The troubles experienced by some shops with their boring-bars and cutters if recorded would make a book of several chapters. This is written with the hope that it will be followed up by some of your contributors with their experiences.
The boring-bar L, shown in Fig. 1, I find unsatisfactory because the hardened edges of the cutter G, at A, cut the softer metal of the bar L. and thereby allow the cutter an uncertain amount of sideplay in the slot, which is always doubled in the hole being bored. When this side-play becomes so great as to allow the cutters to be bore a hole larger than standard size, it is necessary to mill down the flats on the bar and make a new set of cutters throughout, which is very expensive, as the bar has to be out of service while this is being done.
In Fig. 2 we have the boring-bar which is theoretically correct, practically a failure, for the reason that it is next to impossible to drill the hole for the hardened pin M exactly through the center of the bar L. I think any tool-maker will agree with me in this statement, if he has ever tried to drill a 1/2 in. hole through the center of a2-in. round bar of tool steel at right angles with the axis. I have overcome the defects of Figs. 1 and 2 by constructing our boring-bar as shown in Fig. 3.
I make all of our bars, keys and centering pieces of low grade tool steel, and our cutters of Jessop steel. All the work on these bars, except locating the centering pieces, drilling and reaming the taper pin holes J, and finishing the end of the keys K, is performed on the milling-machine.
For grinding-cutters for these bars I use a short bar S, made of tool steel, with hardened centers, and with a slot as shown at Fig. 4. It was necessary to make this bar first, in order to originate what I will call the master cutter M.
The key K, master cutter M, centering piece N, and «crews B and C were hardened and assembled in the short bar S, as shown in Fig. 4. It was next put into the grinder, and the master cutter M was grouud until both ends were trued up. The micrometer height gauge was then set to just touch the ends of the master cutter M, and the reading noted. The master cutter was then changed end for end in the slot and the height again taken. It was then readily seen which way and how much the centering piece W was out of center. By continued grinding, testing and adjusting of screws B and C, we were enabled to set the centering piece N so that the convex projection was within 0.0001 in. of the center.
The same master cutter is used to locate the centering pieces in the boring bars as follows:The boring-bar L is mounted on dead-centers and straightened if necessary; then the centering-piece II, master cutter M, and hardened key K are put in place and the key driven up lightly. The michrometer height gauge is again brought into service, and by tapping the centering piece Hwith alight hammer the master cutter is brought to a position where the variations in the micrometer readings taken from both ends of the master cutter is within the 0.0001 in. limit.
The key K is then driven up tight, and the master M is again tried with the micrometer, and if still within the limit the bar is ready for drilling for the taper-pin J.
After fitting the pin, the centering-piece H is hardened, and if the hole has contracted in the hardening, it is lapped until the taper pin J will enter to the same depth as would previous to the hardening.
If, on the contrary, the hole has expanded, the hole in the bar will have to be reamed larger.
Boring-bars and cutters made in this way will bore within 0.001 in., if the bars are kept straight and are a good fit in the supporting bushings. This, I think, is near enough where holes are reamed by hand after boring. - A. V., in "American Machinist."