Other Dimensions: Growing Gages
October 28, 2009
Despite the implications of the title, this column is not about planting seeds and having some gages grow in your garden. In a recent column I mentioned that steel gages may grow after manufacture and that is one reason why a calibration lab’s data on them may differ from one year to the next.
Two readers e-mailed me about this-one from as far away as Brazil-asking for more information, so I thought it might be worth taking a look at some of the things that cause gages to change after manufacture.
Some gage users would be quite pleased to have gages that keep growing in size to negate wear. Unfortunately, it’s not a controllable situation and will result in your no-go plug gages going oversize on a continuous basis. You win some, you lose some.
How much are we talking? It will vary, but it could be 0.0005 inch or more on a 5-inch diameter plug gage. When it comes to gage blocks, the change in length per inch of length could range up to 2 millionths of an inch per inch in length or 0.05 micron per 25 millimeters in length per year. Flatness also can be affected by up to 3 millionths of an inch or 0.07 micron. The metallurgical composition of the blocks or gages and the way they have been hardened is where the problems come from.
Both of these situations are known so steel makers are onto the problem. With so much of this equipment coming from unknown offshore sources these days, the problem could become more prevalent than in the past.
I’m not a metallurgist so I won’t be treading deeply into those waters. But I am told that retained austenite that has survived the heat treatment process is the culprit.
Where gage blocks and gages are concerned, special steel formulations are used with stability in mind. Careful heat treatment also is a factor and after that, deep stabilizing.
The stabilizing process calls for them to be taken down to sub-zero temperatures for a period of time then slowly returned to ambient temperatures. They are then brought up to a high temperature for a period of time and then allowed to slowly return to normal. This cycle is repeated about three times.
These problems can arise with no external causes, but sudden, extreme temperature changes or physical shocks could trigger them as well. This would suggest that you don’t beat up your calibration source if the numbers are moving around a bit each time the corporate jewels are calibrated.
Modifications made to gages also can change their size no matter how gently your toolmaker grinds steps or flats on them. This is especially so when the gages are case-hardened and the modifications go below the case or hardened skin of the gage.
This is why all size bets are off if a gage has been modified after manufacture.
Uneven modifications to gages can put them out of square or parallel, causing size variations. This is especially so with ring gages. Often, rings are face ground to eliminate a worn diameter at one face for close-to-shoulder gaging. This will send them one way. If you grind the opposite face, they’ll move back the other way, but there are no guarantees as to where they’ll end up.
These problems are one reason gagemakers don’t like to do modifications on items they didn’t manufacture. They don’t know the exact material they’re dealing with and what sort of treatment it has received. What appears to be a simple modification could destroy the accuracy of the gage.
Another situation that gives the impression that a gage is growing occurs most often with thread gages when soft materials such as aluminum are being checked and the gage is forced to engage the work. Some of the product material adheres to the gage like plating so technically it has physically grown in size.
One step you can take to avoid these problems is to know the source of the gages and whether they deep stabilize them. Get them calibrated on a regular basis and review the results for signs of growth. Last but not least, protect them from physical damage or thermal shocks.