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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. Whattypes of hand measurement tools are necessary? What are their purposes? And, what is the correct application foreach?

Squares and Rulers

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

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

These lines are usually marked by a corresponding number, e.g., two orthree. Between each of these numbered lines are ten more equalgraduations or “hundredths“, each line having a value of .010.

Because of the small distances between the lines the next graduationdown, “thousandths” would become just a blur without the aid of some kind of magnifying glass. Therefore, these linesare usually omitted. It is, however, relatively easy to estimate thousandths. With practice, you will be able to measureas accurately as a pair of calipers.

While the rules and square blades stand on their own, the square bladein conjunction with a square or protractor head becomes a much morecapable hand tool.

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

To measure this bend, set your square at dimension. Then, while holdingthe square blade flush to the workpiece, take another blade or straightedge and slide it up edge to edge. If the part is correct the sliding bladeshould meet the set square blade point to point at the bottom edge of theapex.

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

Of course, checking for bend angle with a square or protractor head isalways 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 thescale.

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Figure 4

Figure 5

Figure 6 shows a dimension of1.565

Figure 7

Calipers

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

All three of these are more than adequate for use in the precision sheetmetal shop. All are accurate, but some require a bit of a “practiced eye” toread 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 twoscales, the upper scale reads the outside of the caliper jaws; and thebottom 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 thousandthincrements.

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

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

Dropping down to the slide scale, search for the place where the lines onboth scales form one single line between both scales. I

n figure 6 these lines meet on the bottom scale at .015. To find the totalmeasurement becomes just a matter of addition:

1.000 + .550 + .015 = 1.565.

Dial Calipers

Dial calipers are read directly from the inch / tenths scale on the caliper’s mainbody; 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.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. Dialcalipers need the track brushed out several times daily. Not doing so allows the build-up of dirt and grime which cancause the pinion gear to jump a tooth on the gear rack. If and when the pinion gear jumps a tooth, the caliper will nolonger 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

Figure 8

Figure 9

Figure 10

Figure 11

Keeping the “looseness” in the sliding jaw to an absolute minimum is a must. These adjustments are common to allthree 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 waysmeasurements 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 modernsheet metal shops. Figure 10 shows three styles of protractors. The first is theregular everyday protractor which reads in one degree increments and isfound in most hardware stores. The second is a vernier scale protractor, whichworks in the same manner as vernier calipers; reading in increments of 5minutes. The third type is the digital protractor.

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 thevernier scale (inner). The other two protractors read directly, withthe digital protractor having the capability of both reading indegrees, minutes and seconds; or digital degrees e.g.: 74.74degrees.

Radius Gauges

These tools are used to measure the inside radius of theworkpiece. These come in complete sets of standard Inch orMetric sizes. They can be used to measure the radius of thepunch, 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 thecalculated bend functions will be true and correct.

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

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

The bottom bending column shows radii from the punch nose, and the air forming column shows the radius values foran air formed inside radius. A radius that is “floated” by material type and is developed as a percentage of the dieopening. 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 controldepartment, figure 13.This measurement method could be made using anything that has a diameter: drill rod, barstock, etc. The radius is half the pin diameter.

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

Figure 12

Figure 13

Figure 14

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 ofmovement 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 bemade.

Other attachments to height gauges include center finders that can check hole locations; dial indicators, for plus orminus measurements; even protractors can be attached to aheight 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 aresurfaces that the height gauge and angle blocks work from,and where measurements are made.

Surface plates come in a variety of grades that run fromshop to laboratory in quality, figure 16. Surface plates like alltools require care; in this case, maintenance consists ofconsists of cleaning and protecting.

Clean the plates and blocks regularly with a soft rag, warmwater and mild soap or a surface plate cleaner. The frequency with which you clean theplate is dependent on use. The best practice: clean it at the beginning and end of eachworkday; more if necessary.

If present, wipe up spills, oils or sticky materials immediately. Some chemicals canerode granite surfaces, making it less precise. Do not use volatile solvents such asacetone, alcohol or lacquer thinner; the evaporation can chill the surface and temporallydistort it.

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

Countersink Gauges

Countersink gauges measure countersinks: angles, diameters, or both. The mostcommon are small hand held gauges, manufactured for each of the standardcountersink countersink angles while pin gauges or a hand held optical comparators areused to check diameters. Some gauges have scales printed on the sides for measuring the diameter; figure 17 left,

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Figure 15

Figure 16

Figure 17

Figure 18

show a self made gauge set.

The second style is the dial indicator stylegauge. This tool measures countersinkdimensions on the dial. Each angle will requirea different angle tip for countersink.

Gauge Pins

Gauge pins come in either metric or decimalin 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 thatpasses through the hole. The pins can also be used to check the diameter ofcountersinks.

Whetstones

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

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

Figure 18 is a table top comparator and figure 19 is a reticle. The table topstyle 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 laserduring the set up process. The gauge should move freely through the cut “kerf” with onlyslight 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.

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

CMM’s

Computer Measuring Machines (CMM) come in several styles and from several differentmanufacturers. These machines are built around a surface plate with a gantry, thattransverses the plate on the X, Y and Z axes.

From the gantry one of several different probes can be mounted that in conjunction witha computer can measure accurately in all three dimensions. They can measure hole tohole, 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 and000051

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