Monday, March 17, 2014

How to Re-size a Pipe Cross-section on an Exaggerated Scale Without Thinking

The Scale Command's Tool Tip
I had a profile view with an 18” diameter pipe drawn in cross-section that actually needed to be 24” in diameter.  I know you are thinking, just draw a new circle with a 24” diameter.  Sure, that’s what I would usually do.  In this case though our cross-section was a roadway profile and we draw those with an exaggerated Y-axis scale.  In this case the x-axis scale was at 1:40 and the y-axis was drawn at 1:4.  We do this to help visualize the changes in elevations along the y-axis.  It’s a common practice in civil engineering plan and profile roadway designs.  These types of cross-sectioned pipes are typically drawn with an ellipse.  I could have taken the time to do the math and determine who wide and tall to draw my ellipse.  I also had a pipe drawn in at 18” that was properly scaled in across both axis.  I copied it and used the scale command.  What scale did I use?  24/18.   That’s not a scale.  Sure it is and it enabled me to get this job done in seconds.  Let me explain.

I had a pipe drawn at 18” in diameter.  It was already an ellipse so it was scaled properly in both the x and y axis.  I just needed it to be a 24” diameter pipe.  In order to show an 18” diameter pipe in a 40 scale drawing at a scale factor of 4 we have to draw a 15’ diameter pipe about the y-axis that is also 18” wide across the x-axis.  This makes for a tall and skinny ellipse.  A 24” pipe would be drawn at 20’ tall.  That is because we scale the y-axis “up” to put it at a different scale in the drawing view.  This math can make your head hurt so do the easy thing.



When you scale (by scale here I mean to use AutoCAD’s scale command) something “up” (make it bigger) you have a positive, larger than 1 scale factor.  To scale something “down” you use a positive smaller than 1 scale factor.  Think of it as a percentage if that helps to understand it.  A scale factor of 1, or 100% means it stays the same.  1.5 or 150% means it is half as big or 50% larger.  0.5 or 50% means it is half as small.

Scale Icon
In my case I needed to scale my pipe “up” by 1.33 or 133%.  I determined that scale factor by diving 18” into 24”.  But why take the time to do the math and then enter in the calculated scale factor?  That takes more time and creates several possible points of error.  I can make a math error, I can type something wrong on my calculator, or I can enter the data into AutoCAD incorrectly.  Instead I let AutoCAD do the math for me.  I started the scale command, selected the pipe to resize, selected my base point, then used 24/18 as my scale factor.  AutoCAD did the math and scaled my 18” diameter pipe to a 24” diameter pipe.

It took two seconds to do and I moved on to the next thing.  I use this trick all of the time when I am re-sizing something (like a pipe in this case) or need to change the scale of an object.  It is simple to remember too.  Here is what I do to determine a scale factor:


The top number (the numerator) is the size of the object that I want it to be (in my pipe example it was 24) and the denominator (the lower number) is the current size of the object.

Just make sure that both numbers are in the same units.  Using 2/18 would make my object significantly smaller and would be incorrect.  If you are not sure which number goes where or if you want a quick “check” to see if you are scaling the object correctly, remember that if the top number is smaller than the bottom number your object will get smaller.  If the top number is bigger than the bottom number your object will get bigger.

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