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GD&T Tip - Do Not Mix Basic and Toleranced Dimensions

I receive a lot of questions accompanied

by a drawing I often redraw them so that

they are not a direct copy but the

concept may still be presented in a tip

this tip explains that you should avoid

combining directly tolerance dimensions

and basic dimensions to define a true

profile the only place in the current

standard where it is acceptable and

supported to combine the two types of

dimensions is when tolerancing conical

features i redrew this part so that

readers of the tip could not figure out

who committed this error but you know

who you are

this drawing tried to mix tolerance

dimensions with basic dimensions you'll

notice that this 36.1 one dimension is

coming from the edge of the statum

feature B which is a tolerance to

feature that's not really what the

drawing says to do so this is not

correct this would be correct if we look

at the profile tolerance it tells us

that the profile is located with respect

to the datum is established by features

a and B so a is this face down here and

so that would give me a datum plane and

then B is this diameter and that gives

me a fixed axis and since B is

regardless of material boundary the old

regardless of feature size whether this

diameter comes in large or small were to

Center up and get that axis so that

would be this spot right here that is a

fixed spot that I can measure that basic

from and then see if this surface is in

the profile tolerance so there's my

origin and measurement by the datums

established by

datum features a and B

this is not a fixed origin it didn't

feature beak commit larger or smaller

and it's not fixed so taking a closer

look at the datum reference frame we

have three mutually perpendicular planes

this one is established by datum feature

a and datum feature B gives us an axis

so we would fill this diameter and get

an axis we'd be using what's known as

the related actual main envelope that

axis is the intersection of two planes

two perpendicular planes and here is my

profile tolerance from that origin so

the 1608 basic is from that axis whereas

that other dimension the 3611 would be

coming from this spot on the surface of

the datum feature which is variable so

that would be wrong

now my second and third datum planes are

free to rotate since we've only

referenced a B in fact the feature we're

trying to control here you see is datum

feature C but that's okay what we would

do is we would have to rock or rotate

those datum planes with the profile zone

until we could bring this feature into

that tolerance we make certain the folks

we train will not make mistakes like

this because a techies you know that

Judy and T rules I'll see you next step

you