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drawing precision

lexi

Is there a standard tolerance or precision for cad drawings to be drawn for building construction documents?

 
Mar 4, 09 5:28 pm
spark

Be realistic - no units smaller than the markings on a tape measure!

Remember that the construction workers will be working, through much of the process, in adverse weather or lighting conditions pullling dimensions off a string or a stake or off something that someone else laid out before them. As a general rule, you don't want workers on a construction site to have to calculate anything. Don't overdimension - just make sure all the dimensions they need are there.

Column grids - keep to even foot dimensions if possible

Concrete Masonry - 8" & 16" modules

Wood or Steel Stud framing - try to keep on 4", 16", and 48" modules with the smallest dimension being 1/4"

Window openings 1/8"

If you have a dimension of 17'-9 19/32" on your drawings, be prepared to catch some shit from the guys in the field. If they don't say it to your face, they will definitely say it behind your back.

Mar 4, 09 7:26 pm  · 
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well said Spark. Esp about the tape measure. But I'm all about having the feel of the workers in my construction and have had up to 1 1/4" tolerance before. One day I hope I can give them 6" and see what I get

Mar 4, 09 7:31 pm  · 
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dsc_arch

our cad dwawings tollerances are set to 1/256. if a random fraction higher than 1/8 is found we fix the model and not overright the dimension.

we hate getting backgrounds back that have toleraces of 1/8" it takes us forever to fix it.

Mar 4, 09 7:41 pm  · 
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Arzo

no one measures printed drawings...get over it and put a dimension.

Mar 4, 09 7:45 pm  · 
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toasteroven

don't give masons fractions of an inch unless they have to cut things for some reason. everything is 4", 8", and 16" - even rough openings.

window locations in wood frame construction are typically measured to the center line of the windows. some people do this differently, but I've always done it this way.

there is some debate over whether or not to measure to the face of stud, center line of stud, or face of wall - and people have differences of opinion as to weather or not it matters in wood frame or steel stud frame construction. center-line of stud is difficult when you're getting into rated assembles where you need two studs with an air-gap and one side has 2-layers of gyp and the other only one - so do you go to the center-line of assembly? I'm curious as to what other people do in this situation...

I always try to keep things to whole inches and as minimum 1/4" You can get tighter if you are doing casework, but in general try to stay away from anything smaller than 1/8".

in CAD drawings, nothing drives me more nuts than finding things that are 1/64 or 1/128 off and .0001 degrees off orthogonal.

Mar 4, 09 7:56 pm  · 
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mantaray

extremely fine millwork tolerance and detailling can be up to a 1/16" (it's not uncommon to see 1/32" tolerance on the millworker's own shop drawings, but you should probably never give them anything finer than that yourself. Millwork is your lowest tolerance trade, so outside of that, everything spark said applies.

Mar 4, 09 8:10 pm  · 
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mantaray

sorry --

** you should probably never give a fine millworker a lower tolerance than 1/16" yourself.

Otherwise, I agree with above that nothing else should be less than 1/8", and all rough work whole numbers where possible down to 1/4" where necessary.

Mar 4, 09 8:11 pm  · 
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lexi

In my opinion, there is no need for your cad drawing tolerance to be set higher than the smallest dimension needed for construction (1/16"). It is far more difficult to draw details to the highest 1/256 precision setting and not get a ramdom 1/256,3/256.

I almost feel that it is a matter of office policy. There should be a general note DO NOT SCALE DRAWING. Refer to architect if there is a question about a particular dimension.
Whether you are renovating, adding to existing building, or a new building you wouldn't want someone to pull a dimension off something that you do not intend to detail in that particular drawing or detail.
In my opinion cad is just a drawing tool, you use it to convey your drawings instead of with a pencil.

Mar 4, 09 9:22 pm  · 
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lexi

dsc_arch, if you recieve a background, for instance for an existing building, it would be a waste of time to try to fix it to 1/256 precision. The as built condition may be completely different. If you need to build onto or renovate, you would really have to field measure particular areas anyways and then only put dimensions that are needed for your new construction.

Mar 4, 09 9:29 pm  · 
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lexi

also, in the case of drawing many radiuses and thing with many curlyqs that are less than an exact half circle if your tolernce is too high you won't get a radius anyone can build.

Mar 4, 09 9:36 pm  · 
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dsc_arch

I was talking about kes drawings for new buildings. drawings that were given as bacgrounds to the kes, modified and then returned out of wack. Same goes for some structual drawings.

existing drawings I agree with you!

Mar 4, 09 9:56 pm  · 
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binary

from a trademans/shop/millworkers/exhibit engineers point of view....

pull dimensions from 1 end of the wall with centers of studs. then you can put in additional dims between centers if possible.... reason being is that it's easier to read/mark when a full dimension line and not individual lines that make up the entire dimension...

if you submit dwgs to a millwork shop and then decide that you want to change a few things by and 1/8" here and 1/4" there...forget it...once a project goes to shop, it then gets programmed for cnc/dados/etc... trying to go back and tweek a little here and there will more than likely screw something up depending on how far along the shop is with the project

keep everything to an 1/8" for building. if your making parts for a cnc/assembly then dims dont really matter but for double checking the x,y,z...

make sure you know your lumber dimensions.....actual sizes... also keep in mind the size sheets of laminate/ply that you can buy. most ply you can get up to 5x10 same with lam/veneer. so keep that in mind when designing.

don't ask for a bunch of samples of stains/etc and then not use the contractor/shop to make your product. this will only piss him/her off and more than likely charge you more later on.

Mar 4, 09 10:21 pm  · 
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it's just a drawing. I build shit!

Mar 4, 09 10:40 pm  · 
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holz.box

since i've been working on a lot of remodels lately, most of my drawings say "align with this adjacent piece of shit"

Mar 4, 09 10:46 pm  · 
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binary

question... how many times have you had a v.i.f. and each time the measurement is off by a 1/4"

Mar 4, 09 11:00 pm  · 
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lexi

when there is a V.I.F. dimension, they seem to always be off various amounts. I would give give hard dimensions to locate something, then v.i.f. for things are existing. when you physically pull a tape measure, you may be off a fraction also. or in an existing space, a string of dimensions, and leave a non-critical one out or put vif. for in stance hard dims on things like required corridors or clearances.

Mar 4, 09 11:21 pm  · 
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lexi

In a new building, I can't think off the top of my head where i would put a v.i.f dim.

Mar 4, 09 11:23 pm  · 
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lexi

dsc_arch, cad drawings from engineers are always drawn kinda wacky. I would just use then as reference and dash in their mech/ elec equip. and just draw the column grid according by reading their drawing. or by put notes such as concrete footing, see structural. I would not use their cad drawing.
what do you think of referencing a site survey in your site plan?

Mar 4, 09 11:40 pm  · 
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in other parts of the world you have to reference the site survey, so that it is seperate drawing included in the submission or construction issue of drawings. Typically it is the second drawing behind the coversheet (and is also the joke about why the surveyor always gets paid first).

Mar 5, 09 1:02 pm  · 
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dsc_arch

We tend to do the concept site design and send it to the CE.

The CE drawings are always the wrong size by a factor of 12 (basic unit setup 1 unit = 1'-0" or 1") we scale it up and strip it our of junk - typically it comes to us as 5 meg and winds up as 100K.

Most engineers have never here of purge block!

Mar 5, 09 5:21 pm  · 
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toasteroven

ah - I misunderstood your question - I set CAD tolerances to 1/256 - and then go back and check everything (like dsc_arch does).

I know this is really anal, but I do an overall measurement and then add everything up to make sure nothing is off. I used to get yelled at for this by an old boss (because I was apparently taking too much time to check the drawings), but I caught many dimensional mistakes due to sloppy drafting that saved that office a bunch of money in potential change orders. most interns have been getting better about using whole dimensions in their drafting, but it's always that one kid or senior person who started before CAD who screws things up (i.e. tolerances set to 1/4" or dimensions re-written so that they read whole inches). so - always check your strings and see if they add up.

Mar 5, 09 7:55 pm  · 
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SDR

Agreed.

How many years now is it since offices paid for a "checker" ? I assume this was just another drafter -- was it a separate job description ?

Mar 5, 09 11:16 pm  · 
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