ordinate dimensioning 5

[snowman64]

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?

 

[drawoh]

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

 

[Tunalover]

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

 

[MechNorth]

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
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[TheTick]

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.

 

[Tunalover]

TheTick-
Do I hear an echo?


Tunalover

 

[TheTick]

Thanks for the subliminal suggestion, TL.

 

[CheckerRon]

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.

 

[KENAT]

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 remember that for it to be worth anything you've got to be able to inspect it afterwards.

Plus I've been told that many CnC systems don't like negatives, not that we design just for the manufacture, they should know enough math to work it out, but keep it in mind.

 

[kbro151]

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.

 

[drawoh]

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

 

[kbro151]

drawoh,

I did not mean to imply that that the individual dimensions would each have an indicator. All I meant was that the coordinates were explicitly linked to a particular feature. The feature has a graphical relation to the origin of the coordinate system. For example, if a cartesian origin is used with positive indicators for 2 axes, it is obvious which quadrant the feature is in.

 

[Tunalover]

CheckerRon-
If "width" datums are given to the part width and length then the centerplane is indeed defined by a measurable feature. The length and width of the part are features of size in the truest sense. It is then perfectly fine (in fact desired) to dimension from a centerline passing through the centerplane (the "middle" of the part). This is a great method to prevent dimensional bias to one corner of the part. In many cases it is also a functionally significant method of dimensioning and tolerancing the part and it gives the designer the option of gaining bonus tolerances that vary with the size of the part. Example: A hatch in a bulkhead or a cover over a recessed opening.



Tunalover

 

[KENAT]

tuna

Isn't that what he was saying? His problem seemed to be that people dimension from a center line that isn't related to a width datum, just drawn in.

drawoh, why not use a hole table? Essentially the same thing I guess but with the numbers in the table not off the drawing view, you still define 0,0.

 

[drawoh]

KENAT,

I am on SolidWorks 2003 SP5.0. I cannot do hole tables.

In any case, you still need to define zero intelligently, and you need signs on your dimensions if zero is not at an extreme corner of the part.

JHG