huangyhg

anyone ever see a double positive tolerance?
anyone ever see a double positive tolerance? (e.g 1.000 +0.005 +0.008)
i've always thought the target value must be between the maximum and minumum value; and was selected by the engineer based on maximizing yield depending on how the part would be made, and on maximizing performance of the final assembly. the tolerances could be symmetric +/- 0.003, or they could be asymmetric +0.003 - 0.000, but i've never seen double positive tolerance, or double negative before.
did i miss something in gd&t class?
find a job or post a job opening
usually double positives and double negatives are used for fits (i.e. slip, locational, press, etc.)
i use them all the time.
try to search before posting. this has been discussed before.
here's on example.
i've seen them. i don't like them.
i start seeing them towards the end of the day.
i agree with the others.
chris
solidworks/pdmworks 08 3.1
autocad 08
it never isn't done like that here... but if a double negative is actually a positive... then isn't a double positive actually a negative???

i've used them when i want to convey different information to different groups of readers, e.g. that the shaft takes a 120mm bearing, but it's pressed on.
it usually results in a call from the shop ("am i understanding this correctly?" ... or some rude equivalent).
it's not a bad way to:
- get acquainted with the guys in the shop.
- find out they are really working on your project.

mike halloran
pembroke pines, fl, usa
i'm not that crazy about them, but they do show design intent.
joe
sw office 2008 sp4.0
p4 3.0ghz 3gb
ati firegl x1
double positive (or negative) are part of comprehensive systems for limits and fits. for example, the iso 286 iso system of limits and fits series provides for parts that can be designed with any combination (double positive, double negative, symmetric +/-, etc.).
you can review information on this subject here:
i've used double positive and double negative. sometimes for fits, sometimes because a solid model hasn't been updated to have a nominal within the desired tolerance range on the drawing (yes, there are reasons to do this).
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i agree with above that you see them a lot with shaft fits, especially iso ones.
by extension some german drawings seem to use them even on items that aren't a classic 'fit' situation.
they can cause confusion so i avoid them. also with some cad systems it can be awkward to achieve.
kenat,
a lot of this stuff is very industry-specific. i design custom assembly equipment for factory automation. unilateral tolerancing is really the only thing that makes sense in that context. check a catalog for dimensional drawings of anything - linear shafting, bushings, servo flanges, spacer blocks, dowel pins, raw steel dimensions... the list could go on and on. if there's any sort of fit involved, you will see unilateral tolerances. i'm constantly amazed at people's ignorance of this common practice.
-handleman, cswp (the new, easy test)

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