Page:Practical Treatise on Milling and Milling Machines.djvu/158

152 figures, t" = thickness of tooth and s" = distance from chord t" to top of tooth (See Fig. 61).

These dimensions vary from the standard dimensions of tooth parts shown on pages 321 to 324. The fewer the number of teeth in the gear, the greater the variation.

The Table of Chordal Thickness t" and Distances from Chord to top of Tooth s" on page 325 gives these dimensions for gears of 1 diametral pitch. To obtain t" and s" for any diametral pitch, divide the figures given in the table opposite the required number of teeth, by the required diametral pitch.

Example: Find t" and s" for a gear 5 diametral pitch, 23 teeth.

1.5696 ÷ 5 = .3139 = t".

1.0268 ÷ 5 = .2054 = s".

To obtain t" and s" for any circular pitch, multiply the figures given in the table opposite the required number of teeth, by the addendum s (taking 5 from the Table of Tooth Parts, pages 321 and 322).

Example: Find t" and s" for a $3⁄4$" circular pitch gear, 15 teeth.

1.5679 X .2387 = .3743 = t". 1.0411 X .2387 = .2485 = s".

If number of teeth required is not shown in table, take the nearest number of teeth.

An accurate and convenient tool for taking the measurements of gear teeth is shown in Fig. 62. With this gear tooth vernier, the distance from the top of the teeth to the pitch line, and thickness at the pitch line, can be accurately determined.