Too little attention is given to the selection of means for holding the small sizes of tools in the lathe. This attention becomes all the more necessary in the case of tools used in specialized machines. In making a selection, it is necessary first to consider the conditions under which the chuck is required to operate. If the clearance offered by the work in hand, the jigs, fixtures, etc., is sufficient, then one of the several forms of inserted spring chuck will usually meet the case.
Fig. 1 is a common form of hollow spindle chuck, with draw in spindle engaging from the hack, this being undoubtedly one of the readiest, truest and most secure means for holding this class of tool. Where the use of the draw spindle is impossible or inconvenient, however, a modification of this chuck becomes necessary. Two methods are shown at Figs. 2 and 3. Although somewhat more bulky than the others, that shown at Fig. 2 is an excellent device, and its value increases if the tool to be used are of various sizes. The inserted spring collet - not the simplest part to replace - having no threads, lasts a long time. This cannot be said of either of the devices shown at Figs. 1 and 3. In both cases a stripped thread would give a lot of trouble, while one worn chuck, if sets are used, may cause complications necessitating the making of a new screwed spindle and perhaps chucks as well.
On the other hand, the adjusting cap of the device shown at Fig. 2, being large in diameter, has a greater thread bearing, and consequently increased wearing capacity. Moreover, when worn it is much easier to repair. The chuck shown at Fig. 3 is in every respect identical with that shown in Fig. 1, but of course requires no draw spindle; like Fig. 2, it takes a solid stock spindle or holder, according to the purpose to which it is to be applied. Adjustment of the chuck, Fig. 3, is effected by squaring the projecting portion of the collet for a suitable wrench.
Fig. 4 shows another independent inserted collet-chuck which the writer has used successfully on several jobs where an extended tool was necessary. Its adjustment is rather clumsy compared with that of the others. A hollow punch clearing tool is required to drive it home, and a taper drift passed through the cross-hole to eject it. In construction it is rather plain; about 4 or 5 degrees of taper should be given to the chuck, with the usual three slits for closing.
Figs. 5 and 6 are less elaborated devices, their chief differences from the others being that their use is confined to one size of tool. Still they are extensively used as a convenient form of fixed holder. If they are to be used in the hardened state, they require careful treatment, as they do not lend themselves readily to correction by grinding or other means; for this reason they are often finished and used soft. Fig. 5 is adjusted by a compression screw, and should have two slits; while Fig. 6 has a coned collar and three slits.
Figs. 6, 8 and 9 show chucks which are specially useful for outside clearance or long over-reach. Fig. 7 may be used as an extension for drilling or other operation giving pressure on the cutting end of the tool, and serving to keep the opposite flatted end tight between the two cross-pegs. Fig. 8 is a more familiar form, but not a very good one, the tool being liable to twist off at the back lip, which receives the cutting strain. Hardening throughout and judicious tempering (low spring) of the back of the cutting tool partly checks this tendency.
Fig. 9 has a wider range of usefulness than the last two. The small holding screw should preferably be pointed and the tool slightly countersunk to receive it. Each of these three tools must be made a fit at the commencement, or they soon become unreliable. For a good substantial and reliable chuck, where a minimum of clearance is admissible, there is no alternative but to revert back to the tapered chuck, as in larger tools; see Fig. 10. The taper must be well finished and a good fit.