【转帖】position and basic dimensions

yang686526

position and basic dimensions
i think i know the answer to this but i am not 100% sure. here is a description of the drawing i am checking;
imagine a flat circular plate of steel. datum a is one surface of the part. the secondary and tertiary datums are defined by a central hole (datum b) and another hole on the part (datum c).
we weld a rectangular bracket onto the part. a reference dimension for the width of the bracket is included on the drawing and positional tolerance (no modifiers) w.r.t. a,b,and c is called out inline with the width dimension. this controls the location of the bracket in the "x" direction.
the sticky part is that there is a dimension with a tolerance relating to another hole (not a datum feature) on the part controlling the location of the bracket in the "y" direction.
the question is - with the positional tolerance, is it valid to have a dimension and tolerance for the bracket as opposed to a basic dimension?
i think our drawing is valid because of the location of positional tolerance (inline with width dimension of the welded bracket). if i understand correctly this indicates that the position of the central plane of the rectangular bracket is controlled w.r.t. the defined datums. so this can only control position in one ordinate direction.
of course i do not claim to be a gd&t pro and this wouldn't be the first time i was totally wrong!
thank you in advance for your help.
jbk
quote:
we weld a rectangular bracket onto the part. a reference dimension for the width of the bracket is included on the drawing and positional tolerance (no modifiers) w.r.t. a,b,and c is called out inline with the width dimension. this controls the location of the bracket in the "x" direction.
quote:
the question is - with the positional tolerance, is it valid to have a dimension and tolerance for the bracket as opposed to a basic dimension?
if you're using a positional feature control frame to datums a, b & c for the location of the rectangular bracket then the dimensions that located must come from those datums.  if you're only controlling in one direction you may need to remove a datum from the fcf positional tolerance.  it would help if you posted a picture of the drawing.
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
since you are controlling the rectangular bracket in only 1 direction, use positional tolerances on the width or length of the feature. the dimensions to this feature on this axis would be basic. it would indicate that we are interested in that x axis only and would control the location of the feature on the width, its angle and perpendicularity. the feature would be confirmed using cmm and not checking fixtures.
the y axis is not covered by the feature control frame so the dimension from another hole would have to have a tolerance and not be in a basic dimension.
hope this helps.
dave d.
today is somewhat slow so i blew a couple minutes putting the following site together. i hope the link works, the page should explain the situation.
it is exactly what i have stated but the feature should have a width dimension with tolerances and not as a reference.
dave d.
the width dimension may remain a reference dimension if it is specified elsewhere as a toleranced dimension (such as in a separate detail).
you need to dimension from datum b and remove datum c from the feature control frame.  datum c seems redundant for this simple of a part.
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
heckler:
don't think so. that is removing datum c. we need it for orientation.
dave d.
dave - look at
section 4.4.2 parts with cylindrical datum features.
fig 4-5a-b.
section 4.4.2.1 cyclindrical datum features
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
the center-plane of the width of the bracket (2.042) must lie between two parallel planes spaced .010 apart from a datum reference frame established by a|b|c.
if there is missing information as noted <n1> "majority of drawing and corporate information removed" then those that insist that reference dimensions refer to toleranced dimensions on the same level drawing can assume that the toleranced value exists but is not shown and those that are used to modern product drawings resembling former "process sheets" can assume that all the necessary information to interpret (decode) the callout has been shown.
i'm an old guy i'll let you guess which camp i'm in.
paul
i think i have to have datum c because the bracket needs to be oriented with respect the the 1/2" hole (section 4.4.3 covers this). otherwise the bracket can "rotate" around datum axis b and still be in spec. check out the first sentence in section 4.4.2.3.
if i only use the center hole as a datum it does create the secondary and tertiary datum planes. but as i understand it, the secondary and tertiary datum planes can rotate about the datum axis so the features can float around the axis as well.
we use positional callouts similar to this (only 2 datums) all the time for bolt circles that are evenly spaced about the axis and do not require orientation with a pilot diameter.
the bracket is fully detailed in the drawing of the bracket itself so the width is specified as a reference only dimension in this case. since there are no modifiers in the fcf i didn't figure this would be a problem. i don't really want to get into all the nasty details of the bracket itself.
after some research i don't think the y dimension needs to be basic. examples and info on pages 141-143 of the current standard seem to support this.
for the y dimension to be basic i think i would need to add another positional callout on the thickness dimension of the bracket.
conversely i think if the positional tolerance was called out with a leader to the bracket (and not a size dimension) it would control the position in x and y and require basic dimensions for each.
hopefully my interpretation of the standard is not to far off base!
sorry for the long post and thanks to all for the help!