Drafting dimensions rounding up/down 5

Per the help files:
"The Drafting Application uses the following standard rules for rounding off digits when a value is to be rounded to fewer digits than the total number available.
If the numbers after the required precision (number of decimal places) is less than 5, then there is no change in preceding digits (round down). For example, 2.4634 rounded to three decimal places would be 2.463.
If the numbers after the required precision is greater than 5, the preceding digit is increased by 1 (round up). For example, 4.37652 rounded to three decimal places would be 4.377.
If the number after the required precision is exactly 5, round off to the nearest even number. For example, 8.36500 becomes 8.36 when rounded to two decimal places. 8.35500 also becomes 8.36 when rounded to two decimal places."


This is per an accepted industry standard, but I don't recall the number, and doubt if there is a way to switch it.

OK, I found the following explanation a couple of years ago and I think it makes a good argument for why the current accepted practice, particularly when it comes to engineering, to 'round' using the scheme that we have supported for years with NX/UG, is the correct approach:


Rules for Rounding Off

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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 in 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 preceeding 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.


I hope this helps explain why it's now considered not only appropriate to 'round' in the manner in which NX does, but mandatory if we wish to maintain as accurate a presentation of a models size and shape.


John R. Baker, P.E.
Product 'Evangelist'
UGS NX Product Line
SIEMENS PLM Software
Cypress, CA

John,

Thanks for the explanation, it makes a bit more sense now. I can see why Drafting rounds numbers to the nearest even number when they end in 5, but it has caused quite a lot of head scratching. Also, other software packages like Excel just round up when a number ends in 5.

Is there some way in a future release of NX of being able to choose how NX rounds up numbers?

Thanks,

Matt.

John,

I know that you have had to answer this particular question on a number of occasions, but this is the first time that I have seen the explanation (bold section) in one of your responses.

Enjoy your bright shiny new star !

Thanks for ALL of the help that you provide to us!


Chris Cooper
Senior CAD Specialist
Cleveland Golf / Never Compromise

Matt,

That isn't just John's explaination of the way UG/NX works, it is defined by an ASME standard.

I would certainly hope that Siemens PLM Software would NEVER allow such a request to be entered into their ER database!

What you may want to do is submit an enhancement request to Microsoft to enhance Excel to allow an alternate engineering rounding rule.



"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

Chris,

I ran across it a couple of months ago in some papers that I had in a file drawer. I had forgotten about it, but when I saw it again I made a mental note knowing that it was probably going to come up again, and sure enough, it did. So you're right, this is the first time I've used it, but I expect it will not be the last ;-)


John R. Baker, P.E.
Product 'Evangelist'
UGS NX Product Line
SIEMENS PLM Software
Cypress, CA