Prisoners of Precision
Precision costs money.
Readers of this column are a gentle lot and not likely to run afoul of criminal law but can end up in a similarly frustrating situation that limits their working life. Instead of steel bars restraining them, specifications on setting masters will lock them up and they will only have themselves to blame. The purpose of this column is prevention so you don’t become a prisoner of precision.
The mistake too many folks make is specifying tolerances for dimensional masters that are not realistic thus causing escalating despair and usually increased costs.
Theoretically, the more precise a master is, the better it is, but along with that, the more expensive it will be. Sometimes a tolerance is specified for a master without looking at the direction the tolerance specified will take. For example, if you want a master that is within 20µ” or .0005mm of the specified size, a chart in a gage maker’s catalog will tell you it is XX tolerance. But those charts represent unilateral tolerances ie; all one way plus or minus from the size you want. If the gage maker knows the tolerance is for a master, they will split it in half and apply it bilaterally so if you had specified an X tolerance of 40µ” or .001mm, you would get what you want at a lower cost.
There are more cost increases to all of this beyond the purchase price of the master. Overly precise masters will wear out of limit faster than need be so you’ll be replacing them more often. Such masters are a waste of money if they will be used in an unstable temperature environment or the parts being measured are literally hot off the machine that produced them.
Precision costs money. The more precise you want a master to be, the more it will cost you to get it, but there is a way to reduce the cost: order a lower tolerance but have it calibrated and use the actual calibrated size for setting purposes. This may require any difference between the desired size and the actual size to be offset when adjusting the instrument it is used with. Some digital instruments will allow you to program the zero setting function for this so all the user has to do is push the ‘zero’ button with the master in place and the offset will be accounted for and all readings will be variations from your desired size.
Some folks want masters made to closer tolerances than they need because they suspect the geometry of the master will be better and thus cut down on minor setting variations. While there is some truth to this there are no guarantees. The rule of thumb for plain ring or plug masters used to be that geometry would be held within fifty percent of the diameter tolerance. It would certainly hold true when comparing a ground master as opposed to one that has been lapped to bring it into tolerance but if the lapping is not done properly, the surface finish may be improved but not the overall geometry.
You should also keep in mind that the resolution of the instrument the master will be setting should be in keeping with the tolerance on the master. Overly sensitive instruments can drive inspectors crazy with dancing digits the magnitude of which may mean little in the grand scheme of things.
Our ability to measure masters to high orders of precision has limitations despite the tolerances often quoted in gage maker’s catalogs. This is due to measurement uncertainty of the calibration process being too high compared to the requested tolerance. This occurs most often with setting rings to XX tolerance which leads to battles over what the ‘size’ actually is when two different labs calibrate them. This should explain why if you ask a gage maker for a master with zero variation from the specified size, you may discover that your order will be declined. ‘Zero’ uncertainty is not achievable by any calibration laboratory including NIST.
If you have been imprisoned by some of the problems noted in this column you could always ask for a re-trial or parole to reset the situation and have your lawyer use this column when pleading your case. Take care and I’ll see you when you get out.