A year ago I wrote about the value of buying setting plugs along with adjustable thread ring gages, particularly special thread rings. One point I made was that setting plugs should be used for the resetting of adjustable thread rings in accordance with the standard. Taking measurements of them was not accurate enough.

The problem starts with buyers of special rings who do not wish to pay for proper setting plugs. When it’s time for calibration, the hunt begins for someone who has plugs and of course, it’s unlikely they will be found because calibration laboratories would go broke keeping special setting plugs on hand in most cases. Some laboratories are ready to cash in on this situation by offering to “measure” special adjustable thread ring gages so the technician won’t have to buy the right thing. And, wow, the cost will be a lot cheaper than buying setting plugs.

Makers of specialized calibration devices used for such work all show calibrating thread ring gages, but the key here is that the rings involved are solid, not adjustable and invariably made to looser European specifications. We have such a unit in our lab but would never use it for adjustable thread ring gages because we know the problems that would follow.

Using these devices, simple pitch diameter is all that is measured. Errors in linear pitch and out-of-roundness are either unknown or never measured, so the “functional” or “working” size of the ring is unknown.

Rings with three slots in them will exhibit triangular lobing when adjusted. Those rings with a single slot will go oval but because the open slot is where a second reading has to be taken to determine how bad it is, the error in this regard is usually an unknown. Any adjustment to a perfect ring will result in misaligned threads at the slot.

Some folks will put forward the case that roundness doesn’t matter all that much. Maybe they’re thinking that the gage will now closely resemble an adjustable split die’s geometry, making it even better for cleaning up the thread while “gaging” it.

While a setting plug can have some of the problems noted previously, they are smaller and measurable so they can be kept within limits set by the standards. Setting a ring to them will introduce and include the errors noted earlier, but at least you know the functional size of the ring will be a close match to the plug. You never know what the working size of a ring is if it has been “measured” for simple pitch diameter only.

If direct measurement of adjustable thread rings was a practical method, you can bet every thread gage maker would have at least one universal measuring machine. They would pay for themselves in a year because setting plugs would not have to be made. Another benefit would be shorter delivery times for special rings no longer held up waiting for the plugs to be made.

Some folks promote the idea that variable gages are suitable for calibrating fixed limit gages. Simply put, the theory goes something like, if it can measure the product, it can measure gages—the only difference between them is the tolerance. My take on the theory is that it is an example of the blind leading the blind. And because these are cheaper than the measuring machine mentioned earlier, gage manufacturers would use them if they could, but they don’t because they know better.

Typical of these gages are internal calipers with two arms with ball contacts on them. They are great for measuring components but some folks are using them to “measure” adjustable thread ring gages. They cannot do this with any degree of certainty for the same reasons noted above. In addition, these gages often are set using a plain ring gage so ball roundness is added to the sources of errors in the process. And that alone could introduce errors of close to half the tolerance of the ring.

If the “measurements” you’re getting on your adjustable thread ring gages are not corrected for variations in roundness and linear pitch, for example, you’re getting less than half the story. What you don’t know could haunt you later and no medium that I know will be able to dismiss that sort of ghost of measurements past into the light.