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Aircraft Work Inspection, No. 5 - Hardness Testing (Rockwell)

US Naval Academy, Nimitz Library

Sound | 1944

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  •  The Rockwell machine tests the hardness and strength of metal. 
  •  All metal must be tested for hardness before it can be safely used in a plane. 
  •  These two fittings look exactly alike - the same size, the same material. 
  •  One of the fittings is heat treated. 
  •  In heat treatment, the piece is first heated then quenched in oil. 
  •  This increases the hardness and strength of the metal. 
  •  The fitting which has not been heat treated bends easily. 
  •  It stays bent. 
  •  Its use in a plane is limited. 
  •  Applying the same weight to the heat treated fitting bends it very little. 
  •  It springs back into position. 
  •  A common method of judging steel hardness is by the file test. 
  •  The two pieces are filed with the same pressure and number of strokes. 
  •  The hardness is indicated by the depth of the file cut. 
  •  A more accurate method of testing hardness is necessary. 
  •  The Rockwell hardness tester makes a quick and accurate test for hardness and tensile strength. 
  •  This test shows that the soft piece has a Rockwell number of C-20. 
  •  The heat treated part proves to be harder. 
  •  This is shown by a reading of C-42. 
  •  The Rockwell machine proves that only one of these fittings can be used safely in a plane. 
  •  The inspector must check the hardness of all metal parts used in the plane. 
  •  The working parts of a Rockwell machine when set up for testing are the Indicating Gauge, which registers the degree of hardness, and the Plunger which holds the Penetrator in place. 
  •  The Penetrator is forced into the Work piece or piece to be tested, which is held by the Anvil. 
  •  The Work piece is raised to the Anvil by the Elevating Wheel. 
  •  The dial is set by rotating the Zero Adjuster, then pressure is applied by pressing the Major Load Trip. 
  •  Pressure is withdrawn by pulling the Crank Handle to forward position. 
  •  The Dash Pot is the adjusting mechanism for timing the application of the Load, which is supplied by the weights on the weight arm. 
  •  The first step is to refer to the blueprint, which shows that the required strength of this hydraulic support bolt is 140,000 to 150,000 pounds per square inch, sometimes expressed directly as the Rockwell number. 
  •  The conversion table shows that in this case the corresponding Rockwell number is C-29 to C-32. 
  •  Reference to the instruction book shows that for this range of hardness, a BRALE or diamond point penetrator is required. 
  •  Penetration of the metal is made by the small diamond set in the end and center of the penetrator. 
  •  Clean the stem and the plunger carefully. 
  •  Be sure it seats solidly. 
  •  Tighten the set screw. 
  •  An anvil holds the work for the test. 
  •  A plain anvil is used for testing flat-surfaced work. 
  •  Wipe the anvil with a chamois or soft cloth. 
  •  See that it fits the seat snuggly without binding. 
  •  Weights on the weight arm supply the pressure necessary for the test. 
  •  One hundred fifty kilograms is always used with a BRALE point. 
  •  Readings, when using the BRALE, are always made on the "C" or outside scale. 
  •  The anvil and penetrator must be seated firmly in a dummy test by applying the full pressure of the weight arm on a test piece. 
  •  If this is not done, the first reading may be incorrect. 
  •  Standard test blocks of known hardness are used each day to test the machine for accuracy. 
  •  Clean the test block. 
  •  Seat it carefully. 
  •  This test block has a hardness of C-21 to 23. 
  •  With the crank handle in forward position, raise the anvil with the elevating wheel until a pressure of ten kilograms is applied. 
  •  This is called the Minor Load. 
  •  This pressure is reached when the small pointer comes to the black dot. 
  •  Rotate the zero adjuster until the dial comes to zero or set position. 
  •  Apply the major load of 150 kilograms. 
  •  Penetration is complete when the pointer comes to rest. 
  •  Pull the crank handle forward to remove the major load. 
  •  The dial now registers the difference between the minor and major loads, C-22. 
  •  This tallies with the number on the test block and proves the accuracy of the machine. 
  •  The Rockwell hardness tester indicates on a dial the depth to which a fixed weight will force a penetrator of known hardness into the metal to be tested. 
  •  As the work is raised against the penetrator, a slight penetration of the surface takes place. 
  •  This eliminates the effect of any scale or roughness on the surface. 
  •  This is the minor load. 
  •  Now adjust the dial to zero, or set, and apply the major load. 
  •  This additional depth caused by the 150 kilogram weight, the major load, forcing the penetrator into the metal is now registered on the dial. 
  •  The final Rockwell reading is obtained by withdrawing the pressure of the major load. 
  •  C-22 is an accurate index of the hardness of the metal. 
  •  Now that the machine has been set up and tested for accuracy, we will test the hardness of this hydraulic support bolt. 
  •  First, be sure it is free of scale or dirt. 
  •  In this case, make the test on the end, half way between the center hole and the edge. 
  •  Be sure it is seated solidly on the anvil. 
  •  Elevate the anvil to apply the minor load. 
  •  Adjust the dial to set, and apply the major load. 
  •  Withdraw the major load pressure. 
  •  The final reading is Rockwell C-31 and one half, which as shown by the conversion table, indicates a tensile strength between 140 and 150,000 pounds per square inch, as specified in the blueprint. 
  •  This proves the correctness of the heat treat, and the part can be accepted. 
  •  Different shaped pieces require different anvils to hold them for testing. 
  •  This aluminum fitting is cylindrical in shape, so a roller anvil is used. 
  •  Remove the plain anvil. 
  •  Be careful not to damage the BRALE point. 
  •  Be sure the anvil is clean. 
  •  Seat it carefully. 
  •  Refer to the blueprint for the tensile strength required of the piece. 
  •  This part is aluminum alloy 24S-T. 
  •  The mechanical properties table shows that 24S-T bar stock, of which this piece is made, should have minimal tensile strength of 62,000 pounds per square inch. 
  •  Or, according to the conversion table, Rockwell 69 or over on the B scale. 
  •  The "B" or "inside" scale is used for all tests with the ball penetrator and the 100 kilogram load. 
  •  Reference to the direction book shows that with the 100 kilogram load, a 1/16th inch ball penetrator is used for metal in this range of hardness. 
  •  Inspect the ball point carefully. 
  •  If it has a flat like this, it cannot be used. 
  •  It must be perfectly round and smooth. 
  •  Remove the fifty kilogram weight. 
  •  100 kilograms is used with a 1/16th inch ball point. 
  •  The area outlined by dotted lines represents the difference in the resistance to penetration between a flat surface and a curved surface. 
  •  This allows a deeper penetration and results in a false reading. 
  •  Therefore a flat surface must be ground on round pieces which are less than one inch in diameter. 
  •  Always seat the anvil and the penetrator by running a dummy test on a test piece. 
  •  Be sure that the test is made at a new spot on the flat surface. 
  •  Apply the minor load. 
  •  The starting point, or set, for all readings on the "B" scale is B-30. 
  •  Apply the major load. 
  •  When the dial comes to rest, withdraw the major load. 
  •  The Rockwell hardness number is B-75, which is within blueprint specifications. 
  •  In review, select the right penetrator for the metal to be tested. 
  •  Clean and seat the penetrator carefully. 
  •  Clean the anvil. 
  •  Seat it solidly on the base. 
  •  Be sure to have the correct weight for the major load. 
  •  Test the machine daily for accuracy. 
  •  Clean the work. 
  •  Be sure it is held firmly on the anvil. 
  •  Then make the test. 
  •  By careful attention to these few details, the hardness and tensile strength of metals can be quickly determined on the Rockwell tester. 
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    Made by Dallas's Jamieson Film Company for the Federal Security Agency and the United States Office of Education, this industrial film trains workers to test the strength of metal airplane parts using the Rockwell Hardness Tester. The film provides an explanation of the purpose of the machine, a detailed description of its working parts, and step-by-step instruction for testing different types of items.