3 DIMENSIONAL MEASUREMENT
3.1 Introduction
The engineering firm you work for has been engaged to set up a quality control program for a company that remanufactures engine components. As a first step in the study you must assess the characteristics of various measurement tools and techniques. The findings of the investigation will enable you to formulate realistic goals for the quality control program and to implement appropriate regular checks on the accuracy of measurement instruments. Your report must indicate the accuracy, resolution, range and repeatability that should be expected from each measurement tool as well as special situations which require or prevent its use. Specific results of your test are to be presented in tabular form.
As part of this exercise you will be working with gauge blocks and measurement instruments that are equipped with vernier measurement scales. The following comments on the use of these equipment and should be reviewed before continuing on with the exercise.
3.2 Comments on Gauge Blocks
Gauge blocks are a dimensional measurement standard used in a workshop to calibrate other measurement devices. These blocks come is sets of different grades depending on the accuracy required. Each set has many blocks of incremental lengths. These blocks are stacked together to build a desired length which is then used to calibrate a measurement device. Gauge blocks are not used directly to measure a part, but are used as a reference standard.
In building a reference length using gauge blocks, and blocks are joined together using a process called ‘wringing’. After the reference block is built, the blocks are said to be ‘wrung’ together. This process is shown in the schematic in Figure 3-1 and is as follows:
• Make sure that the blocks are clean.
• Wipe the surfaces of the blocks to be wrung gently across an oiled pad to lubricate the surface.
• Wipe these surfaces on a dry pad, removing as much oil as possible. The small film of oil is used to protect the block, to allow one block to slide over the other and to allow the blocks to be separated after the measurement.
• Slide the surfaces of the blocks together as shown in Figure 3-1. Apply pressure while sliding the blocks. The blocks should slide together without any feel of bumps or scratching, and should adhere to each other strongly after being rotated into place.
• If the blocks fall apart under gravity, then the wringing procedure has not been successful and must be repeated. Take care not to damage the blocks when wringing them together.
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CAUTION: When working with gauge blocks and other finely machined objects, such as the surface table, avoid touching the metal surface with your hands. Salty perspiration from your skin will form acids that rapidly corrode the surface. Wear protective gloves.
Figure 3-1. A schematic showing the process used to wring two gauge block together.
3.3 Reading a Vernier Scale
A vernier scale can be incorporated into any measurement device that has a measurement scale. It allows the user to determine the measurement on the device to within a sub-division of the main scale.
The main scale is shown above the vernier scale. The measurement can be read off the main scale at the location where the ‘pointer’, which is marked as the ‘0’ line on the vernier scale. In the top sketch (Figure 3-2(a)), the pointer aligns with the number ‘2’ (arbitrary units). In this sketch it can also be see that the vernier scale has been shrunk by 10% so that it covers 9 divisions of the main scale so that the 10th division on the vernier scale aligns with it a division on the main scale. In this example, the reading determined with the vernier scale is 2.00. Reading directly from the main scale which has a lower resolution, the reading would be 2.0.
In the lower sketch shown in Figure 3-2(b), the pointer does not align with any division on the main scale. Reading from the main scale, the measurement is greater than 2.3 and less than 2.4. Looking along the vernier scale, the 4th division line aligns with a division line on the main scale. This indicates the amount determined by the vernier scale that can be added to the main scale reading. Hence, in reading to the precision of the vernier scale, the read measurement is 2.34.
Care must be taken when using vernier scale that no parallax error is introduced by the user. This is the error in determining the alignment of division lines on the main and vernier scale if the user is not viewing perpendicular to the axis of the scale. Care should also be taken to not exert excessive force that can alter the reading. The user can
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determine the amount of force required (a light amount) by practicing taking measurements on a known standard, which in this case will be gauge blocks.
(a)
(b)
Figure 3-2. Two sketches of a main and vernier scale. In (a) the measurement reads 2.00, to the precision of the vernier scale. In (b) the reading of the main and vernier scale is 2.34.
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3.4 Dimensional Measurement using a Micrometer
3.4.1 Equipment
Typical micrometers that will be used in this section are shown in Figure 3-3. The parts of the micrometer are shown in Figure 3-3(a) while micrometers of different sizes and styles are shown in Figure 3-3(b). A ratchet is included on the thimble of the micrometer to allow the user to apply a repeatable amount of torque to the spindle when measuring a part. This is usually two ‘clicks’ of the ratchet.
Other equipment that will be required include:
• gauge blocks
• steel balls
• steel bolts
• engine crankshaft
RatchetDistanceMeasuredSpindleLockFrameAnvilBarrelThimble
(a)
(b)
Figure 3-3. Typical micrometers. In (a) a schematic of the parts is shown and in (b) three micrometers of different types and sizes.
3.4.2 Micrometer - Calibration
To become familiar with reading a vernier micrometer, check for zero, anvil and screw error. Use gauge blocks and a steel ball to check for anvil error. Use three different gauge blocks to check screw error over the range of the micrometer. Try to minimize error due to torque variations by using the ratchet on the micrometer. Each group member should measure the dimension the same gauge block to determine micrometer precision. Tabulate your results.
The three sources of error that should be typically checked when using a micrometer are:
1. Zero Error. Does the micrometer read zero when the anvils are fully closed?
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2. Screw Error. Using gauges blocks ensure that the micrometer reads correctly over its range of measurement. Error from the standard (gauge block reading) can be linked to damage in the thread or bending of twisting of the frame of the micrometer.
3. Anvil Error. The spindle face and anvil should be parallel, aligned and meet when the micrometer is reading zero. Anvil error can be determined by using the gauge block and a steel ball as shown in Figure 3-2. Anvil error is typically caused by twisting of the frame of the micrometer.
(a)
(b)
Figure 3-2. Method used to determine the anvil error of a micrometer.
3.4.3 Micrometer Measurement 1 - Measuring a Sample Set of Bolts
Each group is to measure the diameters of one sample of 20 bolts, from a larger population, to determine size variations (bolts in each sample have been identified with a different colour of paint). Using a micrometer, each group is to check the diameters of one set. Account for zero error in the micrometer, but ignore screw and anvil error. In addition to recording the dimension of the bolts in your sample, obtain the readings from another group in your lab section (40 readings total). Record your results in Table 4.
3.4.4 Micrometer Measurement 2 - Measure Bearing Journal
Using the 50-75 mm micrometer measure the journal diameter of each of the four throws on the crankshaft. On each journal measure at two locations, 90o apart. Record your results in Table 2.
3.4.5 Micrometer Measurement 3 - Measure Disk Brake Rotor
Measure the variation in thickness of the disk brake rotor provided. Measure at least four positions on the rotor to determine the maximum variation. If the variation exceeds a manufacturer’s tolerance the rotor must be resurfaced or replaced. Record your measurements in Table 3.
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3.5 Dimensional Measurement using a Caliper
3.5.1 Equipment
A vernier caliper is versatile linear dimensional measurement tool. As seen in Figure 3-3, the vernier caliper can be used to carry our inside, outside and depth measurements. Most varieties of vernier caliper will also measure in both metric and imperial units.
The following equipment will be required in this section:
• gauge blocks
• vernier and digital calipers
• metal part
Inside MeasurementOutside MeasurementDepthMeasurementLockFineAdjustmentMetric ScaleImperial ScaleVernierMetricVernier ImperialOutside JawInside Jaw
Figure 3-3. A schematic of a vernier caliper showing parts
3.5.2 Caliper Calibration
Check the inside and outside jaws for nicks and burrs. Ensure that when the caliper is closed that the outside jaws close together and that the caliper reads zero. Do this zero error check for both digital and vernier calipers.
Using at least three different gauge blocks calibrate the calipers over a 0 - 150 mm range. When completing these measurements, ensure that the outside jaws are properly mated to the gauge blocks, and that there is no error in the measurement due to misaligning the caliper. This will occur if the jaw faces of the caliper is not parallel to the face of the gauges blocks. Check the precision of the vernier and digital calipers by having each MecE-301 3-6
member of the group measure the same gauge block. Note what types of measurements the calipers can be used for. Note the range and resolution of each type of caliper.
3.5.3 Caliper Measurement
For this exercise you will have been provided with a metal part to be measured with the vernier caliper. To fully describe the part inside, outside and depth measurements of the parts features will be required. Record these dimensions on a sketch, neglecting the zero and scale error. The sketch will be a neat, hand drawn and will have all the dimensions need to have the part manufactured.
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3.6 Dimensional Measurement using a Surface Table
3.6.1 Equipment
A surface table is a precession piece of equipment that is often found in tools rooms and work shops that carry out fine machining. It can be used in conjunction with a variety of other measurement devices. Surface tables are recognizable by their large flat surface that has a protective cover. They are often made from granite, however those used in this lab are made of cast iron and have a ground surface. This surface provides a standard datum from which other devices can be used to measure from. These devices include such instruments as a vernier calipers (depth measurement), vernier height gauges and dial indicators. Surface tables can also be used to measure angles in conjunction with a sine bar as shown in Figure 3-4.
Figure 3-4. The calibration of an inclinometer using a sine bar a surface table as a reference.
The following equipment will be required for this exercise include:
• gauge blocks
• sine bar
• surface table
• dial gauge with magnetic base
• engine camshaft
• inclinometer
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3.6.2 Surface Table - Linear Measurement of a Camshaft
The object of this exercise is to take measurements of an engine crank shaft. The performance of the crankshaft in the engine depends on whether the shaft has been manufactured within tolerance. The exercise here will be to determine the concentricity of the central journal bearing to the two end bearings. This same procedure would be used to test a crank shaft that has been in service to determine if it has been damaged and whether is can still be used. The ‘run-out’ of the central journal bearing will be determined by using a dial indicator to measure the relative distance from the top of the surface table to the bearing surface as the crank shaft is rotated in the vee-blocks.
The first requirement in this exercise is to calibrate the dial indicator and to ensure that it has been mounted perpendicular to the surface table. Mount the dial gauge and magnetic base on the surface table. Check the accuracy of the dial gauge as follows. Wring together two or three gauge blocks to make a stack approximately the same height as the camshaft when mounted in vee-blocks. Note that the stack of blocks serves only as a reference point for calibrating the dial gauge and ensures that the gauge is calibrated at the same point in the range that will be used for measurements. (It is not necessary to calibrate over the entire range if the instrument is only to be used in a small portion of the range.) After zeroing the outer units scale on the dial gauge, add a small gauge block (approximately 1 mm) to the stack and record the change in dial gauge reading for this known displacement (this allows you to determine accuracy). Repeat this procedure a few times to establish precision for the dial gauge.
Carefully remove the gauge block stack and replace it with the camshaft. Determine the ‘run out’ of the centre bearing by carefully rotating the camshaft under the dial gauge and determining the difference between maximum and minimum readings. Deviations from true can be caused by wear, a bent shaft or poor machining. Making this type of measurement allows one to decide whether the camshaft is usable. Determine repeatability of the procedure by having each member of the group set up the dial gauge and determine the run out.
3.6.3 Surface Table - Calibration of an Inclinometer
A sine bar is used to align machine tools or set surfaces to a specified angle for calibrating other angle measurement instruments. Check the surface table for level. Using the sine bar and gauge blocks construct three angles between 0 and 45 degrees as specified by your instructor. Use the angles constructed to calibrate the inclinometer. Be sure to list all the block sizes used in constructing each angle so an estimate of the uncertainty of the constructed angle may be made. Choose one angle and have each member of the group reconstruct it to determine precision. (Remember that to determine precision the stack of gauge blocks should be taken apart and re-wrung.) Record the inclinometer manufacturers accuracy specification.
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3.7 The Report
The report for this lab will be of a short formal style. The following sections are required to be included in the report.
1. Prepare a table showing the accuracy, resolution, precision and range of the calipers (both digital and vernier), micrometer, inclinometer, and dial gauge/surface table system. For the dimensional measurement instruments, indicate accuracy using units of length (ie + 0.002 mm) rather than using a percentage of range or percentage of reading.
2. Discuss considerations which would require the use of a particular measurement instrument and indicate potential sources of error.
3. You measured various automotive parts using micrometers. Calculate the mean and standard deviation of each set of measurements. Are any points suspect? Can you expect the same repeatability uncertainty when using the 50-75 mm micrometer as with the 0-25 mm micrometer?
4. Calculate the mean and standard deviation of each sample of bolts. Are any data points suspect? Are the two samples of bolts from the same population? If so, estimate the mean of the population. Show the uncertainty associated with this estimate. Where does the uncertainty come from? Include the analysis in the report Appendix.
5. Prepare a scale drawing of the metal part measured using the vernier caliper. On the figure, show all dimensions as well as the uncertainty associated with each. (Assume the uncertainty in each measurement is the greater of the accuracy or the repeatability uncertainty).
6. Make a schematic drawing of the method used to measure the camshaft run out. The uncertainty in your estimate of run out can arise from several sources - procedural or instrument calibration usually being dominant. Estimate the run out and the uncertainty associated with this estimate. What do you think is the dominant source of uncertainty?
7. Sketch the set up used to construct the required angles on the sine bar. Estimate the probable error in constructing each angle assuming L = 127 mm ± 0.05 mm and class B gauge blocks, which have a ±2.0 × l0-4 mm uncertainty. Does the inclinometer meet the manufacturers specification for accuracy?
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