Round off rules 1

[EngJW]

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.)

 

[ewh]

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.

 

[looslib]

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

 

[EngJW]

What is a better system than Solidworks?

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

 

[ajack1]

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.

 

[ewh]

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.

 

[Heckler]

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.

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[looslib]

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

 

[ringman]

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.

 

[EngJW]

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.

 

[looslib]

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

 

[ctopher]

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

 

[EngJW]

????

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.

 

[ctopher]

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.

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

 

[looslib]

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

 

[btrueblood]

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.

 

[templetonc]

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.

 

[looslib]

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

 

[EngJW]

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.

 

[looslib]

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