【转帖】perp callout - relative to 2 datums

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perp callout - relative to 2 datums
on page 182 of asme y14.5, 1994, it show a perpendicullarity callout relative to 2 datums.  datum a primary and datum b secondary.  if i am setting up a gage to check this do i pick up the whole surface of datum b or can i use two points and use it basically for clocking.  (i never knew you could have this secondary datum....)  thanks in advance.
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chancey,
   use datum b for clocking.  there is no reason to assume that the entire face of datum b is perfectly perpendicular to datum a.
                       jhg
use the entire surface for the seconday datum feature unless it is specified as an area or target points.
david strole
engineering systems administrator
gdtp s-0132
drstrole,
   the datum plane b is exactly perpendicular to datum plane a, by definition.  the second face of the fabricated part is out of perpendicular by, hopefully, a small amount.  
   ideally, the datum a surface locates at three points, and the datum b surfaces locates at two points, and in this case, controls rotation.
                          jhg
i agree with drawoh on this one. b is the secondary and should be treated as such.   
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
first of all, you are going to have a difficult time setting up an attribute gauge for this plane. attribute gauges can be set up on holes or pins (features of size) as long as the fcf is in mmc. otherwise, go measure.
theoretically, datum a should sit on a plane and not 3 points unless it has a high spot in the centre. now it would wobble and a measurement cannot be taken. in this situation, i would suggest a 3 point set up but only as a last resort.
datum b is for clocking as other people have suggested.
here is how i would measure this requirement.
place an angle plate on a granite surface that is located in the tool room. now datum a is clamped to the vertical surface of the angle plate so that, in theory, the feature in question should be parallel to the granite plate. zero up (rotate) datum b so that if one ran a dial indicator on a stand along the feature, it should read zero all the way.
sweep the feature with the dial indicator and report the tir or fim which should not exceed 0.12 mm as per page 182.
perpendicularity of a plane is a combination of angle and flatness.
hope this helps.
dave d.
just arbitrarily picking two points may not yield the two high points of the secondary datum feature. you do not know where the two high points are. what the original post is describing is using datum target points as a secondary datum feature, and these must be noted as such on the drawing. the drawing needs to specify the placement of these targets.
per the illustration in figure 6-35 on page 182. the secondary datum feature is shown as a plane. the two-point contact for the secondary datum feature is at 90 degrees to the primary datum feature. the two high points on the part would come into contact with the simulated datum plane (say an angle block at 90 degrees to datum plane a).
if it is desired to have only two fixed points as a secondary datum feature to prevent rotation, they should be noted as datum targets.
  
david strole
engineering systems administrator
gdtp s-0132
the points are not arbitrarily picked. they could vary from part to part though. this is not necessarily a problem and it is also the premise by which the 3-2-1 restriction of degrees of freedom works. see 4.4.1 in the standard. it is only when you want to communicate where the 2 points of contact are do you use datum targets, otherwise whatever 2 points make contact establish the true geometric counterpart of the secondary datum feature.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
yes, i agree with powerhound except the 2 points are usually at the ends of the secondary datum plane just inside the edges. that is usually the optimum set up.
dave d.
unless i am misreading what everyone is saying (which could very well be) 4.4 explains what i am stating. what i am reading is that a fixture with two points (datum targets) is being used.
if the entire surface of the secondary datum feature (fig 6.35 page 182) is being specified as the datum feature, then the set up would need to be two perpendicular planes (say a surface plate and an angle block) to establish the drf. the two high points from the part would contact the datum simulator, but the simulator needs to be a plane. that is what figure 6.35 is showing.
if only two target points or areas at fixed locations are intended to be used, then the secondary datum plane needs to be specified using target points or areas.
  
david strole
engineering systems administrator
gdtp s-0132
the two points will be established by the actual geometry of the part. if the high points are closer to the center of the part then that's where the 2 points of contact are.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii