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2-75.  MACHINING OF STEELS (GENERAL)

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T. O. 1-1A-9 HEAT TREATMENT For maximum stress-rupture life in range 1560°1740°F, fully heat treat solution treat, and double age as follows: Solution treat  21020F  4  hours,  air  cool.    Double  age  19220F,  16  hours,  air  cool  and  1526°F,  8  hours,  air  cool.    Where  stress rupture strength above 1562OF is not the important property, but tensile strength, elongation and impact strength up to 1292OF is desired, the following heat treatment is recommended. Solution treat 21040F, 4 hours, air cool. Age 15620F, 16 hours, air cool. 2-75.  MACHINING OF STEELS (GENERAL) 2-76. There are five basic factors affecting machinability as related to steel: They comprise (1) the capacity and rigidity of the machine tool; (2) cutting fluids; (3) design composition and hardness of the cutting tool; (4) cutting condition with respect to feeds and speeds; and (5) the structure of the steel to be machined. 2-77.    The  cutting  tool  angles  (back  rake,  side  clearance,  front  clearance,  and  side  rake)  are  highly  important  in  the machining of metals.  The range of values based on general practice for the machining of steel and steel alloys, are as follows: (1) back rake angle, 8-16 degrees; (2) side rake angle, 12-22 degrees; (3) front clearance angle, 8-13 degrees; and (4) side clearance angle, 10-15 degrees. 2-78.  Regardless of the material of which the cutting tool is made, the cutting action is the same.  The main difference is the cutting speed.  The carbon-steel tool cuts at low speed.  The high-speed tool cuts at twice the speed of carbon-steel, the  cast  alloys  at  twice  the  speed  of  high-speed  steel,  and  the  sintered  carbides  at  twice  that  of  the  cast  alloys.    The cutting  speeds  listed  in  Table  2-4  are  approximate  speeds  using  high-speed  steel  tools,  and  are  to  be  used  only  as  a basis from which proper speeds for a particular part may be calculated.  These speeds are based on SAE  1112 steel, which is assigned a machin2bility rating of 100%.  In order to obtain an approximate starting speed for different steels, select the type of operation, the width, depth or diameter of cut and obtain the recommended cutting speed for SAE 1112 from Table 2-3 then refer to Table 2-4 for the percent rating of the metal to be machined, and multiply the SFM value from Table 2-5 by the rating in Table 2-4.  The result is the recommended surface feet per minute (SFM) for the cutting operation.    For  a  known  diameter  and  surface  feet  per  minute  (SFM)  to  be  used  for  an  operation,  the  corresponding revolution per minute (RPM) can be obtained from Table 2-5. 2-79.  The term cutting feed is used to express the axial distance the tool moves in each revolution.  A course feed is usually used for roughing operations, and a fine feed for finishing operations.  In general, the feed remains the same for different cutting tool steels, and only the speed is changed.  Approximate cutting feeds are listed in Table 2-3.  For tool corrections when improper machining on an operation is encountered, refer to Table 2-6 for recommended checks. 2-80. The use of a proper coolant (cutting fluid) often results in an increase of cutting speed for the same tool life, and also acts as a lubricant giving better cutting action and surface finish.  Recommended cutting fluids for steels are lard oil, mineral oils, sulphurized oils, and soluble or emulsifiable oils. 2-66

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