Other Dimensions: Calibration Mistakes-Thread Plug Gages
Editor’s note: This is the fourth installment in a four-part series on common calibration mistakes.
Unlike plain plug gages, thread plug gages are more complex-something that will be noticed by even a casual observer. Calibrating them is more complicated as well, as there are more than a couple of simple features on them which have to be verified. The more complex the gage, the more room there is for mistakes to be made during calibration, some of which are noted here.
THE SPECIFICATIONSMost of the standards dealing with screw threads are quite detailed, so a current copy of the relative ASME B1Series document should be reviewed before calibrating gages. Many people rely on their trusty machinist handbook for this type of information, but, as noted elsewhere in this series, their data may be out of date. A common mistake in this area is to assume that the handbook information is all you need to do the job, but this is rarely the case.
These specifications often include calibration process information that is critical to doing the job properly. A mistake frequently made is to apply information on the measurement of threads to other specifications, particularly foreign threads where it does not usually apply.
THE EQUIPMENTThe most popular device for measuring thread plug gages is a high accuracy bench micrometer-type device. A variety of lower cost devices may be used, but they rarely deliver the performance required with respect to measuring forces and measuring face flatness and parallelism. A mistake that occurs often is selection of the wrong measuring force.
A comparator stand with vertically mounted measuring head or indicator may be used for this type of calibration. It often is favored because of the large worktable on which two of the thread wires can be placed when taking pitch diameter measurements.
Two mistakes made using this method are to forget that the measuring force will vary with the size of the gage and there is no way it can be adjusted. The indicator has to be replaced for one with a different load. The other mistake is failing to have the worktable checked for wear.
A critical part of pitch diameter measurement involves the thread measuring wires. Their size and roundness must be within specification in order to do their job, but a common mistake in their calibration is to verify their size while roundness is ignored.
Another mistake in respect to thread wires involves the mathematical constant required for their use. Charts and tables in various publications show the theoretically perfect constant for each pitch of wires, and many people make the mistake of using values from those sources. The correct constant is based on a calculation using the calibrated diameter of the wires and this is why, when new, the manufacturer provides this value on the wire container.
Laboratories calibrating thread wires often make the mistake of providing the wire diameter in their reports, while ignoring the most important number for the wire user: a value for the constant based on the calibrated wire size.
Other mistakes respecting thread wires used in this calibration work is encountered when wires are retained in holders that prevent them from freely locating in the thread.
At this point you must be thinking there is little more I can do to beat up on thread wires, but there is. The mistakes involve forgetting that wires in a 60-degree thread form will have their errors incorporated in the size measured on a 3:1 basis because of it.
American standards for thread gages require, among other features, that the major diameter of the plug gage be calibrated. Nothing exciting about that-it’s a relatively simple measurement. The mistake that comes into play is not following the standard that requires the measurement to be made using the same measuring force used for the pitch diameter measurement.
Another mistake in thread plug calibration is forgetting that thread angle and linear pitch need to be verified if the gage is to be properly qualified. Unfortunately, most companies will not pay for these checks so they remain a mystery that can haunt the users of them later.
All the rules I’ve noted elsewhere in this series in respect to gage block buildups as masters also apply to thread plug gage calibration. Hopefully, the mistakes associated with them will be avoided as there are so many possible which are specific to this process on its own.