PRECISION TOOL MAKING 



399 



feeler has a constant pressure of 7 or 8 ounces against the work, thus 

 eliminating the "sense of touch" factor. Variations of the size of a 

 part within a range of ± .0035 in. may be read directly to .00005 in. 

 and by interpolation to within one or two hundred-thousandths of 

 an inch. 



The instrument just described illustrates the use of light for making 

 precise measurements by the optical lever method. Another method 

 is by the use of a lens or projection system, whereby beams of light 

 are controlled in such a manner as to form on a screen enlarged images 

 of objects with a high degree of geometrical similarity between the 



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Fig. 25— Optimeter and "Johanssen" gage block set 



image and the object. As the errors or variations in the object are 

 magnified the same amount, they become correspondingly easier to 

 observe and measure or check against a standard template, contour 

 plate, limit chart, or accurately made scale drawing of the object. 

 With a magnification of 250 diameters an error of only a thousandth 

 of an inch will appear as a quarter of an inch and an error of one 

 ten-thousandth can be readily observed. This method is particularly 

 adaptable for measuring to close limits irregular shapes and contours, 

 screw thread and profile gages, gear teeth, etc. The instrument 

 used for this purpose is the contour measuring projector, the magnified 

 image of an object being projected either on a vertical screen or the 

 horizontal table attached to the instrument. 



