Fig. 23. Example of Work illustrating Accumulation of Errors
When doing precision work of this kind, the degree of accuracy will depend upon the instruments used, the judgment and skill of the workman and the care exercised. A good general rule to follow when locating bushings or buttons is to use the method which is the most direct and which requires the least number of measurements. As an illustration of how errors may accumulate, let us assume that seven holes are to be bored in the jig-plate shown in [Fig. 23], so that they are the same distance from each other and in a straight line. The buttons may be brought into alignment by the use of a straight-edge, and to simplify matters, it will be taken for granted that they have been ground and lapped to the same size. If the diameter of the buttons is first determined by measuring with a micrometer, and then this diameter is deducted from the center distance x, the difference will be the distance y between adjacent buttons. Now if a temporary gage is made to length y, all the buttons can be set practically the same distance apart, the error between any two adjacent ones being very slight. If, however, the total length z over the end buttons is measured by some accurate means, the chances are that this distance will not equal six times dimension x plus the diameter of one button, as it should, because even a very slight error in the gage for distance y would gradually accumulate as each button was set. If a micrometer were available that would span two of the buttons, the measurements could be taken direct and greater accuracy would doubtless be obtained. On work of this kind where there are a number of holes that need to have accurate over-all dimensions, the long measurements should first be taken when setting the buttons, providing, of course, there are proper facilities for so doing, and then the short ones. For example, the end buttons in this case should first be set, then the central one and finally those for the sub-divisions.
Fig. 24. Special Arbor for Turning Eccentrics
Eccentric Turning.—When one cylindrical surface must be turned eccentric to another, as when turning the eccentric of a steam engine, an arbor having two sets of centers is commonly used, as shown in [Fig. 24]. The distance x between the centers must equal one-half the total “throw” or stroke of the eccentric. The hub of the eccentric is turned upon the centers a—a, and the tongued eccentric surface, upon the offset centers, as indicated by the illustration. Sometimes eccentrics are turned while held upon special fixtures attached to the faceplate.
When making an eccentric arbor, the offset center in each end should be laid out upon radial lines which can be drawn across the arbor ends by means of a surface gage. Each center is then drilled and reamed to the same radius x as near as possible. The uniformity of the distance x at each end is then tested by placing the mandrel upon the offset centers and rotating it, by hand, with a dial indicator in contact at first one end and then the other. The amount of offset can also be tested either by measuring from the point of a tool held in the toolpost, or by setting the tool to just graze the mandrel at extreme inner and outer positions, and noting the movement of the cross-slide by referring to the dial gage of the cross-feed screw.