Hello,

I'm working on a series of sheet metal parts to come up with a K-Factor for various thicknesses of sheet metal, and various number of bends. We started with some sample pieces (6" x 6") in various thicknesses from 16GA to 3/8" Hot Roll. In each thickness, we made several samples, (single bends, double bends, etc.) and took measurements of the bent shapes, including the resultant bend radius. I then took these measurements and used them in a mathematical equation that I've found on-line for accurately defining a K-Factor for a single bend, but when the same K-Factor is used on a piece with multiple bends, the total length of the flat pattern does not equal the 6" that we started with on our sample piece. Below is the equation that I used to find a K-Factor for single-bend pieces for each gage size and had great success with the flat pattern equalling 6.000". A and B are the overall outside lengths of each leg and Total Flat Length is the 6" flat sample piece.

**Bend Deduction = A +B – Total Flat Length**

**Outside Setback = (Tan(Bend Angle/2)) * (Thickness + Bend Radius)**

**Bend Allowance = (2 * Outside Setback) – Bend Deduction**

**K-factor = (-Bend Radius + (Bend Allowance / (π * Bend Angle / 180))) / Thickness**

Here is the eqaution with my numbers(in inches) plugged in for 16 GA:

**Bend Deduction = 3.054 + 3.056 - 6 = .110**

**Outside Setback = (Tan(90/2)) x (.060 + .060) = .120 (where (Tan(90/2) = Tan 45 = 1)**

**Bend Allowance = (2 x .120) - .110 = .130**

**K-Factor = (-.060 + (.130 / (3.14 x 90 / 180))) / .060 = 0.38 (where 3.14 x 90 / 180 = 1.57)**

I followed this formula for each GA thickness and, with the resultant K-Factor, had a flat pattern of 6.000" each time. But when I tried using the same K-Factor of 0.38 for a 16 GA piece with two bends, with an overall outside width of 3.595 and two legs with overall outside lengths of 1.320/1.321, the flat pattern length grows to 6.016". To get the flat pattern to equal 6.000", I had to use a K-Factor of 0.295. Why is there a difference when going from one bend to 2 bends when starting with the same size flat pattern sample piece? I have yet to start working with pieces that have 3 or more bends, because I have been stumped at 2......

Thank you in advance for any input,

Mike Knauss

Mike,

I've been bending a long time and tried to better myself by better understanding what happens to minor changes in the bend setup, allow me to share some of that with you. While there are some on this forum that only need/desire to produce models that may be close to actual production models, I (and apparently you) need to produce accurate models for our shops. This also eliminates errors in programming.

That said, while K-factors exist, they are simply an approximation. You also cannot use a K for 16 gauge and 3/8" thick stock and expect the same results. Not that they couldn't be obtained though. This would depend on the talents (read patience) of your press brake operators and available tooling.

Air bending is one of the bending styles available, coining is another. Air bending does not generate a bend radius equal to the tool radius, except maybe for a small part of a 90 degree bend. Coining does however. Each bend style has a different K factor as does each material given similar material thickness and die width ratios. 10 gauge stainless has a different K than 10 gauge CRS.

As you have found your operator came in with a different result, no matter the reason. I found I had less to no further occurances of that nature when I specified a bend deduction. A brake operators job is to achieve the specified bend deduction. When he cannot, the programmer/designer has made a mistake. Of course we can sometimes cheat and achieve the bend deduction, but all that does is make the job harder and most often a request (should I say demand) to fix it for the next time.

No doubt for many materials and bend radii your operator can tell you what to use as a bend deduction, I would ask. When there is no information, a simple test bend in scrap of the same material, gauge and grain direction will give you an accurate bend deduction. I always do at least three samples. Measure the flat blank or test sample before the bend, then make the bend. Now measure the two flanges and subtract the sum from your original blank size. That is your stretch or bend deduction.

Don't get me started on how SW handles jogs or offsets! LOL!

Bernie