Boring Double Tapers.—To prevent end play in journal bearings where it is essential to do so, the form of journal shown in [Fig. 1150] is sometimes employed, hence the journal bearing requires to be bored to fit.
Fig. 1151.
Fig. 1152.
[Fig. 1151] represents a bearing box for such a journal, the brasses a, b having flanges fitting outside the box as shown. The ordinary method of doing such a job would be to chuck the box on the face plate of the lathe, setting it true by the circle (marked for the purpose of setting) upon the face of the brasses, and by placing a scribing point tool in the lathe tool post and revolving the box, making the circle run true to the point, which would set the box one way, and then setting the flanges of the box parallel with the face plate of the lathe to set the box true the other way; to then bore the box half way through from one side and then turn it round upon the face plate, reset it and bore the other half; thus the taper of the slide rest would not require altering. This plan, however, is a tedious and troublesome one, because, as the flanges protrude, parallel pieces have to be placed between them and the lathe face plate to keep them from touching; and as the face of the casting may not be parallel with the slide ways, and will not be unless it has been planed parallel, pieces of packing, of paper or tin, as the case may be, must be placed to true the ways with the face plate, and the setting becomes tedious and difficult. But the two tapers may be bored at one chucking, as shown in [Fig. 1152], in which a represents the lathe chuck, and b is a sectional view of the bearing chucked thereon, c, c being the parallel pieces. Now it will be observed that the plane of the cone on the front end and on one side stands parallel with the plane of the cone on the back end at an exactly opposite diameter, as shown by the dotted lines d and e. If then the top slide of the lathe rest be set parallel with those lines, we may bore the front end by feeding the tool from the front of the bore to the middle as marked from f to g, and then, by turning the turning tool upside down, we may traverse or feed it along the line from h to j, and bore out the back half of the double cone without either shifting the set of the lathe rest or chucking the box after it is once set.
In considering the most desirable speed and feed for the cutting tools of lathes, it may be remarked that the speeds for boring tools are always less than those for tools used on external diameters, and that when the tool rotates and the work is stationary, the cutting speed is a minimum, rarely exceeding 18 feet per minute, while the feed, especially upon cast iron, is a maximum.
The number of machines or lathes attended by one man may render it desirable to use a less cutting speed and feed then is attainable, so as to give the attendant time to attend to more than one, or a greater number of lathes. In the following remarks outside work and a man to one lathe is referred to.