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Round off rules

Round off rules

Round off rules

(OP)
What round off rules do you all prefer for decimals? If the decimal ends in 5 we have always rounded up if it comes after an odd number and down if it is even.

For example, a length is 1.125 inches. If from the start I intend it to be a 2 place decimal to use default tolerances, I draw it as 1.12. However, if later on I change my mind I can either go back and redraw it at 1.12 or accept the cad default of 1.13. Unfortunately, that bugs the hell out of the checkers because it breaks the rules.

With Autocad or Solidworks I don't think you have the option of changing the rule. I would just as soon take the default as long as it does not affect the function of the part.

(Related to this, you would not use .70 for a dimension when you could use .69, because it is equivalent to 11/16. I don't see a lot of reason for perpetuating this.)

RE: Round off rules

There is a standard covering this.  It used to be an ANSI standard, but now may be a ASME standard.  Sorry, I don't know the standard number.

RE: Round off rules

Round off rules are covered by an ASME standard when it comes to engineering drawings. The dimensions is always rounded to the even number.
If your CAD system doesn't conform to ASME drafting standards, get a better system.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

(OP)
What is a better system than Solidworks?

There might be a way to change it but I haven't found it yet. Still learning.

RE: Round off rules

I was always taught to round down for 1,2,3 and 4 and round up for 5,6,7,8 and 9, as far as I am aware all CAD systems do the same.

RE: Round off rules

A quote from UG help documentation (which follows the ASME standard):

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.

RE: Round off rules

According to ANSI Y14.5M - 1982 section 1.6.4 conversion and rounding of linear units....see ANSI Z210.1

I'm not sure if it's covered in ASME Y14.5M....my copy is at home since my company hasn't updated.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 4.0 & Pro/E 2001
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      o
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"There is no trouble so great or grave that cannot be much diminished by a nice cup of tea"  Bernard-Paul Heroux
 

RE: Round off rules

ASME Y14.5M-1994 section 1.6.4 Conversion and Rounding of Linear Units.
For information on conversion and rounding of U.S. customary linear units, see ANSI/IEEE 268.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

EngJW,

Im not sure that I correctly read your statement about not using .70 when .69 equates to 11/16. But,if I did,I would disagree with that statement.  Also with regards to the length of 1.125.  How did we arrive at that when 1.20 would be easier to deal with?

I have been retired for a few years now, but I seem to recall a Standard that we should design to decimal parts of an inch rather that fractional equivalents.  Has that been superceded or just merely forgotten and discarded.  It certainly seemed to make sense at the time?

It would tend to eliminate some of the confusion with the rounding up or down also.

RE: Round off rules

(OP)
Mr. Ringman,

Most of our old drawings were based on fractions. If I design a part based on the old one and see an 11/16 dimension, I usually round it off to two places and make the new part .69. Our default tolerance is plus/minus .010 for two place, so I make everything two place unless more precision is needed. Now, for a totally new part there would be no reason to use .69 when a nice round number of .70 would work. However, some checkers who think in terms of fractions seem to have fits about this.

One guy will take 11/16 and draw the part as .6875 and let Autocad round it off to .69. I can't see designing a part out to 4 places if that kind of accuracy is not needed, but that's just my preference.

Actually I like designing in metric better. I do everything in increments of .25mm which is .010 inches. It still gets the default tolerance, and no more hangup about fractions.

RE: Round off rules

If your part is meant to be 11/16, then designing it at .7 is out of tolerance, if your tolerance is +/- .010.

Always design the part in a CAD system to the actual nominal dimension of the part. Tolerenaces are for manufacturing, not for design.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

Designing a part should not be based on what you prefer, it should be what will work.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

RE: Round off rules

(OP)
????

Is there a difference between what you prefer and what will work?


As to making a part .70 instead of .69, isn't it .07+/-.010? I mean a new design, not an old one redrawn.

RE: Round off rules

If you prefer to make all dims 2 places at a certain tol, then someone else designs a part that a 3 places dec is a preference, there may be a problem. All parts and the tolerances have to be designed to work together.

Quote (EngJW):

As to making a part .70 instead of .69, isn't it .07+/-.010? I mean a new design, not an old one redrawn.
????

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP3.1 / PDMWorks 05
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

RE: Round off rules

The original dimensions was 11/16 which is .6875.
If you redraw the part at .70, you are already out of tolerance.
Even if you draw it at .69 +/- .01, it can get out of tolerance from the original .6875 +/- .01.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

Why not show the dimension as a fraction, then add a a tolerance (+/- 1/64?) or a "default tolerance" to your drawing (title block or notes) stating what tolerance to apply to fractional dimensions.

RE: Round off rules

What does ANSI/IEEE 268 say about rounding anyway?

I use CATIAv5 R15 (and It will round .125 to .13) Is that not the standard? Do I need to modify the drafting standard in CATIA?

Also if your design intent was .125 the only reason I can think of to use .13 is to use the larger block tolerance. If you want to preserve your design intent can you just put the larger tolerance on that one dimension?

What standard says you are not to use decimal equivalents of fractional inches. I was promoted off the floor of my factory and I have no GD&T training (I have theatrical drafting experience.) Almost all of my design work I use fractional units because that is what I have always used. Is that not proper form?

BTW I do try to learn the standards. I read them as much as they will allow me to at work.

RE: Round off rules

see the 6th note in this thread.

.125 should round to .12

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

(OP)
About decimal equivalents to fractions-

I am laying out out a part and I look at my pocket scale and say, I think I'll make that section about 3/16 thick. So I draw it at .1875 but that sounds kind of tight, so I put a plus/minus .010 tolerance on it. Naw. I'll just round if off to .19, draw it as that, and use the default drawing tolerances.

RE: Round off rules

Now put these parts into an assembly and scartch your head wondering why the CAD file looks right, but the real thing isn't.

Always use nominal dimensions, especially when it comes to stock material thickness.

"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.

Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

RE: Round off rules

(OP)
Not sure I explained myself too clearly, but if one part is designed as .19 instead of .1875, a part next to it is designed as .34 instead of .34375, and they are placed in the assembly at .06 apart instead of .0625, they ought to go together. The only rounding off is in my head. The real part or the drawing is not rounded off.

RE: Round off rules

Fractions are silly. Thinking in fractions is ok, but modeling and drawing in fractions is silly. The whole system of fractions has trouble converting to decimal (some are .XXX, some are .XXXX, and so on) so why use it when I can't express that kind of accuracy?

This is my thinking. Except for halves and fourths, all .XX and .XXX dimensions are approximations of the actual value of the fraction, so why fill my drawings with approximate values? Unless my design requires .69 I will tend to use .70. What is so sacred about a fraction anyway? I had a boss who wanted to shave down or nudge up my dimensions .001 or .002 to be closer to a fraction. Oy-VEY! In mechanical engineering we typically express lengths as thousandths even if it is only a 2 place decimal. I never think in terms of 16ths or 32nds of an inch, and all my scales have a fractional side and a decimal side--I use the decimal side to measure with. The only time a fraction enters into my design is when I have to match a part or hole pattern that was designed in fractions.

Part of my aversion to fractions is that I often have to work in Metric (no fractions) and another part is that practically no one is machining with equipment that uses fractions. It is a decimal world! I also don’t express degrees in minutes and seconds. I don’t know many who do (actually, I only know one person, and I think he fought in the Civil War).

When I worked on AutoCAD I always used a .05 snap grid. I find SolidWorks grids annoying and useless. Since I have to type in the dimension anyway, what's the point?

End of rant. You may now return to your regularly scheduled programming.

RE: Round off rules

(OP)
Excellent rant, Wgchere. There should be nothing sacred about .69 when .70 will do. We had a checker that had a fit because someone used .13 instead of .12, and since it was an Autocad drawing they went back and fudged the text. If it were me, I would have redrawn the whole thing and let someone else account for the wasted time.

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