Thread plug gages are fascinating products when you consider the technical details that go into their manufacture and the precision they embody. To some, they look like fancy bolts and are treated accordingly. To others who appreciate what it takes to make them, they represent a lot of precision for their low purchase price. The calibration of them presents similar approaches.
To the uninitiated it’s as simple as measuring the thread on a bolt. Those who know what they are doing understand there’s a lot more to it. In this column I’ll look at what is required to measure the most common elements of a thread plug gage.
There are many devices capable of measuring a diameter but only a few are precise enough for thread plug gage calibration. The most popular is a bench micrometer which functions like any other except it requires larger than normal micrometer anvils to span the thread wires used when measuring pitch diameter on the gage. The basic construction of the unit has to be substantial to ensure the anvils are flat and remain parallel to a high degree under varying measuring forces. Since gage standards call for different measuring forces for different thread pitches, some means of obtaining them has to be a feature of the device.
Universal length measuring machines (ULMs) are another device used for this work provided the inherent level of precision is suitable. Large, coarse pitch threads make some folks believe that a coordinate measuring machine (CMM) could do the job, but most are not precise enough, don’t provide changeable measuring forces and use spherical contacts instead of wires, all of which will produce inaccurate results.
The level of precision required for pitch diameter and major diameter of the gage is quite high so the basic device used should have resolution in the order of .00001” or better.
Thread Measuring Wires
The ASME standard for thread measuring wires over which pitch diameter measurements are taken is quite specific and very precise for size and geometry. Variations in these features have a three-to-one effect on the measurement. Substituting vee-shaped anvil devices or wires of other than the recommended nominal diameters will produce incorrect measurements. The wires have to be calibrated and the ‘constant’ applied in using them has to be based on the actual wire diameter, not some theoretical value from a chart. If this is not the case, the measured size will be incorrect.
Gage blocks that have been calibrated to fine limits are the most used masters for setting the measuring device. In some cases, companies will use calibrated cylindrical masters for setting purposes to overcome minor problems with anvil parallelism etc. In either case, the actual calibrated values of the masters are used as opposed to the ‘class’ tolerance they were made to.
The most common material for such masters is steel since the item being measured is usually made from the same material. This reduces the errors introduced by variations in thermal expansion or deformation when other materials are used.
These devices are used to check thread form at specific magnifications as outlined in the standard as well as minor diameter clearance. While many people attempt to use them for measuring the linear pitch of the gage, they are not precise enough to do so. Special instruments designed for this purpose have to be used to obtain the level of precision required by the standards.
Many measuring devices have computers and offer software so the user doesn’t have to wade through standards tables for specifications. All the user has to do is enter the size and pitch of the thread involved and the computer will do the calculations. Unfortunately, a study done a few years ago determined that approximately 15% of the calculated values will differ from the published standards which were done years before computers were in popular use. The computer programs will probably be technically correct but when such differences arise, the published standard data are all that count.
All dimensional calibration work requires a controlled environment without which the measurement uncertainty will be too high, making the measurements of doubtful value.
This brief listing should get you started calibrating thread plug gages or cause you to have second thoughts about getting involved. Either way, there’s more to it all than listed here so don’t say you weren’t warned.