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Sheet Metal Bending | Bend Chart

 

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What k-factor should I use?

 

Recently I have received a number of comments from our article about bend allowance. They all have one thing in common. “What is the correct k-factor to use?”

And the answer is, I can’t tell you. It really starts with all of the variables involved in the manufacturing process. Number one is the material being bent. How hard or soft is the material? Then there is the process, are you air bending or bottom bending?

Next is the radius of the tooling in your press brake. That’s right, don’t just sit in your office and say I want a .06 radius on this bend. Get out in the shop and ask what radius punch will be used when the bend is formed.

Can you start to see why this isn’t an easy subject? Well how about some answers. Referring to “Tool and Manufacturing Engineers Handbook, Volume 2”, I found a chart on page 10-4 which shows some data for air bending and it includes a bend allowance based on the other data included in the chart. Using the Bend Calculator, I found the k-factor for 90 degree bends to be as follows:

• Gauge - Thickness - Inside Radius - k-factor
• - - 20 - - - - 0.036 - - - - - - 0.039 - - - - - 0.3314
• - - 18 - - - - 0.050 - - - - - - 0.059 - - - - - 0.3224
• - - 16 - - - - 0.060 - - - - - - 0.078 - - - - - 0.3340
• - - 14 - - - - 0.074 - - - - - - 0.098 - - - - - 0.3963
• - - 12 - - - - 0.104 - - - - - - 0.137 - - - - - 0.3967
• - - 10 - - - - 0.134 - - - - - - 0.176 - - - - - 0.4016

So how about a rule of thumb and from there we will discuss this further in the Manufacturing Forum.

• Thickness > Inside Radius - - - - k-factor = 0.25
• Thickness = Inside Radius - - - - k-factor = 0.33
• Thickness < 2 * Inside Radius - - - - k-factor = 0.42

Any questions? I’ll see you in the forum.

 

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