Designing spindle bearings and boxes. Thrust bearings. The Lodge & Shipley form. Ball bearings. Proper metal for boxes. The cast iron box. Early form of boxes. The cylindrical form. Thrust bearings for a light lathe. Experiments with different metals on high speeds. Curved journals. The involute curve. The Schiele curve. Conical bearings. Adjustments to take up wear. Split boxes. Line-reaming boxes. Lubrication of spindle bearings. The plain brass oil cup. The use of a wick. Oil reservoirs. Loose ring oilers. Chain oilers. Lodge & Shipley oil rings. Neglect of proper lubrication. Back gearing. Varying the spindle speeds. Triple gearing. Theory of back gearing. Back gear calculations. Triple gear calculations. Diagram of spindle speeds. Faulty designing of back gears and triple gears. Four examples. A 14-inch swing lathe. A 19-inch swing lathe. A 17-inch swing lathe. A 30-inch swing triple-geared lathe. Explanation of the back gear diagrams. Essential parts of the triple gear mechanism. "Guesswork" in lathe designing. Wasted opportunities. Designing the head-stock. Cone diameters. A homely proportion. The modern tendency in cone design. Proportions of back gears. Driving the feeding mechanism. Reversing the feed. Variable feed devices. Rapid change gear devices.

Great care ought always to be used in the design of the bearings of the spindle and the boxes in which they run. To a great extent these determine the life and usefulness of the lathe, for with an improperly made spindle or poor boxes, either of design or quality of material, the lathe is soon worn so much out of true as to be prac-tically worthless.

Mention has been made of the thrust bearing at the rear box. It is important that this should be well designed and constructed, as the quality of the lathe's work, particularly face-plate and chuck work, depends upon its proper performance.

One form is shown in Fig. 73, which is a style used for a number of years on the New Haven lathes. It consists of a hardened steel ring B, forced into an annular groove in the end of the hollow spindle A. The rear end of the bronze box C is extended as shown and tapped out with a fine thread. Fitted to this is the thrust sleeve D, whose forward end bears against the ring B. The sleeve D is adjusted by means of two slots (one of which is shown) cut across its face, and is held in position by the check-nut E. This device is much improved by the addition of a hard bronze ring, loosely interposed between the thrust sleeve D and the hardened ring B. The sleeve D was made of a steel casting, as was also the check-nut E, which had holes drilled around its circumference for the accommodation of a spanner for adjusting it. The device was very successful in practical use.

Fig. 73.   New Haven Lathe Thrust Bearing.

Fig. 73. - New Haven Lathe Thrust Bearing.

Fig. 74.   Lodge & Shipley Thrust Bearing.

Fig. 74. - Lodge & Shipley Thrust Bearing.

The form of thrust bearing used on the Lodge & Shipley lathes is shown in Fig. 74, and is constructed as follows: Upon the spindle A is keyed the cast iron ring B. Next to this is a bronze washer C; next a hardened steel washer D; then another bronze washer E, which in turn rests against the faced end of the rear box F, which in this case is formed of the head-stock casting itself.

While this is an efficient form of end thrust it has the disadvantage of occupying some space inside the rear box and consequently increasing the distance between the front and rear boxes, increasing the length of the head-stock by just its own width, or the space occupied by the cast iron collar and the three friction washers. Unless covered by a projecting portion of the casting, or by a special guard over it, there will be more or less trouble on account of dirt working in between the washers. This, however, is easy to prevent by a proper design and construction.

The popularity of the ball bearing and its successful application to many different uses no doubt suggested it as a proper device for the thrust bearing of a lathe. It has been objected to in a lathe designed for fine work, on account of the possible influence of any slight vibration caused by the rapid rotation of the balls, owing to any inaccuracy in their perfect spherical shape or diameters. Yet the device is in apparently successful use on many lathes at this time. The construction is shown in Fig. 75. Upon the spindle A is fixed the collar B, having a ball-race cut in its rear side as shown. Fixed to the end of the box, or the inside of the rear housing, as the case may be, is the collar C, which also has a ball-race formed in it, and set deep enough to form a sleeve which projects out over the balls and the collar B, so as to protect the balls from dirt. This thrust is open to one of the objections urged against the form shown in Fig. 74, namely, the space it occupies on the lathe spindle.

It is entirely feasible, however, to place this device, or the one shown in Fig. 74, near the rear end, or even at the center of the rear box if so desired. In this location it would have the added advantage of position for ample lubrication and absolute protection from dirt.

There has been a great deal of discussion on the question of what is the proper metal, and what is the proper form for a lathe spindle box. Any number of different metals have been used for this purpose, from cast iron at a cost of two and one half cents per pound to a fine quality of nickel-bronze worth thirty one cents per pound.

It is an old and a true saying that with a good, true, and well finished journal, and the bearing kept free from dirt, always clean and well lubricated with good oil, a cast iron box is as good as anything that can be made. Every practical shop man of even moderate experience can cite instances of the excellent record of the old-time cast iron box, and the fact that it is still used by some of the oldest and best lathe manufacturers is certainly a strong argument in its favor. But one condition is always insisted upon: it must be kept clean and free from dirt. It will not stand dirt. Under adverse conditions many bronze boxes withstand successfully dirt, grit, and poor lubrication that would put the cast iron box out of business in a few hours.

Fig. 75.   Ball Thrust Bearing.

Fig. 75. - Ball Thrust Bearing.