Other Dimensions: Grunt and Groan Gaging

June 4, 2012
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There are ways to reduce variations between gage users.



Like many gage manufacturers, my company has an extensive library of specifications for plain and threaded go/no-go or fixed limit gages. These documents contain endless charts of data and drawing details that you would think cover anything even remotely connected to such devices, and they do for the most part. However, missing in all of this is the most critical detail: how to use them.

Oh, yes, they do explain that if the go gage enters the part while the no-go does not, you’ve got a keeper. The names say it all but rarely describe how to get there. Sure you “test” the part with the gage or engage the two (no pun intended) but when you get to the “how” part, what started out as a simple but effective system often becomes a nightmare. This is especially so when the gage is rejecting some of those perfect parts you’re making.

It usually comes down to how much force or torque should be applied to a plug or ring gage. Since the gage is a decision-making device and everyone is looking for a “good” decision, the process may be accelerated using brute force, hammers, wrenches or other tools. There are automated systems that include controls for the grunt and groan aspects to it all but most gages are manually applied, meaning unaided hands should provide the only forces involved.

Since most hands are connected to people they vary considerably in strength and determination, thus putting a variable in a process that shouldn’t have one. The only stable part of the game may be the weight of the gage involved, which is useful for plain gages but of lesser value for their threaded counterparts. Despite some of the problems involved in their use, there are some ways to reduce variations between gage users, as I hope the following will point out.

Having a death grip on the gage practically guarantees that too much force will be used. Where practical, let the weight of plain plug and ring gages provide the force. When the hole being checked is not in a vertical position use fingertips to hold the gage in the same way you hold a pen or pencil. When larger gages are involved, use the fingers on one hand to support the gage and the tips on the other to apply it to the work. Similar rules can be applied when using plain rings.

When very close tolerance work is being checked, you may find it advantageous to have a large chamfer or specially made pilot on the gage to assist in aligning it with the hole.

Gravity can be helpful when using thread gages, but only the fingertips should provide the torque needed to engage the thread for most applications. Gravity is very useful when go/no-go plain snap gages are being used; in fact, that’s the only force needed for most applications whether setting the gage to gage block build-ups or using it to check components. The gage is applied vertically with the frame of the gage on top. The fingers are used to suspend the gage over the component and guide it. The fingers slowly release the gage while the weight of the gage causes it to contact the component and pass over it-or not.

When a snap gage cannot be applied vertically, the same cautions used for plain ring gages being used horizontally need to be observed. While the snap gage frames are usually shaped for maximum gripping power, it’s that maximum power that causes wide variations in results between users. To make matters worse, the application of too much power can cause frame deflection, adding further errors to the results.

Training is the key to consistency but few people receive any training beyond, “Stick it in the hole. If you can’t, it’s no good.” People have to be reminded that gages are for checking, not machining the part feature even though this can occur with some materials. One way to get consistency with fixed limit gages is to show by example using your gages on your work. Better yet, take some photos of various gages being applied to the work so folks know the way to hold and apply them to a part feature. Set aside an area on the workbench for the gages so they don’t get dropped onto other tools and equipment and get damaged in the process.

As usual, I leave the most critical element to the last: remove all hammers and wrenches from any area where gages are being used.

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