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Few topics in dimensional metrology will inflame the passions of a metrologist, but the subject of this column-thread gages vs. thread comparators-is one of them. In the gaging industry, when asked to choose the best method to qualify a screw thread, few ride the fence. And those who try to stay neutral become targets for both sides.
The subject is fraught with misconceptions, misunderstandings, horror stories, politics and junk metrology. The fervor can border on such a level of fanaticism that common sense is completely lost.
I'll try to rise above the fray and provide practical information so you can make a decision on which method is best for your application.
What do you want to know?
The first thing you need to know, is what do you want to know? Dumb question? No. If the primary interest is in bolting parts together, fixed-limit gages will do the job. However, if the parts are being plated and you want something for process control of the plater, a comparator could be more useful. Should the thread be employed like a lead screw in an assembly, you may want more information than that provided by fixed-limit gaging of the mating parts.
A comparator is automatically needed if the thread is not particularly critical, if it is going to be a long production run or if variable data for statistical process control (SPC) is wanted. Comparators are more helpful when setting machines that will be single-pointing a thread so they can be set where wanted in the tolerance band.
Of course, if it is a short run and the data is to gurgle through a computer as well, then a typical comparator system may cost too much. It may be better to use thread measuring wires and a micrometer, but that requires someone to carefully take the measurements. You may not have that someone, or if you do, he may not have enough time in the production cycle. You could try an end run with pitch micrometers, but they have their own problems and so we'll leave them out of the debate.
At this point it can be seen that it is not a matter of which system is ‘best,' but what type of information is needed to control the process and qualify the end product and, of course, what tolerances are involved.
Comparators, being what they are, require a threaded setting master to function. They also require a set of rolls for the pitch of the thread being inspected. These rolls can be used for left or right hand threads and for preplate dimensions. They can also be used for a range of sizes. Segment type contacts, or those used for internal threads, are size and pitch dedicated and not as versatile. Some argue that comparators are more accurate than fixed-limit gages that rely on the operator's ‘feel' when using the gage. Others claim comparators are easily misread or can cause false rejects because of dirt and cuttings in the thread.
Those in the fixed-limit gage camp say that most comparators do not replicate the mating part when roll contacts are used and only marginally so when segments are used. Thus, the numbers obtained from them do not give a full picture. Simplicity of use and a lower initial purchase cost are cited as benefits of using fixed-limit gages. In addition, they can take more abuse than comparators.
As far as I'm concerned, both methods have their benefits and disadvantages, so I would make my decision based on how much information is really needed. Sometimes a combination of the two is a good solution. And I most definitely would not perch indicators with 1 micron or 50-microinch resolution on comparators when the uncertainty of their results could be up to 10 times that or greater.
Now that I've set myself up as a target, I'm going to hide under the desk until the shooting stops.