ONE WEEK METROLOGY AND HARDNESS TESTING
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METROLOGY
Class Connection: Monday Week 1 (7/14). During this class session we discussed how to read a micrometer. Reading from Course Text: Chs. 2, 44.3 Background The micrometer is a widely used and very accurate measuring device, the most common form of which consists of a spindle and a C-shaped anvil. Figure 1 shows the functional and metrological features of a micrometer. The Figure 1: Metric Micrometer reading 5.83mm as spindle is moved relative to the fixed anvil by shown in Class Session 1. means of an accurate screw thread. On the metric micrometer used for this experiment, each rotation of the spindle provides 0.5 mm of linear travel. Attached to the spindle is a thimble graduated with 50 marks around its circumference, each mark corresponding to 0.01 mm. The micrometer reading is obtained by adding the readings obtained from the barrel and the reading obtained from the thimble. Figure 2 shows the methodology for taking micrometer readings. Laboratory Demonstration 1. The instructor will briefly review how to use the metric micrometer as this was discussed in the class session prior to the practica. Laboratory Procedure 1. Procure a micrometer, examine its components, and practice taking sample readings. Observe proper procedures in handling the instrument and in taking readings. The only precaution to be remembered when reading a metric micrometer is to add the half millimeter graduations of the lower row of graduations when required. This will be demonstrated by your instructor. Record your measurements on the data sheet. You will need this to complete the HW assignment. Record the measurements of another student on your data sheet. Note that you will be asked to determine if the two parts are made to the same specification. Make enough measurements so that you are confident of your decision.
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Homework Connection Homework Number One asks you if the two spindles were made to the same specification.
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HARDNESS TESTING
Class Connection: Wednesday Week 1 (7/16). During the class session we discussed hardness testing. Reading from Course Text: Ch. 11
Background Hardness is defined as the relative resistance, (A) (B) (C) to penetration, to abrasion, or to scratching, or
It is one of the most misunderstood mechanical properties. Unlike ultimate tensile strength and ductility which measure the average response of the cross section of the specimen to a mechanical force, hardness measures only the response of a small surface layer of the material to a mechanical force. The hardness of a material often varies from the outside to the centerline and from the top to the bottom. Since we deal mainly with metallic materials in our laboratory, we will principally use the Rockwell "B", and Rockwell "C" test apparatus. Although all are indentation hardness testers, they achieve their sensitivity in a limited range by manipulating (1) the size and shape of the indentor and (2) the magnitude of the load. 15 Indeed, hardness scales can be generated on the Rockwell test apparatus by manipulating the indentor and load. Rockwell B readings are valid between 30 and 100, Rockwell C readings are valid between 20 and 70. There is a slight overlap. For this reason we will convert all our readings to Rockwell A using the chart in the lab. Laboratory Demonstration 1. The instructor will demonstrate the correct set-up procedures and operating procedures for the Rockwell "B" hardness test apparatus. The instructor will demonstrate the correct set-up procedures and the correct operating procedures for the Rockwell "C" hardness test apparatus. The instructor will demonstrate using the chart to convert from Rockwell B or C to Rockwell A..
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While the instructor is demonstrating the Rockwell hardness testing machines, you may want to refer to the operating instructions for each machine at the end of the experiment.
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Hardness Testing Guidelines 1. Check the LOAD and the INDENTOR on the Rockwell hardness tester before using it for either the Rockwell "B" or Rockwell "C" scale. For Rockwell B the load should be 100kg and the indentor a 1/16 inch ball. For the Rockwell C the load should be 150 kg and the indentor should be a diamond shape cone. Be sure that the indentor on the Rockwell hardness tester is seated firmly in the holder. Be sure that the specimen sits flat on the anvil. Laboratory Procedure 1. Obtain a hardness test specimen from your instructor. Record the identity of the material and the hardness specification on your data sheet.
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Figure 2: Rockwell Hardness Tester
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If you have the steel sample, measure the Rockwell "C" hardness at different locations on the sample and record the values on your data sheet. If the har...