huangyhg

ordinate dimensioning
in a drawing, with the orgin of ordinate dimensions located in the middle of the part geometry, is it proper to have positive dimensional values going in each direction from the orgin?
snowman64,
according to asme y14.5m-1994, "coordinate dimensioning must clearly indicate which features of the part indicate [your coordinate zero]." the standard does not show an example doing both ways.
manually placing a minus sign in front of the dimensions on one side is bad cad practice. you and every other co-worker who access the drawing will have to manually check these signs every time you move dimensions around.
if you make your zero point obvious, you should be okay.
jhg
it's fine and done all the time. if you have a small ordinate dimension to either side of the origin it's good to add a jog to emphasize which side the feature is on.
tunalover
an instructor once said to me "what is a negative dimension? if you can measure it, you can use it." all ordinate dimensions are +ve regardless of where the origin is.
jim sykes, p.eng, gdtp-s
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if dimensions are too close to zero to tell, give them a noticeable jog.
for positive vs. negative, you could sketch arrows and add notes to indicate positive and negative direction. this would indicate the sign for dimensions in each quadrant without forcing one to maintain and check signs on individual dimensions.
thetick-
do i hear an echo?
tunalover
thanks for the subliminal suggestion, tl.
referring to drawoh's comment, i hope that origin in the center is a relating to a real, measurable dimension, such as the center of the part width defined as the datum.
i am having a problem with designers that often dimension this way to a theoretical centerline that dissapears as soon as set up is broken and the part is taken off the machine, since it doesn't relate to any feature. they are relating their model to the model datum center planes, and forgetting the real part datum features. ordinate dimensions are ok and dimensioning from each way is fine as long as it is related to a measurable datum feature.
amen to checker ron.
i'm not a fan of ordinates but that's pretty much just a personal prejudice.
however, whatever dimensioning scheme you use re
dimensions on a drawing are meant to indicate distance, not direction. when you look at a drawing, the directional components are indicated graphically, by the orientation of the dimension. remenber, a picture is worth a thousand words.
kbro151,
i am not aware of a cad package that attaches arrows to your ordinate dimensions. dimension 0 (zero) tells you where your origin is, and all dimensions, regardless of which side they are on, come from it.
i love ordinate dimensioning. i do all sorts of parts with dozens of holes and other features in them, where anything other than ordinate dimensioning would be an unreadable mess.
once you resort to the solution of ordinate dimensioning, all the standard drafting rules apply. pick appropriate origins as per checkerron. make sure dimensions are readable and that you can see which line connects where. on occasion, i have moved features in my design to assure readability on my drawings. often, i am able to place milled feature ordinates on one side of the part, and hole coordinates on the other side.
your drawing is your primary communication with the outside world. a good drawing is like a good written paragraph. you follow the rules of grammar, and you organize for clarity.
when i started out, i was told that machinists liked drawings to show all features from the same datum. it allowed them to zero their milling machines and manually move from coordinate to coordinate. confronted with any other drafting style, they would re-calculate all the coordinates. today, with most machine shops going immediately to cnc, i would think, this was much less important. before you slap pitch circles all over your plate, think about your inspectors.
jhg

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