Page:Practical Treatise on Milling and Milling Machines.djvu/189

Rh Graduating. Another use to which the milling machine may be put is that of graduating flat scales and verniers. It is possible to obtain very accurate results, and when required, odd fractional divisions can be easily spaced.

This operation requires the use of the spiral head and a single pointed graduating tool which is held stationary in a fly cutter arbor, mounted directly in the spindle, or can be fastened to the spindle of a vertical milling or rack cutting attachment. The scale to be

Fig. 74

graduated is clamped to the surface of the table parallel to the table T slots. No power is required for the operation, as the lines are cut by moving the table transversely under the point of the tool, and this can be easily done by hand. The spiral head spindle is equal-geared to the table feed screw as shown in Fig. 74, and indexing for the divisions required is accomplished by means of the index plates, the index crank being turned in the usual manner for each division.

It has already been explained that one turn of the index crank moves the spiral head spindle $1⁄40$ of a revolution, and if equal gearing is employed between this spindle and the table feed screw, the feed screw will likewise make $1⁄40$ of a complete revolution. The lead of the feed screw being .25", it is apparent that one turn of the index crank will advance the table an amount equal to .25" × $1⁄1/40$, or .00625".

Suppose it is required to graduate a scale with lines .0218" apart. Now, if one turn of the index crank moves the table a distance of