For the production of accurate plane surfaces the use of the straightedge is not sufficient. Such surfaces should be compared with standard surfaces, called surface plates, Fig. 41. A surface plate is a cast-iron plate strongly ribbed on the back to prevent distortion, and supported on three points to insure a uniform base. Their production and use will be described under the head of "Scraping". They may be had in sizes varying from 3 inches by 4 inches to 36 inches by 72 inches. Work Bench. The machinist's bench at which hand work is ordinarily performed should be of substantial character, about 2

Fig. 41. Cast-iron Surface Plate.

Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island.

Fig. 42. Work Bench.

Courtesy of Brown and Sharpe Manufacturing Company, Providence, Rhode Island feet 10 inches from the floor and 2 feet 3 inches wide, Fig. 42. For the sake of economy it is usual to have a 21/2- or 3-inch plank at the front to which the vises are fastened and on which all the heavy work is done, while the rear of the bench is made from 1-inch lumber. Maple and birch are preferred as materials for a bench, although ash makes a very good substitute.