The starrett book for machinists apprentices

Laying out work preliminary to machining is transferring blue-print instructions on to the metal. While the blue-print gives dimensions accurately, without any great precision in the drawing itself, lines laid out on the metal are to be worked to and must therefore be accurate. No one can consider himself a skilled machinist unless he can lay out his own work and, when called upon, lay out work for the less experienced.

To become skilled in laying out should be the aim of every apprentice. Possessing this skill gives more opportunity to show ability than the running of a machine. It is a qualification one must have for advanced positions such as toolmaker, foreman, or superintendent.

But laying out requires some knowledge of mathematics, some skill at mechanical drawing, and an acquaintance with machinists' fine tools and shop operations. Attention to details and extreme care are of utmost importance. Increased labor cost, as well as material wasted because of errors in laying out, are the penalties of mistakes.

The apprentice, then, should lose no opportunity to make himself capable of laying out work. Close observation of pieces laid out by skilled machinists is one way of becoming acquainted with the art. The fortunate apprentice may also have opportunity to observe a skilled machinist while laying out various jobs.

The number of measuring and laying out tools or instruments now purchasable is very great and the apprentice must become familiar with practically all of them. He must know what he can accomplish with each so that he will instinctively select those best suited to the job in hand.

Economy of time in laying out is another element of success. Time-saving tools, such as the dial test indicator, quick-acting micrometer, and combination set, should be among those ready for use. The combination set, for instance, combines a rule, square, miter, protractor, center square, depth gage, height gage, and level. The fewer the tools used, provided the ones at hand are really good ones, the less the bench will be littered with tools which may be used only occasionally.

The tools in a machinist's tool-box are a sure indication of his ability. A well-fitted kit of fine tools helps him hold a job in hard times and is one of the best assets a man can have when applying for a job. The possession of many fine tools indicates a love for accurate work, freedom from the borrowing habit, and a determination to do work which will demand recognition. Next to having a complete outfit of fine tools is the disposition on the part of the apprentice to add the best tools as rapidly as he can afford them.

DRILLS. A drill is an end-cutting tool, consisting usually of two cutting edges set at an angle with the axis. The more common types of drills are flat flat twisted straight-fluted spiral-fluted and gun-barrel. The most common, and for most purposes the most efficient, type is the spiral-fluted, known as a twist drill.

Twist drills are made with two, three, or four cutting lips. The two-lip drill is used when drilling solid stock. The three and four lip drills are used for enlarging holes previously cored or drilled. When drilling solid stock with a two-lipped drill, the point of the drill controls the cutting edges, and if the drill is correctly ground the resulting hole will be reasonably round, straight, and the size of the drill. When a drill is used for enlarging holes already made, either by coring or by previous drilling, the drill is guided by its sides and a three or four fluted drill will give better results.

FORM OF POINT. In the types referred to all except gun-barrel drills are cone-pointed on the cutting end. The gun barrel drill, used when especially straight, round, and true holes are essential, has a blunt end with a single cutting lip.

A cone-pointed drill of two or more cutting lips depends for its efficient working upon four factors:

(a) All the cutting lips shall have the same inclination to the axis of the drill.

(b) Cutting lips should be of exactly equal length.

(c) A proper lip clearance of the surface back of the cutting edges.

(d) A correct angle of lip clearance.

Figs. 1, 2, and 3 show the result of careless free-hand grinding. Figs. 4 and 5 show how to test the length of the cutting lips, also their inclination to the axis.

After sharpening a drill free-hand, use the hand-feed at first and observe (a) the chips made by the cutting; (b) the size of the hole. If the cutting lips are shaped to a proper clearance, the chips will curl as they start from the cutting edge; but if the cutting lips lack a proper clearance the resulting chips have the appearance of being ground off rather than freely cut. If the cutting lips are of uneven length the hole will be enlarged over the diameter of the drill. Drillings from cast iron should look as in Fig. 6, and those from steel as in Fig. 7, if the drill is properly sharpened.

Free-hand grinding results are usually so disappointing that in most machine shops the drills are sharpened in a special drill-grinding machine. The design of this machine is such, that when it is set for grinding any size of drill the cutting lips are made of equal length and of the correct form. Fig. 8 shows how the cutting lip is located to correctly grind the edges.

FEEDING THE DRILL. To get the best results from drills and drilling machines, the drill should advance into the work a definitely regulated amount for each revolution. The distance which the drill advances per revolution is termed the FEED, and must be adjusted to suit the conditions under which the work is being performed. Table No. 2 gives the feeds per revolution recommended by one manufacturer of drills. They are recommended for average conditions; they can be greatly exceeded under some conditions, but must be reduced for others.

Feeding the drill freehand, if skilfully done, may answer in certain cases, but is less effective than power feeds, except for small wire drills.

DRILL SPEED. This is the surface or peripheral speed of the drill in feet per minute, and is rated at the outer diameter. Under average conditions the peripheral speed recommended for carbon steel drills is thirty feet to forty feet, and for high-speed drills seventy feet to one hundred feet. Working conditions may at times cause a change in these figures. When the extreme outer corners of the cutting edges wear rapidly it is evidence of too high a surface speed.

Table No. 3 gives the revolutions per minute at which to run drills for various cutting or surface speeds. For example, with a 1-inch drill and seventy feet as the selected cutting speed, read across from 1-inch in the left-hand column and under heading 70' find 267, the revolutions per minute.

CUTTING COMPOUNDS. To maintain high cutting speeds, it is necessary to use a lubricant. Those recommended have stood the test of service:

For hard and refractory steel, turpentine, kerosene, or soda water.

For soft steel and wrought iron, lard oil, or soda water.

For brass, paraffine oil.

For aluminum, turpentine, kerosene, or soda water.

For cast iron, a jet of air if anything is used usually worked dry.

LAYING OUT. Locating the centers for drilled holes upon the body of the work is termed "laying out." On the smaller jobs, laying out and drilling are usually done by the workman. Larger amounts of work warrant a skilled "layer out."

Laying out for drilling comes under two heads, viz.: APPROXIMATE and ACCURATE. Unless the holes when drilled are to match up with other holes or with fixed studs, it is enough if the center is laid off with a chalk pencil and a steel rule. For jig, tool, and experimental work, the centers must be accurately laid out and scribed upon the surface of the work. The practice is to scribe two or more lines which intersect at the exact desired point as shown in Fig. 9. Assume that the link is to connect two studs. Proceed to scribe two intersecting lines upon one of the hubs, as shown in Fig. 9, using a combination square fitted with a center head. At the intersection accurately place a light center-punch indentation. Place one leg of a spring divider with its point in the center mark and adjust the other leg to have its point touch the edge line of the hub and note the concentricity of the center. If correct, close dividers to scribe a circle the diameter of the required drilled hole, setting the points by the scale graduations upon a steel rule. Locate light center-punch marks on the scribed circle as shown in Fig. 10.

When the work is laid out by another than the driller, a second circle, having a slightly greater diameter, should be scribed. This check will show whether the hole was drilled to the original lay out. If no importance is attached to the center to center distance of the holes proceed as before with the second hub. Where the center to center distance is important, set the points of the universal dividers to the center length, and with the point A, Fig. 11, in the previously located center mark scribe on the opposite hub. Scribe a short line across its face afterward, proceeding as before.

For all accurate work use the automatic center-punch, Fig. 12, and for heavy work the machinists' center-punch, shown in Fig. 13.

PREPARING THE SURFACE. For accurate laying out, clean the machined surfaces and wet the portion to be worked upon with the copper sulphate (blue vitriol) solution., When dry, the surface will distinctly show any lines which are made upon it. Chalk well rubbed into the surface is sufficient for the less accurate jobs.

STARTING THE DRILL. After laying out and previous to drilling, greatly enlarge the center holes with a center-punch to assist the starting of SCRIBING CIRCLES WITH DIVIDERS the drill. Start the hole with drill point in the enlarged center, using hand feed until a reasonable dimple is made in the work. Observe if this is central with the scribed circle, and if not central use center gouge, as in Fig. 14, and repeat until accurate.

TO DRAW A DRILL. When starting a drill it often has a tendency to slide or crowd off to one side. Where it is essential that the drilled hole coincide or center with some previously scribed circle or layout, the drill must be brought back into the correct position. This is accomplished by the use of a small gouge-pointed chisel, sometimes called a center chisel, and the process is termed, "drawing the drill." First, note toward which side of the small dimple left by the drill-point it is necessary to shift the drill. Then chisel a small groove in that side of the dimple.