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Is there a chart of k-factors I can use?

 

Every now and then I get email asking, “Where can I get a K Factor Chart? Original chart, not someone's calculations.” This one came from Noel this week. So I thought it might be a good time to answer it publicly.

To the best of my knowledge this chart does not exist. You can find several technical manuals offering their theoretical chart.

Back in the 70's I was taught a rule of thumb:
Radius < Thickness, K=.25
Radius < 2 * Thickness, K=.33
Radius > 2 * Thickness, K=.5

While this is just a rule of thumb, try the math on a couple of your material thicknesses. The difference in the answer if you use a k-factor of .25 vs .33, it's only a couple of thousandths. Here a quick formula for Bend Allowance (BA) of a 90 degree bend is 1.57 * (radius + thickness * k factor). So for .048 thickness with a .0625 inside radius, with k = .25 the BA = .117, and with k = .33 the BA = .123, a difference of 0.006.

The real issue is that the k-factor varies not only with the hardness of the material, but also with the tooling and method you use on your press brake. The k-factor which works for one shop does not necessarily work for the next shop.

You may want to talk with people you know in other shops to see what they are using. Or perhaps a tech support rep for a press brake manufacturer. They should have a chart to get you started. From there, it is a matter of experience and adjustment. You must find out what works for you.

If you have a k-factor chart you would like to share with others, email to me and I will post it.

 

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