Determining the K-Factor for Sheet Metal

I notice that the K-factor is basically a reference, of which you would adjust it to your equipment, and metals. Keep the communications open between engineering/design and the fab shop. They will help you on this.

Its been a while but I had the chart for k-factors on a 3 1/2" floppy, I'll see if I can find it, hope I can access it, have to bring one of my computers out of retirement that has a 3 1/2 is floppy drive.

Ditto on Phoenix911's advice about communications. You always want to draw the bent piece and, if possible, let the metal shop do the development drawing. Otherwise, you can end up where purchasing changes vendors and you get to eat a whole bunch of incorrect parts (been there, done that).

If you have to do a development drawing, have the vendor do some test bends. Use that K-factor to do a development drawing, mark that drawing "for reference only for xyz vendor", and make sure the PO specifies that you are ordering the bent pieces.

The trouble you can get into is that K-factors vary for such things as grain direction in steel, etc. Not much, but a little. Normally, if you're doing 2 decimal, or fraction, sheet metal, you'll never notice the errors. But, there are people now doing 3 decimal and it will matter (Don't ask me why anybody is dumb enough to do 3 place, though I used to have an engineer working for me who did that for relay frames - it's expensive).

this link should help.

http://www.ciri.org.nz/bendworks/bending.pdf

Not sure if this is in machinist handbook or not. its at work. I'm not.

milo

Hello JJSEEKER:

This site looks pretty helpful. It has a K-Factor calc as well as other things you may need.

http://www.sheetmetalguy.com/

http://www.sheetmetalguy.com/bend-calculator.htm

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Formulae

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

Bend Compensation = Bend Allowance – (2 * Set Back)

Inside Set Back = tan (Angle / 2) * Radius

Outside Set Back = tan (Angle / 2) * (Radius + Thickness)

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Sheet Metal Thickness (Gauge) Chart in Inches

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Good luck, and I hope you find what you want.

like others said, you should only provide dimensions for the flat state of parts FOR INFORMATION ONLY.

all binding dimensions and tolerances should be specified on the fabricated part! mark that explicitly in your drawings.

but to start you on the road of building insights:

1. the inside radius of the bend (depending on the tool) varies between tools and shops. unless you specifically need a certain value for it, don't dimension it at all. with thin sheet, you will often have difficulty identifying ANY inside radius at all...

2. Both K-factor and spring-back angle depend HEAVILY on the direction of anisotropy: everything depends on the orientation of the part as cut (i.e. punched) from the original sheet. you realise, of course, that you can't be a control freak and specify this orientation, or even the dimensions of the original sheet (1.5x3m, 2x3m etc etc...)

3. rest assured that EVERY shop makes a prototype, even using your provided details, which it then measures and corrects lengths, angles & other process parameters incrementally. then they make a second prototype etc etc, until they meet your inspection tolerances (these you MUST provide, and be reasonable please!). because everything is on CNC, the production from that point on is completely (...) reproducible and series production begins.

4. So, you see, it is actually wrong to specify the details that you seek (and will be perceived as naive, or even be expoited by your contractors)

5. I do commend you on wanting to learn, though, and you should. so: a) stick around in one such shop, witness and get a feel for the process I described to you. b) get a good book; our posts here are hardly sufficient

best of luck!

a correction: in point 2 I meant bend allowance, not K-factor, which really doesn't vary...

a 2nd correction: disregard the 1st correction (I am having a bad day...)

No problem. Sounds like one of my posts where I change my mind in the middle.

Hello Guest,

I was waiting for the third post!.........Dam!!!

All my days are like that!!!!! Kinda glad I am not the only one like this................

Bend allowance with a K-Factor is calculated as follows:

BA=P(R + KT) A/180

where:

You'll have to solve for K but this is basically it

The estimated number if it close to a sharp inside corner is (minus 1 1/2 the material thickness), always a good ballpark number to remember.

example .060 thickness ----------- = -.090

example .100 thickness ----------- = -.150

Good Luck !

Thanks for the info guys, I had found a lot of the same websites during my own search but get tripped up by the K-factor portion of the equation.

I think the best advise is to leave it to the professionals at the shop to take care of the specifics..at least I can talk to them a little more inteligently...despite myself, thanks again.

JJ