24 THUE SURFACES AND ACCURATE MEASUREMENTS ROBB. 



and to fit closely. The bar fitted the bush so well that while it 

 could be shoved into its place when free and clean from oil, when 

 oil was applied it refused to move except by using considerable 

 force, showing" that there was not room for the oil. The shaft 



o 



required to be reduced 2/1000" in size to make room for the 

 lubricant. Heating of journals is frequently caused by neglect 

 of this point. In many kinds of machinery no attempt is made 

 to true the surfaces carefully, or indeed to do more than get them 

 approximately round or flat, as the case may be, or to make 

 them to any exact size nearer than can be measured by the eye 

 or an ordinary box wood rule. The result is that journals and 

 seats have to work out their own salvation or destruction, by 

 wearing the high parts down until the low parts approach near 

 enough for the oil to support the whole journal, consequently, 

 much care and patience must be exercised in working new 

 machinery until it is worn to a bearing, otherwise the metals will 

 abrade and heat by friction until the surfaces are completely 

 destroyed. 



The foregoing will emphasize the importance of being able 

 to make minute and accurate measurements, as much depends 

 upon the certainty with which the mechanic can measure the 

 inaccuracies of his work in order to bring it to the necessary 

 state of perfection. The ordinary system of measuring by a 

 rule graduated to 16ths, 32nds or perhaps the 64ths of an inch, 

 the use of which leaves room for an error which is too great for 

 the production of good machinery, has been superseded in 

 machine shops, where accuracy is aimed at, by the " Micrometer 

 Caliper" and hardened steel gauges of various kinds, by means 

 of which measurements of 1/4000 of an inch can be made as 

 easily as 1/16 inch can be measured by the ordinary rule." 



" The Interchangeable System," first used by American work- 

 men in the production of watches, fire arms, and other machines, 

 having a number of small parts, any one of which should fit any 

 other, has done much to introduce refined and rapid methods of 

 measurements. Steel gauges, which are hardened and then 

 ground, are now produced by manufacturers of tools which are 



