CAD Precision Snafu

A furnituremaker describes how a little imprecision in drawing corrections messed up his dimensions. (News flash kids these days can fix your foul-ups. Film at 11.) April 19, 2011

I designed a cabriole leg for a piece of furniture I have to build using AutoCAD2000. The top of the leg is 2" x 2" and the total width needs to be 52". I think we all know 2" + 48" + 2" = 52". My problem is I'm getting 51 15/16". Am I just having a bad day in CAD or am I doing something wrong? I've redrawn the face frames and the legs several times and am still get the same dimension.

Forum Responses
(CAD Forum)
From contributor C:
I get that once in awhile, too. What I do is stretch the item about .01 or .02 until the dimension reads 52". It will also list it as 52". I guess somehow a tolerance is not set right or something - I really don't know. I just cheat it a hair and move on.

From the original questioner:
Thank you. Below are three drawings of what I have. I tried the stretch but it didn't work. I also tried to make another drawing with these legs and still have the same problem. I've been using these legs for over 10 years and the only changes I make in them is to change the height of the post to fit the face frames on a new design. The top drawing is done by shooting a dimension from the outside of the left leg and then attaching the right leg to it. I then shoot a 48" line from the left leg.

Just the opposite on the middle drawing. On the bottom drawing I made two 2" x 2" squares, same size as the legs, did the dimensioning, and they're dead on the correct dimensions. The top two drawings are done by cropping the legs. I have no clue as to what the problem is - never had it happen before.

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From contributor T:

Try setting your units and your dimensions to the highest precision. You may find out that one of the items is just a hair under what it should be. I run across this sometimes and it is hard to pin down. It helps to use a construction line and offset all your points from one side to the other. Once you've done that, just zoom in tight to check your lines.

From contributor H:
Are you typing the line coordinates or snapping the line endpoints?

From the original questioner:
Thanks guys for all your suggestions - I've tried every one of them. Here is what I found today. I have several templates set up in model space. I have two that I design furniture in, two for cabinetry, and two for construction. The ones for furniture are the same except one is for white lines only and one is colored lines. I did a redraw on the face frames on the color template. The first one was done on the white template. They both measure 48" long on either template. I can take the colored frames and then insert the white frames on either template. When I connect the two face frames together the blue frames are 1/16" longer than the white frames, but they're both measuring 48" long. I can then insert the legs into the model space template and put them 52" apart. When I put the white frames between the legs, it likes 1/16" connecting with the other leg.

When I measure I get 51 15/16". When I put the blue frames in, I get the correct dimensions and the blue frames connect with both legs. I have no clue as to why it's doing this and it's never happened to me before.

From contributor S:
Check your Z axis. It may be possible that two points are located on different planes in regard to Z and so are measuring differently. Put your drawing in front or side view or just distance (DI) from different points and look at Z. And as suggested, set your Units to higher precision and to decimals instead of Architectural.

From the original questioner:
Thanks. Late last night I turned this problem over to my 16 year old daughter and went to bed. When I came back to my desk this morning, I found a note taped to my monitor that said, "mud in your face dad."

This leg has been worked a lot the last ten years with changes to meet the Chippendale Style furniture designs I do, and I think this caught up with me on this project. She started doing some serious zooming and checking where the knee of the leg meets the post of the leg. What she found is in the images I've uploaded, the one on the right being her redraw of the leg knee.

During all the changes in the leg, I had more or less corrupted it, never even thinking about rechecking the knee after each change. All I was doing was changing the height of the post, and doing this I had gotten where the knee joins the post out 1/16", I guess on a recent change I had made in the leg post. The left side of the post was also 1/16" out. She deleted the post and redrew the knee and put it back where it's supposed to be. She then took the blue frames I drew yesterday and inserted between the two legs at 48" apart. The measurements were then dead on what they're supposed to be.

When I started this design I put the legs at 52" apart outside to outside and then drew the face frames to the legs, not realizing I had a bad leg now. She removed the old legs from the side panels, placed her new leg redraw and those measurements were correct.

Because of both the front face frames measuring 48" and the blue frame being 1/16" longer, it makes us wonder if we still have a problem elsewhere. We're going to take all your suggestions and see what we find.

Now that I've got mud in my face, I've got to got to go buy my daughter a new 27" monitor so she can have more room to correct my mistakes now . Got to love them, and she loves CAD.

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