How To Use Test Indicator

MAXNOVO MACHINE Tips : Every production engineer uses a test indicator, for there are few, if any, instruments with wider applications in practical engineering. This may account for the variety of names it goes by-dial indicator, clock indicator, clock gauge, and so forth. Some engineers call it a comparator, for that is what it is-the common workshop comparator, used by turners, millers, grinders, and inspectors. In specific applications, its function is to compare the shape and alignment of faces, the location of faces on components, the alignment of slides, and work on machine tools. You see these best by examples. When you test the running truth of round stock in the independent chuck by bringing a tool near to it, you compare parts of the surface. If the stock wobbles, there is a high point and a low point. You see the gap at the tool. When a test indicator is applied to the stock, the error is shown in thousandths of an inch.

By this principle, you can test diameters for circularity and concentricity; and in resetting part machined components, you can ensure that the surfaces which have still to be machined will be true with those that are already completed. When you mark off work on the surface plate, you make sighting lines to which the surfaces are to be machined. You test the surfaces afterwards with scribing block or surface gauge, looking carefully for variations. But you make a far more accurate check with a test indicator on the surface gauge, its sensitive plunger resting on the surfaces.When you set up a vertical slide by graduations, you chance whether the machining will be really accurate. And so you may mount a fixed point and traverse the slide past it, again looking carefully for variations-and, of course, correcting them. Here, too, you can mount a test indicator, and adjust the slide so that its face is exactly parallel to the line of movement.

One job which every turner must do from time to time is to true the tailstock for parallel turning between centres. On some lathes, the tailstock can be adjusted. On others, an adjustable centre is employed. When a lathe is in good condition, the basic setting can be made quickly, with a mandrel and a test indicator, as at A . The mandrel is turned in mild steel, brass or aluminium-alloy, to choice, with its two diameters made the same size. On a worn lathe, the tailstock should not be moved after the setting has been made. The test indicator is mounted on the top-slide. Another job on which accuracy is essential is the turning of a taper. Only great good luck would give a true setting from topslide graduations. To get precision you use a test indicator with its plunger at centre height and set at right-angles to parallel work when the topslide is adjusted to angle. You need a variation in reading, in thousandths, on the test indicator, when the topslide is moved 1 in. by its feed-screw. The variation is the sine of the half angle of the taper. You can take it from trigonometry tables. These are what you need for common angles: 1 deg. O.O174in., 2 deg. 0.0348 in., 3 deg. 0.0523 in., 4 deg. 0.0697 in., 5 deg. 0.0871. Diagram B shows the setting.

Sometimes there is need to set up work to a centre punch indentation. You have marked the work on the surface plate and punched at intersections of lines. You make a set-up on the faceplate, on the angle plate, or in the independent chuck, as at C. To true the centre punch dot, use a centre finder-the subject of a correspondence in Postbag. It is a true piece of rod, pointed at one end and centred at the other. The test indicator must maintain a steady reading as the work is rotated. To check settings of a vertical slide, at right-angles to the lathe axis and parallel to this axis, you mount the test indicator as at D and E, and then traverse the slide along its plunger. You can use a test indicator as the basis of a master square by mounting it, as at F, on a straight-edged, three footed base. The test cylinder you machine parallel in the lathe.

In mechanical engineering all components and machines have a basis of geometry which settles the shape and alignment of surfaces. For the most part it is elementary geometry visible and tangible, consisting of plane surfaces, diameters and right angles, all of which can be proved in straightforward ways with a test indicator. But in proving visible features you often prove those that are invisible -except on drawings, where they form the framework as axes and centre-lines. To ensure accuracy in draughting, axes and centre-lines are put in first on drawings. Then you design components, in the flat, around them. When you test three dimensional components, you prove the basic geometry.

Take as an example a connecting rod on which the axes of big-end and small-end are parallel. On a drawing, they are two parallel lines. In testing the connecting rod, you put a well-fitting mandrel in each bearing and support the connecting rod on a surface plate. Both ends of a mandrel should then give the same reading on a test indicator, when the connecting rod is lying horizontally, and when it is standing vertically. If there is an end-to-end difference in the height of a mandrel, it is shown by a variation in the reading on the test indicator, and you know that the axes are not parallel. (You forcibly true a misaligned connecting rod through a corrective twist or by applying pressure opposite the bend.) Geometry offers us many opportunities for halving errors in seeking accuracy, and for doubling the amplitude of errors the better to find small ones. An example on the drawing board is the way that you check celluloid square. Holding it to the edge of the T-square, you pencil a vertical line. Then you turn the square over-and any error is doubled. It is the same when you test a steel square by scribing lines on a straight-edged metal plate.

By a similar principle, the flatness of a lathe faceplate can be verified and the alignment of the cross-slide checked, with a test indicator used in a holder on the top-slide, as at A. Turn the faceplate to discover wobble and then locate the run-out vertically. Using the cross-slide, run the test indicator across the near half of the faceplate U. With a long holder, repeat the test on the remote half V. If the faceplate has been machined on the lathe, an error is concurrent between the faceplate and the cross-slide for the near half U. And so the first test reveals nothing unusual. But on the remote half V. an error runs counter to the one on the cross slide. The second test shows this clearly. A universal attachment, or a lever attachment, equips a test indicator for use in bores and on outside diameters where clamps obstruct direct access. Examples of use are at B and C (clamps omitted). When work is held in a chuck B, its face as well as its bore must be true. Place packing at low jaws; and for tapping the face of work, use a lead or hide hammer. If a face-plate wobbles. pack at the low point with paper. To true a button C, tap the edge of work with a brass or aluminum punch-the clamps can set a vertical slide to any angle just gripping. Then tighten them firmly.

Diagram D illustrates how you for a milling operation. The test indicator is mounted in a chuck or on a driving plate for its plunger to bear on the blade of a protractor. You can set a vernier protractor to a fraction of a degree, and place the stock to the vertical slide with the blade across the lathe axis. To hold the protractor level, clamp it to an angle plate. Set the vertical slide so that the test indicator shows a steady reading by cross-slide movement X. Two more typical uses of a test indicator are shown at E and F. To set a fly cutter, test over the bar for height Y and add the projection of the tool Z. Set a home-made height gauge to Y, and for Z place two turned rollers under the gauge. To test concentricity of the pitch circle of a gear put a roller in each tooth space in turn, and rotate the gear under a test indicator.

It is a principle in engineering that accuracy leads to accuracy. Conversely, from one error can come several others. Consider a shaft which runs in a pair of plumber block bearings that are bolted to a flat surface. The two bearings must have the same rise to centre, and the axis of each must be parallel to the base. Then the shaft runs freely without strain or binding. But you are put to a great deal of trouble when errors have occurred in the machining of plummer blocks. The lower one of a pair must be packed up to the other. A Plummer block whose axis and base are not parallel must be corrected by careful filing and then by packing. When two plummer blocks are inaccurate, both must be corrected to prevent the tilting and binding of the shaft. This is an elementary case-an example of the implications contained in a single error in a simple assembly. Obviously, the greater the number of errors among components in an assembly, the more formidable become the combinations of faults.

With inaccurate plummer blocks, the fault lies in the setting of the angle plate on which they are machined on the faceplate. It is not a difference in centre height when two have been machined, one after the other, at a single setting of the angle plate. The fault is that the angle plate is not square, with the result that an error from parallelism occurs between the base and the bearing axis of a Plummer block. Similarly, facing a component on an inaccurate angle plate introduces an error from squareness between the base and the rising face. You can avoid errors by packing the angle plate square on the faceplate, after finding the direction of error with a test indicator. You mount the instrument on the slide, and run it along the angle plate, as at A, to see whether the angle is more or less than 90 deg. According to the direction of error, you pack the angle plate top or bottom, using a strip of shim-stock to the faceplate. It is advisable to test all unproven angle plates, as errors can always be corrected like this.

Unless you are sure of a cross slide and vertical slide, it is advisable to make a check, as at B, with a test indicator. It should be mounted on a driving plate, or in a chuck with an off-set, so that it can be turned on the face of the vertical slide. Any difference in reading from where the indicator is shown to point X reveals an error which you can correct by packing the base of the slide at Y or Z. A dropped centre caused by wear of the base of the tailstock is not uncommon when a lathe has been in use for several years. Test for it as at C. Mount the test indicator on a driving plate, as shown left, and turn it on the tailstock centre. By this method, you can see if there is a horizontal error-the kind that causes a taper in turning between centres, which can be corrected by setting over the tailstock (when it is adjustable), or by using an adjustable centre.

The extent of error with a dropped centre can be found from the extremes of reading when the test indicator is mounted with its plunger square on the centre, as C right. If the centre has a parallel part which can be held in the independent chuck, the error can be repeated in setting up for grinding. Then the centre must be put in the tailstock with the lift upwards. The same can be done with a holder for a silver steel centre. To turn this taper before hardening and tempering it, set up as at D with the error on the indicator. Mark the centre and fit it with the lift upwards. Concentricity of the base circle of a cam can be verified, as at E, with a test indicator, turning the cam from one flank (1) to the other (2). By the same method with a circular protractor or division plate, you can check the opening angle. Diagram F shows another use for a test indicator-in a built-up caliper for testing the splay which is given to the bearing caps of some car differentials when the bearings are adjusted.

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