Measurement Tools

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Measurement tools

To insure that parts being produced are correct, you must be able to measure them, and measure it correctly. What types of hand measurement tools are necessary? What are their purposes? And, what is the correct application for each?

Squares and Rulers

Squares, blades and rulers are common to many industries which have their own special needs. A wood shop, for example will use rulers with fractional scales.

In precision sheet metal, a decimal scale is the norm, figure 1. The view is of the second inch of an R16 decimal inch scale or the metric equivalent. Notice that each inch is divided into ten equal graduations or “tenths” equal to .100 between these larger lines.

These lines are usually marked by a corresponding number, e.g., two or three. Between each of these numbered lines are ten more equal graduations or “hundredths“, each line having a value of .010. Figure 1 Because of the small distances between the lines the next graduation down, “thousandths” would become just a blur without the aid of some kind of magnifying glass. Therefore, these lines are usually omitted. It is, however, relatively easy to estimate thousandths. With practice, you will be able to measure as accurately as a pair of calipers.

While the rules and square blades stand on their own, the square blade in conjunction with a square or protractor head becomes a much more capable hand tool.

Figure 2 shows a 45 degree complementary bend. For this example, the workpiece is dimensioned to the outside apex of the bend.

To measure this bend, set your square at dimension. Then, while holding the square blade flush to the workpiece, take another blade or straight edge and slide it up edge to edge. If the part is correct the sliding blade should meet the set square blade point to point at the bottom edge of the apex. Figure 2

A square set can be set one inch longer and the required dimension now be directly read. In the example shown, the edge of the workpiece should just cover the 1.000 inch mark if the measurement is correct, figure 3. These are by no means the only ways that the square set can be used, just a couple of good examples; be creative.

Of course, checking for bend angle with a square or protractor head is always an option, figure 4. Note that the marks on the blades have width, the reading is taken from the edge of the mark closest to the zero end of the scale. Figure 3

000047 Calipers

There are different styles of calipers in common use: vernier, dial and digital.

All three of these are more than adequate for use in the precision sheet metal shop. All are accurate, but some require a bit of a “practiced eye” to read them well. The “vernier caliper” is that case in point.

The Vernier

In figure 5, we see an enlarged view of a vernier scale. There are two Figure 4 scales, the upper scale reads the outside of the caliper jaws; and the bottom scale, between the jaws.

Using the lower scale, look at the divisions written on the body of the caliper. It measures inches and is divided into hundredths, and .050 thousandth increments.

The slider scale is graduated from 0 to 50, with each having a value of .001 of an inch. In the example shown in figure 7, the bottom scale zero rests somewhere between .550 and .600 inches.

Reading first from the top scale, one inch, five hundred and fifty thousandths plus. (1.550-inches plus) Figure 5

Dropping down to the slide scale, search for the place where the lines on both scales form one single line between both scales. I n figure 6 these lines meet on the bottom scale at .015. To find the total measurement becomes just a matter of addition:

1.000 + .550 + .015 = 1.565.

Figure 6 shows a dimension of Dial Calipers 1.565

Dial calipers are read directly from the inch / tenths scale on the caliper’s main body; and then from the dial for the thousandths reading.

Figure 7 shows a standard dial caliper.

Digital Calipers

Digital calipers read instantaneously from an LED screen on the caliper face. Figure 7 Depending on the brand, these may even read to ten-thousands of an inch.

All the tools must be cleaned and adjusted regularly to maintain their accuracy; calipers are no exception. Dial calipers need the track brushed out several times daily. Not doing so allows the build-up of dirt and grime which can cause the pinion gear to jump a tooth on the gear rack. If and when the pinion gear jumps a tooth, the caliper will no longer zero in the correct location. Vernier and digital calipers only need to be wiped off and checked for adjustments.

The most important caliper adjustment is done to the head, keeping the head and the body of the tool parallel. 000048 Keeping the “looseness” in the sliding jaw to an absolute minimum is a must. These adjustments are common to all three styles of caliper and should be checked regularly.

Most calipers can measure with more than just the primary jaws. Figures 9 a, b, c, and d, show the different ways measurements can be made: “Inside Dimension” jaws located above the main jaws (9a); between the jaws (9b); behind the main caliper’s head (9c); and a depth measurement can be made with the “stinger” (9d).

Protractors

There are three different types of protractors currently being used in modern sheet metal shops. Figure 10 shows three styles of protractors. The first is the regular everyday protractor which reads in one degree increments and is found in most hardware stores. The second is a vernier scale protractor, which works in the same manner as vernier calipers; reading in increments of 5 minutes. The third type is the digital protractor. Figure 8 The vernier protractor reads left or right with 360-degrees on the outer scale. On the inner scale the increments read from the right or the left, from zero to sixty minutes. All measurements are read from the vernier scale (inner). The other two protractors read directly, with the digital protractor having the capability of both reading in Figure 9 degrees, minutes and seconds; or digital degrees e.g.: 74.74 degrees.

Radius Gauges

These tools are used to measure the inside radius of the workpiece. These come in complete sets of standard Inch or Metric sizes. They can be used to measure the radius of the Figure 10 punch, the final bend radius (internal or external) or even 90- degree bend angles. Figure 11 shows the most important use: matching the inside radius to the calculated radius.

If the radius gauge can be seated in the same manner as shown in figure 11, all of the calculated bend functions will be true and correct.

Unfortunately, radius gauges do not come in all possible sizes, which creates a measurement issue.

Generally, the inside radii produced by air forming will not match anything from a radius Figure 11 gauge set; note the bend deduction chart, figure 12, has two columns, one for air forming and one for bottoming.

The bottom bending column shows radii from the punch nose, and the air forming column shows the radius values for an air formed inside radius. A radius that is “floated” by material type and is developed as a percentage of the die opening. Notice that they do not have any radii in common.

This may require a different tool to check the radius.

The next best option for measuring inside radius of bend is a gauge pin set commonly found in quality control department, figure 13.This measurement method could be made using anything that has a diameter: drill rod, bar stock, etc. The radius is half the pin diameter.

Figure 14 shows how a pin is used to measure the radius. 000049 Height Gauges

Height gauges, just like calipers, come in the three basic measuring styles: vernier, dial and digital.

The gauge has a large solid base, precision ground for quality and solid measurements while allowing ease of movement across the surface plate. Figures 15 a, b, and c show a height gauge in action.

Used in conjunction with an surface plates and angle blocks, which are extremely precise pieces of granite; flat, squareand perpendicular within millionths of an inch.

And by holding workpiece to the angle block perpendicular to the surface plate an accurate measurement can be made.

Other attachments to height gauges include center finders that can check hole locations; dial indicators, for plus or minus measurements; even protractors can be attached to a height gauge.

Precision Hand tools

Surface Plates

Surface plates are large precision ground pieces of granite, from four to ten inches thick.

These plates are flat within millionths of an inch and are surfaces that the height gauge and angle blocks work from, and where measurements are made.

Surface plates come in a variety of grades that run from shop to laboratory in quality, figure 16. Surface plates like all tools require care; in this case, maintenance consists of consists of cleaning and protecting. Figure 12 Clean the plates and blocks regularly with a soft rag, warm water and mild soap or a surface plate cleaner. The frequency with which you clean the plate is dependent on use. The best practice: clean it at the beginning and end of each workday; more if necessary.

If present, wipe up spills, oils or sticky materials immediately. Some chemicals can erode granite surfaces, making it less precise. Do not use volatile solvents such as acetone, alcohol or lacquer thinner; the evaporation can chill the surface and temporally distort it. Figure 13

Once cleaned, allow one to two hours for the plate to dry completely before using it again. To protect the plate, cover it when it is not in use to reduce the amount of dust landing on the plate; dirt and dust are abrasive and will wear away the surface. As silly as this sounds, the surface plate is not an anvil.

Countersink Gauges

Countersink gauges measure countersinks: angles, diameters, or both. The most common are small hand held gauges, manufactured for each of the standard Figure 14 countersink countersink angles while pin gauges or a hand held optical comparators are used to check diameters. Some gauges have scales printed on the sides for measuring the diameter; figure 17 left, 000050 show a self made gauge set.

The second style is the dial indicator style gauge. This tool measures countersink dimensions on the dial. Each angle will require a different angle tip for countersink.

Gauge Pins Figure 15 Gauge pins come in either metric or decimal in all standard graduations, with each pin .001 larger than the last.

The pin gauge is used to check hole diameters by finding the largest pin that passes through the hole. The pins can also be used to check the diameter of countersinks.

Whetstones

The whetstone or sharpening stone is used to dress burrs or galling from the tooling surfaces. Make sure the stone is a very fine stone; “dressing off” a surface like the bead of the press is all that is necessary. A coarse stone can alter the surface.Optical comparators come in both table top and hand held varieties. Regardless of model, comparators all work the same way.

A piece of glass call a “ Reticle” has been etched with the pertinent information for the task. For example, the reticle can be used as a way to check diameters, radii, linear distance, angle, etc. Figure 16

Figure 18 is a table top comparator and figure 19 is a reticle. The table top style magnifies the image 300% or more for accuracy.

Feeler Gauges

Standard feeler gauges are used to measure the width of the cut “kerf” at the laser during the set up process. The gauge should move freely through the cut “kerf” with only slight drag; the next gauge should not pass through the cut.

The last gauge to freely pass through the kerf is the width of the cut. Figure 17

They can also be used to measure between surfaces, if that’s where the dimension is called.

CMM’s

Computer Measuring Machines (CMM) come in several styles and from several different manufacturers. These machines are built around a surface plate with a gantry, that transverses the plate on the X, Y and Z axes. Figure 18 From the gantry one of several different probes can be mounted that in conjunction with a computer can measure accurately in all three dimensions. They can measure hole to hole, edge to feature, formed flange to edge, etc.

Scanning Systems

There are also computerized optical scanner, figure 20; 2D machines that optically scan a flat part for size and 000051 000052