Other Dimensions: Pushing Your Luck
July 29, 2010
The nice thing about fixed limit or go/no-go gages is their simplicity. They are easy to use in pretty much any situation, check all the basic features to ensure mating parts will assemble and are relatively inexpensive. Of equal importance is that very little training is required for them to be used effectively. This latter characteristic often is taken to mean no training is required so anything goes when it comes to applying them to the work.
Too often it becomes a matter of issuing tools and gages to the shop and then sitting back waiting for the magic to happen. The parts get made, everyone swears they passed the gages so all is well, but, like all magic, the end result is really an illusion. How they were passed by the gages remains unseen until parts don’t go together, and since that happens further down the road, assigning blame can be a problem.
The folks machining the parts understand that the go gage should enter the part and the no-go should not when it comes to checking holes. Unfortunately, if the go gage does not enter the part, it means that it is scrap-something no self-respecting machine operator wants to experience. This is not a major problem for small, cheap parts, but large, expensive parts are another story and could cost the machine operator his job. Faced with such a problem, some folks figure that all the go gage needs is a little assistance to accept their “perfect” part.
“Assisting” the gage to do the right thing may take many forms, including the use of brute manual force to push the gage into the work. On other occasions, mechanical assistance in the form of a hammer may be employed. Remember, the operative word is “enter” so for some, as long as that occurs the hole is good. When thread plug gages are involved, it is not uncommon for wrenches to be used to attain this state of bliss.
The gage handles will usually show marks indicating the degree of assistance used. In some cases, the ends of single-ended gage handles look like a mushroom due to repeated assistance being applied. Occasionally so much assistance is provided that the gage locks up in the workpiece and the handle splits open, rendering it useless. Help may be needed to remove the gage.
Battles ensue over just how much force should be used with plain gages and torque when the discussion moves into thread gage territory. Unfortunately, there are no controlling devices for these elements built into manually used gage handles so the battles are rarely settled.
One suggestion to obtain uniform force is to use the weight of the gage. This works reasonably well when the hole being inspected is vertically oriented. When this is not the case, careful handling to keep force to a minimum is required. One way to do this is to ensure the gage is held like a pencil-you know, those wooden sticks with a graphite rod in the middle used for writing before computers came along. Holding the gage in this way reduces the amount of force that can be applied.
When large gages are involved, one hand can be used to support the gage while the fingers of the other hand apply it.
Checking threaded holes provides similar problems, but you can’t use the weight of the gage to act as a fixed force in doing so. All you can do is hold the thread gage in a similar manner to a plain plug gage, keeping two or three fingertips on the body of the handle so the torque generated by the operator is limited.
Another argument when it comes to thread gaging is, “How many turns can the no-go member enter the product before it should be rejected?” The thread standards allow for up to three turns, but your company may have another policy, particularly when the number of threads being checked is quite low.
Whatever your situation, you should establish a policy in these matters and ensure it complies with your customers’ expectations or policies if they have any.
One thing is certain: using tools such as hammers or mallets to assist in the gaging process is definitely a no-no and all operators should be told this.