huangyhg

true position using bolt circle and angle
hi,
quick question.
imagine you have a square part with 2 holes.
one hole is right on center and it is datum a.
the other hole is drilled somewhere else, on the same face.
it is dimensioned using a bolt circle and one angle.
now, the only thing i need to control tightly is the radial distance from datum a. so i apply a positional tolerance of .001" to datum a.
my question is this...do both dimensions need to be basic? or just the bolt circle?
the angular position can vary by 50 degrees so i do not want to include it in the tolerance.
know what i mean?
thanks,
j
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to use positional tolerance the locating dimensions need to be basic.
you could perhaps use a bilateral positional tolerance more like you typically would with a slot. so separate fcf for the radial displacement v the angular. i've never seen this with the dimension scheme you are proposing but have with rectangular dimensioning schemes.
kenat,
so, how then, can i control the radial only position of the hole and not control the indexing of the same hole?
is their even a way using gdt?
i added a sketch. the only thing i want to control is the dimension shown.
the small hole shall be free to rotate about the center.
now, i know i could simply put a linear tolerance on it, but i am curious if gdt has a way to do it.
j
oh, btw, this is not a real life situation, just something i thought up on a lazy thursday afternoon.
jaydenn,
let's see if we understand your design and requirements.
your plate is attached using two screws. both screws must pass through their clearance holes. the locatation and rotation of the plate is much less critical.
apply a dimension and ± tolerance between the two holes. a gd&t positional tolerance controls two dimensions, and so far, you are controlling one. for screws, as opposed to bolts, your tolerance should be ±<screwclear>/2, or ±<boltclear>.
you could apply a tolerance to your angle, but this would be hard to inspect, and meaningless if you do not apply it carefully. i would apply a gd&t positional tolerance to your second hole, using the base, the first hole and some clocking feature as datums. this would control only the rotation of your plate, so it would not be very accurate.
do not bother to draw the pitch circle. it has no meaning if there is only one hole on it. your angle would be a basic dimension. i would be tempted to replace the angle with a linear dimension from your clocking feature, which also would be basic.
jhg
i just tried to give you the gd&t answer. you split the radial and rotary tolerances and put in separate fcf. take a look at figure 5-42 in asme y14.5m and paragraph 5.9.2.
if you don't have the standard to hand i'll try and post an example.
kenat,
what if you gave the small hole x,y dimensions from your datum a, or large hole, and then apply positional tolerance and reference datum a. this will maintain the distance you desire and since there is no clocking datum, the small hole can orbit around datum axis a.

if you use a positional tolerance then your dimension between the two holes must be basic. a +/- tolerance would be illegal. if this is one part of a mating assembly then using position would probably be the best way to go. if the holes are used to mount 2 separate objects that are not really related to each other then a +/- tolerance might be okay but either way, you should drop the bolt circle, as drawoh said, as it is pretty much meaningless when there's only one hole.
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i'm pretty sure the op's real question is fully answered by figure 5-42 or the standard and the related text.
attatched is an approximation of that figure (don't ding me for minor issues, i rushed it).
we had one like this a while back.
large ring with 32 ea .500 dowel holes to hold circular thompson rail to position.
t/d had tp dia of .005, and some of the holes were failing the cmm survey.
changed the cmm output from xy tp to radial +/-.0025 and every hole passed.
t/d changed the call-out to rxx.xxx +/-.0025
hgmorgan,
as i noted above, a gd&t postional tolerance controls error in two dimensions. most of the time, including your case, this is what you want to do. your original positional tolerance of ?.005" does this. i suggested a ± tolerance for the op because he does not require this for his particular application.
your ±.0025" tolerance on the radius does not control the angle. nor is your ±.0025" equivalent to a positional tolerance of ?.005". by my figuring, your postional tolerance reflects a clearance around your pins of .010". the required tolerance on your pitch diameterfff"> would be .0035". the tolerance on your radius should be half that.
your angle tolerance is a function of the pitch circle diameter. for a 12" pitch circle, you need an angle tolerance of approximately one arc minute.
it is an imaginary situation for the purpose of understanding gd&t application.
good post though drawoh.
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