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Difference Between MOW and Ring Gages On Threads--Why?

munruh

Hot Rolled
Joined
Jan 3, 2011
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Kansas
I machine a lot of parts with OD threads in a production setting. What I mean by that is that often I am running 1,000+ parts per setup. Several times I have run into this: My NoGo ring gage will not screw on, but the MOW is is on the low side or actually a couple thousands under the minimum for MOW. This happens to me on several sizes of threads. I am using thread gages that we have checked and certified yearly. I am using Sandvik threading inserts that are the correct inserts for the thread. Any ideas?
 
I machine a lot of parts with OD threads in a production setting. What I mean by that is that often I am running 1,000+ parts per setup. Several times I have run into this: My NoGo ring gage will not screw on, but the MOW is is on the low side or actually a couple thousands under the minimum for MOW. This happens to me on several sizes of threads. I am using thread gages that we have checked and certified yearly. I am using Sandvik threading inserts that are the correct inserts for the thread. Any ideas?

I thought that the no go gage was not supposed to screw on.

CarlBoyd
 
I machine a lot of parts with OD threads in a production setting. What I mean by that is that often I am running 1,000+ parts per setup. Several times I have run into this: My NoGo ring gage will not screw on, but the MOW is is on the low side or actually a couple thousands under the minimum for MOW. This happens to me on several sizes of threads. I am using thread gages that we have checked and certified yearly. I am using Sandvik threading inserts that are the correct inserts for the thread. Any ideas?

Assuming you mean the go gage will not screw on, there are 3 possibilities I can think of, 2 involving threadform. 1) if the tool is not 90 degrees to the work, 2) if the tip of the tool has too much radius and the root is not deep enough, 3) burrs.

CarlBoyd
 
More info required. What thread type and diameter?
Are you sure you are using the correct wires?

1.375-12 thread. I've had the same problem on a 1"-12 thread and 1.125-28 thread. UNF Threads. I have the right size of wires. The Go Gage goes on fine. The No Go Gage functions normally and does not go on. MOW is right at the bottom or underneath tolerance by .001-.003.
 
I also use Sandvik full profile inserts almost exclusively.

Does the insert pass a visual check after cutting a bad thread? Only time I've seen this is when the point is gone and the operator doesn't notice.

A faulty profile on the insert is not out of the question, although I have never experienced such on a Sandvik insert. I HAVE on a few others (Vardex for one)

Assuming that the insert is not faulty, and assuming that the gauge is not faulty, the only possibilities are the root or the threadform, as the full profile insert rules out an oversize major. Pitch error should allow the gauge to screw on so far and then stop.

Maybe you are aware but the Sandvik threading system is different to everyone else in that there are FOUR inserts per pitch, not two. External left and right, and internal left and right. Most other systems just use two inserts - External right/internal left, and external left, internal right. Are you absolutely certain you're using the correct one?

There is one other possibility - your lathe may not be syncing the passes perfectly axially. The result is a wider vee - major and minor dia's remain correct, but the wires will sit lower in the thread and pitch diameter will measure undersize.
 
Suggestion: mic your wires and see if they are the size you think they are. We have found that our wires are not exactly the same size as stated on the package. When we input the actual diameter of the wires, readings make more sense.

As others have stated, the form of the thread could complicate matters. Go/NoGo gaging is good in my opinion, but it only checks how the pitch diameter seats. It doesn't check minor, major, flank angle, thread crest, etc.
 
Perhaps a simple explanation?

There are 3 PD tolerances. 1A, 2A and 3A. A go ring is basically the same for all 3 but the nogo is NOT. Use 2A on a 1A or a 3A on a 2A and you Will get what you experience.

Suggestion - get you nogo rings calibrsted.
 








 
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