Forums >> General Discussion >> Revit Project Management >> Unit tolerance

jasonkinnard

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Is there any way to restrict actual model dimensions to match whatever the unit tolerance is?

We have a model set to 1/2" tolerance, but when we sent the CAD export to a subcontractor, they pulled dimensions off of the file to 1/16" accuracy, which showed several different dimensions.

Other than making sure in the future to model everything at 1/16" tolerance before switching unit tolerance to 1/2", what can we do? I don't need to have elements accurate to 1/256", but just changing the unit tolerance does not prevent the model from being created in smaller fractions (which may happen unintentionally as component construction develops).

Thanks.

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WWHub

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There are two units in REVIT.  Project units are what you draw to.  Dimension units are what it is dimensioned to.  If you have your project units set to 1/2", you will find the problems you have. 

We set our project units to 1/256" ... then set the dimension tolerance to what we want. If you don't do that, you will also find dimension strings that don't add up.

We have found that REVIT is always more accurate then AutoCAD.  Because of that, we never trace a cad drawing.

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jasonkinnard

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Thanks for the response. The situation you describe is exactly what the problem is. Our project units are at 1/256", and our dim units are at 1/2". However, suppose you have 8 elements that are modeled to be 10'-0 5/8" apart. That is the actual dimension of the model, but the dimensions shown in a dim string will show 10'-01/2" for each. Then the overall dim string would show 80'-4". Yet the actual dimension since there is an 1/8" acutal difference, would be 80'-5".

While I foresee the future being that we will share Revit models with contractors and subs, right now the closest is to export to CAD for them to assist in their layout. We have an instance where a steel joist manufacturer has taken our file into CAD and they have a dimension tolerance of 1/16", which shows some variations with those dimensions noted on the official construction documents.

I can't think of any way this could work because of dimensional constraint abilities in Revit. For instance, if you wanted to space several items equally, you can dimension and constrain equally. When it constrains them equally, it may not model them to the dimension units, because the length and constraints may not be divisible into equal units consistent with the dim units.

The onus is on us to make a more precise model if we are going to share it with consultants and subcontractors.

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WWHub

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First - we have several different dimension units depending on what we are dimensioning.  1/2" tolerance is not good for everything just as 1/16" isn't.   We would never dimension steel, grids, overall building to 1/2" tolerance.

Second - If items are laid out as an equal spacing, we use the EQ dimension.

Third - If tolerances are acceptable, don't dimension a full string when another overall controls.  Leave off the last section.

Last - when laying out multiple spaces (elements), make all of them true dimensions that dimension w/o a tolerance and let the last one float.  Why have elements @ 10'-0 5/8" when you really want 10' 0 1/2" and the last one can be 10' - 1"?

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jasonkinnard

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Thanks. That is good and practical. I am still curious as to the implications going forward to the extent where the BIM becomes a construction document, given to the contractor(s).

More rigor is required in the modeling process, but all of us have instances where we don't model correctly, but instead just do a "work-around" for expediency, to get something to show up as we want it to on the drawing set. This behavior will be potentially detrimental when we start issuing to the BIM for contractors to use in the field.

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WWHub

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We are still adjusting how we do things.  Because REVIT is so precise and because everything comes out of the model, the old AutoCAD "work arounds" become a real pain!  When you cheat, it comes back and bites you.  We are having to be more precise in how we work and you really have to think furthur ahead.

When you lay things out so they are visually OK, you have problems in CD.  The wall wants to dimension as xx 1/8" so you move it but when you try to do that, something else wants to move so you hit disjoin and all of your tags go away!!!

Our thoughts .... have a working plan (never placed on a sheet) where you lay things out and have working dimensions on all the time.  FIX IT EARLY!!!!   As you are developing the model, change the temporary dimension from one that has a x/256" fraction to a real dimension.

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SideOutClemson

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I don't think you read anywhere in my post that I set my snap settings to 1/256".   I certainly don't set my snaps to that.  Dimension settings are as I noted and I do that precisely for the resaons you discuss.  The fact is that Revit accurately dimensions and you need to work accordingly.

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SideOutClemson

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I think you still have some things to learn about Revit.  What Revit does is that it understands we do buildings, not watches.  Thus the 1/256" rather than CAD's .oooXXX? whatever. 

What this means to you is we don't fudge dimensions like everyone did in CAD.  And when a line is parallel, it really is.  You will discover that when you bring and AutoCAD drawing into Revit, it will not be accurate because it will draw to those very small tolerances yet dimension to whatever setting you have. 

The result is you will grow to hate CAD imports.  

Set your snaps as you want.  Mine start at 1/4"... and so when I place something that is my smallest snap but it will be 1/4" - not some rounded off decimal.

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SideOutClemson

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I will admit I have a lot to learn in the program, however, I have worked in it long enough at this point to know that I would like a minimum working unit option. Unfortunately my terminology may be a bit off as I have bounced between AutoCAD and Microstation depending on the firm I'm with over the years. It's tough to take 16 years of engrained CAD terminology mixed in with new Revit terms and keep everything strait. I don't suppose calling it a grid snap with a grid that can be adjusted in spacing would help clarify it any? Even with a Revit snap setting in place your cursor can freely move so a line drawn in an unrelated location could end up with a 1/256" dimension seperating the two.

I've never let any of the drafters or interns on my teams fudge dimensions and often go into the drawings myself to double check them or add more information. I also often do my master planning in CAD which is now transitioning to Revit. Dimensions in Healthcare Architecture don't allow for much wiggle room and are highly regulated for most spaces and details so I can't afford to have dimension string errors.

In the end, when you get down to it, it's great that Revit is accurate to within 1/256" but in reality if you can't read it on a tape measure anything more is just useless in architecture.

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