Fig. 363.
When the tap does not pass through the hole it may be employed with a guide which will keep it true, as shown in [Fig. 363], in which w is a piece of work, t the tap, and s a guide, the latter being bolted or clamped to the work at b. In this case the shank of the tap is made fully as large in diameter as the thread. In cases where a number of equidistant holes require tapping, as in the case of cylinder ends, this device saves a great deal of time and insures that the tapping be performed true, the hole to receive the bolt b and that to receive the tap being distant apart to the same amount as are the holes in the work.
Fig. 364.
Fig. 365.
In shops where small work is made to standard gauge, and on the interchangeable system, devices are employed, by means of which a piece that has been threaded will screw firmly home to its place, and come to some definite position, as in the following examples. In [Fig. 364] let it be required that the stud a shall screw in the slide s; the arm a to stand vertical when collar b is firmly home, and a device such as in [Fig. 365] may be employed. p is a plate on which is fixed a chuck c to receive the slide s. In plate p is a groove g to hold the head h at a right angle to the slideway in c, there being a projection beneath h and beneath c to fit into g. The tap t is threaded through h, but not fluted at the part that winds through h when the tapping is being done, so as not to cause the thread in h to wear. h acts as a guide to the tap and causes it to start the thread at the same point in the bore of each piece s, and the stem will be so threaded that the screw starts at the same point in the circumference of each piece.