【转帖】indication of projected tolerance zone for thru holes

yang686526

indication of projected tolerance zone for thru holes
asme y14.5m-1994
if you have a thru hole that you are using a projected tolerance zone with do you need to show which direction the projection goes with a chain line & dimension as in 5-34?
initially i was thinking you did (as just puting it in the fcf doesn't tell you which side the zone is projected) but then started to think that it doesn't matter if you inspect the projected tolerance on the front or back it will control the 'perpendicularity' of the hole the same.
in my case the holes are for dowels that are being used as linear guides for another part to slide along, although i don't think it makes a difference, the same would be true for a threaded hole.
kenat, probably the least qualified checker you'll ever meet...
man, you got me thinking there.  i would say since the standard specifically shows how to detail it, then do so.  it seems 5.5.2 is pretty clear on this.
as far as your reasoning goes, i guess it wouldn't make a difference is the projected zone at the same place in both directions, but how often is that going to happen?  5.5.3. says the positional tolerences only applies to the height of the projecting portion of the stud or pin after installation.  unless you insert your pin so that it protrudes equally on both sizes, i don't see how this is possible without detailing with a chain line, especially if the pin sticks out on both sides unequally (which isn't an uncommon event).  
this is just some initial thoughts give you something to bounce your question off of.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
if in doubt, show it. both sides, if neccessary. a blind hole can only be interpreted one way. a threaded to a specific depth can only project one way.
however, a thru dowel hole, you might want the lengths indicated on both sides (same length or different).
a 0.2mm diameter zone for a part thickness of 10mm yeilds = .02 tangent.
add a 10mm projected tol. zone it becomes 0.2/20 or 0.01 tangent.
if you add 10mm projected tol. zone on both sides, it becomes 0.2/30 = 0.0067 tanget.
ok, i've convinced myself it does matter which side, i'll be adding it in (or back in, i put it once then deleted it).
it doesn't make a differenct to the 'angle' of the hole but does effect the location so is needed.
fcsuper & weavedreamer thanks.
kenat, probably the least qualified checker you'll ever meet...
although the standard is not explicit about this, you will find in fig 5-33,34 that direction and orientation is directly related to the primary datum of the drf as specified in the fcf.  i have found that when i am considering applying the ptz, i must make sure that i have identified a "functional" drf that not only conveys function of the component/features but captures the assembly sequence/process which is the premise behind calling out a ptz. so if all understand this per the standard, a chain line is not necessary, however if it helps communicate to others who don't, a chain line is an acceptable alternative method of specification.  
as a side note please be aware that the ptz is applicable to the projected axis of the ame of the hole as a resultant of the manufactured feature hole from the drf specified in the fcf, not the actual axis of the hole, although derived from.  if perhaps a "fastener" is required on both sides of the hole another datum needs to be specified on the the opposite side and a second callout with this datum applied as primary in the fcf is necessary.  i hope this helps.
vwm
good point xplicator, my datums are set up that way however i don't have confidence that the machine shops we use will be as knowledgable as you so i think the chain is probably appropriate in my case.
kenat, probably the least qualified checker you'll ever meet...