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Flat pattern cutlength is inaccurate?

Hey guys,

       

              I created a cylinder in a sheetmetal and used flatten option,

 

            The cut length in the y direction seems inaccurate as  the arc length measure to be                 around 313mm but the cut length is 309 mm.

            The arc length should be equal to the linear length right ?

            I am sure 3-4 mm wont make much of a difference but just wanted to make sure i am             not making a mistake

 

            Am i missing something? Pls help

 

Thanks,

 

Balaji

              

9 REPLIES
Solution
Solution
Accepted by topic author balaji1991
‎02-16-2016 04:02 AM

Re: Flat pattern cutlength is inaccurate?

Hi,

 

Ohh no. The cut length depends on "Bend Equation". And Bend Equation depends on following parameters:

Bend Radius; Neutral Factor; Material Thickness; Bend Angle

 

In your case Neutral Factor (what can be 0-1) will modify your cut length!

 

BR,

Imics
http://solidedgest.wordpress.com/

Re: Flat pattern cutlength is inaccurate?

Ty for the help @Imics

 

I made another shape and i tried to flat pattern it but i am not getting a smooth and symmetric geometry, not sure what am i doing wrong here.

 

 

 

 

Thanks,

 

Balaji

Solution
Solution
Accepted by topic author balaji1991
‎02-16-2016 04:02 AM

Re: Flat pattern cutlength is inaccurate?

[ Edited ]

To further add to this, the arc length will not be equal to the linear length.  When you bend metal, the metal stretches.  This stretch is what the "bend equation" that @Imics mentions will determine.


There are many resources on the interwebz that discuss this.  Here is one such resource that does a great job of explaining the bend equation:

 

http://sheetmetal.me/formulas-and-functions/bend-allowance/

Re: Flat pattern cutlength is inaccurate?

[ Edited ]

Hi there @balaji1991,

 

What you have modelled there is not a typical sheet metal model. A "blank" is a new tool for developing complex forms that are likely being stamped.

Your "hopper" [for want of a better description] design would normally be made as two separate parts....a cylinder, then a cone, and then put together as an assembly.

Sean Cresswell
Design Manager Streetscape Limited
Solid Edge ST10 [MP0] Classic [x2 seats]
Windows 10

Re: Flat pattern cutlength is inaccurate?

Ty so much @uk_dave

Re: Flat pattern cutlength is inaccurate?

Hi @SeanCresswell

 

TY for the help.

 

 Yes i am aware it is made in two parts. I was thinking if there was a way to make it as a single part. 

 

Thanks,

 

Balaji

Re: Flat pattern cutlength is inaccurate?

[ Edited ]

OK,....could also do that, although an unconvetional approach, but modelling tools will allow it.

 

You can use two contour flanges First one for the barrel, then the second, a inward line for the cone section.

 

Sean Cresswell
Design Manager Streetscape Limited
Solid Edge ST10 [MP0] Classic [x2 seats]
Windows 10

Re: Flat pattern cutlength is inaccurate?

For radii that large i typically set the neutral factor to "1"

Re: Flat pattern cutlength is inaccurate?

[ Edited ]

My wheel house:

 

Flat patterns are calculated along the neutral axis. This is the plane/sheet where there is no stress in the bent part. Typically K=0.33 is a good approximation. Meaning 1/3 of the thickness is in compression and 2/3" of the thickness is in tension. The compression side is the inside of the bend.

 

K=0.33 (Think thickness = 1 unit-less) is typically the best approximation until you start to have bends past 165 deg, then K = 0.29 is considered a better approximation. Very small bend (under 10 deg) are better off using K=0.5, but the difference in value is not worth keeping track.

 

Now you have what is needed to look at the many reference about bend calculations on the web.

 

I have not found a problem with those numbers for Carbons, Stainless, and Aluminum.

 

The real issues is bend radius. Radius you have to assume from practical measurements or information about you're tooling. The radius drives everything about the bend. The radius is measured on the inside of the bent part.

 

The main rules of thumb there is the desire to have R = or larger than 2 X thickness. And never smaller than the thickness. Stainless tends to have very large spring back numbers (another can of worms). Aluminum is more likely to crack as the bend radius approaches the thickness.