Some machinists are required to work to tolerances of ±0.0002 inch, others to 0.002 inch. There is a vast difference between the two. Many components are manufactured to very close dimensions. Therefore, a machinist cannot afford to make a mistake while measuring or machining workpieces. Once the piece has been machined, there is no turning back or adding on. It is, therefore, important for him to become familiar with precision measuring tools, instruments, and with the types of fits, allowances and tolerances required for his job.