CHAPTER X.

Milling-Fixtures for Accurate Work.

FACTORS IN THE SUCCESSFUL USE OF ACCURATE MILLING-FIXTURES.

WE are now about to take up a class of milling-fixtures of a different type from those described in the preceding chapter, in that they are more intricate and are also capable of producing more accurate results. When designing these tools there are three questions to be considered: First, are the parts which are to be machined required in large quantities? Second, must they be finished very accurately, so as to be interchangeable? Lastly, can the parts be handled and finished to the best advantage in the milling-machine?

The first two questions can be answered in very short order. But in deciding the answer to the last one, the knowledge and skill of the designer, who is often the constructor as well, are put to the test. If it is decided that the milling-machine is most suitable for the work, the following points must then be considered after the shape and type of fixture have been determined: The surface by which the pieces are to be located; the devices for fastening the work, and the most practical way of presenting the surface to be machined to the cutter or cutters, as the case may be.

As types of the most reliable class of milling-machine fixtures for duplicating small and medium machine parts, there are here shown five examples which are well designed for the particular pieces of work for which they are intended. The devices also are suggestive, in that many of their features can be so modified as to be applicable to work of other kinds. Methods for constructing the fixtures will be described-explaining how they can be produced within a reasonable length of time and at moderate expense.

FIXTURE FOR THE FIRST PIECE OF WORK.

The fixture shown in three views of Fig. 140 is used for facing the flat surface of the work, Fig. 141. The finishing of the ends of the piece is accomplished in the lathe, the parts e e, dd, and the threaded portions being interchangeable. The fixture, Fig. 140, for facing the flat surface F true with the turned por

tions of the work, is of few parts, and holds the work rigidly. As the method of construction is not very intricate, and can be understood from the illustrations, a slight description will suffice.

The fixture proper consists of the body castings G, the standards II between which the work is located, the back projection I for the fastening- and locating-screws N N and 0 0 respectively, and the two clamping-lids JJ. The lid clamping-screws LL are fastened in the slot in the standards, as shown in the face view, by means of Stub steel pins, so that they may be fastened and released as rapidly as possible. The lids JJ are hinged as shown at K K. The locating-screws are of tool steel and are reduced at the ends as shown at P, in the end view, and hardened

and equipped with jam-nuts. The tongue T is let into a slot in the body casting G so as to be perfectly in line with the turned portion of the work when within the fixture.

The boring of the standards and lids to size, and the facing of the surfaces M M so that the work will fit between them

F

d

FIG. 141.

snugly, is accomplished in the following manner: The base is first planed and the body casting strapped to an angle-plate on the drill-press table. A boring-bar is then used with the end running in the bushing in the table, and the holes are bored and the shoulders faced. The two screws N N for forcing the work against the two locating-screws 0 0 have knurled heads with a spanner hole as shown, are threaded to screw freely in the tapped holes, and are also equipped with jam-nuts.

When using this fixture it is clamped on the miller-table with the tongue T in the slot nearest the spindle. The two lids JJ

are then thrown back and the work located as shown, first tightening the lids, and then forcing the work against the two locating screws 0 0 by means of the knurled head-screw NN, and fastening the nuts to keep them tightly against the work. The cress-feed of the miller-table is then clamped so that the cutter

will remove the amount of stock required; and the face is milled, using a large face-cutter, running it so that the cut will be downward, thereby taking the strain off the fastening-screws N N and keeping the work against the locating-screws 0 0. The facing of work of this class in fixtures of the type shown can be accomplished to a greater degree of interchangeability and in less time than by any other means known to the author.

FIXTURE FOR USE IN MILLING THE SECOND PIECE.

In Figs. 142 and 143 we have a milling fixture of a more intricate type, and one which for rapid locating, fastening, and releasing of the work when finished, would be hard to beat, as

d

a

C

d

one turn of the screw fastens or releases, as required. This fixture is constructed for the accommodation of two pieces at a time, and could, if required, be constructed for twelve on the same principle. The fixture was designed for milling work of the shape shown in Fig. 144. The piece was of machine steel and was finished, all but the milling, in the turretlathe, and was used as a part of an electric clothcutting machine which was being manufactured in large numbers. The milling consists of a slot through the stem at a and a flat at either side of the largest circular portion, as shown at bb.

Zb

FIG. 144.

The fixture consists of two castings, P and E, and springchuck devices, of which II are tool-steel pieces screwing into the casting E and carrying the spring-jaws K. These jaws are forced out against the work by the expanders L L, which screw into threaded holes in I I. The one point in the construction of this fixture most worthy of a detailed description is the manner of finishing the locating-depressions F F in the part E. This part is of cast-iron, with a projecting lug at M which is used when finished as a gauge for setting the three cutters which mill the work. This cast-iron block is first planed on all sides, and one side N finished dovetail, to fit tightly into the dovetailed channel milled in the body casting P. This channel, by the way, was milled on the front of the casting and faced, after the base had been finished and the groove for the tongue was milled,

on the machine on which the fixture was to be used, to guard against inaccuracy.

The block E was driven into this channel and fastened by two screws, shown at R. The position of the centres for the locating-depressions F F F F were then located so as to be dead in line with each other by the "button" method described in a previous chapter. The depressions were finished and holes bored and threaded at the back by strapping the block E on the lathe face-plate, truing the "buttons," boring the holes, finishing the formed depression to exactly the shape and depth by means of a forming-tool, and then reversing the work and enlarging and finishing the holes at the back, as shown.

When the fixture is in use, the work is held down on the locating-face FF by hand, and the expander given a turn by the handle J. This causes the spring-chuck K to grip the work and draw it down on the locating-face. The cutters are then set by

the gauge M and the work milled.

DESCRIPTION OF FIXTURE FOR THE THIRD PIECE.

H

9

In Fig. 145 there are two views of a piece which is an ideal job for the milling-machine. It is a cast-iron spindle-bracket, and the milling operation consisted of facing the fronts and backs of the two bosses, and finishing the projecting rib H at a certain distance from centre of hole Q and at a right angle with the hole K. Before miliing, the hole Q is bored and one side of the hub faced in the turret-lathe. The opposite side is then faced and the two holes drilled through g g and

FIG. 145.

one through K. The side j is faced in a special jig and all points machined are interchangeable.

The milling fixture shown in Figs. 146-147 is designed to hold two pieces of work at once, and can be constructed for the accommodation of a dozen, if desired. One casting, A, is all that is required for this fixture, and is in the shape of an angle-plate