1/16” off, replace the punch in the wrong punch mark. Slant the punch, pointing it in the direction of the correct location. Gently tap the end of the punch to move it along until the proper spot is reached, then move the punch to a perpendicular position and make a punch mark.
If the incorrect punch mark is off more than ignore it and make a new punch mark at the correct spot. To avoid confusion and error, the incorrect punch mark should be erased by gently tapping the metal back into the depression with the peen, ball hammer or with a round-pointed punch.
The most commonly used type of drill is the twist drill. Figure shows a twist drill with its flutes and its parts named. For several reasons, the twist drill is excellent for drilling of holes in almost any material. The angle of the lips can be varied to cut either hard or soft materials. The chips are removed in the form of curls which occupy very little space; this is important, for waste may jam a drill so that it cannot rotate. The flutes permit these curls to come out readily and make it possible for the lubricant to flow readily over all parts of the drill point.
Figure (soon) shows the various types of shanks twist drills may have. The straight shank is intended to be held by the three-jaw chuck of the hand drill or breast drill. The chuck is that part of the machine that holds the drill. The square-shank (or square-tang) twist drill is planned to fit the two-jaw chuck of a brace, as a wood bit does. The tapered-shank twist drill is intended to fit the chuck of a drill press, lathe, or similar machine.
Twist drills come in a great variety of sizes, or diameters. There are three series, and no drill of any one series has the same size as any drill in any other series. One series is numbered from 1 (which is 0.288 inch) to 80 (0.0135 inch) in diameter. Another series begins with 1/64" and increases in steps of 1/64" up to 1/2" diameter. Stilt another series is sized by letters from A, 0.234", to Z, 0.413".
Sizes of larger drills are stamped on the shank. Since there is no room on the shanks of the smaller drills to stamp the size, it is determined by fitting the drill into the hole of a drill gage or by using a micrometer and the table of drill sizes.
A hole 1" or less in diameter is generally called a drill. A hole larger than 1" in diameter is commonly called a bore. However, standard drills are made up to 3 1/2" in diameter.
Selection of drill size
The particular drill must be selected to make a hole of the required size. Whether the hole to be drilled should be threaded or unthreaded also determines the drill to use.
Determining drill speed
Speed is the rate (revolutions per minute) at which a drill turns. When holes are being drilled in certain materials, such as steel, wrought iron, and hard cast iron, the cutting speed and the life of the twist drill will be increased if a lubricant is used. Its chief purpose is to keep the cutting surface of the drill cool and thus keep its temper. Any light machine oil, kerosene, lard oil, cutting oil, or turpentine will serve, but heavy bearing oil should never be used. Cast iron, bronze, brass, and bakelite should be drilled dry. Kerosene is a good lubricant for drilling aluminum.
Carbon-steel drills, very low in cost, are used for ordinary work, especially in hand or breast drills. If used with high speed they burn, or lose their temper, and soften. High-speed drills, as their name indicates, are used for fast production work, especially as power drills.
When using a hand drill there is no need to consider speed, because the drill cannot be turned fast enough to cause trouble. It is when using power drills that the mechanic must be on his guard. Examples of proper speeds are: hard steel, 350 rpm (revolutions per minute); cast iron, 532 rpm; brass, 912 rpm.
The mechanic should always follow three speed rules:
The feed is the rate at which the drill moves into the stock. Naturally this depends partly on the speed the drill is turning, but mainly on the amount of pressure applied on the drill. Therefore we are more concerned about feed pressure as it relates to power drills than as it relates to hand drills.
The feed depends on
Speed and pressure should vary as the hole is drilled. Begin slowly with little pressure; end in the same manner so that the drill point does not suddenly push through the metal.
Since experience is the best guide, only one general rule can be given for feed pressure: Keep just enough pressure for the drill to remove material.
When the drill is to be taken out of the hole, keep it turning in a cutting direction but reverse the feed pressure; that is, pull instead of press on the drill until it is completely out of the hole.
The underneath part of the head of a flathead screw slopes at an angle of 82°. The beginning of a drill hole to receive such a screw must be finished with a similar slope.
Whether they are carbon-steel or high-speed, or whether they «ere used to cut soft or hard materials, drills will get dull with use. Since 90 per cent of all drill failures arise because of poor grinding, the mechanic should quickly master the skill of proper sharpening.
Figure shows how a drill should be ground for general purposes. However, certain materials require different cutting angles. Figure shows how the drill should be ground for special purposes. It is no simple matter to sharpen a drill correctly. A special drill-grinding attachment is low in cost and should be used. When possible, drills should be ground on a power grinder, not on a hand grinder. However, care must be taken not to burn the drill. When grinding a carbon-steel drill, dip it frequently in water to keep its heat below that of its temper. Always check the cutting angles of a sharpened drill with a drill gage.
Use of drills
Most of what has already been said about drilling applies to hand drilling. The following is a summary, with some additional suggestions.
Portable electric drills
Portable drills are made in a great many different types. However, they may be classified as light-duty and heavy-duty drills. Light drills take points up to 1/4" in diameter and generally have a pistol grip. Heavy drills take points up to 1", are quite heavy, and have an extra handle to hold the machine steady. Ordinary straight-shank drills are used, as in the hand drill. The size of the drill is given as the largest size drill it will hold.
Cheap, light, portable drills have the same chuck as the hand drill, the type that is tightened with the fingers. Better light drills and most heavy drills have the chuck that must be tightened with a key. This kind is best and safest because it allows for a tight grip on the drill point.
In addition to these special points of information, all the previous advice about drilling by hand applies here as well.
In addition to the general instruction already given, the following applies particularly to the use of the drill press. Before putting on the power, check for certain things. The drill point is brought to the work by pushing down a lever. Check to see if the drill will reach the work. If it does not, the clamp knob holding the table in place should be loosened, the table raised to the correct height, and the knob tightened again.
If the hole is to be drilled only part-way through the work, the stop gage for feeding the drill should be checked. If the hole is to go through the work, the drill point should reach just a trifle beyond. A piece of wood should be placed between the work and the table, or the hole or slot in the table should be properly aligned with the drill. The purpose in either case is to prevent the table from being marred with drill cuts.
Small pieces should be held by pliers or clamped to the table in a vise. Otherwise there is danger of the work's spinning and causing damage or injury. The V-block is used on the table to hold round work, such as tubes and bars.
In the beginning just drill a small part of the hole and check to see whether it is located properly. If it is not, make a cut with a cold chisel, and move the work so that the drill hole will be properly placed. The chisel cut permits the drill to cut at the new spot without walking. If the hole to be drilled is large, first drill a small hole, about 3/32”, as a pilot for the large drill.
Drive the tapered drill into the sleeve or socket by tapping it with a soft-lead or brass hammer. Never hit it with a machinist's hammer or a piece of steel.
Do not use a file or wedge to pry out a drill from a sleeve or socket. Do not permit the drill when it comes out to strike the metal table. The correct procedure is to use a drill drift to "drift out" the drill and let it fall on a piece of wood or other soft material. Oil the press as directed by the manufacturer's manual at least twice a day.
If no drill point is available and a hole must be made, a brad or nail of suitable gage can be substituted. It can be driven into the work and removed. The hole will not be accurate or smooth, but it may suffice temporarily.