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Posted by: mbend

03/04/2009, 22:20:01

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I need clarification as to the use of a plus minus ping gauge set. Say if I had a hole with a tolerance of .195 +.001 -.000. What would I use for the Go gauge and what would be the no go gauge? I keep getting conflicting opinions. Some say the go would be .195 plus and the no go would be .196 minus and others say that the go would be .195 plus and the no go would be .197 minus. Which is correct







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: gauge pin plus minus -- mbend Post Reply Top of thread Engineering Forum
Posted by: randykimball

03/05/2009, 14:36:16

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It is fine to use pin gauges to use to check locations of holes if you fit them to the holes. If you test a +.001 / -.000 hole for size tolerances with a pin gauge you stand a good chance to hang the pin or damage both the pin and the hole. If / when I do the check you are posting I use .1950 as a GO and a .1958 as a NO/GO pin. This assures the hole is within tolerance although it actually will reject holes that are at the extreemes of but are in tolerance. My method provides a safety margin for error coverage, .. however, I by far prefer the below method.

To check a hole of this size and tolerance it is wise to use an air gaging system. In simple terms, with an air gaging system a pin of a few thousands under the hole size is inserted into a master ring gauge of the correct size. Air is passed through the pin and out two small holes aprox .2 from the end (one each side of the center) and an air pressure gauge is calibrated accordingly to the results by reading the pressure drop. The pressure of the supply air is carefully regulated and calibrated. The tighter the fit the less the pressure drop. The size of the hole is read on the pressure drop gauge after calibration. Many holes can be checked quickly and nothing ever touches the hole. These air gaging systems sets are VERY accurate and can provide up to a .000005 resolution.





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Modified by randykimball at Thu, Mar 05, 2009, 15:13:21


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Posted by: traingdt

03/04/2009, 23:46:00

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Basic answer: the go gage pin would be .195 and the no-go gage diameter would be .196.

But if you're asking about tolerances on the gages themselves (sometimes you'll hear about a 10% tolerance for the gage) then that gage tolerance would be on the positive side of the above numbers, for the go gage pin and also for the no-no gage.








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Posted by: rikas

03/05/2009, 03:22:27

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hello,
if we dont consider the positional tolerance, what u say is right. If the hole has a pos.tol., then virtual boundary shud be calculated which is the go gage size.

sir, i need clarifications in GD&T, as ur name is traingdt i think u cud help me.








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Posted by: rwolfejr

03/05/2009, 09:35:44

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Go - NoGo Gages cylindrical gages are made to 5 standard tolerances. (See ANSI B1.2-1983)

You need some allowance for encroachment when deciding on a gage diameter.

If you have a perfect .1950 hole you won't be able to insert a gage that is also .1950. Not by hand anyway. So if you want to take advantage of the entire tolerance window, you make your gage "Go" .1948 or .1949. The size you select will depend on the application and machining methods. Same goes for your No-Go. If it is fairly critical but you need to check quickly you won't be able to fit a .196 pin into a .196 hole but it might try to start into a .1961 hole.

When you tighten up your tolerance window... temperature starts to become more critical. A hundred degree swing will move your .195 by .0001. The international standard temperature is 68 degrees F.

I see some crazy tolerances on non critical dimensions all the time. They probably come from folks with no machining experience or no understanding of what processes are required to attain higher levels of precision. It's easy to draw the perfect part but it can be another thing to build it.

If it isn't critical I'd go .1948 Go and .1962 No-Go.








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Posted by: traingdt

03/05/2009, 10:15:14

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rikas: If the gage pin were meant to check the position tol., then yes, that virtual boundary would have to be considered, and a more elaborate gage with walls representing the datums would be required. But it sounded like he was only asking about a gage pin for size. (If you have GD&T questions, feel free to post them as a new item in this forum. Whether it's a general question or very detailed, that's OK.)

rwolfejr: I don't have the ANSI B1.2 standard in front of me, but can you clarify something? I agree with you that a .1950 gage pin won't fit into a .1950 hole, but wouldn't a .1948 go gage allow a bad part to perhaps be accepted? (A hole of .1949 and pin of .1948 will fit.)

Since the borderline part of a hole at exactly .195 is tricky, I've always heard -- at least in my work in the automotive industry -- that we should err on the safe side: it's better to reject a good part once in a while (if the hole is .1950) rather than accept an occasional bad part of .1949.

That's the reason for my initial answer, but maybe I'm mixing apples and oranges in this discussion...








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Posted by: rikas

03/06/2009, 01:06:04

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hello all,
this topic is very informative for a young engineer like me.
i come across this satement "Funcional gages cannot be designed for features specified at LMC".
i need to understand this...?







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Posted by: traingdt

03/06/2009, 22:45:33

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Fixed-size functional gages only work for features modified with the MMC symbol because that gage is really checking the "virtual condition," which is a constant value only when the MMC symbol is invoked. Think of what the M symbol does: it means that the geometric tolerance gets larger as the feature size deviates away from the MMC value. The virtual condition is a formula that combines the feature size and the geometric tolerance.

Let's use the example of a pin with a diameter of 10 mm +/- 0.3, and a position tolerance of 0.1 at MMC. The virtual condition formula is: VC = MMC + geo tol = 10.3 + 0.1 = 10.4.

But if we make another pin at 9.7, realize that a "bonus" factor adds into the position tolerance. Thus, that 9.7 pin gets a position tolerance of 0.7. Well guess what: 9.7 +0.7 equals 10.4, which is the same virtual condition in the paragraph.

This all shows that the MMC symbol results in a constant value for the virtual condition, so we can create a constant-size functional gage to check the position.

You had asked about LMC -- that does NOT result in a constant virtual condition that a gage can mate with. Example: suppose the pin described above still had a size of 10.0 +/- 0.3 but a position tolerance of 0.1 with the LMC symbol. The functional gage that we would try to create for the smallest pin of 9.7 would have to be 9.7 + 0.1 = 9.8.

The gage for the largest pin must be 10.3 + 0.7 = 11.0. (This is because the stated position tol of 0.1 is tagged with LMC, which was 9.7, but our bigger pin picks up a corresponding "bonus" on the position tolerance).This value of 11.0 is NOT the same value as the virtual condition in the paragraph, and therefore a single-size functional gage cannot do the job.

Sorry for being so wordy -- a sketch might help as you reread the explanation. Again, the key is to realize that the functional gage is checking virtual condition.







Modified by traingdt at Sat, Mar 07, 2009, 19:02:24


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Posted by: rikas

03/09/2009, 02:17:00

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correct explanation..In ur example, when LMC is specified, 9.8 is allowed to have 0.1 pos.tol. but it can move 1.4 as virtual condition is 11.0. i got struck at this point...thank you so much...







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Posted by: Kelly Bramble

03/05/2009, 14:11:12

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In practice I have seen features inspected with gage pins on both sides of the MMC boundary. Depending on application, a gage pin at .1952 may be just fine. I have seen many applications where the inspection required a .1948 diameter pin for the sizes indicated.

Ultimately, judgment is the word here, I personally cringe when I see a feature rejected for being over size by 0002". In general, most folks over specify tolerance requirements (too tight) anyway.

As everybody knows, smaller gage pins will bend with applied force over long distances. Verifying dimensional conformance using a gage pin is a touch and feel thing.







Modified by Kelly Bramble at Thu, Mar 05, 2009, 14:11:54


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Posted by: rwolfejr

03/05/2009, 16:10:37

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Traingdt... Randy and Kelly... and the fellow who initially asked the question.

Regarding... "but wouldn't a .1948 go gage allow a bad part to perhaps be accepted? (A hole of .1949 and pin of .1948 will fit.)"

My bad...
Technically yes. But the person doing the gaging would have to work at it to get the pin in, even with 2 tenths clearance. As Kelly mentioned... using a gage pin requires a feel. We use a ZZ tolerance fit on most of our ring and plug gages to avoid some of what Randy mentioned. This tolerance class has the largest allowance of the lot. We choose this class for our parts because we are working with larger diameters and it keeps our guys from fighting the gages. Try taking a perfect 2.500 diameter pin or ring gage and mating it up with a .00032 clearance part and you'll see what I mean. Most folks won't fight it that hard to "make" it a good part. The .00032 is the ZZ encroachment allowance for parts ranging from 1.51 to 2.51 diameter.

Regarding "at least in my work in the automotive industry -- that we should err on the safe side: it's better to reject a good part once in a while (if the hole is .1950) rather than accept an occasional bad part of .1949."

You're right. Statistical process control frowns heavily on running at the edge of your limits... especially in the automotive industry. For the parts we produce for automotive... heavy truck specifically... we gage to error to rejecting a good part as well. This keeps us under an allowable bell on their charts. As was mentioned... It's a judgment call. (Oops... did I say judgement? I think that's like swearing in the world of ISO and SPC and all!!)

Randy you've got my attention on the air gages. Going to look those up after I type this. Thing that pops to mind right off on those would be... What effect might surface finish have on that type of gage and I'm figuring it might not be accurate at the entrance and exit of a hole?








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Posted by: traingdt

03/06/2009, 00:15:58

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Thanks for the follow up! I hope you don't think I was being critical -- I just wondered if there was something written in stone about all these gaging sizes.

It's true that sometimes it's a judgment call, and it's also true that many parts are overtoleranced. I've been in engineering and manufacturing almost 20 years, and sometimes I'm still amazed that we all build the darn things that we do, what with all the smoke and mirrors that sometimes get utilized in quality checks!








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Posted by: randykimball

03/05/2009, 16:52:42

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Kool...

"Randy you've got my attention on the air gages. Going to look those up after I type this. Thing that pops to mind right off on those would be... What effect might surface finish have on that type of gage and I'm figuring it might not be accurate at the entrance and exit of a hole? "

Yes, surface does have an effect but not as much as you would expect. A hole worth checking with an air gauge system had best already have qualified for surface roughness. No.. you can't check the edges of the hole... which is a huge concern after processing, another good reason to toss out the near limit toleranced product. (or tighten tolerances.... depends on how you operate)





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