【转帖】dimensioning repetitive patterns 9opinions0

yang686526

dimensioning repetitive patterns (opinions?)
just looking for opinions on proper procedure.
i'm dimensioning a row of holes that measure 2.0625" between centres in the model (don't ask me why it isn't just 2"!)
should i dimension the pattern:
(a) 5x 2.0625 (=10.3125) also indicating the first dimension as reference (2.0625)
(b) 5x 2.06 (=10.31) also indicating the first dimension as reference(2.06)
the problem is that the math doesn't work in (b) due to rounding (should i care?) but (a) is far more precise than my shop is used to seeing.
thoughts?
fyi: i have an aerospace background and am familiar (not expert) with gd&t but my new company (mining) hardly uses +/- tolerances let alone gd&t (ie. big sloppy holes make for easy fits) and they need some serious help with tolerancing to increase quality.
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what's the standard dim tol on the drawing?
i have been lucky that i have never run into this problem before, but since the overall dimension is only for reference (hence the parenthesis) you should be able to round it to whatever decimal accuracy you want.
"art without engineering is dreaming; engineering without art is calculating."
what is the tolerance for the dim? someone made it 2.0625 for a reason.
chris
solidworks/pdmworks 08 3.1
autocad 06/08
maybe i should just simplify the question:
is it acceptable to have a mismatch on a drawing like in (b)where the pattern 5x 2.06 does not equal the indicated total 10.31?
ctopher:  i would like to be able to give you a tolerance but no standard tolerances exist here (that's what i'm attempting to develop)  makes me wonder how any of our parts actually fit together!
if there is no tolerance, then 5x 2.06 equalling 10.31 is acceptable. but i suggest working out the tolerance or the holes most likely will not line up with the mating part(s).
chris
solidworks/pdmworks 08 3.1
autocad 06/08
i might try to dimension it as:
4 equal spaces = 10.31
(5 holes x 2.0625 apart)
i know this is not to any standard, and is probably wrong/confusing for a number of reasons. however, if your dimensions/tolerances are based on what the shop likes, i don't think they can expect much better. the "what the shop likes" in places of a standard is far too common of a practice (including at my company), but that's the reality of it. either try to get it changed or work with it.
on second thought, if the basis for your tolerancing is what the guys in the shop would like to see, maybe ask them how to tolerance it. i would start with drawing with dimensions and tolerances per standards/good practices, and then take it to them and let them complain and demand it get changed... at least then if it comes out not functioning you did it right and it was changed by others.
-- mecheng2005
i would do it similar to what mecheng2005 recommends - a total dimension "(5x 2.062 =)10.31"
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

gmurr,
   apply a tolerance to the 2.0625 dimension.  usually, with a sloppy tolerance, you do not want to show four decimals.  in this case,  you want the arithmetic to work.
   5x 2.0625±.05 = 10.3125
   alternately, you can use some gd&t equivalent.  it is not as if you do not need the tolerance!
                        jhg
consider fig. 1-53, 1-54, 1-55, & 1-56 on pg. 21 along with the supplemental text contained in sections 1.9.5, 1.9.5.1 & 1.9.5.2 per asme y14.5m-1994
thanks all.
it should be interesting when our shop sees an actual tolerance on a dimension.  can't wait to introduce gd&t and then maybe we will look at revision control (what a concept!)
the fig 1-54 example (asme y14.5m-1994)that weavedreamer references is basically the same as your (a) example.
if you choose to use the two place example (b), then make it =10.30 to eliminate the obvious math error.

if your holes are bolt holes for 1-7/8 dia or less, then tolerance accumulation shouldn't be a problem. (example: pipe flange holes, which are 1/8" bigger than the bolt.) if it can be, then i would use a "hard" overall dimension as ewh and mecheng suggests.
in the piping industry, (similar to mining) fractions were common, thus 2.0625" could be 5x 2-1/16" = (10-5/16").  
however, if your canadian for which the word "centre" is a clue, then maybe fractions are an american thing.