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| surface profile on a spherical radius |  |  | |
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| Posted by: joebeck ® 06/18/2009, 08:56:43 Author Profile eMail author Edit |
Hi - My Quality Engineer and I are arguing about this and I wanted to get input from the forum in an attempt to squash the confusion. Basically, we have a print depicting a cross section of a sphere, with a surface profile callout of .005 (so, +/- .0025 from the spherical radius basic). My Quality Engineer is multiplying his measurement by 2, with the logic that it's a sphere. I say, he's not supposed to multiply anything. The result is the result. (i.e. if the callout is for a 5.000 spherical radius with a profile of .005, and he measures 4.999, then his profile is .001). He's saying his profile in that example would be .002. The profile callout is to the radius (half of a circle). I say it is what it is and there's no need to multiply anything. Hope my description makes sense. Anyone else have input on this? Agree / disagree? Let me know. Thanks,
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| Posted by: fadelgad ® 08/20/2009, 19:53:11 Author Profile eMail author Edit |
Per ASME Y14.5M-1994, Section6.5.2, there is no need to multiply the tolerance. The issue is that he is using a diameter and you are using a radius. For a SR5.000 the diameter will be SD10.000. However the measurement to verify will be half the difference in the measurement. Per your example a SD9.998 shows a .001 tolerance, even though there is a .002 difference from basic. |
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| Posted by: traingdt ® 06/18/2009, 11:10:46 Author Profile eMail author Edit |
If the spherical radius is given as a basic of 5.000, and the profile callout gives .005, then the actual radius can be 4.9975 to 5.0025. (This means that the full diameter of the sphere can be 9.995 to 10.005.) Now, if you measure a radius to be 4.999, then it would pass a tolerance spec on the drawing of .002 or greater because profile is presumed to be equal bilateral; this is probably what your QE is thinking and in a sense he's right. But it can be confusing, so I if find it easier to translate the profile spec into max/min limits, and then find out if the actual measurement conforms to the limits.
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| Posted by: joebeck ® 06/18/2009, 11:19:20 Author Profile eMail author Edit |
"If the spherical radius is given as a basic of 5.000, and the profile callout gives .005, then the actual radius can be 4.9975 to 5.0025." > I agree here. "(This means that the full diameter of the sphere can be 9.995 to 10.005.)" > I agree here. "Now, if you measure a radius to be 4.999, then it would pass a tolerance spec on the drawing of .002 or greater" > I'm a bit confused here. If it's measured to be 4.999, then the measured profile would be .001, correct? "I if find it easier to translate the profile spec into max/min limits, and then find out if the actual measurement conforms to the limits." > Same here. I think we're both in agreement here, just want to make sure. thanks,
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| Posted by: traingdt ® 06/18/2009, 13:27:33 Author Profile eMail author Edit |
Sorry if I confused things -- let's see if this helps... If the measured profile is 4.999, that means that there was an error of .001 to one side of the nominal. But a callout of .002 allows a deviation of .001 on each side, so the sample part can deviate no further. So your QE is right in the sense that a .001 error is at the very edge of a .002 total tolerance. It's just a semantics thing: 4.999 is off by only .001, so you were right about that, but we speak of it as equivalent to a .002 profile tolerance, because the profile numbers given on a print are assumed to be half to either side, unless otherwise specified. John-Paul Belanger |
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| Posted by: joebeck ® 06/18/2009, 13:51:18 Author Profile eMail author Edit |
Either you've confused things some more or I'm just getting more confused by the minute (probably the latter, ha ha ha). "But a callout of .002 allows a deviation of .001 on each side," > But this is a radius, not a diameter. There is no, "other side" to be concerned with. Plus, according to your last reply, if we're multiplying the result by 2, then a 5" radius with a .005 profile really means my true tolerance is 5.000 +/- .00125 in order to meet the .005 profile. (i.e. my profile callout of .005 is really a profile callout of .0025). My whole point here is that this isn't like accounting for plating on a hole or something where .001 build-up actually creates .002 smaller hole (i.e. each side of the diameter). This is merely the tolerance for the sphere size at any given point. Am I missing something? Maybe a description of the part would help? Take a globe, and divide it in to 4 cross sections parallel with the equator. Take the 2nd to last cross section (so, open top, open bottom) and that's what we're talking about. It's called a, "spherical shell". |
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| Posted by: joebeck ® 06/18/2009, 15:13:46 Author Profile eMail author Edit |
........additionally, it's already being measured all the way around (two circles top and bottom). |
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| Posted by: traingdt ® 06/18/2009, 16:33:50 Author Profile eMail author Edit |
Uh-oh now I'm getting confused  To keep it simple: It doesn't matter how the curve is defined. It could be spelled out as a radius or a diameter or a goofy mathematical formula, or just left as a CAD model (math data) with no dimensioning. Here's the important part -- wherever the perfect surface should be, the profile spec on the drawing constructs an imaginary "bandwidth" superimposed on that perfect curve. The width of the bandwidth is the exact number shown in the GD&T callout. And the bandwidth is centered around the perfect curve. So maybe sketch out a bandwidth for these next two sentences: if we decide to speak in terms of a 5" radius with a profile of .002, the actual radius could be 4.999 to 5.001 (bandwidth of .002). But if you speak of a diameter of 10" with the same .002 profile, the bandwidth is the same size, but because it's wrapped all the way around, the measurement could be dia. 9.998 to 10.002.
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| Posted by: joebeck ® 06/18/2009, 16:54:53 Author Profile eMail author Edit |
"if we decide to speak in terms of a 5" radius with a profile of .002, the actual radius could be 4.999 to 5.001 (bandwidth of .002). But if you speak of a diameter of 10" with the same .002 profile, the bandwidth is the same size, but because it's wrapped all the way around, the measurement could be dia. 9.998 to 10.002." Right on - That's what I've been saying. We are definitely on the same page here. And with that said - the same 5.000 radius with the same .002 profile callout, yielding a measurement of 4.999 equals a profile measurement of .001. NOT .002. Correct? |
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| Posted by: traingdt ® 06/18/2009, 17:49:47 Author Profile eMail author Edit |
Yes -- I think we're on the same page now. I might phrase it: a profile "deviation" or a profile "error" of .001 just so everyone knows that that's how far from nominal it is. |
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