Sign necessary on ordinate dimensions?

Jim Canale

I'm the only designer / draftsman here, and as such, much of my work is "self taught".   That means I have some doubts that I do things to industry standards....

My question regards ordinate dimensioning of 2D layouts.    I design a lot of panels with a high concentration of holes.   Linear dimensioning would make the drawing extremely busy, and hard to read.   So I have grown accustomed to using ordinate dimensions to locate holes and other features.  

If the panel is large, I usually make it's exact center, my UCS datum (0,0).   That way, if there is not sufficient travel on the  milling machine table, the machinist can flip the panel over, re-index the datum, and machine the other holes. 

My question is, should I be applying a sign (+/-) to indicate which side, (Left / Right, or Up/Down) a hole falls from the datum?  (The ordinate dimensioning tool does not apply a sign.)   Since I machine most of the panels myself, the lack of a sign is not a problem.  However, what about a layout that contains a bunch of holes very close to either or both axes of the 0.0 datum?  If I send this out to a shop, are they going to notice which side of zero the hole lies?  

Perhaps there is a LSP file or setting that would automatically assign signs to the ordinate dimensions, depending on their location relative to the UCS origin?

Scott McKenzie

I always use the negative sign when appropriate - I usually use ordinate dimensions to show the elevations of a house, where 0'-0" is top of finished grade, in my mind the sign adds clarity to the dimension.  That being said, I think that ordinate dimensions should be set so that they do not show negative values (0,0 on bottom left); I would then use ordinate dimensions to locate the machining datum which would be marked with a datum symbol (circle with x inside and two opposing triangles shaded).  The pitfall here is now your machinist is doing math, which could lead to errors and more time on the machine as they now have to crunch numbers if the plate needs to be flipped or rotated. 

Jason Bourhill

If your Machinist is Rotating/Flipping the sheet, then the sign on the Ordinate dimension would no longer be valid. The Machinist in affect has changed the UCS of the sheet :-). Have you talked to the machinist as to how he is setting up, and how he would prefer to work? Unless there is a hole in the middle of your sheet I would suspect that he is aligning to this datum point indirectly by using the edge of the sheet. Normally you would select a datum for ease of set-up, typically at one of the corners of the sheet. Also CNC machines most often don't use the same direction as CAD for X/Y. That said it would be quite easy to add a sign by simply selecting the dimensions, and adding a prefix from the property bar.

The other consideration is how you deliver the information to the machinist, and the actual purpose of the drawing. Its quite common to provide a DXF in this situation, here the Machinist will make up his own mind about datum and how he will go about the job. The drawing in this case may be used indirectly such as for check measuring.

Ordinate dimensions measure relative to your current UCS, so it is possible to have more than one datum on your drawing. Whether you would want to do this in practice is another thing. Attach a sample drawing to illustrate.

Regards, Jason Bourhill CAD Concepts 

OrdinateDatum.dwg

Joe Dunfee

 Jason Bourhill said;

Have you talked to the machinist as to how he is setting up, and how he would prefer to work?

That is, by far, the most important thing to do.  While it is good to adhere to widely used drafting standards, your main customer is the guy in the shop or in the field.  Doing what is an efficient use of time is also a factor.  The more widely used your drawings are across disciplines, or perhaps even other countries, then the more important it is to stick to standards.

I am always very hesitant to override what the CAD program put in the dimension. If you edit a value to add the "-" sign, what if you create a hole pattern that later gets stretched so that what was in a negative quadrant, is now in a  positive one?  The negative values will still be negative, when they should be positive.  The draftsman may catch it, and may not, since we normally expect the number in a dimension to automatically update.

If I explode or modify a dimension, I try to leave a visual clue by changing the color of part of the dimension, such as changing the text to be red.  This would serve as a warning to future users of my drawing that something is different.

If some of the objects are near enough to the origin, but fairly far away from the origin mark, then it may not be clear if it is positive or negative.  In that case, I would suggest extending a line from the origin to beyond the dimension in question.

-Joe

Joe Dunfee

 Jason Bourhill said;

Have you talked to the machinist as to how he is setting up, and how he would prefer to work?

That is, by far, the most important thing to do.  While it is good to adhere to widely used drafting standards, your main customer is the guy in the shop or in the field.  Doing what is an efficient use of time is also a factor.  The more widely used your drawings are across disciplines, or perhaps even other countries, then the more important it is to stick to standards.

I am always very hesitant to override what the CAD program put in the dimension. If you edit a value to add the "-" sign, what if you create a hole pattern that later gets stretched so that what was in a negative quadrant, is now in a  positive one?  The negative values will still be negative, when they should be positive.  The draftsman may catch it, and may not, since we normally expect the number in a dimension to automatically update.

If I explode or modify a dimension, I try to leave a visual clue by changing the color of part of the dimension, such as changing the text to be red.  This would serve as a warning to future users of my drawing that something is different.

If some of the objects are near enough to the origin, but fairly far away from the origin mark, then it may not be clear if it is positive or negative.  In that case, I would suggest extending a line from the origin to beyond the dimension in question.

-Joe

Joe Dunfee

 Sorry for the double post.  The web page would not give an acknowledgment of the post, and only said to try again later.

-Joe

Jim Canale

Sorry for my late response..  I have been tied down with other work.    I have 2D drawings with multiple objects dimensioned with ordinate dimensioning.  I could save the UCS origins, but I would then have to come up with a descriptive name for each.      So I always remember to re-set the origin whenever I work on a particular object.

I machine 70% of the work I design.  My ancient manual milling machine does not have enough travel to cut all the holes in the panels I design in one setup.  So I have to rotate or flip the panels to finish the machining.    It is much easier for me to set the UCS origin at the center of the panel, and put a target there for the milling machine zero.   Then after I have machined everything to the left of the target, I can rotate the panel, re-zero the mill, and machine the other side of the panel.   If I didn't use the center as the UCS origin, I would need multiple corner origins, and multiple sets of dimensions.  That would be confusing.

My concern is the other 30% of the time that I send work out.  I think the ORDINATE tool should always assign a sign, relative to the current UCS origin.  

Incidentally, I understand that introduce errors in the accuracy of machining when I rotate the panels.   A panel might not  be completely square.    But lacking a mill with a larger table, it is the best I can do.

Jim Canale

Forgot to thank everyone for their reply!  My bad....