EPICYCLOIDAL TEETH.
[All wheels having from 12 to 21 teeth have a special cutter for each number of teeth.][6]
| Cutter correct for | ||||||||
| No. of teeth. | ||||||||
| 23 | Used on | wheels | having | from | 22 | to | 24 | teeth. |
| 25 | „ | „ | „ | „ | 25 | to | 26 | „ |
| 27 | „ | „ | „ | „ | 26 | to | 29 | „ |
| 30 | „ | „ | „ | „ | 29 | to | 32 | „ |
| 34 | „ | „ | „ | „ | 32 | to | 36 | „ |
| 38 | „ | „ | „ | „ | 36 | to | 40 | „ |
| 43 | „ | „ | „ | „ | 40 | to | 46 | „ |
| 50 | „ | „ | „ | „ | 46 | to | 55 | „ |
| 60 | „ | „ | „ | „ | 55 | to | 67 | „ |
| 76 | „ | „ | „ | „ | 67 | to | 87 | „ |
| 100 | „ | „ | „ | „ | 87 | to | 123 | „ |
| 150 | „ | „ | „ | „ | 123 | to | 200 | „ |
| 300 | „ | „ | „ | „ | 200 | to | 600 | „ |
| Rack | „ | „ | „ | „ | 600 | to rack. | ||
[6] For wheels having less than 12 teeth the Pratt and Whitney Co. use involute cutters.
Here it will be observed that by a judicious selection of pitch and cutters, almost theoretically perfect results may be obtained for almost any conditions, while at the same time the cutters are so numerous that there is no necessity for making any selection with a view to taking into consideration for what particular number of teeth the cutter is made correct.
For epicycloidal cutters made on the Brown and Sharpe system so as to enable the grinding of the face of the tooth to sharpen it, the Brown and Sharpe company make a separate cutter for wheels from 12 to 20 teeth, as is shown in the accompanying table, in which the cutters are for convenience of designation denoted by an alphabetical letter.
24 CUTTERS IN EACH SET.
| Letter | A | cuts | 12 | teeth. | Letter | M | cuts | 27 | to | 29 | teeth. | ||
| B | „ | 13 | „ | N | „ | 30 | „ | 33 | „ | ||||
| C | „ | 14 | „ | O | „ | 34 | „ | 37 | „ | ||||
| D | „ | 15 | „ | P | „ | 38 | „ | 42 | „ | ||||
| E | „ | 16 | „ | Q | „ | 43 | „ | 49 | „ | ||||
| F | „ | 17 | „ | R | „ | 50 | „ | 59 | „ | ||||
| G | „ | 18 | „ | S | „ | 60 | „ | 74 | „ | ||||
| H | „ | 19 | „ | T | „ | 75 | „ | 99 | „ | ||||
| I | „ | 20 | „ | U | „ | 100 | „ | 149 | „ | ||||
| J | „ | 21 | to | 22 | „ | V | „ | 150 | „ | 249 | „ | ||
| K | „ | 23 | „ | 24 | „ | W | „ | 250 | „ | Rack. | |||
| L | „ | 25 | „ | 26 | „ | X | „ | Rack. | |||||
In these cutters a shoulder having no clearance is placed on each side of the cutter, so that when the cutter has entered the wheel until the shoulder meets the circumference of the wheel, the tooth is of the correct depth to make the pitch circles coincide.
In both the Brown and Sharpe and Pratt and Whitney systems, no side clearance is given other than that quite sufficient to prevent the teeth of one wheel from jambing into the spaces of the other. Pratt and Whitney allow 1⁄8 of the pitch for top and bottom clearance, while Brown and Sharpe allow 1⁄10 of the thickness of the tooth for top and bottom clearance.