These differences might easily be due to the difference in the light at the time the colors were observed. It must also be remembered that even a thin coating of oil will make quite a difference and cause confusion. It is these possible sources of error, coupled with the ever present chance of human error, that makes it advisable to draw the temper of tools in an oil bath heated to the proper temperature as shown by an accurate high-temperature thermometer. Another table, by Gilbert and Barker, runs to much higher temperatures. Beyond 2,200 deg., however, the eye is very uncertain. TABLE 26.--COLORS FOR TEMPERING TOOLS ----------------------------------------------------------------------- Approximate | color and | Kind of tool temperature | --------------|-------------------------------------------------------- Yellow | Thread chasers, hollow mills (solid type) twist drills 430 to 450 deg.F.| centering tools, forming tools, cut-off tools, profile | cutters, milling cutters, reamers, dies, etc. --------------|-------------------------------------------------------- Straw-yellow | Thread rolling dies, counterbores, countersinks. Shear 460 deg.F. | blades, boring tools, engraving tools, etc. --------------|-------------------------------------------------------- Brown-yellow | Taps, Thread dies, cutters, reamers, etc. 500 deg.F. | --------------|-------------------------------------------------------- Light purple | Taps, dies, rock drills, knives, punches, gages, etc. 530 deg.F. | --------------|-------------------------------------------------------- Dark purple | Circular saws for metal, augers, dental and surgical 550 deg.F. | instruments, cold chisels, axes. --------------|-------------------------------------------------------- Pale blue | Bone saws, chisels, needles, cutters, etc. 580 deg.F. | --------------|-------------------------------------------------------- Blue | Hack saws, wood saws, springs, etc. 600 deg.F. | ----------------------------------------------------------------------- CHAPTER X HIGH-SPEED STEEL For centuries the secret art of making tool steel was handed down from father to son. The manufacture of tool steel is still an art which, by the aid of science, has lost much of its secrecy; yet tool steel is today made by practical men skilled as melters, hammer-men, and rollers, each knowing his art. These practical men willingly accept guidance from the chemist and metallurgists. A knowledge of conditions existing today in the manufacture of high-speed steel is essential to steel treaters. It is well for the manufacturer to have steel treaters understand some of his troubles and difficulties, so that they will better comprehend the necessity of certain trade customs and practices, and, realizing the manufacturer's desire to cooperate with them, will reciprocate.