【转帖】fractional dimensions

yang686526

fractional dimensions
my company has a long history of designing all in fractional or architectural dimensions, except the rare case where you need a tolerance to the thousandth. we deal with mostly castings (sand castings) and fabricated (welded) items. being a younger engineer i was only exposed to working in decimal until i started here. my personal rule of thumb up to this point has been to design anything machined all in decimal and anything fabricated, especially big stuff (some of the fabrication you could fit inside) in feet & inches (architectural). i am dealing with more and more castings now and am note sure if i should continue to use fractional measurements or decimal. because of the loose nature of sand casting only two place decimal is really needed. the head of our department doesn't care which method i use. i am curious what the collective experience here has to say about this choice. what is your personal or your company's position? pros & cons of either choice?
fractions don't lend themselves well to inspection, tolerances, and significant digits.  this is one of the reasons why they are not used in complex mechanical systems like automobiles.
regards,
cory
asme y14.5m-1994 (1.6) only deals with decimal dimensioning, doesn't mention inch fractional that i'm aware of.
you talk about only holding to 2 decimal points but this is meaningless withouth the title block tolerances.  i've sean tol blocks where 2 dp was +-.005, more typically +-.010.  
the number of decimal points only indicates tolerance if your title block is set up that way.  in my last company in the uk the number of decimals had no significance.  we assigned a singe general tolerance, typically +-.25mm and every dimension that required a different tolerance had it indicated on the dimension.
you can have a 1.375 dimension and apply +-.050 to it if it makes sense.   
kenat, probably the least qualified checker you'll ever meet...
i continue to be confused at why people believe that there is some association with the format or number of significant digits needed to describe a nominal dimension and the necessary tolerance of that dimension.
tolerance is the amount of part-to-part variation that is acceptable without preventing proper function of an individual part, or its parent assembly.  it is a design function, and is separate from and unconnected to the nominal dimension.
if i have a part that must have a feature within the range of 20.200 and 20.206 to function the requirement does not change if i call the nominal dimension 20.203 or 20-13/64.
or maybe the part must be between 19.997 and 20.003 to properly function.  the nominal is 20.  
the fixation on number of decimal places "defining" the required tolerance - as associated in a typical "standard title block" - is simply laziness, indicating that the engineer or designer could not be bothered to actually engineer the tolerances, and has simply made a guess, or more likely given no thought to tolerance at all and has simply let the cad program variable the sets the number of significant digits decide the tolerance.
i would never use fractions. they can be rounded either way to the nearest .xx, .xxx or .xxxx, depending on who is working with them. some machinist's don't know how to convert them to decimal.
i agree with others comments above.
chris
solidworks 07 4.0/pdmworks 07
autocad 06
"the fixation on number of decimal places "defining" the required tolerance - as associated in a typical "standard title block" - is simply laziness"
give us a break...
block tolerances are usually found to be reasonable manufacturing tolerances, and are no different than if every dimension had its own tolerance.  the design should allow for this.  it serves no purpose to put a tolerance behind every dimension if the part was designed with the noted tolerances in mind.  drawings are supposed to be concise, not verbose.
when and where a block tolerance is not appropriate, then call out something different.
you did not write the y14.5 standard.  if you had, i'm sure that it would be a very different animal.
the laziness is if you don't verify the block tolerances make sense/support/are driven by function, which sadly is very common.  
lots of parts here either have potential interferences or unnecessarily tight tolerances because people don't think it through and just use the default 3dp our cad system places which invokes +-.005.  on an older format 3dp was +-.002, we had large invar pieces which could easily have been +-.01 or more on overalls held at .002 because the person doing the drawing didn't think about it!
using block tolerance and checking to see if any dimensions can/need to be different is not laziness, just trying to keep a tidy drawing.
you think standard tolerance block is bad look at iso 2768, especially the last paragraph in the notes.
kenat, probably the least qualified checker you'll ever meet...
if my memory serves me correctly, we were directed by mil-std-100, and some other standards, some 50 years ago to use decimal dimensioning without making it an equivalent to a fraction.  that is: a new designed feature would be .30 or .32 perhaps rather than .312 which is the dec equivalent for 5/16.  the exception being that if the feature were driven by a fractional tool size.
this was not fully applied then and most probably not applied today by cad operators.
anyone else recall this?
asme y14.5m-1994 only talks about metric and decimal inch, it doesn't include fractions, as i posted before.  is this kind of what you mean?
kenat, probably the least qualified checker you'll ever meet...
i should have mentioned that decimal dimensioning was not directed to apply to architectural nor civil engineering drawings.
kenat,
not quite.  the standard is directed towards metric dimensioning for the most part, i believe.  
briefly stated:  throw away the fractional scales and use only decimal-inch scales.