Why Does a Thread Plug No Go Gage Mean Fail?

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#2
All 'go / no-go' gauges operate on the same principle. The 'go' end of the gauge should pass through, verifying that the aperture is within specification (the 'go' end is typically machined to bottom tolerance). The 'no-go' end of the gauge will be made so that it only passes through if the aperture is out of specfication (so the 'no-go' end will typically be machined to top tolerance).

If the 'go' end of the gauge passes through the aperture it shows the aperture is above bottom tolerance
If the 'go' end of the gauge does not pass through the aperture it show the aperture is below bottom tolerance
If the 'no-go' end of the gauge passes through the aperture it shows the aperture is above top tolerance
If the 'no-go' end of the gauge does not pass through the aperture it shows the aperture is below top tolerance

Hope that helps.
 
S

Shasta Ell

#3
Another related question.

When I have pieces (a bolt for instance) that fail the NoGo as they should, and pass the Go, what does it mean when they do this AND they are able to wiggle back and forth slightly and make a clicking noise inside the Go gauge? Does this mean the minor diameter is too small? Is this not a failure as well?
 
D

Dave Dunn

#4
Shasta Ell said:
Another related question.

When I have pieces (a bolt for instance) that fail the NoGo as they should, and pass the Go, what does it mean when they do this AND they are able to wiggle back and forth slightly and make a clicking noise inside the Go gauge? Does this mean the minor diameter is too small? Is this not a failure as well?
As long as your parts are going in the go gage, and not going in the no-go gage, they are acceptable. The difference in size between the gages take into account the specifications of allowable size for the part, the so-called "wiggle-room". The gages represent the extremes of the specification, therefore anything in-between them is ok.
 

Wes Bucey

Quite Involved in Discussions
#5
I think you misunderstand the characteristic being measured by the GO portion of a ring gage. The GO ring represents the MAXIMUM allowable diameter which may pass - ergo every undersized shaft will pass through the GO portion, but that's only half the inspection. If the shaft passes through the GO portion, it must also be tested on the NO GO portion which represents the minimum diameter of the shaft specification. If it passes through, it then is a NO GO. (Think trying to drop a 1/4 inch shaft through a 1/2 inch GO/NO GO ring gage with +/- tolerance of 0.010 where the diameter of the "GO" is 0.510 and the NO GO is 0.490 - obviously it will pass though BOTH Go and NO GO.)

ASIDE:
Until I saw this post, I never realized the terminology could be confusing, but now I see it can be. Do not feel bad about being confused. Many of us old timers learned by on-the-job training and may very well have asked such a question, but we always had some older old timer to set us straight. In this context, NO GO means it should not go through. Both GO and NO GO must be tested to assure the part is in spec..
 

Jim Wynne

Staff member
Admin
#6
Wes Bucey said:
I think you misunderstand the characteristic being measured by the GO portion of a ring gage. The GO ring represents the MAXIMUM allowable diameter which may pass - ergo every undersized shaft will pass through the GO portion, but that's only half the inspection. If the shaft passes through the GO portion, it must also be tested on the NO GO portion which represents the minimum diameter of the shaft specification. If it passes through, it then is a NO GO. (Think trying to drop a 1/4 inch shaft through a 1/2 inch GO/NO GO ring gage with +/- tolerance of 0.010 where the diameter of the "GO" is 0.510 and the NO GO is 0.490 - obviously it will pass though BOTH Go and NO GO.)

ASIDE:
Until I saw this post, I never realized the terminology could be confusing, but now I see it can be. Do not feel bad about being confused. Many of us old timers learned by on-the-job training and may very well have asked such a question, but we always had some older old timer to set us straight. In this context, NO GO means it should not go through. Both GO and NO GO must be tested to assure the part is in spec..
The question was about thread rings, but you're essentially saying the same thing Dave Dunn said. A threaded fastener must fit the "go" ring and not fit the "no go" ring--Dave Dunn's answer is correct, as far as it goes. There could be defects in the threads that are not detected by pitch diameter gages. Also, in the example of shafts, if the rings are calibrated to the exact specification limits, they will reject "good" parts at the upper and lower limits. In the example given, a .490 shaft won't fit in a .490 gage, and ditto for the upper limit.
 

Wes Bucey

Quite Involved in Discussions
#7
JSW05 said:
The question was about thread rings, but you're essentially saying the same thing Dave Dunn said. A threaded fastener must fit the "go" ring and not fit the "no go" ring--Dave Dunn's answer is correct, as far as it goes. There could be defects in the threads that are not detected by pitch diameter gages. Also, in the example of shafts, if the rings are calibrated to the exact specification limits, they will reject "good" parts at the upper and lower limits. In the example given, a .490 shaft won't fit in a .490 gage, and ditto for the upper limit.
True - in that regard, some pin gages and some ring gages have an additional marking of "+" or "-" to indicate the extra "smidge" to allow a 0.490 or 0.510 pin to pass through the 1/2 inch ring gage.

Without real research, though, I have no idea how the "smidge" is decided, I merely know it exists.
 

Jim Wynne

Staff member
Admin
#8
Wes Bucey said:
True - in that regard, some pin gages and some ring gages have an additional marking of "+" or "-" to indicate the extra "smidge" to allow a 0.490 or 0.510 pin to pass through the 1/2 inch ring gage.

Without real research, though, I have no idea how the "smidge" is decided, I merely know it exists.
Meyer has a good explantory page here: Tolerance of Gages. The same general principles apply to rings.
 

cncmarine

Quite Involved in Discussions
#9
Fyi

Preferred method for reporting non-conforming conditions of threaded holes that acceptance no-go plug gage.

Equipment:
• Thread plug gage
• Dial indicator with appropriate mounting base and knuckles

Instructions:
1) Select a dial indicator with an appropriate mounting base as to indicate the top of the thread gage once engaged in the threaded hole.
2) Thread the go plug gage into the hole until fully engaged.
3) Mount the indicator and register on the end of the thread gage
4) Without rotating the gage, force it towards the bottom of the thread and zero out the indicator on its end.
5) Carefully pull the gage up along the thread’s axis and measure the movement with the indicator. Note and record the indicator reading.
6) Add the noted indicator reading to the pitch diameter recorded on the gage handle. This is the reportable pitch diameter of the thread.
 

Wes Bucey

Quite Involved in Discussions
#10
Let's make it clear we are talking about plug and ring thread gages as a "screening" tool, not a measuring tool. Products at either end of the spec limits will be tight or loose, but we can only guess at the actual dimension with a ring or plug gage.

I recall using "3-wire systems" for actually measuring different characteristics of a thread versus an attribute check of whether the thread was go/no go.

I suppose vision systems can also measure the characteristics of external (male) threads. Dealing with internal (female) threads is a really difficult task, compounded when the thread hole is "blind" versus "through" because of buildup of debris in the bottom of the hole.
 
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