Sentence:  All these methods remove metal and can, in fact, remove a few <WSD>thou</WSD> from the surface. For accurately machined parts, therefore, none of these methods are suitable but wet blasting with a fine alumina which gives a polishing–cleaning action may be operated within the required tolerances. 
Meaning of the word marked with <WSD> tags:  A unit of length equal to one-thousandth of an inch.