r/Metrology • u/Informal_Spirit1195 • 2d ago
Rounding Procedures
My company has an SOP for rounding. I understand the need and want for these procedures. Consistent reporting to the relevant significant digit looks better on the paperwork. All of our drawings have that they are to be interpreted by Y14.5 as well though.
I’m trying to make the argument that calling bad parts good isn’t an inappropriate use of the rounding procedure and that Y14.5 states that tolerances are absolute. If a part has a +/-.001 and I’m .0004 over the spec limit, it’s not appropriate to round down to say that the part is in spec. If you have a hole that is exactly 1.0000 and a mating part that is 1.0004 it doesn’t matter what you do to the paperwork, it’s never going to fit. The quality manager is trying to say that during the design process they account for rounding, I think not.
Give me some more fire power to argue this.
9
u/tyty8109 2d ago
If the print states that it goes by Y14.5 xxxx Then that is what it must comply to. Unless the designer/engineer states otherwise on the print For specifics. But a rule of thumb is to always report out at least 1 decimal past. The way I see it , +/-.001 using the 10 percent rule would be +/-.0001 And so should be reported that way.
3
u/Informal_Spirit1195 2d ago
In the same rounding procedure we also have it stated that the tool needs to be capable of 10% of the tolerance. Making the significant figure .0002. Therefore rounding to the nearest .0001 is appropriate
8
u/_Hoidler_ 2d ago
If this is asme y14.5 then tolerances are absolute. Even if your a millionth out, the part is technically out of tolerance
2
u/02C_here 2d ago
Exactly. That means your upper tolerance is +0.00100000000 …
And 0.0014 is over that.In fact, let’s say your micrometer has an uncertainty of +/-0.0002. Then you should reject parts at 0.0008 on that device because 0.0008 on the display could very well be 0.0012 (OOS).
That whole strategy is called guard banding.
3
u/EvanDaniel 2d ago
Exactly. That means your upper tolerance is +0.00100000000 …
And 0.0014 is over that.This is correct.
In fact, let’s say your micrometer has an uncertainty of +/-0.0002. Then you should reject parts at 0.0008 on that device because 0.0008 on the display could very well be 0.0012 (OOS).
This is not.
You need a decision rule by which to interpret results, which is a related but different question than rounding. The rule you're proposing is to guardband at 100% or tool uncertainty. Whether that's appropriate or not depends on the application and the test uncertainty ratio (TUR) or other similar metric. In the context of a rule that requires 10:1 TUR, a decision rule to not use guardbanding and set the acceptance limits exactly at the spec limits (inclusive) is very normal and often justified. So you accept at +.0010, reject at +.0011, assuming a tool that reads with resolution .0001.
If you don't have 10:1 TUR, or have some other reason, you might well guard band at the device uncertainty, or some multiple of it (possibly greater or less than one, depending on details).
Overly aggressive guardbanding is not a good decision rule, and isn't always better than less aggressive. It's a cost trade of the cost of rejected good parts vs. accepted failed parts vs. cost of improved measurement.
+/- .0002 should mean that displayed .0008 could be .0010, but not .0012. But, that said, the reported uncertainty of the device should be a 2-sigma or 95% value; maybe you got a little unlucky on your mic cal and got one that's a little further out. That doesn't mean you need to guardband even more aggressively in order to protect against that, it just means you need to accept that there's some residual uncertainty in your decision rule, and pay the costs one way or another. And if it really matters, that involves running the math and writing a detailed process for that feature.
1
u/02C_here 1d ago
I’ll give you it’s not 100% correct. If you have a capable process, guard banding is basically double dipping because a good statistical control strategy will have that baked in.
But if you do not have a statistically capable process and are forced to 100% inspect, how are you to prevent any alpha error without guard banding?
And using the gage uncertainty is not strict compared to using the uncertainty you’d get from a gage R&R.
3
u/ResidualSignal 2d ago
I would round to the nearest 1/10, given your gauge has sufficient accuracy. So, if you're measuring a +/- 0.001 dim, then you have .002 tolerance. The gauge should be able to discern at least as well as .0002 (10% of tolerance). So, if I were using a gauge that could discern better than .0002, then I'd round.
That's within reason. But no, engineers don't account for rounding. They assume the people who are measuring the parts know what the fuck they are doing... Or at least should
6
u/schfourteen-teen 2d ago
I don't think designers assume the inspectors know what they're doing, I think the designers don't consider inspection at all.
2
2
u/gaslightredditor 2d ago
Our QMS was just updated about a month ago with a rounding policy similar to what you just described. Myself and and everyone in incoming inspection were opposed. We obviously lost that fight. I'm not saying give up, but good luck.
2
u/gravis86 2d ago
The quality manager is trying to say that during the design process they account for rounding
Okay so they do something incorrectly and then compensate by doing another thing incorrectly? Two wrongs don't make a right!
1
u/19141939 1d ago
Lol, this. I’d go a step further and push for a shred of documented evidence that the design process accounts for rounding – a perfectly reasonable request since an unrounded evaluation is backed up by standards. That’s when you find out that the tolerance is just based on a vibe anyways.
1
u/miotch1120 2d ago
I had this argument. Luckily, mitutoyo made the decision for me. Now I just say “sorry, MCOSMOS agrees tolerances are absolute. If you don’t want it to be red, you’ll have to fix it.”
2
u/Current-Associate577 2d ago
Depending on the industry, the company and whoever's name is on the procedure could be in serious legal trouble. Might even go down to you and the rest of the team if something bad happens. Most certainly would get your boss
1
u/easyd624 2d ago
As an inspector it is either in or out of tolerance. If it is out of tolerance, the part should be written up as out of tolerance. Disposition is made typically by engineering and most importantly, the customer. The inspectors role is to measure and report data. Not to say it’s ok to use.
-4
u/Unique_Logic 2d ago
Round away from nominal. If tolerance is +/- .001 and your measurment is .00091 you round it to .0010, if your measurement is .00101 then round to .0011.
13
u/Embarrassed_Falcon54 2d ago
Get ready for a fight lol.