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.)

 

[EngJW]

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.

 

[wgchere]

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.

 

[EngJW]

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.