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Rounding in drafting

Master
Master

I am in an inch drawing with say a dimension of 2.625", I put it to 2 place dimension and it rounds down to 2.62" rather than up to 2.63". Anyone have an idea how to change this behavior? Thanks

Shags72 Mfg. Eng.
Windows 10 Pro 64bit
NX 11.0.2.7 MP2 Teamcenter 11
Dell M7720 i7-6820HQ 16GB RAM,
NVIDIA Quadro P3000
12 REPLIES

Re: Rounding in drafting

Gears Esteemed Contributor Gears Esteemed Contributor
Gears Esteemed Contributor

This is the standard rounding behavior in NX (rounding to the closest even number); you cannot change it. For more info search the NX help file for the term: Round-off rules for Drafting.

Re: Rounding in drafting

Master
Master
Ok I guess, seems funny to round down from a 5 though. Thanks
Shags72 Mfg. Eng.
Windows 10 Pro 64bit
NX 11.0.2.7 MP2 Teamcenter 11
Dell M7720 i7-6820HQ 16GB RAM,
NVIDIA Quadro P3000

Re: Rounding in drafting

Gears Esteemed Contributor Gears Esteemed Contributor
Gears Esteemed Contributor

This rounding scheme helps to reduce accumulated error because you don't always round up. It is assumed that the digit before the 5 has roughly a 50% chance of being odd. In the real world this isn't a bad assumption.

 

As an example, let's say you have a linear pattern of holes, the distances between successive holes are: 0.125, 0.135, 0.145, and 0.155; the total distance (first hole to last hole) wold be 0.560. Using the NX rounding scheme, the 2 place dimensions would be: 0.12, 0.14, 0.14, 0.16; for a total (first hole to last hole) of 0.56. Should you round up each time, the distances would be: 0.13, 0.14, 0.15, and 0.16 for a total of 0.58. So if you placed a reference dim from the first hole to last hole, NX would give you 0.56, but if you added the individual dimensions together, you would get 0.58. You've thrown off the calculation by rounding up each time (accumulated error).

Re: Rounding in drafting

Siemens Phenom Siemens Phenom
Siemens Phenom

I have posted the following treatise on this subject many time, both on the old BBS and on Eng-Tips, but this might be the first time on the Community site, so here goes:

 

 Rules for Rounding Off

--------------------------------------------------------------------------------
Ever since the calculator replaced the slide rule, people have been able to get results to six or more places, therefore it's critical that we know how to round the answers off correctly. The typical rule taught back in elementary school was that you round UP with five or more and round DOWN with four or less.

SORRY, BUT THIS RULE IS WRONG!

However, please don't rush off to your elementary school teacher and read 'em the riot act!

The problem lies in rounding "up" (increasing) the number that is followed by a 5. For example, numbers like 3.65 or 3.75, where you are to round off to the nearest tenth.

OK, let's see if we can explain this. When you round off, you change the value of the number, except if you round off a zero. Following the old rules, you can round a number down in value four times (rounding with one, two, three, four) compared to rounding it upwards five times (five, six, seven, eight, nine). Remember that "rounding off" a zero does not change the value of the number being rounded off.

Suppose you had a very large sample of numbers to round off. On average you would be changing values in the sample downwards 4/9ths of the time, compared to changing values in the sample upward 5/9ths of the time.

This means the average of the values AFTER rounding off would be GREATER than the average of the values BEFORE rounding.


THIS IS NOT ACCEPTABLE.

We can correct for this problem by rounding "off" (keeping the number the same) in fifty percent of the roundings-even numbers followed by a 5. Then, on average, the roundings "off" will cancel out the roundings "up."

The following rules dictate the manner in which numbers are to be rounded to the number of figures indicated. The first two rules are more-or-less the old ones. Rule three is the change from the old way.

When rounding, examine the figure following (i.e., to the right of) the figure that is to be last. This figure you are examining is the first figure to be dropped.

If it is less than 5, drop it and all the figures to the right of it.
If it is more than 5, increase by 1 the number to be rounded, that is, the preceding figure.
If it is 5, round the number so that it will be EVEN.

Keep in mind that a zero is always considered to be EVEN when rounding off.

 

Anyway, I hope this helps.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

Re: Rounding in drafting

Master
Master

Thanks John!

Shags72 Mfg. Eng.
Windows 10 Pro 64bit
NX 11.0.2.7 MP2 Teamcenter 11
Dell M7720 i7-6820HQ 16GB RAM,
NVIDIA Quadro P3000

Re: Rounding in drafting

Phenom
Phenom

Boy, I sure wished I read your disclaimer...I did tell me daughter that she was rounding incorrectly and her teacher actaully emailed me on the topic. I could have used your arguement logic as my wasn't as elegant as yours! Needless to say, she still got the problem marked wrong!

Re: Rounding in drafting

Siemens Phenom Siemens Phenom
Siemens Phenom

That's unforgivable that they're still teaching this incorrect approach (and yet we wonder why America is falling behind other countries when it comes to the educations that our kids are getting and how they are unprepared for today's high-tech jobs).  I would suggest that perhaps you think about seeing either your daughter's teacher or someone else at the school since this sort of thing needs to be nipped in the bud..

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA

Re: Rounding in drafting

Phenom
Phenom

I won't argue with you about the need for better skills to prepare our kids for high-tech jobs. But I will have to say that is very difficult to the method that teachers are using to teach our kids. The process of rounding numbers is just the tip of the iceberg. There are plenty of things that should be changed. Remember, that every parent out there, including myself have been taught to round numbers the incorrect way. It will take a generation of the correct way to make a change!

 

I am a huge advocate of "flipped" classrooms and using technology in the class rooms. And I don't mean buying each kid an iPad mini and calling it technology! We just had a huge technology referendum pass and that was the first thing the school district decided to do. Each kids gets an iPad. Well, the school year is technically over and I asked my daughter what she used this expensive iPad mini for and she said for doing about 5 assignments in health class. That was it! See technology can be added to the classrooms but changing the curriculum and teaching methods are hard to do and not to mention time consuming for teachers (in my opinion) that are underpaid. Sorry, I'll get off my soapbox, now.

Re: Rounding in drafting

Experimenter
Experimenter

If the rounded portion of the dimension is more complex than just a 5, say 0.1452 instead of 0.145, would NX take that into account and round up instead of rounding off?