Comparative ID/OD Gaging

A wide variety of gaging methods can be used to measure inside and outside diameters. At the low end, a hole could be measured with a scale or a fixed go/no-go gage. At the other extreme, any number of precision measuring machines, including CMMs and optical or vision machines, are available.

Usually the big decision comes down to choosing between making measurements with either a caliper/micrometer or a comparative bench or portable gage. Working on the production floor, there are two critical points to think about: gage performance and speed of measurement.

Calipers/micrometers are very labor intensive gages and subject to operator influence in the way the operator places the hand tool on the part and applies force to make the measurement. Offsetting the placement of the caliper just a bit or applying too much gaging pressure will increase the variation of the measurement.

Comparative gages are set up for a specific measurement. The operator simply places the part on the gage. There is virtually no operator influence, the gage measures with the same force every time, and it measures the part the same way every time. Thus the gaging process is not only faster but much more accurate. The Tolerance Chart in Figure 1 shows how much less uncertainty of measurement is associated with a comparative type gage as opposed to a hand tool. For these reasons comparative gages are the gages of choice in the production environment.

There are two basic types of ID/OD comparator gages: bench top and portable. Choosing between these two depends mainly on part size and whether the part will be brought to the gage or vice versa. Bench top comparative gages are typically used on small parts and generally restricted to measuring single dimensions less than 255 mm (9 in) in diameter and not more than 25 mm (1 in) deep. Portable ID/OD gages can go as large as 2500 mm (8 ft) and as deep as 125 mm (5 in).

Both are meant to be used in high volume, high performance applications by operators with shop-floor level skills. Because they provide a comparative measurement, these gages require a master to set a zero reference point. They also have relatively limited travel which allows the use of high resolution dial or digital indicators, or very high resolution electronic probes with amplifier readouts.

Bench Top Gages

Bench top gages are capable of higher precision than portables, with resolutions of .0005 mm (.00002 in) readily achievable. Typical of these are the “plate gages” seen in the bearing industry. Recognized by their tilting stage plates – used to set and locate the part being gaged – this basic design has been around for over 50 years. Plate gages are convenient for fast, comparative gaging of flat, relatively thin-walled parts, such as ball and roller bearing rings, where diameter measurements must be made in a plane parallel to at least one of the faces, and sometimes at a particular depth on the ID or OD.

There are two types of contact arrangements in these gages: a “T” plate and a “V” plate. The “T” plate is the most common. Since the reference contact and the sensitive contact are in line, the gaging principle is the same as in a portable snap gage. There is a difference, however, in that the contacts are not flat and parallel but curved or donut-shaped. This means the gage may not necessarily pick up the MAX or MIN diameter of the part every time, and some slight “swinging” of the part through the contacts is necessary. The second reference contact on the “T” can help locate the part. However, it should be set to produce a reading slightly outside of the MIN or MAX value. Otherwise – if it is set to be exactly on the “zero” diameter – any other position will produce a chord reading, and not the true diameter of the part.

The “V” plate incorporates two reference stops, one at the top of each arm of the “V,” that must be adjusted symmetrically to assure that the part is staged on the center plane of the “V.” This double stop has a locating effect similar to a vee block, and provides positive and precise location of the part on the gage. This greatly speeds up the measuring process, taking some of the operator involvement out of the measurement, and is especially useful on parts with odd lobing characteristics from the machining process.

However, there is a drawback to this type of contact arrangement. Since the sensitive and reference contacts are not in a direct line, there is not a one-to-one relationship between sensitive contact movement and the diameter. Rather, the measurement result is determined by a multiplier based on the angle between the reference contacts, just like when using a vee block. In most cases this angle is 60 deg and the ratio is 4:5. Thus, for every four units seen by the indicator, five units come out (which is another way of saying the sensitive contact is multiplied by 1.25 to get the correct result).

Portable Gages

If the part is large or awkward to manipulate, or if it is set up on a machine and you want to measure it there, then a portable beam-type gage is needed. Beam-type gages are available with maximum capacities from 125 mm (5 in) to about 2500 mm (8 ft), the largest ones used to measure bearings and castings for jet engines and other large precision parts. Range of capacity is typically about 150 mm (6 in), while the measurement range is determined by the indicator installed.

Most portable ID/OD gages lack centralizing stops, so they must be “rocked” like a bore gage to find the true diameter. A special fixture with sliding stops at major increments is used to master large ID measurements. Gage blocks are inserted in the fixture to “build out” the desired dimension. For OD measurements, calibrated “end rods” are often used. When mastering and measuring large dimensions, the gage, the master, and the part must all be at the same temperature. Otherwise, thermal influences will throw off the measurement.

Even so, don’t expect very high precision when measuring dimensions of a foot or more. Most indicators on these large-capacity gages will have minimum grads of 0.01 mm (0.0005 in). This is adequate, given the inability of most machine tools to hold tolerances much tighter than about 0.05 mm (.002 in) for parts that large. But beware the gage claiming to measure more than 900 mm (3 ft) ID/OD capacity with .001 mm (.00005 in) resolution: it’s probably not capable of repeatable measurements.

To accelerate the gaging process with either bench top or portable gages, mechanical dial indicators can be replaced with electronic indicators. The dynamic measurement capabilities of the latest generation of digital indicators enable them to capture minimum or maximum readings, or calculate the difference between the two. This frees operators from having to carefully monitor the motion of a rapidly swinging needle on a dial indicator when rocking a portable gage, or checking for deviation on a bench top version.

Special Feature ID/OD Checks

Sometimes, small or large ID/OD checks are complicated by parts that do not present themselves in a straightforward fashion. Usually these checks are on the inside of some type of large bearing – measuring the diameter of an internal surface behind a shoulder, for example – where the entry diameter is smaller than the diameter being measured; or measuring a ring groove in a bore; the pitch diameter of an internal thread; the effective diameter of a barrel roller bearing; or the included angle of a tapered bore.

One way to achieve this result is to use a long-range digital indicator as part of the measuring frame. This has the benefit of actually referencing the measurement based on the accuracy built into the gage’s own long-range slide. Of course, the contacts must be designed to handle the depth and gaging pressures, but the beauty of this gage is that its long range offers the ability to measure any number of depths within its range.

In the future (possibly near term) we will probably see this capability extended with longer range ID/OD gages that can measure similar sized parts with less mastering for set-up. This could eliminate the need to have one gage for each size and reduce master costs. This will require higher performance long range indicators, precision slide mechanisms, and designs that assure precise alignments of the contacts over their full measuring range.

George Schuetz is the director of precision gages at Mahr Federal Inc., 1144 Eddy Street, Providence, RI 02905, 401-784-3100, Fax: 401-784-3246, george.schuetz@mahr.com, www.mahr.com.