An excellent tool for inserting studs of this kind to the proper distance is shown in [Fig. 394]. It consists of a square body a threaded to receive the stud whose end is shown at c. The upper end is threaded to receive an adjusting screw b, which is screwed in so that its end d meets the end c of the stud. It is obvious that b may be so adjusted that when a is operated by a wrench applied to its body until its end face meets the work and the stud is inserted to the proper depth, all subsequent studs may be put into the same depth.
Fig. 395.
When the work pivots upon a stem, as in [Fig. 395], the bolt is termed a standing pin, and as in such cases the stem requires to stand firm and true it is usual to provide the pin with a collar, as shown in the figure, and to secure the pivoted piece in place with a washer and a taper pin because nuts are liable to loosen back of themselves. Furthermore, a pin and washer admit of more speedy disconnection than a nut does, and also give a more delicate adjustment for end fit.
In drilling the tapping holes for standing bolts, it is the practice with some to drill the holes in cast iron of such a size that the tap will cut three-quarters only of a full thread, the claim being that it is as strong as a full thread. The difference in strength between a three-quarter and a full thread in cast iron is no doubt practically very small indeed, while the process of tapping is very much easier for the three-quarter full thread, because the tap may, in that case, be wound continuously forward without backing it at every quarter or half revolution, as would otherwise be necessary, in order to give the oil access to the cutting edges of the tap—and oil should always be used in the process of tapping (even though on cast iron it causes the cuttings to clog in the flutes of the tap, necessitating in many cases that the tap be once or twice during the operation taken out, and the cuttings removed) because the oil preserves the cutting edges of the tap teeth from undue abrasion, and, therefore, from unnecessarily rapid dulling. With a tap having ordinarily wide and deep flutes, and used upon a hole but little deeper than the diameter of the tap, the cuttings due to making a three-quarter full thread will not more than fill the flutes of the tap by the time its duty is performed. We have also to consider that with a three-quarter full thread it is much easier to extract the standing bolt when it is necessary to do so, so that all things considered it is permissible to have such a thread, providing the tapping hole does not pass through into a cylinder or chamber requiring to be kept steam-tight, for in that case the bolt would be almost sure to leak. As a preventive against such leakage, the threads are sometimes cut upon the standing bolts without having a terminal groove, and are then screwed in as far as they will go; the termination of the thread upon the standing bolt at the standing or short end being relied upon to jam into and close up the thread in the hole. A great objection to this, however, is the fact that the bolts are liable to screw into the holes to unequal depths, so that the outer ends will not project an equal distance through the nuts, and this has a bad appearance upon fine work. It is better, then, in such a case, to tap the holes a full thread, the extra trouble involved in the tapping being to some extent compensated for in the fact that a smaller hole, which can be more quickly drilled, is required for the full than for the three-quarter thread.
The depth of the tapping hole should be made if possible equal to one and a half times the diameter of the tap, so that in case the hole bottoms and the tap cannot pass through, the taper, and what is called in England the second, and in the United States the plug tap, will finish the thread deep enough without employing a third tap, for the labor employed in drilling the hole deeper is less than that necessary to the employment of a third tap. If the hole passes through the work, its depth need not, except for cast-iron holes, be greater than 1⁄8 inch more than the diameter of the bolt thread, which amount of excess is desirable so that in case the nut corrodes, the nut being as thick as the diameter of the tap, and therefore an inch less than the depth of the hole at the standing end, will be more likely to leave the stud standing than to carry it with it when being unscrewed.
Fig. 396.