Figure 75.
For countersinking, to let in the heads of these bevelled forms of screws, a tool is used like the one as shown in [Fig. 75]—the stem serving as a guide when inserted in the work, and a slot to admit the screw-driver, for turning them, being made in the large end. This tool is made about two inches in length, the head being about half an inch in diameter.
The length of the tools in Figs. [72], [73] and [74], where they are held in the vise, is about an inch and a half or an inch and three-quarters; the width from half an inch to five-eighths, according to size; the thickness one-quarter inch. The round part with the cutting teeth is about three-quarters of an inch in height from the flat portion, and is about the same diameter. The teeth may be in number five, six or eight, as most convenient to make. The tools for bevelled heads are best made with five teeth.
Making Small Taps.—The best manner of making large taps is to turn them to size in a lathe, and then cut the screw by the same means of with a die plate. Small taps cannot very well be so made. A very good way is to get good steel wire—generally sold of all sizes, under the name of Stubs’ steel wire—and from this make taps; the thread can be cut by means of a die stock.
When the thread is properly made, there are several ways to form it so as to make it a cutting tool. One method is to file it with four sides, making it a square. In this case it scarcely cuts, but rather “jams” up a thread. Another one is to file it triangular or “three square.” This form makes it rather a better form, as regards cutting quality; but as in case of the square form, it will be observed that the inclination of the sides is such that they would not be selected for a cutting tool if such an angle were made as a tool for that purpose. If two flutes or grooves be made of angular form, with a square file, or two hollows be cut with a round file, these flutes running lengthwise with the tool, the edges then present more of the aspect of a cutting tool; but the distance is so great between the flutes that there is great friction, and breakage will result if the hole to be tapped be too small, or too much force be applied. If three or four flutes be made the evil of friction will then be remedied. Be sure to cut the flutes deep enough to receive all cuttings that may be removed or the tap will become clogged, and can with difficulty be turned out, or perhaps may be broken.
A cheap and good way to finish a tap is, after the thread is made, to file away one-half of its diameter nearly the length of the thread. This will give great clearance and space for the cuttings, and at the same time the cutting edge is very sharp and is strong. It will be found to cut very easily. If a tap of this make becomes dull it can be sharpened by grinding on the flat formed by filing it away. If the tap be too large it can be made smaller by thus grinding.
Large taps work equally well made in this way, but care must be taken in starting them in a hole or the thread may be made crooked.
Reamers, both large and small, straight or taper, can be made in this manner, and are effective as well as cheap to make, and can be kept sharp by grinding them on the flat side.