Thread: allowance for a wired edge?

Actually I realsie I'm being over pedantic about it, got caught in a worry loop. I'm fretting over 1mm!
I'll got with using pi

Times 2.5 would be closer, you aren't going all the way around and you get most of the first 1/4 of the circle from the lenght of the side(if that makes sense). If its to long it won't roll right back under. But then it's been that long since I've done a wired edge I wouldn't have remembered 2.5 times if you hadn't said it.
Stuart

thanks Stuart i figure I've got to start my metal working somewhere, may as well be tinsheet.

OK now I might be stating the obvious but

Circumference of a circle is pi times diameter of circle

so the bigger the roll, the more material...

For a rolled edge - lets say around a piece of 3mm diameter wire (1/8 inch approx) there will need to be 3 x 3.14159 - lets say that is 9mm of material

For a bigger rolled edge - lets say around a piece of 6mm rod (~1/4 inch) there will need to be 6 x 3.141159 - lets say 19mm of material

If we use 2.5 then as stated, the roll will be 2.5/3.14159 (ie 0.8 or 4/5ths) of the circle with a gap with no metal - which may be exactly the effect wanted but my efforts at getting the metal to all neatly end in the same place might point me to want to hide the edge!

.. now how to actually do it and make it look nice - well as hinted I think there are others more practised and skilled than I!

If this is obvious then please just read the next post

Glad to try and help techgirl

As I said HavinaGo, you get some of the material you need for free. I did a little picture to try and explain. So you only need to cover 270 degree of the wire. (this is leaving a few things out but for tin plate on small wire won't matter.)

Stuart

As always the picture speaks a thousand words.

thanks Stuart

As a curious fellow this now has me asking

• Is the wire always the same size?
• How does one roll the metal onto it so it looks good?

Happy to be pointed to a reference or left to figure it out

"Is the wire always the same size"? No.
Do you mean "Is the wire allowance always the same size"?
In theory No. In practice pretty much. As the Dia of the wire goes up the allowance should get bigger. As the sheet thickness goes up the allowance should get bigger. But if you are rolling thick sheet with a hammer you may stretch it enough that you have to reduce the allowance. oh and good luck with anything over about 1.2mm

What thickness are you trying to roll?
Use a soft hammer. Working along as much of the length as you can fold it just a little each pass, trying for as long as possible to have the wire on the anvil. If the wired edge is straight you can use wedged shape anvil (can't remember what they are called "knife" maybe) You want to stop the wire from moving down, once its moved it a pain to get back. Lots of little hits. Once its over as far as you can get it with the wire still on the anvil, flip it on its back. Holding the wire in with your thumb, screw driver or vice grips, depending on how thick the sheet is. Hitting back towards the wire roll the sheet over about every 100mm until it is holding the wire. Then you should be able to join these up without the wire moving. I hope my "drawing" helps.

Check out the following link:

Sheet Metal Calculator Bend Allowance Equations and Calculator - Engineers Edge

Yes it is in inches but the units can be easily substituted for millimeters. Also, any good hands-on engineer should be bi-lingual anyway!

What has been said about the 3/4 circumference of the wire is correct but the allowance for extra material should be calculated on the neutral plane of the sheet being used rather than the inside face. The neutral plane for mild steel is about 0.43 of the thickness on the inside of the fold. For harder materials such as stainless I believe it is somewhat less but I will leave you to do your own research.

For a wired edge, the calculation should use a radius which is a combined total of the radius of the wire plus 0.43 of the thickness of the material. For instance , if using a 2.0mm diameter wire with 1.0mm mild steel, the radius becomes 2.43 mm (although rounding to 2.5mm would probably be close enough).

The neutral plane is that distance that does not change regardless of how a flat pattern is folded. If you can master this concept you will hardly ever (I won't say never!) stuff up a sheet metal development.

If calculating the allowance for a wire edge on the inside face of the fold (or 'roll') I suspect it would come up short.

I have never had the opportunity to make cake tins but I have done a few 'hems' (safety folds) where the edge is folded back on itself. For this I had access to a pan brake, and folded the edge over to about 135 degrees (or as far as the pan brake would go) and then pressed it down the rest of the way with the clamp leaf of the pan brake.

If I were creating a wire edge I would insert the wire before squashing it down, and perhaps finish it off with a few taps with a hammer on a solid surface using a forming tool (say a short piece of 5-10mm thick flat bar).

A good idea is to do a few experimental practice runs on scrap. You might need to use a packer between the top surface of the sheet and the underside of the pan brake clamp leaf to give you the required inside fold radius to fit the wire in.

You can't be over pedantic about this. I wish I had been a bit more pedantic about building my pan brake and now I could go out and test the advice I have just given. I designed it, had all the bits laser cut, but haven't been able to find one of those rare 'roundtoit' things to put it together....

Originally Posted by Gerbilsquasher

Check out the following link:


You can't be over pedantic about this.

Well if we are going to be pedantic, the neutral plane moves depending on the radius

Originally Posted by Stustoys

Well if we are going to be pedantic, the neutral plane moves depending on the radius

...not until it is folded! The neutral plane will move closer to the inside fold radius as fold radius decreases, but this is more to do with the distortion of the metal by the tooling, when fold radius is less than material thickness. Which is a bad idea as strength is reduced.

Whether the neutral plane ends up closer or further away from the inside fold radius depends on the tooling in the machine in question. This where the 'K factor' comes into play... an individual workshop should determine how much distortion a given set of tooling creates for a given thickness of metal and use this factor to accurately develop folded items.

An assumed neutral plane of 0.43 of thickness is good enough for most situations until it gets to some amateur hack brake press operator who doesn't know what they are doing.

Exactly. If we are going to be pedantic then the only way to find the allowance is to try a sample piece on the tooling we intent to use. If that tooling includes a hammer there will most likely be some stretching involved. 0.43 is just another approximation.
The approximation 2.5 x Dia already includes all the fudges you can think of, its an approximation. You think that we should add the approximation 0.43 to the approximation 2.5?
2.5 is not 3.14 x .75. But then the .75(3/4 circumference) is just a guess to start with as the "free" material I talking about is 1 radius, which is not 1/4 of the circumference. But then it doesn't matter because you will can't roll around the full 360 degrees anyway.
But then I've never tried to roll 1mm around 2mm Dia wire.

Stuart

P.S. On cake tins you should think about using double safety folds

P.P.S how much is strength reduces by a radius less that material thickness? Sure the fatigue life goes out the window, but strength?

Thank you Stuart - the picture makes sense and your writing about taking it slowly is important for my natural inclination to thump too much at one go.

You asked what thickness - I'm an interloper in this thread and I'm hoping techgirl will fill you in on the current project

For me -- my attempts with tin plate have tended to be old fruit tins being turned into other objects. The latest was a small model rain water tank. The current one is a light shade from an old spray tin. With more thinking I G I think the stretch and shrink in a roll will be too much to I might just solder a piece of wire to the edge or if I can find a bass pipe fitting, maybe even turn a piece and again solder it on.

Thanks again for the descriptive pictures and for the enlightening discussion.

Wow, what an informative bunch you are. Those are some really detailed explanations.

You all might be interested in this website that i stumbled across, it's full of very old books on all sorts of subjects.


haveinago I'd love to see a pic of your creations with scrap. I suspect that is what the majority of my projects will be made from.

No problem HavinaGo, hope I have helped

Coffee cans is where I get most of my tinplate, but I've been known to use fruit cans as well.

You want to put a wired edge on a cylinder? yes that's a little harder, its harder to stop the wire moving because you can't support in on the anvil as long(unless you want to make up a special anvil) and you have to stretch the flange first(Can't say I've ever done that on tinplate). Still it should be doable if you take your time and you use fairly small wire size,(not sure how big a flange you will be able to put on tin plate with a Dia of the cylinder I think you are talking about). If you really want to make your life hard, put the wire on the inside of the cylinder... also doable and as are you are shrinking the tinplate flange the size shouldn't matter.


If you dicide to give either a go I can try and give you some hints.

Stuart