huangyhg

we need a gd & t guru!
our company is training for gd & t, and i have a couple of questions for all you gurus out there.
1)what is the correct interpretation of an area indicated by section lines inside the phantom line?
2)which symbol should be used to express coplanarity of two or more features? (there's a cup of coffee riding on this one!)
3) can anybody refer us to a pocket-sized reference guide for use on the production floor?
thanks in advance for any guidance offered.
i've expressed multiple surfaces being in relationship to each other by using the note "nth surfaces". one can also note these surfaces together using a center line. i would just use the asme spec by itself.....i have a couple of pocket guides i've gotten from training classes but i use the spec.
best regards,
heckler
sr. mechanical engineer
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do you trust your intuition or go with the flow?
to group coplanar surfaces, you can use a composite surface profile. there is no "coplanar" callout.
ok guys, if the surfaces are all co-planar, how about just labeling the plane as a datum and let that datum encompass the surfaces? or, if that is not acceptable (it is the way i would do it), how about using parallelism?
for gd&t pocket guides, check this page:
1.datum target
2.profile of a surface (with a note below stating how many surfaces and all are identified clearly)
1. i believe the area cross hatched represents the size of the target area, when specifying datum targets.
2. check figure 6-20 in the y14.5 text for an example. i believe that will work for your case.
karman303,
there is no co-planar symbol in gd&t. you do not need one. assign a datum to surface a. draw surface b co-planar to surface a, and use a profile tolerance to control surface b with respect to surface a.
connecting the surfaces with a phantom line and applying a profile tolerance should do it too.
i had a pocket guide to asme y14.5m-1994, but i passed it on to someone. i forget where i got it from. why not just buy a copy of asme y14.5m-1994? it is quite readable. you might as well go to the source.
ewh,
labeling the multiple surfaces as a datum does not control flatness, co-planarity or anything else. the actual inspection datum is the set of three points that make contact with the reference surface.
the parallel control does not work at all, since it allows gross displacement of the two surfaces. i think i see what you are getting at. let's use my solution, above. the part is located by datums_a, b and_c. your reference surface is datum_d. your profile tolerance from datums_a, b and_c is 0.5mm. your profile tolerance from datum_d is 0.1. the flatness of datum_d must be controlled to 0.1mm. joining everyhing with a phantom line and applying the flatness specification should work too.

your surfaces are located within a 0.5mm tolerance zone. they are co-planar to within 0.1mm.
jhg
i've seen the flatness callout for multiple-point surfaces, whether as a datum plane or not. an explanatory note is never a bad idea. a confusing obfuscatory note, however ...
flatness is a low level control which can only be applied to a single feature. parallel is mid level orientation and form control which must be aligned to a datum. profile is the top of food chain which can control form, orientation and location. for multiple features that are coplanar it's the only control which can be applied.
type26owner,
i assumed that the surfaces are parallel to each other. i made no assumptions about the primary datum. whether two surfaces are connected or not has a lot to do with the format of the drawing. if i draw a rectangular plate and i cut a notch in one side, i would regard the resulting two surfaces as co-planar to within some tolerance. if the shape is more complex and the two surfaces are not so obviously connected, i would look for some way to make the drawing clear.
i _am_ making the assumption that there is a plus/minus tolerance or a profile tolerance available to control the co-planarity, but that this is not accurate enough. in my example in the previous message, the surfaces can be out by 0.25mm, but they must be co-planar to 0.1mm total.
jhg

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