June 5, 2012

Employing Precision Measuring Instruments

Close enough may be okay in many fields, but to advance in gunsmithing, precision is a must. Here are some tools to help you.

Gunsmiths need two types of measuring instruments, direct and indirect (or comparison). The indirect, or comparison, instruments include tools found in the caliper family, such as inside, outside, and hermaphrodite calipers, and dividers. Direct measuring devices include the rule, various micrometers, and dial-faced measuring tools.

Tolerances can be held surprisingly close with calipers, and measurements taken with an outside or inside caliper can be transferred to a rule or to another measuring tool. Unless a measured distance is required, the caliper family is generally useful as a “snap gauge” to transfer an old part’s dimensions to a new one during reproduction.

Direct measuring tools such as outside micrometers, depth micrometers, dial, and vernier calipers are almost an absolute requirement when manufacturing parts to close tolerances. When calculations are needed in machining, these instruments provide the linear measurements needed.

The dial indicator, which does not measure length like a micrometer or dial caliper, is indispensable in “setting up” mills and lathes and monitoring machining processes to close tolerance.

Some of these precision tools are versatile enough to perform several functions and may be easier to read than single-purpose instruments, thus are used more often. The downside to multi-function tools like dial calipers is that some accuracy may be sacrificed to provide this reading ease and multiplicity of use.

Measuring-instrument quality, like any other gunsmithing tool, is directly related to cost, and your purchases should be guided by the item’s expected use. For example, a 6-inch plastic dial caliper retailing for $17 to $20 is fine as a snap gauge for measuring cartridge cases or the occasional bench job. But if that piece is used constantly on daily eight-hour shifts, it will soon wear to the point of being useless. On the other hand, a 6-inch Starrett Satin Chrome Master Vernier Caliper at $270 or more is certainly not the tool to purchase if all you measure is the width of slings or recoil pads.

An important rule of tool purchasing is to buy the best quality tool you can afford for the expected use. But be sensible about it. Don’t go overboard and buy more quality than you will need just because your budget allows it.

Another rule is to safeguard your investment once it’s purchased. Regardless of how much you spend, you need to keep your tools in good shape by wiping them clean before storing. When storing precision measuring tools, leave a small gap between the measuring surfaces.

This allows parts of different metals and shapes to expand and contract at different rates as the temperature changes, preventing stresses that could possibly damage the tool’s accuracy.

The Dial Caliper

Practicing gunsmiths we consulted with overwhelmingly recommended the 6-inch dial caliper as their first choice in precision tools. Only a few old-timers voted for vernier calipers, which were once the standard for machinists.

Verniers are still available and are useful and accurate instruments, but learning to use vernier scales can be more difficult than the dial versions, and extra practice may be needed to read the scales accurately. Dials, especially for those of us who are overdue to visit an optician, are far easier to read.

The dial caliper has three standard uses: for outside, inside and depth measurement. Dial calipers usually measure to a thousandth of an inch (0.001) and are generally accurate enough and acceptable for most gun work. In those cases where 0.001 inch is not close enough, and a “tenth,” or one ten-thousandths of an inch accuracy, is required, a “tenths” micrometer should be used.

Precision Measuring Instruments

Courtesy, American Gunsmith

Inside and outside calipers, dividers and a pocket-slide caliper are accurate but can’t be considered precision measuring instruments.

A dial caliper’s depth measurement is perhaps not as accurate as it would be from a depth micrometer because holding the caliper exactly parallel or vertical to the measured surface is more difficult. However, adapters are available and can be attached to the caliper to provide a more stable measuring platform.

The major caliper parts are the fixed and movable jaws, slide, dial, scale, and jaw-adjustment knob. The dial has an adjustable bezel or face and a bezel lock. Most, if not all, dial calipers have a lock to keep the jaws from accidently moving while reading or transferring measurements. The dial indicator is available in two standard types on which a full revolution of the needle can equal either one tenth (.100) or two tenths (.200) of an inch. The .200 dial will have zeros at its top and midpoint. Of the two, the .100 revolution dial provides wider spaces between graduations and is easier to read. Hint: Before you take any measurements, clean and close the jaws of the caliper, then adjust the dial bezel to the zero point.

Apply only enough pressure to the jaws or depth stem to assure that contact with the work has been made. With a little practice, you will develop a feel for this. Consistent pressure will become a habit, contributing toward consistent measurements. Quality calipers have an adjustment knob that “slips” when reading pressures have been reached and automatically provide the consistency needed from one measurement to another.

Many calipers, dial and vernier alike, have scales that read in millimeters and inches. Since most of our measurements will be in inches, be sure it is the inch scale that is used. Do not mix the scales.

The inch scale is divided into two types of graduations. The largest lines and numbers are inches; the smaller are hundredths of an inch. The dial also divides each tenth into thousandths of an inch.

To read the dial caliper, remember to add the measurement from the scale in increments from left to right, from largest to smallest. Convention says that any numbers smaller than an inch are to be converted to thousandths of an inch to eliminate confusion. One inch plus six tenths plus fifty three thousandths equals 1.653 and is read as one and six hundred fifty three thousandths of an inch. If these measurements are not precise enough, the micrometer with tenths capabilities will be needed.

Dial Indicators

The next precision measuring tool mentioned by gunsmiths was the 0.001-inches dial indicators used primarily to prepare milling machines and lathes for specific jobs. They are also used to center items in a four-jawed chuck, to monitor machining progress, and to work to precise dimensions. If quality and precision machining is in order, the dial indicator cannot be overlooked. Costs for the basic indicator will vary from around $20 to more than $250, while an indicator stand will run an additional $20 to $100.

The stands come in many styles, but the magnetic base is the most popular. The holder arms may be solid or flexible, depending on personal preference and available funds.

Precision Measuring Instruments

Courtesy, American Gunsmith

Transfer the measurement you get with the outside calipers to your rule.

The dial indicator differs from the caliper and micrometer in that it produces readings of variance and motion, or continuously monitors the distance of travel from one given point to another. Neither the dial caliper nor micrometer can do this.

From its at-rest position, the dial indicator’s needle will indicate in one direction (clockwise) only. The number of needle revolutions around the dial is determined by the length of the stem, which can be 0.050 inches or less to several inches. Some dial indicators have replaceable stems of different lengths, but a stem with 1 inch of travel is probably versatile enough for your needs. These instruments are generally available in 0.001 inches accuracy.

Movable bezels are generally graduated in 0.100 inches per needle revolution. To ease failing memories, pointers are provided and can be set as reference locations or to mark a variation spread.

Some confusion exists between two similar indicator instruments, the dial indicator and the dial test indicator. The latter has capabilities to read either left or right from its normal state. The test indicator’s range of motion is usually limited to a short amount of travel, and it has dial markings graduated plus and minus right and left of the zero. It is used to measure vertical runout or wobble. The dial indicator reads to the right, its readings limited only by the length of the stem. It can be used for both runout and linear motion.

In lathe work, if we mount the indicator on the lathe bed so it contacts the saddle, we can accurately determine the length of any given longitudinal cut or amount of saddle travel along the bed. Contact with the cross feed can determine very precise cutting depths for finished work. If centering work in a four-jawed chuck, the indicator is mounted with the stem bearing on the work. Any errors in centering are removed by revolving the chuck 180 degrees and removing half the indicated error by loosening and tightening opposing jaws. The chuck is again rotated 180 degrees and checked again.

After removing this runout, rotate the work 90 degrees and start the process again. The work is centered when the needle does not move as the work is rotated 360 degrees against the stem.

For milling machine work, the indicator is used to “swing” or “sweep” the table to set the mill head absolutely vertical to the table. This is necessary to produce parallel cuts and prevents “shelving” in successive cuts across wide surfaces. The dial indicator is also required for set ups, like scope-mount drilling or slabbing the sides of a barrel, that must have the axis of the work parallel to bed travel.

In both of these examples, the bore must be held absolutely parallel to the direction of table travel. This set-up is best done with the dial indicator.

To “swing” or “sweep” the table in adjusting the milling head to vertical, hold the indicator on the end of a bar held at 90 degrees in the spindle, the stem pressing 0.025 inches, more or less, against the table. A reading is taken on the long axis of the bed, the indicator rotated 180 degrees and the reading compared. Any change indicates the milling head is not vertical in the Y axis and needs to be adjusted by half of the variation. As this process continues, the indicator is swung back to the starting point. The variation should become smaller on each swing. Additional tests are taken on the X axis (90 degrees from the original setting) and the head adjusted so no variation exists in the Y axis. When the readings are equal in all four quadrants, the head is perpendicular to the table.

Precision Measuring Instruments

Courtesy, American Gunsmith

Key tools you will use include outside micrometers with standard and digital scales and a depth micrometer with extension rods.

If you want to square a vise or work to the table, clamp one end of the vise or work to it. Set the stem of the indicator to get an 0.025 inches reading on the fixed jaw of the vise or on a flat edge of the work. Note the position of the needle on the dial and traverse the table. If the needle shows any motion from its original reading, tap the vise or work in the direction needed to bring the needle back to its original position. A couple of passes should square the vise or work to plus or minus 0.002 inches or less.

If the lathe is used to chamber, the dial indicator can be set up to read motion in 0.001-inch increments as the reamer is fed into the chamber. Reaming distances of less than 0.001 inches can be controlled in this manner and holes drilled to exact depths in the same manner.

Remember that direct-measurement instruments—the dial indicator, outside micrometers and depth micrometers—produce actual linear measurements. The dial indicator measures variation, or motion from one point to another, and is not an instrument that is generally used to measure length directly.


The final family of precision measuring tools needed for those more precise jobs are micrometers. A 1-inch outside micrometer with tenths capability is an excellent choice. A depth micrometer with 1- to 6-inch capabilities is extremely useful for barreling and chambering.

The major components of micrometers are, from top to base, the thimble, barrel, spindle, anvil and yoke. Most micrometers are now equipped with a pressure-regulating thimble or ratcheting top to provide equal force from measurement to measurement. (Don’t let customers handle your micrometers. No matter what some people believe, micrometers are not precision “C” clamps.) The cautions of cleaning and verifying the zero apply to micrometers as well as dial calipers. If a zero reading is not obtained when the spindle touches the anvil, follow the manufacturer’s instructions for adjusting the micrometer barrel.

Note that the value of the micrometer scale differs from that of the dial indicator. Each large graduation on a micrometer barrel represents 0.100 inches, the same as the scale of the dial caliper. Within each of these major divisions are three smaller graduations producing four spaces from one major line to the next. The distance between these smaller lines represents 0.025 inches.

The thimble is also marked around its circumference with 25 graduations, each representing 0.001 inches. One full turn of the thimble equals 0.025 inches.

To arrive at an outside micrometer reading, use the same principle as we did with the dial caliper, adding from large to small, left to right. As the micrometer is adjusted, the reading is taken from the last graduation that appears under the barrel on the base line.

All graduations on the micrometer, like the dial caliper, are read in thousandths. 0.100 is read as one hundred thousandths, 0.025 as twenty five thousandths, and 0.003 as three thousandths. Together they add up to a total reading of 0.128 or one hundred twenty eight thousandths.

The “tenths” micrometer has a set of markings parallel to the barrel numbered from 0 through 10. These lines break a single thousandth of an inch (0.001) into 10 parts to provide the ability to read in ten-thousandths of an inch.

Precision Measuring Instruments

Courtesy, American Gunsmith

Dial calipers and a dial indicator with an extension adapter and indicator stand vary widely in expense. The indicator alone can run from $20 to $250, the stand as much as $100 more.

If a graduation on the thimble does not line up on the base line, look at the tenths lines on the barrel; one of the graduations on the thimble will line up exactly with one of those lines.

The number of “tenths” is read on the barrel scale, not the thimble, and the number is added to the end of the reading. For example, 0.1287 is read as one hundred twenty eight thousandths and seven tenths.

The depth micrometer scale is handled differently than an outside micrometer, but it uses the same additive principle. In both instances, the scale starts at zero, but as the thimble on the depth micrometer is turned, the thimble covers the graduated lines on the barrel instead of revealing them. Remember, the last covered line is the graduation to read. If you use the graduation line that is visible, the measurement will be long by 0.025 inches.

Depth micrometers are especially useful in measuring receiver-ring and barrel-shank lengths, when reaming during chambering to monitor progress, and for seeing how many more thousandths remain before the GO gage is fully seated.

The Tools In Use

You’ll find these instruments invaluable if you ever have to reproduce a part for a customer’s gun. We need the part’s dimensions, overall length, width, depth of retaining slots, and so forth. Many of these measurements can be taken directly from the old part without difficulty, but the new part’s dimensions may have to be altered or adjusted to compensate for wear that may have occurred. Special consideration must also be given to those dimensions that concern safety, such as firing pin–tip protrusion, diameter, or loss of length through battering at the rear of the pin by the hammer.

Tip diameter should be matched to the existing hole, not to the old pin. Wear may have taken place on both the pin and in the hole. If this compounded error is not eliminated, the newly manufactured firing pin could indicate a headspace condition by letting the primer flow into the space around the tip and hole. The tip must be long enough to ignite the primer without piercing it. These lengths vary from shotguns to .22 rimfires, so published data should be followed in making them up.

Measurements that you take from old parts need not be drafted professionally into a full-blown blueprint. They are only working guides for reproduction, though it is a good idea to record and save them for future reference just in case another one is needed. Several laws of gunsmithing suggest that if you do not record the data, the next job will be identical and the measurement will have to be taken over again!

A little practice in using dial calipers, micrometers and dial indicators can give the gunsmith an increased ability to make precision measurements. But the quest for accuracy does not end here. The best measuring skills are useless without the ability to transfer and work to those measurements. Without a high level of competence using precision measuring instruments, the mastery of mechanical skills needed to work to close tolerances becomes a nearly impossible task.

If your reputation is important—measure with a dial indicator, mark with a finely pointed scriber, machine and file to within 0.001 inches. It will not be long before you are known as a superior craftsman and quality gunsmith!

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