The pressure of the cut increases as the tool point loses its sharpness, and this makes sufficient difference for the amount of tool spring in light boring tools or in long holes to cause the tool to bore a larger hole at the beginning than it does at the end of its feed traverse; or, in other words, to bore a taper hole, whose largest end is that at which the cut was started. If, therefore, the cut is traversed from the front to the back of the hole the latter will be of the smallest diameter at the back, and conversely if the cut proceeds from the back to the front of the hole the front will be of smallest diameter. The amount of the taper so caused (or in other words the error from parallelism) will obviously increase with the length of the hole.
To obviate this taper, the slide of the rest should for the finishing cut be set up firmly, and the tool after being sharpened should take a finishing cut through the hole, and then let traverse back, which can be done providing that care be taken not to bore the hole too large.
A boring tool will take a smoother cut and chatter less if the final cut be from the back to the front of the hole, and for the following reasons: When the tool is fed in, the strain or pressure of the cut is in a direction to partly compress and partly bend the steel which is being pushed to its cut, but when it is fed in the opposite direction it is pulled to its cut and the strain is in a direction to stretch the steel, and this the tool is more capable of resisting, hence it does not so readily vibrate to cause chattering.
In consequence, however, of the liability of a boring tool to spring away from its cut, it is far preferable to finish holes with standard cutters, reamers, or bits, in which case the boring tool may be employed to rough out and true up the hole, leaving a fine cut for the finishing cutter or bit, so as to wear its cutting edge as little as possible. To further attain this latter object, the cutter or bit should be used at a slow cutting speed and with a coarse feed.
If cutters or bits are not at hand, tool holders are desirable, and the forms of these depend upon the nature, or rather the diameter, of the hole to be bored. In all cases, however, the best results will be obtained when the diameter of the tool holder is as near that of the hole to be bored as will give it clearance. This occurs on account of the rigidity of the holder being greater than that of the tool.
For large work tool holders are desirable, in that the tools, being short, are easier to forge, to handle, and to grind.
For example, a tool holder of a cross section of two inches square may contain a tool whose cross section is 1 by 3⁄4 inch, in which case it is necessary to forge, grind, &c., the small tool only, whereas in the absence of the holder the tool would require to be of a cross section equal to that of the holder to obtain an equal degree of rigidity.
Fig. 1118.