having trouble with dimensions not adding up

[felixb]

Im making a window that is 300” I divide it up into 9 equal parts autocad lt 2012 divides it up @ 31 1/8” that is 280 1/8”

The mullions are two inches wide and there is 10 of them that equals 20” the caulk joints are 3/8” there are two of them that equals 3/4"

So 280 1/8” + 20” = 300 1/8” + 3/4 “ = 300 7/8”

[felixb]

anyone?

[hwalker]

Doing addtions like that will always confuse me

You state the window is 300" long yet you seem to be adding extra to it.

Think of it as 300" to the EXTERNAL edges of the caulking.

also when dividing your window into equal parts remember that th mullions will need to be 1" either side of the division.

EDIT: I've attached something I quickly knocked up.

[felixb]

here it is

[hwalker]

As with my attached drawing (See my previous post) I would work first with the 300" line then put the two 3/8" edges in and THEN divide the shorter line into 9 equal parts, also is your dimension fixed to 1/8" if so change it to 1/16"

To get you window right the spacing between the centre of the mullions needs to be 2'9"

What you did was divide the whole line without taking into account the caulk joints at the end.

Work it out this way.

(300"-(20"+6/8"))/9

[nestly2]

I didn't look at either drawing, but if you want each glass sections to be exactly equal, you should subtract the frames and the caulk from the overall width then add one mulion width back in (because you're trying to find the centerlines of the mullions)  I come up with 297.25 divided by 10 = 29.725 for the mullion centerlines  I did that with a calculator, not AutoCAD, so be aware of rounding depending on the Precision settings in UNITS.

[felixb]

I use the div command to divide in equal parts put it is rounding. i have it set to 1/16 idk whats up

[pendean]

DIVIDE command is not the proper method for your task: re-read the two long detailed responses you've received so far. They are using OFFSET as we as good math about the product being divided.

[nestly2 · Accepted solution]

I should have looked at the PDF as I misunderstood the number of divisions, It's still the overall width (300) minus the caulk and one mullion (2-3/4") = 297.25.

297.25 / 9 = 33.02778  ie 31-1/32" between mulllions (to nearest 1/64th)

It appears to me AutoCAD did the division properly, the problem is that you're relying on the Dimension to report the correct dimension, which it probably will if you turn it up to 1/256th precision.  Curently, each of the 9 window dimesions are being rounded up to the nearest 1/8, which totaled individually, adds up to a pretty signifcant rounding error.

Post the drawing and we'll be able to see for sure if its the mullion placement, or the dimstyle settings causing the discrepancy.

[Charles_Shade]

Probably should change Units to Decimal and not Fractional.

Much more accuracy.

[pendean]

But they still need to be aware of AutoCAD's rounding error when using DIVIDE command: it will present an inaccurate division at the two ends otherwise.

[nestly2]

Can you discribe the issue with DIVIDE command?  That's actually the command I would have used in AutoCAD after determining the length to be divided.

[felixb]

LOOK AT THE PDF ON THE OTHER PAGE ADD ALL THE 31 1/8" AND THE 2" MULLION UP IT ADDS A 1/8 INCH

[nestly2]

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@Anonymous wrote:
LOOK AT THE PDF ON THE OTHER PAGE ADD ALL THE 31 1/8" AND THE 2" MULLION UP IT ADDS A 1/8 INCH

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I'm not sure how to explain this any clearer...  the distance between mullions is not 31-1/8" (31.125)  it's only 31.02777777777778

If you used DIVIDE to locate the mullions, you either didn't start with the right line length, or your dimstyle is using a precision that's insufficient to reflect the actual distance between mullions  (Ie your dimstyle is rounding the dimension)

If you will post the drawing, I'm sure we can make it crystal clear.

[felixb]

Im having trouble attaching the drawing thats why i put it in pdf

[nestly2]

Try putting the dwg in a zip file and attach the zip.

[felixb]

thanks everyone for the help!!!

[nestly2]

I see you marked the topic solved, but I'll go ahead and demonstrate anyway. 

Best viewed in full screen mode. -->   http://www.screencast.com/t/vUHGZunlAo21

I'm not suggesting you dimension your drawings with 1/256 precision (because the guy that's building the windows will surely come after you with power tools) but rounding is something you have to be aware of.  Personally, I use 1/16 dimensions, but I always have length and angle precision in UNITS as high as they can go so small errors can be detected using the DISTANCE and MEASUREGEOM commands.   Lastly, I'll point out the 3/8" caulking dimensions at either end.  That's likely due to using direct distance entry, but your mouse was not tracking precisely horizontal when you entered 3/8 <enter>.  ORTHO and/or Polar Tracking would likely prevent those type of errors.