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K-factor and bend allowances 5
- Thread starter macatac
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There are several theorys on sheetmetal bending, and it all boils down to how precise you need to be. I have found the only error free method is to test bend the material on the machine it will be made on. If you can't do this, try "The Machinist Handbook". A good website is
Hope this helps.
Hope this helps.
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- #3
The easiest way is to draw the profile of what you want to bend. Give it square inside corners and then use you inside edge dimensions to get your layout. Essentially this is the same as using a 0 bend allowance. I reasearched the bend allowance and k-factor and spent ages measuring the actual internal bend radius that was coming off our machines etc. I started drawing parts in Solidworks with accurate internal bend radiuses etc then the corner notching became to difficult. I eventually just went out into the shop and asked the guys how they do it an this was the answer was simply use the inside edge dimensions to get the layouit size. If it is only 16 guage it will work fine. It is when you get above 3mm or 4mm sheet and plate that things get tricky. Keep it simple an you will get good results.
Another thing that will come in handy is corner relieving. If the parts are being punched or laser cut hit the corner with a 3mm punch or hole. this is for edge to edge welded parts. The hole gets welded over and you don't end up with a sharp point that needs to be sanded off.
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- #5
There are several formulas to be had. You can have charts that give you bend allowances for different gauges. The problem is, it also depends on the tooling and process for your company uses.
One of the companies that I worked for chose their die by multiplying the material thickness by 8 times. This gave us the die width to use. Then we had a chart that told us which bend allowance to use depending on whether it was stainless steel, aluminum, or mild steel. All we used there was air bending.
Another company I worked for multiplied the material thickness by 6 to give us the die width. We used strictly coining, but air bending when absolutely required. We used the same formula for bend allownaces regardless of the material being used. The charts there used a different formula and k-factor for computing the bend allowances.
Then you get into deep discussions about the difference between bend allowances and bend deductions.
After all is said and done, the best way is to cut a small blank, maybe 4" by 6". Debur it to have accurate dimensions. Measure the 4" width with calipers, and write each dimension on the corresponding side. Form it at 2". You now have 2 legs roughtly 2"+ material thickness apiece. Again use calipers to to measure the legs. Add the length of the 2 legs. Subtract the length you wrote down from the combined formed length. Do the same for the other side. You now have the bend deduction for that material thickness using that die width and that punch radius.
Here is a link for an incredible FREE little program that helps you calculate bend deductions and even lets you reverse engineer the k-factor like my long winded explanation above.
Flores
One of the companies that I worked for chose their die by multiplying the material thickness by 8 times. This gave us the die width to use. Then we had a chart that told us which bend allowance to use depending on whether it was stainless steel, aluminum, or mild steel. All we used there was air bending.
Another company I worked for multiplied the material thickness by 6 to give us the die width. We used strictly coining, but air bending when absolutely required. We used the same formula for bend allownaces regardless of the material being used. The charts there used a different formula and k-factor for computing the bend allowances.
Then you get into deep discussions about the difference between bend allowances and bend deductions.
After all is said and done, the best way is to cut a small blank, maybe 4" by 6". Debur it to have accurate dimensions. Measure the 4" width with calipers, and write each dimension on the corresponding side. Form it at 2". You now have 2 legs roughtly 2"+ material thickness apiece. Again use calipers to to measure the legs. Add the length of the 2 legs. Subtract the length you wrote down from the combined formed length. Do the same for the other side. You now have the bend deduction for that material thickness using that die width and that punch radius.
Here is a link for an incredible FREE little program that helps you calculate bend deductions and even lets you reverse engineer the k-factor like my long winded explanation above.
Flores
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- #6
The last post is good advice. The best way to get the K factor is to use your material and process and find the change in material length to develop the "K" factor. It’s the only way I have found to accurately provide RBA's. After establishing a RBA, I always recheck buy doing a simple drawing using the new RBA and have it run through our process. So far all have come back accurate. We run 30+ materials through our shop weekly.
If your interested I have a spreadsheet I've developed to do just what smcadman described in the last post.
If your interested I have a spreadsheet I've developed to do just what smcadman described in the last post.
- Thread starter
- #7
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- #8
I usually use the "Neutral Bend Axis" method when starting out. This is the theoretical part of the bent material that niether stretches nor compresses.
"When the inside bend radius is 2x the material thickness or GREATER, the bend allowance is equal to the inside radius plus 1/2 the thickness"
"When the inside bend radius is LESS than 2x the material thickness, the bend allowance is equal to the inside radius plus 1/3 the thickness."
This works well with most sheet metal and tubing. Some materials have a tendency to elongate slightly more or less than others.
"When the inside bend radius is 2x the material thickness or GREATER, the bend allowance is equal to the inside radius plus 1/2 the thickness"
"When the inside bend radius is LESS than 2x the material thickness, the bend allowance is equal to the inside radius plus 1/3 the thickness."
This works well with most sheet metal and tubing. Some materials have a tendency to elongate slightly more or less than others.
- Thread starter
- #10
I've wriiten a program for figuring BA and press brake set back. The formulas used in it are taken from over 10 years experience in the metal fabrication industry. It is used in the plant I work for and the engineers there found it very useful. If anyone is interested in it please let me know. I'd be happy to share.
htaed@comcast.net
htaed@comcast.net
The most acceptable formula for calculating bend allowance, and used throughout Aussi aviation industry is
BA= 2 pi x (inside radius + half thickness) x angle of bend in degrees / 360, as a formula 2 x 22/7 x (R + T/2) x A/360 if you are working on outside radius formula becomes 2 x 22/7 x (R - T/2) x A/360.
Works exceptionally well when completing repairs where a doubler must exactly fit a Z section on the corners.
In your application K = R+T.
Grue
BA= 2 pi x (inside radius + half thickness) x angle of bend in degrees / 360, as a formula 2 x 22/7 x (R + T/2) x A/360 if you are working on outside radius formula becomes 2 x 22/7 x (R - T/2) x A/360.
Works exceptionally well when completing repairs where a doubler must exactly fit a Z section on the corners.
In your application K = R+T.
Grue
Pbotimer, the formulas for plate would be pretty much the same as sheet metal if you use the same material-thickness:die-width ratio. For air bending, we use 8 x material thickness to get our die width. We have bent H.R.S. up to 1/2" thick using the same formula. This page has a few references on the bottom you might want to look into:
Flores
Flores
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