BASIC MEASUREMENTS Read the following BEFORE getting started: Ruler: A ruler, or rule, is an instrument used in geometry, technical drawing and engineering/ building to measure distances and/or to rule straight lines. The foot scale measures a maximum of 30cms or 12 inches. The least count for the metric side is 1mm or 0.1cm. the Least count for the imperial side is 1/32 or 0.02. Vernier Calipers: Vernier calipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and depending on the manufacturer, depth measurements by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools. The vernier scales may include both metric and inch measurements on the upper and lower part of the scale. Vernier calipers commonly used in industry provide a precision to a hundredth of a millimeter (10 micrometers), or one thousandth of an inch. Operation of a Vernier Caliper: The vernier scale is constructed so that it is spaced at a constant fraction of the fixed main scale. So for a decimal measuring device each mark on the vernier would be spaced nine tenths of those on the main scale. If you put the two scales together with zero points aligned then the first mark on the vernier scale will be one tenth short of the first main scale mark, the second two tenths short and so on up to the ninth mark which would be misaligned by nine tenths. Only when a full ten marks have been counted would there be an alignment because the tenth mark would be ten tenths, that is a whole main scale unit, short and will therefore align with the ninth mark on the main scale. Now if you move the vernier by a small amount, say, one tenth of its fixed main scale, the only pair of marks which come into alignment will be the first pair since these were the only ones originally misaligned by one tenth. If we had moved it 2 tenths then the 1
second pair and only the second would be in alignment since these are the only ones which were originally misaligned by that amount. If we had moved it 5 tenths then the fifth pair and only the fifth would be in alignment. And so on for any movement, only one pair of marks will be in alignment and that pair will show what is the value of the small displacement. Micrometer: A Micrometer, sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precisely measuring, along with other metrological instruments such as dial calipers and vernier calipers. Operation of a Micrometer: The accuracy of a micrometer derives from the accuracy of the threadform of the screw that is at its heart. The basic operating principles of a micrometer are as follows: 1. The amount of rotation of an accurately made screw can be directly and precisely correlated to a certain amount of axial movement (and vice versa), through the constant known as the screw's lead. A screw's lead is the distance it moves forward axially with one complete turn (360 ). (In most threads [that is, in all single-start threads], lead and pitch refer to essentially the same concept.) 2. With an appropriate lead and major diameter of the screw, a given amount of axial movement will be amplified in the resulting circumferential movement. 3. The markings on the spindle usually are broken into increments of 0 to 25, 0 to 50 or 0 to 100. The spindle markings related to the marks on the shaft. (e.g. for 0 to 25 there will be 3 marks between whole numbers and for 0 to 50 only 1 mark between whole numbers on the shaft. For example, if the lead of a screw is 1 mm, but the major diameter (here, outer diameter) is 10 mm, then the circumference of the screw is 10π, or about 31.4 mm. Therefore, an axial movement of 1 mm is amplified (magnified) to a circumferential movement of 31.4 mm. This amplification allows a small difference in the sizes of two 2
similar measured objects to correlate to a larger difference in the position of a micrometer's thimble. Dial Gauge: Dial indicators are instruments used to accurately measure a small distance. They may also be known as a dial gauge, dial test indicator (DTI), or as a "clock". They are named so because the measurement results are displayed in a magnified way by means of a dial. Dial indicators may be used to check the variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. The definition of small depends on the observer however a range between 1 mm (0.040") and 50 mm (2") may be thought of as typical with a travel of 10 mm (approx 0.5") being perhaps the most common. Operation of Dial Gauge: Probe indicators typically consist of a graduated dial and needle (thus the clock terminology) to record the minor increments, with a smaller embedded clock face and needle to record the number of needle rotations on the main dial. The tool may be graduated to record measurements between 0.01 mm (.001", which is not a direct unit conversion) down to 0.001 mm (.00005") for more accurate usage. The probe (or plunger) moves perpendicular to the object being tested by either retracting or extending from the indicator's body. The dial face can be rotated to any position, this is used to orient the face towards the user as well as set the zero point, there will also be some means of incorporating limit indicators, these limit tabs may be rotated around the dial face to any required position. There may also be a lever arm available that will allow the indicator's probe to be retracted easily. Other tools of measurement: 3
Sine Bar: A sine bar is a tool used to measure angles in metalworking. It consists of a hardened, precision ground body with two precision ground cylinders fixed at the ends. The distance between the centers of the cylinders is precisely controlled, and the top of the bar is parallel to a line through the centers of the two rollers. The dimension between the two rollers is chosen to be a whole number (for ease of later calculations) and forms the hypotenuse of a triangle when in use. The image shows a 10 inch and a 100 mm sine bar. Slip Gauges : A gauge block (also known as a gage block, Johansson gauge, slip gauge, or Jo block) is a precision ground and lapped length measuring standard. It is used as a reference for the setting of measuring equipment used in machine shops, such as micrometers, sine bars, and dial indicators Height Gauge: 4
A vernier height gauge is a height gauge with the additional refinement of a vernier scale for greater accuracy in reading or setting the tool. These measuring tools are used in metalworking or metrology to either set or measure vertical distances; the pointer is sharpened to allow it to act as a scriber and assist in marking out work pieces. They may also be used to measure the height of an object by using the underside of the scriber as the datum. The datum may be permanently fixed or the height gauge may have provision to adjust the scale, this is done by sliding the scale vertically along the body of the height gauge by turning a fine feed screw at the top of the gauge; then with the scriber set to the same level as the base, the scale can be matched to it. This adjustment allows different scribers or probes to be used, as well as adjusting for any errors in a damaged or resharpened probe. Spirit Level: An engineer s spirit level is generally used to level machines, although they may be used to level large workpieces on machines such as planers. Spirit levels are also used in building construction, by carpenters and masons. 5
Measurement Experiment Objectives: a) measure all dimensions of several objects within tolerance of device b) Understand how to calibrate these instruments c) Understand how to take inside and outside measurements using a micrometer, Vernier caliper, and/or indicator gauge d) Understand how to use a dial indicator to take measurements with respect to a given datum e) Draw these objects on AutoCad or SolidWORKS with dimensions and tolerances noted Procedure: 1. Record all necessary measurements in your lab notebook 2. Draw the objects in a(n) AutoCAD or SolidWORKS drawing a. Note tolerances (measurement error) b. Label important measurements (i.e. OD, ID, radius to center of hole) In the report be sure to: Explain how each device works and its measurement error Insert the drawing into the results section with proper citation Have the drawings notations be readable in the report 6