Pipe flattens to wrong dimension

[willpsill]

I make a pipe @ 12mm, meaning a circumference of 37.7mm.  I verify the measurement using the inspect tool (after bisecting with a .1 degree angle section).  

However, when I use the flatten command, I get a flat section of 34.8mm width (losing ~3 mm from the circumference).  This is very odd behavior.  I also tried changing materials to canvas and paper with the same results.

 I can't understand why flattening changes the dimensions this much... 3mm is significant on a 12mm pipe.

FYI my purpose is printing pipe-cutting templates and I did manage to get the scale right using QCad.. meaning this is the only preventative issue.

Any help appreciated!

[TheCADWhisperer]

This is rational and logical real world behavior.

When you bend metal it compresses on the inside of the bend and stretches on the outside of the bend. (After all - the circumference of the ID is smaller than the circumference of the OD.) 

But in the flat state that length is equal.

This is referred to as Bend Allowance (or Bend Deduction).

The neutral plane (no compression or stretching) is generally not midplane in the thickness.  Often somewhere around 45% from the inside.  This is referred to as the k-factor.  If you are attempting to make a paper template for wrapping around the pipe to  cut "bird mouth" set your k-factor to either 1 or 0 (I forget which).  Now you will get Pi*dia=circumference as I think you were expecting.

[willpsill]

Thats true for metal.. but canvas, paper? Is there a workaround?

[jhackney1972 · Accepted solution]

If you want the flat pattern length measure to equal, come very close, the the circumference of your pipe, you will need to change the K-Factor of the material to 1.0.  @TheCADWhisperer mentioned, this is an unrealistic procedure as per his description.  If you cut a template using a flat pattern developed this way, you will have an inaccurate cut due to the same reasons he mentioned.

[TheCADWhisperer · Accepted solution]

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@willpsill wrote:
Thats true for metal.. but canvas, paper? Is there a workaround?

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I added additional information to my previous response on how to get a correct paper (or thin material) template.

[willpsill · Accepted solution]

Well that's a great solution for a printed template to trace onto an
existing tube 🙂

However modifying the k-factor wasn't straight forward, but I had another
idea based on your suggestions. I changed the tube thickness to .01 and
that got me within a tenth. Thanks!

[davebYYPCU]

Then you totally missed the point of the previous replies.