This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
The old and almost universally used method of trueing up work in a lathe chuck or when held on centers by using a piece of chalk to indicate the high spots is only good for the first adjustment of rough work. For trueing up work where the finish must be accurate, test indicators have been devised which show minute variations from truth of running in an unmistakable manner. Two forms, and the method of using them, are shown at Fig. 329.
Fig. 328. - Tools for Knurling in the Lathe.
The "Last Word" test indicator at A employs a dial gauge and enclosed multiplying mechanism, the Starrett device at C has a simple multiplying lever and one end of the lever serves as a pointer to mark the error on the scale carried on the body of the device. The gauge at A is shown installed on a ball joint tool post shank. The auxiliary clamp provided with this instrument to permit of setting it up on the scriber member of a surface gauge at B is also shown at A.
The ball joint connection between the tool post shank and the indicator makes it possible to apply the device to a great variety of work. The contact lever has a hardened taper-head stud for bearing, which provides adjustment for wear. There is a small tapered hole through the contact ball, and contact points of special shape can be fitted into this hole to meet the requirements of special classes of work. In many cases the ordinary contact ball is quite satisfactory without providing any auxiliary point. The mechanism of this indicator has been worked out to give the magnifying power of 100, and at the same time the instrument is of remarkably small size, which will be appreciated when it is known that the weight is only 1 1/4 ounce. The dial is graduated to read in 0.001 inch. This instrument can be quickly changed from the tool post shank to the needle of a surface gauge to adapt it for surface plate work or back to the tool post shank for lathe, shaper, planer, grinding or milling-machine work. It is adaptable for use on a great variety of tool room and machine-shop operations.
Fig. 329. - Indicators and Their Use.
The Starrett indicator is also light and very simple in construction. The indicating lever is so proportioned that it will multiply the error about 15 times which is sufficient for all ordinary work. The method of using the indicator is so clearly outlined at D that no further description is necessary. Many other uses for a device of this nature will suggest themselves to the practical workman.