# Machinery's Encyclopedia; A Work of Reference Covering Practical Mathematics and Mechanics, Machine Design, Machine Construction and Operation, Electrical, Gas, Hydraulic, and Steam Power Machinery, Metallurgy, and Kindred Volume 5

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This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 edition. Excerpt: ...angle of the flute may need to be slightly modified at the time when these flutes are milled. In order to relieve hobs with a regular relieving attachment, it is necessary that the number of teeth" in one complete turn along the thread be such that the relieving attachment can be geared to suit this number. In order to obtain a number of teeth that can be relieved, the angle of the flute is often varied slightly, so that the number of teeth in one complete tum of the thread will either be a whole number or at least some number for which the relieving attachment can be geared. This change in the angle is comparatively slight, so that the flute will be approximately at right angles to the thread. The effect which a comparatively slight change in the angle of the Fig. 7. Diagram illustrating how Angle of Flute aflects Spacing of Teeth along Thread Helix would be in a single turn of the hob thread if the flutes were milled exactly at right angles to the thread. This number equals the actual number of flutes in the hob divided by the square of the cosine of the thread helix angle (angle between thread and axis of hob). The first step is to obtain the thread helix angle. The tangent of this angle equals the lead of the hob thread divided by the circumference. Thus 1.851 ] 6.676 = 0.277, which is approximately the tangent of 15 degrees 30 minutes. The cosine of I5 degrees 30 minutes equals 0.9636; therefore, the number of teeth in one turn of the thread, in this particular case, equals 12 + 0.9636' = 12.92 teeth. An ordinary relieving attachment cannot be set for this number of teeth, but, by making a slight change in the angle or lead of the spiral flute, 13 teeth can be obtained in one turn of the hob thread, instead of 12.92....