Once again, it’s time to talk about tolerance.

Time flies when you’re having fun-something I was reminded of when I decided on this month’s topic. About 10 years ago I wrote about the subject of gage users, assuming that the gagemaker’s tolerance belonged to them rather than the gagemaker. At that time I called the column “Stealing Tolerances,” and felt that I had done my duty to sort things out.

What brings the subject up again is the fact that too many gage users still think the same way. The supplier is asked to provide gages in the top half-or less-of their tolerance so they have “room” to wear. What is really being asked is for the gagemaker to give up half or more of the tolerance the specifications allow without any price increase to cover the costs in doing so.

For this to happen, the gage involved has to be made to suit and this makes it a one-off special. Alternatively, if it’s a stock gage, someone has to take the time to measure a quantity of items from inventory in the hope of finding what the customer wants and that takes time. In either case, time is required on the shop floor or in the lab and time does cost money.

Since thread gages in particular are so detailed in their relevant standards, everyone-okay, most folks-just reference the thread spec on the part drawing or gage record and call it a day. Few deal with the actual specifications or add a factor for wear. The absence of this information means the default acceptance tolerance is the same as for a new gage.

Following this line of thinking, a gagemaker can correctly supply a gage that is on the bottom limit. But after little use, it wears a tad outside this limit and if this is discovered, the gage is considered scrap because “new” tolerances are being applied. It’s rather like getting rid of your new car because the tires have worn a bit.

Engineering and quality should review thread applications and determine how much of the product tolerance can be given up for gage wear beyond the new gage limits. This could be in the form of a percentage or all of the gage tolerance or a range of dimensional values based on thread sizes.

Manufacturing will balk at the thought of losing some of their tolerance-just like the gagemakers do when it’s their tolerance being whittled away. But it will all become academic in the future. Why? Because the folks who prepare the standards involved are aware of the problem and will be working towards changing dimensions to include a wear allowance plus the gagemaker’s tolerance. This type of standards overhaul will take a lot of time, considering the magnitude of the industry.

When I first commented on this subject I noted that it was going to be dealt with in the near future, but ten years have gone by and it hasn’t been yet. I can’t complain too loudly about this because I sit on a couple of these committees; I know how difficult it can be to make significant changes and cover all the various interests involved. The changes will come in due course. In the meantime, readers should review their applications as noted earlier and establish wear allowances they can live with.

Now, just to make your day more interesting, remember that measurement uncertainty has to be considered when talking about calibration to determine the state of wear on your gages. In the case of thread plug gages, it could mean that even though you’ve requested a gage to the high end of the gagemaker’s tolerance, by the time uncertainty is taken into account, the real size of that gage could be at or near those limits.

If you want a wear allowance on your gages, you should discuss the situation with your gagemaker, who can advise you on different ways this can be done. This can ensure you get the maximum amount compared to a little when you’re attempting to “borrow” it from the gagemaker’s tolerance.