Counterbores are tools used for enlarging a hole without changing its relative position. For an emergency job and for a small number of holes, it is advisable to make as cheap a form as is consistent with the work to be done. Probably the cheapest counterbore that will do satisfactory work is the one shown in Fig. 141. This can be forged so as to require but little machine work. After forging, it is turned to size, and the shank A and pilot B finished with a fine file before being taken from the lathe. The cutting edges CC should be faced true and smooth. The necking between the pilot and the body should be cut with a tool having the corners slightly rounded, to decrease the liability to cracking when the counterbore is hardened. The flat sides D of the body may be finish-filed; the edges should be drawfiled, and more stock removed on the back than on the cutting edge, to prevent binding. File the cutting edges for clearance, as shown at E. The pilot and the body should be hard the entire length, or they will wear and rough up so that they cannot cut a smooth hole. Draw the temper to a full straw color. Unless intended for accurate work, the tool need not be ground.
Fig. 141. Flat Counterbore.
For permanent equipment, counterbores are usually made with four cutting edges, as shown in Fig. 142 and Fig. 143; Fig. 142 represents a taper-shank counterbore for a taper collet, while Fig. 143 has a straight shank to be used in a chuck or collet having a straight hole the size of the shank.
Fig. 142. Typical Counterbore with Taper Shank.
Fig. 143. Typical Counterbore with Straight Shank.
Counterbores for screw holes are usually made in sets of three - one for the head of the screw with pilot, or guide, of body size; one for the head with pilot of tap-drill size; and one to enlarge a tap-drill hole to body size.
The following instructions apply to counterbores with either straight or taper shanks.
Take stock somewhat larger than the finish size of the counterbore. Turn a roughing chip all over the piece; turn the necked portion between the shank and body to size, and stamp the size of counterbore and pilot as shown in Fig. 143; turn shank C, body A, and pilot B .015 to .020 inch above finish sizes to allow for grinding. In the case of the taper-shank counterbore the tenon should be milled.
The counterbore is now ready to have the grooves milled to form the cutting edges. One method is to cut them with a right-hand spiral of from 10 degrees to 15 degrees; the other method is to cut the grooves straight. The former has the effect of running chips back from the cutting edges, and works very well on wrought iron and steel; while the latter method is considered more satisfactory for brass and cast iron, though it too works well on wrought iron and steel. The cutting edges are given clearance by filing, as shown at A in Fig. 144. If the counterbore is to be used for brass, it is necessary to give clearance to the lands also, as shown at AAAA, Fig. 145.
Fig. 144. Sketch Showing Clearance of Cutting Edges of Counterbore.
Fig. 145. Sketch Showing Clearance of Lands.
When centering counterbores, or any tools whose centers are not to be used after the tool is finished, the drill should be small, and the countersinking no larger than is necessary for good results in machining. If large centers should, by accident, be put in the ends, the one on the end to be hardened should be filled with fire clay moistened with water to the consistency of dough, or with graphite mixed with oil; this prevents steam from forming in the hole and cracking the tool when dipped in the bath. If the piece is to be heated in lead, the filling should be dried thoroughly before immersing.
Fig. 146. Sleeve for Counterbores with Holes Larger than Pilot.
Solid counterbores can be used with holes larger than the pilot by forcing a sleeve over it, as shown in Fig. 146. B and C are two views of the sleeve which is to be forced on to the pilot A.
After hardening, the counterbore may be ground to size on the shank, body, and pilot; the shank should be ground first, as the length is greater, and, in the case of a counterbore having a straight shank, the grinder may be adjusted to perfect alignment by measurement.
Fig. 147. Taper Hole in Line with Hole Already Drilled.
Two-lipped counterbores are sharpened by grinding on the flat faces marked D, Fig. 141; a four-lipped counterbore is ground on the flat side of the groove, as D, Fig. 146.
It is necessary many times to produce a hole of a given taper extending into a piece of work, as shown in Fig. 147, where the hole must be exactly in line with a drilled hole already in the piece. This can be done by using a counterbore of the design shown in Fig. 148. At other times, it is necessary to produce an impression of special form which must be true with a drilled hole. In such cases a counterbore may be made whose pilot is the size of the drilled hole, and whose body has the form of the desired impression, Fig. 149. As the cutting edges of this counterbore cannot be ground after hardening, they must first be backed oft" for clearance with files and scrapers, and special pains taken during the hardening to prevent springing. This can be done by heating the piece in a muffle furnace and turning it frequently to prevent uneven heating; or by placing the tool in a piece of gas pipe in an ordinary fire, quenching it in lukewarm water, and drawing the temper to a full straw color, 460° F. Better results follow if the tool is pack hardened, and then quenched in raw linseed oil or cottonseed oil.
Fig. 148. Counterbore For Producing Holes Shown in Fig. 147.
Where a limited number of holes are to be counterbored, the tool shown in Fig. 150 may be made. All that is necessary in making this tool is a piece of stock, A, the size of the hole to be counterbored, and a piece of drill rod for the cutter B; the latter is filed to a cutting edge, hardened, and driven into place.
If accuracy is essential, the piece of drill rod must be cut off somewhat longer than the diameter of the required hole; it should be driven into the hole in the bar leaving an equal length on each side, then turned to the correct diameter and filed to shape. If several cutters are to be used in the same bar, or if the tool is to be used as a facing bar to square a shoulder inside a piece of work, Fig. 151, the cutter B is removed from the bar; after the bar is in place, it is inserted and held by a set screw C.