Topic: How Precise is CAD?

[craigr]

If I draw a Rectangle at 0,0 & 400miles,400miles - then zoom way in at one of the corners, will it be accurate down to the 1/8" ??

craigr

[JCTER]

400 units isn't very much.

Sure it will be.

400 inches, 400 miles, 400 parsecs... doesn't matter what the unit is.  Autocad just believes it's 400 long.  It has no idea what units are... you tell it that when dimensioning, and as far as Autocad is concerned, it's just text anyways.

400 is 400 is 400.

[craigr]

So when zooming in on floor plans, one would think that walls should be 3-1/2" plus 3/8"(x2) drywall = 4-1/4" thick.

Right?

When people draw a house for example, do they consider all of this?

Then, when the house is built, do the (good) construction guys follow it THAT close?

[JCTER]

I don't bother worrying about the thickness of a wall.  Stuff changes too often for me to go back and revise my drawings based upon every little finish change, like gyp board thickness.

Why would you ever draw that minutia level of detail on a floor plan?  That wouldn't -ever- show up on paper.

I draw my walls 4" thick or 6" thick, even, depending on stud width.  3 1/2" stud (or 5 1/2") + 1/4" gyp on both sides... it's a rough thickness, but it's good enough.  No way I'm ever going to worry about drawing every little thing exactly perfect in Autocad.

As for construction actually matching it... hell no.  They'll do their best, but the nature of wood construction and the methods involved, added to the fact that they can't spend all day worrying about fixing that last little trailing 1/16" off the end of their wall... no.

Where's this question coming from?  What are you trying to get at?

[craigr]

Just how things work.

I often use floor plans from outside sources and need to know the overall bldg dimensions as well as room dimensions for Electronic signal ranges.

Sometimes those plans are not drawn to scale or the scale has been changed for some reason. So, I try to find a door or something that I can 'make a guess' at it's size, then scale the entire floor plan off of that.

Sometimes I get bit doing it this way and it can cost us money. The last time I got bit, it costs us 2k.

So, I am always trying to learn how things work, in and effort get more accurate with my 'guessing'.

Thanks for the replies,
craigr

[JCTER]

I have no idea why they would scale their floor plans :\  Should always draw full scale.

But anyways... the "important" things get drawn accurately.  Out to out of building (or out to out of concrete foundation, or steel line to steel line) gets drawn accurately.

[Birdy]

You mean the important "to me" things get drawn accurately.
Craigr, that's why you get bit. Because other people dont draw things accurately.... because it isn't important to them.

So you should charge your customers more money to cover yourself from other peoples carelessness.

[craigr]

That happens to be one of my faults, I expect things to be done right.

Some of my coworkers tease me, they call me 'anal' (obsessive), about getting things right.

I have a difficult time with 'that's good enough'.

Of course, this means that I spend more time on tasks, and as we all know, 'time is money'.

[JCTER]

How would it have been drawn on the board?  Would your floor plan worry about the difference between 3/32 gyp board and 1/4 gyp board?

I don't draw my trim to be 100% accurate on the sections, either.  I just draw a quick representation of generic trim.  Not my job to design/pick the floor trim or moldings out.  So it's not my job to draw it either.

[craigr]

How would it have been drawn on the board?   Would your floor plan worry about the difference between 3/32 gyp board and 1/4 gyp board?

Good Point.

[Birdy]

3/32" gwg?
are you serious? 

[Birdy]

on the board it would've been redrawn.
that's the point.  CAD is supposed to allow us to collaborate and share the design data.

[JCTER]

3/32" gwg?
are you serious? 

I was thinking font heights, apparently 

[JCTER]

on the board it would've been redrawn.
that's the point.  CAD is supposed to allow us to collaborate and share the design data.

If it's a 3D BIM model where the linework is regenerated when you update the wall family with a different construction assembly, then sure, but I'm not going to go through a whole building and change every partition wall's linework in a floor plan so that the thickness of the walls reflect the owner's last second decision to switch the gyp board thickness to save a few bucks.

That would incur a bucket load of costs, incur a change order, or swallowing a bunch of cost that unnecessary.  No one is going to be able to tell the difference when the drawings are printed on paper, anyways.  No one should be scaling the wall thickness to determine anything.

[Birdy]

If it's a 3D BIM model where the linework is regenerated when you update the wall family with a different construction assembly, then sure, but I'm not going to go through a whole building and change every partition wall's linework in a floor plan so that the thickness of the walls reflect the owner's last second decision to switch the gyp board thickness to save a few bucks.

Whose job is it to change that stuff, regardless of how "last minute" it is?  If the drawing isnt changed, or spec noted, now all my door jambs are the wrong size!

That would incur a bucket load of costs, incur a change order, or swallowing a bunch of cost that unnecessary.  No one is going to be able to tell the difference when the drawings are printed on paper, anyways.  No one should be scaling the wall thickness to determine anything.

We are talking about electronic CAD files, not printed drawings.

edit:  if you know the stud is 5 1/2" why on earth would you draw it anything else?

[Greg B]

No one is going to be able to tell the difference when the drawings are printed on paper, anyways.  No one should be scaling the wall thickness to determine anything.

Strong assumption there and a big old should or should not.

What's the point of using a computer to draw something if you can't be spot on accurate?


The only variables you are going to contend with is actual material size (it is going to be a little different) and the people actually doing the building (no one is perfect).

In the end...your drawing can be perfect, the labor and materials aren't going to be.  As long as your drawing is perfect anything that goes wrong can't be pointed back to you.

[craigr]

That 2k mistake mentioned above, was pointed at ME because I am the one that did the electronics layout. - Even though the floorplan I was given was either 'scaled down' to 2'x2' or was originally drawn that way.

Accracy!!!

That is a problem that will NEVER be solved. Different people draw different ways, either by lack of knowledge or they just don't care.

Since that last goof, I double and triple check my scaling. In fact, I have been asking the Salesmen get me an accurate measurement or more in the physical bldg.

[Birdy]

I've been using CAD for almost 15 years, and have learned to never trust anyone else's drawigns for just that reason craigr.

Currently I am on a house that checks in at just over 33,000 sq.ft.  We are charging tens of thousands of dollars for us to verify the framing is to the archietects drawings.  They are off here and there, at most 1/4", but overall quite good.  For our millwork, those 1/16th's that others dont care about are vital.

[Keith™]

I understand the concerns as well as the lack of concern for changing something that can and will likely be changed in the field. Understanding that in architectural work in residential homes, finishes are seldom specified on plans unless there is some specific reason they have to be. Homeowners and contractors too often change things like GWB thickness and add wainscote and a whole host of other things that we don't anticipate.

In reality a drawing with 1/2" GWB plotted at 1/4"=1' will be so insignificantly small as it would be nigh impossible to distinguish between 1/2" and 5/8" anyway ...

We draw sans finishes on all walls, mainly because we dimension to the edge of the walls .. it makes for more accurate wall placement when there is a close tolerance on clearances.

All of our walls are drawn at 3 1/2 and 5 1/2 with no exterior finishes applied either, this has been pretty standard regardless of who I have worked for .. although occasionally someone would use 4 and 6 because they are easy round numbers to deal with.

Now I have to take issue with one thing posted earlier, because it is true only to a point .. this is merely to clarify the issue ... If your drawing units are inches (most common architectural work) and you draw a 400 mile square, that isn't 400 units ... that is 400*5280*12 or 25344000 units .... this is why some people (namely architypes and surveyors) have issue with using one anothers drawings ... scale up the drawing by 12 .. scale down the drawing by 12 .. adnauseum

So Craig, if the drawings you receive are not to your scale, I would suggest asking the originator what units they are using for the drawing .. also, if you are talking about houses and such, knowing that exterior doors are typically 36" wide and walls are 4" or 6" (nominal width), you should be able to scale an exterior door to 36" wide, check the thickness of the wall and if it is something weird, then use the two to come up with a best estimate. Also, it might be useful to put a few dimensions on the plan you get so you can verify that they make some kind of sense .. like a standard tub is usually 5'-0" long. Given the usual suspected dimensions, (and making sure that you don't have some whacked out exterior dimensions like 49'-3 5/8") you should be fine.

[t-bear]

We're mechanical drafters here, working with steel channels and beams.  For accuracy, we DRAW to 8 decimal points, but dimension to the nearest 16th.  We do all our mech drafting in 3D, so if a 40' pipe run with two or three X-Y-Z elevation changes is not to 8 decimals, and we shorten/lengthen it by 10-20', the new end point can be 1/8"+/- off.  That in itself may not matter, but the accumulative effect overall can be a real mess.  You might get away with "fudging" in 2D but 3D is unforgiving.  AutoCAD functions to 8 points, so we draft to 8 points.

[craigr]

What do you mean that you draw of 8 decimal points?

Where is that adjustable?

[Bob Garner]

Typically wood framed plans are dimensioned to the face of studs, aren't they?  So finishes shouldn't matter.

If you are anal and like accuracy, you should try steel detailing.  It's pretty much got to be dead on accurate.

[sinc]

If I draw a Rectangle at 0,0 & 400miles,400miles - then zoom way in at one of the corners, will it be accurate down to the 1/8" ??

I guess it depends on what you mean by "accurate"...

In the real world, a 400mile X 400mile square would be distorted by the fact that we're on a round planet, but your Autocad drawing would not have the same distortion.

[Birdy]

Typically wood framed plans are dimensioned to the face of studs, aren't they?  So finishes shouldn't matter.

Depends on what world you live in. 
In my world, we see doors and windows in an awful lot of the houses, so yeah, it really does matter.
Do I need to see 1/2" GWB on the plan? No, but I do need to know if it is 1/2" or 5/8" GWB, not to mention if there is 1/8" skim coat over top or not.

But if there is a finished wall line in the plan at 1/2" off face of stud, and spec is 1/8" skim coat plaster on 5/8" GWB, then someone should be unemployed if they were to lazy to make the change.
 :pissed:

[t-bear]

What do you mean that you draw of 8 decimal points?

Where is that adjustable?

Type "UNITS" at the command line.  A dialog box pops up and you can set your units of measure there. For us, both length and angle are set to "type" decimal and "precision" 0.00000000.  Therefore, when you run the distance command on a line, it will measure 84.00000000 inches in the X and 0.00953989 inches in the Y, or some such.  When extruding pipe runs, we need this to be 84.00000000 in the X and 0.00000000 in the Y. That .0095etc might seem insignificant, but turn that into a 3D pipe, stretch that pipe to 60 FEET.....now the ends vary by .11447868...almost 1/8". Can you see where this is going?

[SDETERS]

There is a tolerance with a Cad Packages.  It is very small for example our 3D software is only accurate down to .000394.  So if you try to change a dimension by .0002 the software does not really see the change.  You have to change it bigger then adjust back to the dimension you wanted to change .0002 by.  I do not know the Autocad tolerance but I am sure there is one.  It is probably so small it is insignificant.  You can only take out decimal points places so far until ones say I had enough.

[t-bear]

There is a tolerance with a Cad Packages.  It is very small for example our 3D software is only accurate down to .000394.  So if you try to change a dimension by .0002 the software does not really see the change.  You have to change it bigger then adjust back to the dimension you wanted to change .0002 by.  I do not know the Autocad tolerance but I am sure there is one.  It is probably so small it is insignificant.  You can only take out decimal points places so far until ones say I had enough.

Shane...AutoCAD goes to 0.00000001........not that I worry TOO much about those last two zeros.....LOL

[craigr]

There is a tolerance with a Cad Packages.  It is very small for example our 3D software is only accurate down to .000394.  So if you try to change a dimension by .0002 the software does not really see the change.  You have to change it bigger then adjust back to the dimension you wanted to change .0002 by.  I do not know the Autocad tolerance but I am sure there is one.  It is probably so small it is insignificant.  You can only take out decimal points places so far until ones say I had enough.

This is mhat prompted my curiousity. I just didn't know how to ask it.

[mjfarrell]

So when zooming in on floor plans, one would think that walls should be 3-1/2" plus 3/8"(x2) drywall = 4-1/4" thick.

Right?

When people draw a house for example, do they consider all of this?

Then, when the house is built, do the (good) construction guys follow it THAT close?

only if you are really lucky....

[Krushert]

Even though everyone came and left, there is still some beer and food here to be consumed.

I my office we spec 5/8" GWB for 96% of the time. 3.98% is for the 1"  GWB liner panel for shaftwall framing. The other 0.02% was for the one time that we used 1/2" drywall for some special reason that I can not recall at the moment.  Furthermore we draw our wall from face of GWB to face of GWB and we dimension to the center of the wall.  The last recent retired old timer (and he was the president of the firm) drew based upon the stud sizes.  We had to go behind and change the lines out to face of GWB lines.  The reason is for ADA clearances and for casework.  Personally IMO if you draw the walls to face of gwb and then when it is time for adding casework or ADA clearances there is less thinking involved.  Yes stuff changes, that is the nature of our work and why we use electronic pencils and erasers.   

My two cents and you could have added some more ice to the beer before you left.

[Bakerman]

Here is a pretty good explanation of AutoCAD's accuracy:


http://www.drack.info/wpsmoke/accuracy/accuracy.html

[JohnK]

> How Precise is CAD?

* Se7en raises hand excitedly: Oh,O, 32?!

[Bob Wahr]

Now I have to take issue with one thing posted earlier, because it is true only to a point .. this is merely to clarify the issue ... If your drawing units are inches (most common architectural work) and you draw a 400 mile square, that isn't 400 units ... that is 400*5280*12 or 25344000 units .... this is why some people (namely architypes and surveyors) have issue with using one anothers drawings ... scale up the drawing by 12 .. scale down the drawing by 12 .. adnauseum

International feet or US Survey feet?

[Greg B]

Now I have to take issue with one thing posted earlier, because it is true only to a point .. this is merely to clarify the issue ... If your drawing units are inches (most common architectural work) and you draw a 400 mile square, that isn't 400 units ... that is 400*5280*12 or 25344000 units .... this is why some people (namely architypes and surveyors) have issue with using one anothers drawings ... scale up the drawing by 12 .. scale down the drawing by 12 .. adnauseum

International feet or US Survey feet?

One has sandals the other has blisters?