【转帖】question on perpendicularity

yang686526

question on perpendicularity
i got a call from a vendor about a drawing i checked a few months ago.
i鈥檝e attached a very simplified sketch giving you a rough idea of the situation.
on the prototype, which was copied from a similar older drawing, the central bore was (incorrectly) identified as datum and the bottom face was perpendicular to it.  (the real part has some other diameters coaxial with the central bore so it was unclear what the datum was 鈥?small iso
i would go with your propose rev b as long as you are locating to real, measurable surfaces, not the centerline shown in your first sketch. i don't see why it would make a great difference unless the vendor has a cnc and maybe a cmm program set up and doesn't want the expense and bother of changing it.
depending on your design. you want the hole perp to the end or the end perp to the hole?
chris
solidworks/pdmworks 08 1.1
autocad 06
thanks ron, i'll make sure on rev b that the datum is the central bore which from a function point of view is what it should be.  i got the impression he thought he'd have trouble inspecting rev a with cmm.  also he had some manufacturability concerns.
ctopher, that's what i'm trying to get my head around.  what difference does it make if the hole is perpendicular to the end face or if the end face is perpendicular to the hole?
kenat, probably the least qualified checker you'll ever meet...
it would make a difference with the machinist for steps to machine it.
you show a hole related to once surface. do you want the hole through the center axis, or perp to the one surface?
if perp to one surface, the hole could not be down the center of the axis.
chris
solidworks/pdmworks 08 1.1
autocad 06
kenat-
think about one of those nifty gd&t blocks, and you can visualize the basic difference. if the bore is the datum, then that is the feature that is used to constrain the part first. the bore will be straight and the plane will be off-kilter. however, if the plane is the datum, that will be straight, and the bore will be off-kilter. the difference comes into play when you consider other features that may be referenced from one or the other.
v
mechanical engineer
"when i am working on a problem, i do not think of beauty, but when i've finished, if the solution is not beautiful, i know it is wrong."
- r. buckminster fuller
kenat,
   for the sake of argument, i assume that your drawing is approximately to scale.  
   my first thought that the .001 perpendicularity tolerance on reva is not equivalent to the .001 perpendicularity tolerance on revb.  the bore is longer than the face, therefore, the perpendularity calls up a tighter fixturing angle.  for reva, consider opening up the perpendicularity tolerance.  
   my second thought is that, as per asme y14.5m-1994, both reva and revb are valid callups.  the fabricator is required to fixture to the primary datum first.  it sounds like his fixture picks up the bore, and locates the part while he machines the accurate face.  your reva at the very least, requires him to redesign his tooling.   
   how does the part work?  datum_a ought to be the primary mounting face.  
                    jhg
thanks ctopher & vc66, i've been having those thoughts.
drawoh, you're right that the length of bore is significant'y longer than the face so that makes a difference, the light bulb has just come up over my head.
i asked the designer about this when i checked it but dont' recall getting a satisfactory answer and at the time didnt' think i could justify digging further.  (now the designer is out on maternity leave.)
it was based on function that i made the face datum a as this is how it mounts on the instrument in service.
this whole issue is confused by the fact the designer has a thing against gd&t and i made the mistake of trying to meet her half way on this one and am now regretting it.
kenat, probably the least qualified checker you'll ever meet...
yes, that is the trap alright. is this a certain swiss engineer we both know?
might be.
kenat, probably the least qualified checker you'll ever meet...
people that are against gd&t are the ones that don't know it, or well enough.
chris
solidworks/pdmworks 08 1.1
autocad 06