Don't ignore the measuring force.
In order to simplify pitch diameter measurement of products and gages, a number of the variables that could affect the results are stipulated in the standards. One of these is the diameter of the measuring wires that are used, as noted in last month’s column. 
One of the more critical elements in the process is often ignored and that is the measuring force to be used. You may get away with this for some product measurements, but it can make a big difference where thread gages are involved and the tolerances are much tighter. Despite this, I have seen a number of gage manufacturing operations where it is dealt with improperly and could cause errors of more than one third of the pitch diameter tolerance. 
The measuring force situation regarding thread gage calibration has come up for discussion by those in the business from time to time over the years but, in most cases, no one had any data to refer to. It was always much easier to do what the standard called for so you didn’t have to worry about it. 
Incorrect measuring force can be a major source of errors in pitch diameter measurements.
I was never happy not knowing what to expect when the force involved in a pitch diameter measurement was not as required in the standard. About ten years ago I decided to see what happens. I was looking for orders of magnitude as opposed to precise values and organized some simple tests in my lab to find out. The results more or less confirmed what many experienced gage makers and inspectors suspected.
Each standard thread pitch has a measuring force linked to it in the standards so I used that as the starting point or “zero.” For example, an 8 pitch thread plug gage was calibrated using the prescribed 40 ounces of force. The measurement was repeated using 16 and 8 ounces of force and the differences noted. At 16 ounces, the pitch diameter reading was 0.00010 inch larger and, as would be expected, when only 8 ounces of force was applied it measured a bit bigger at 0.00012 inch. Similar results were observed for 16 pitch measurements.
The same test was done with a 40 pitch thread gage only it was set up with 16 ounces of force for the starting point. Changing the force to 8 ounces and then 4 ounces meant the readings of pitch diameter increased 0.000035 inch and 0.000085 inch respectively.
Going in the other direction, a 28 pitch gage with 16 ounces of force became the base dimension and the force was increased to 40 ounces and then reduced to 8 ounces. When the force was increased to 40 ounces, the reading of size was reduced by 0.00009 inch while it was increased by 0.00004 inch when only 8 ounces was used.
For consistency, the inspectors who did the work made sure the wires were applied in the same manner at the same location on the gage. The measuring forces were created by dead weight loading and were applied horizontally so the weight of the gage did not interfere. This setup is similar to how most bench type micrometers are used for this work except they rely on an adjustable spring mechanism to deliver the force. 
The results of this study make it clear that incorrect measuring force can be a major source of errors in pitch diameter measurements. In some cases, they can equal 50% of the tolerance on this feature. 
It’s true that much of this is due to the significant differences in the forces used. Sometimes, however, a little difference in the force can produce a significant variation in the measurement. For example, a 4 ounce difference measuring a 40 pitch thread will produce a variation of 0.00005 inch. When measuring close to gage limits, that’s all it would take to cause a false accept or reject decision.
If you follow the standards regarding measuring forces you should not have any problems. But be sure the device you are using is delivering the correct force. This means that calibration of your equipment should include verification of the measuring forces it is supposed to provide, especially when that is achieved using a spring type mechanism.
After all, if you want the force to be with you it will only happen if the galaxy and the earthly elements are aligned with you in your quest.