National Physical Laboratory

What is rake correction and how do I calculate it? (FAQ - Length)

The complete inspection by direct measurement of parallel screw threads necessitates the measurement of the effective diameter which, in the case of external threads, is obtained by measurement across cylinders placed in the threads on either side of the screw. This measurement does not give the effective diameter directly. Corrections are made for the fact that particular cylinders do not necessarily contact the flanks of the thread at the effective diameter and also, because of the helical nature of the thread, the cylinders do not contact the flanks in an axial section and are thrust further from the axis of the screw. This latter correction is known as the rake correction. In the case of internal threads cylinders cannot be used and the effective diameter is usually measured in a comparator with ball-ended stylus points. Again, because of the helix the balls are displaced, in this case toward the axis of the thread, and a rake correction should be made.

A method of calculating rake correction is given in Rake Correction in the measurement of parallel external and internal screw threads by R. S. Marriner and Mrs. J. G. Wood Proceedings of the Institution of Mechanical Engineers 1958 Volume 172 Number 19.

Last Updated: 25 Mar 2010
Created: 18 Jul 2007

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