Thoughts on bending metal

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Ol'fogasaurus
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Thoughts on bending metal

Post by Ol'fogasaurus » Sat Sep 17, 2016 1:38 pm

Since I am not sure just where to put this I am sticking it here and if it is considered important enough it can be moved.

Last week our freezer we keep in the garage had it’s thermostat go out again. Since it was a ’89 unit and since there were some sales going on we purchased a new one. My wife, being a person of great perspicacity in some areas keeps track of things and acts accordingly. She was out last night checking the new freezer out… again when she noticed some tape on the rear corner of the freezer. Being a person of action she pulled the tape off and found what she thought was a dent and she came in irked and complained about the dent to me.

I finally gave up and went out to check. Looking at it then checking the other side I found the same thing. What it was is an error in manufacturing and design. I mention stress risers a lot and the so called dents are indication of there being one at each of the top corners of the bent “tin” and this was a good example of just that.

(for more information of stress risers do a search on either stress risers or stress concentration (https://en.wikipedia.org/wiki/Stress_concentration). You can also do some research on the de Havilland Comet Comet and the discoveries made after the three crashes and the putting an airframe in a big water tank then pressurizing it through many cycles then finding out that square windows were the problem. This was in the late 40 or early 50s. (http://lessonslearned.faa.gov/ll_main.c ... LLTypeID=2)

These pictures are something I posted several years ago on how to relieve stress fractures and/or stress deforming. Basically it is drilling a hole at the apex of the corner cutaway where the flanges meet.

Image

This is the plate I was making flanges on on all four sides. I sprayed Bluing on all the sides to use for scribing my bend. I didn’t go to the detail of marking OMLs or IMLs as I didn’t think that in this case it wasn’t that important as my bends were not to be 90° but 45°. The flanges were not to be structural as much as keeping liquids in the tray if I ever added liquid cooling.

Image

Here you can see the corners (I double checked parallelism and found a slight error hence the double lines) then drilled a hole the diameter of which was double the thickness of the material. It could have been bigger but this was the minimum. After the bending then I welded the legs together as well as the relief hole.

Image

This shows the pattern and the cut lines to remove the overlap now added for the bend.

If you don’t do the relief the corner of the bend will want to rise and you can see where the bend is pinched to the max on the underside of the material and stretched to the max on the outside of the bend. The radius of the bend as it gets to the corners will hump rather than lay flat and that is what caused the dent/dimple my wife thought that had happened.

As I have said before, when you bend metal the inside of the bend compresses and the outside of the metal stretches. You can only go so tight before either cracks start happening. If you bend something and the outside of the bend takes on a gray color those are stress cracks starting to form. In the inside of thin material the bend can start to fold over on itself kind of like a Pittsburg lock. Flat stock and tube both can act like this.

Square stock is a bit different as the inside of the bend will form a bowl. If you look at tube furniture you can see this if the bend isn’t done correctly. When I was doing some heat bending of square tube if I pulled the sand out too quickly the bowl would form. I had some square tube deform (the bowl) after sitting OK for over a year.

For what it is worth… before I forget all this stuff.

Lee

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Jadewombat
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Re: Thoughts on bending metal

Post by Jadewombat » Sat Sep 24, 2016 5:37 pm

Can you show the piece after you bent it/folded it? Those diagonal lines marked out are the excess where you cut before doing the bends?

Ol'fogasaurus
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Re: Thoughts on bending metal

Post by Ol'fogasaurus » Sat Sep 24, 2016 5:47 pm

I will try as the last picture did not post. I am having big troubles with Photobucket right now and I can't down load to them or use the pictures that are there for either here or email. Give me a bit, I just got out of Photobucket trying to up load some pictures and that was spotty. Trying to get the missing pictures up loaded now.

Lee

Ol'fogasaurus
Posts: 13099
Joined: Tue Nov 14, 2006 1:17 am
Location: Just north of Seattle

Re: Thoughts on bending metal

Post by Ol'fogasaurus » Sat Sep 24, 2016 5:57 pm

Image

This is the table before mounting. The table is this shape for both strength and if a cooling system was to be added.

Image

This is the bottom of the table.

I don’t know why I am having so much trouble with Photobucket. As long as STF is up and running Photobucket won’t work. If it is up first then it is a best guess if it will work. I am now using word and doing the write up here then copying to STF (probably shouldn’t have said that as now that won’t work!). I use copy now rather than cut and paste as even at times I can't get the cut piece to come over so I copy so I have a second or third chance.

Lee

Ol'fogasaurus
Posts: 13099
Joined: Tue Nov 14, 2006 1:17 am
Location: Just north of Seattle

Re: Thoughts on bending metal

Post by Ol'fogasaurus » Sat Sep 24, 2016 7:58 pm

Jadewombat wrote:Can you show the piece after you bent it/folded it? Those diagonal lines marked out are the excess where you cut before doing the bends?
I drew flat patterns as part of my job. The first picture shows the piece of flat stock already cut to shape including the material for the bent up flanges. I did not show the rest of the marks I would normally had to add for the pros to use.

It gets complicated when you are making super tight tolerance bent up parts; the table was not that big of a deal so I wasn't that concerned. What I was trying to show was the need for the corner relief and the slight trimming for a close, non-interfering bend.

Thanks for taking the time to read and especially to ask questions.

Lee

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Jadewombat
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Re: Thoughts on bending metal

Post by Jadewombat » Sun Sep 25, 2016 2:25 pm

Ah OK, I see now. Is that 16ga.? I've played with stuff much thinner on my brake, but still don't fully understand the parts that stretch and shrink. This helps a lot.

Ol'fogasaurus
Posts: 13099
Joined: Tue Nov 14, 2006 1:17 am
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Re: Thoughts on bending metal

Post by Ol'fogasaurus » Sun Sep 25, 2016 3:21 pm

I haven't been able to justify getting a brake but I sure have thought about it.

http://www.grizzly.com/products/12-3-in ... ne/T26472Z

This is the one I have been thinking about. I don't usually need more than 12" wide and don't have room for 30" or larger machine. These combo machines are a trade off but I have heard OK things about this. There are several companies that sell similar or the same thing under their name.

http://www.swagoffroad.com/20-TON-Press ... _p_40.html

This is the other style, a press style. This is a kit and requires thick metal welding which has slowed me down this way.

In a way you can compare bending metal to using a two wheel dolly. If you rotate the dolly on its center the outside wheel moves forward but the inside wheel moves backwards and this is kind of what metal is doing. If you put a stop behind the inside wheel then the outside wheel has to travel farther. When bending: the outside material stretches and thins out while the inside of the bend material compresses (to a point and this is the minimum bend radius that is so important. For example: http://www.wisetool.com/bendradius.htm or http://www.bjg-design.com/designbook/shbend.htm) and the compression can affect the stretch on the outside quite a bit.

When I was working with AL I had tables to follow for bending that covered brake, hydro or Yoder Roll forming. If you do a search on-line there is a simple formula that is used for nonferrous materials (an example: http://sheetmetal.me/formulas-and-funct ... allowance/).

Sorry if this is too detailed but I need to share before I loose it. Getting old it the pits!

Lee

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