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.)
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
RE: Round off rules
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
There might be a way to change it but I haven't found it yet. Still learning.
RE: Round off rules
RE: Round off rules
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
I'm not sure if it's covered in ASME Y14.5M....my copy is at home since my company hasn't updated.
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RE: Round off rules
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
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
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
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
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
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
????
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
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
RE: Round off rules
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
.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
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
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
RE: Round off rules
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