Can someone please help me understand bend radius?

[Bubbleboy]

Im at the stage of bending 4130 Chrome Moly for my Pietenpol project. I understand that when bending these parts, you bend across the grain, allow at least 1 x the thickness of the piece as the bend radius. I know I cant just clamp the piece up in my vice and smack it with a hammer to bend it...as easy as that would be to do

 

Im struggling to get my head around the bend radius. As an example, if I had a piece of 3mm Chrome Moly I need at least a 3mm bend radius. To achieve this, would I need to bend it around a 3mm rod or 1.5mm rod? Radius being half the diameter.

 

Have I confused you yet?

 

Also does anyone know of a commercially available bending block with various radius made on each corner? I believe a beveled block is best so you can do a 90 degree bend with out the spring back effect. I know, I could make one but it would be a bit hit and miss I would reckon.

 

I know there are some very talented and knowledgeable folk on here who probably think I am a doodle for not understanding this but im never afraid to ask for help when im out of my depth!

 

Scotty

 

 

[turboplanner]

If it's the inside radius, then it has to match the rod radius - so 3 mm inside radius = 3 mm rod radius.

 

Have a look on google for home made folders - will take you a little time to make one, but then each fold is perfect, tales 30 seconds.

 

There are cheap folding blocks, I just can't remember where from, so maybe google again.

 

 

[Bubbleboy]

Thanks Turbs!...I have a bending brake here but some of the parts have tabs so cant use that. So I was on the right track! Thanks for the help

 

Scotty

 

 

[facthunter]

Calculating the length requires a bit more complexity. If you can trim the length after bending it is easier. Nev

 

 

[metalman]

Not sure what type of folder you have but usually you adjust bend radius by moving the front apron or in some cases the clamping blade, if your using a press brake then modifying a blade by taking the edge off to give a bigger radius works as well

 

 

[facthunter]

Are you bending solid? Nev

 

 

[Deskpilot]

Remember to double the inside bend radius to get the 'rod' DIAMETER. Never confuse the two.

 

 

[damkia]

Im struggling to get my head around the bend radius. As an example, if I had a piece of 3mm Chrome Moly I need at least a 3mm bend radius

 

bend it around a 3mm rod or 1.5mm rod?

If my logic is correct you would be running it over a 6mm diameter rod to achieve a 3 mm bend radius

 

 

[Bubbleboy]

Oh...it looks like I am not the only one here who is struggling with this! Now I am confused!

 

Nev...it is solid 4130 sheet.

 

Doug...yes I was thinking that as radius is half of diameter right?

 

Scotty

 

 

[old man emu]

Scotty,

 

Does this help?

 

http://www.nirvanatec.com/support/bend/kfactor.htm

 

OME

 

 

[Bubbleboy]

Scotty,Does this help?

http://www.nirvanatec.com/support/bend/kfactor.htm

 

OME

Hiya Mark...thank you for that. I guess what I am trying to ask is what Damkia said above. If im bending a 3mm thick piece to say 90 degrees, do I bend it around a 3mm or a 6mm rod? Radius is half of diameter so to get a 3mm bend radius I would have though I would need a 6mm rod?

Still unsure...hhmmm...

 

Scotty

 

 

[turboplanner]

A piece of 4x2 is not 4"" x 2". a piece of 25 mm rod may be nominal 25 mm only depending on the manufacturer and Industry tradition - we still have people making sheets with one dimension metric and the other imperial (metrication came to Australia in 1972.

 

So we talk about radius or diameter rather than a piece of "3 mm rod"

 

Best way to see it is to lay it out on a piece of paper

 

Draw roughly a 25 mm radius circle (A radius is from the centre to the drawn circle)

 

Draw a piece of sheetmetal around the circle maybe 5 mm think, so the radius is about 30 mm

 

You can use a compass, but hand drawn will show you

 

That's how you would draw as a draftsman, the radius is what you'd be working to.

 

Now, you can see the inner circle of the rod and the inner surface of the sheetmetal is the same - so the ratio is one to one.

 

The diameter is twice the radius, so the diameter of the rod you want has to measure radius (25 mm) x 2 = 50 mm.

 

 

[facthunter]

If the bend follows the shape of the former you are using and usually it will spring back some and you end up with a slightly larger radius. This is a little academic but happens. Principle... IF you want an inner radius of 3mm you use a former with THAT radius ie a 6mm Dia former. Nev

 

 

[Bubbleboy]

Thank you guys! Thats confirmed it for me!

 

I was on the right track but couldnt pin point an explanation like that on Dr Google!

 

Bending will begin!

 

Scotty

 

 

[djpacro]

usually it will spring back some and you end up with a slightly larger radius.

Not really, radius is unchanged but the angle of the bend is less.From my old Aircraft mechanics Pocket Manual, the bend allowance is

 

BA = (0.01743 x R + 0.0078 x T) x N in inches

 

where

 

BA - length of material required to make the bend

 

R - bend radius

 

T - material thickness

 

N - number of degrees or angle of bend

 

3 0r 4 degrees allowance for springback in steel sheet seems to be the go.

 

If it is annealed 4130 then the minimum bend radius for 3 mm or 1/8" thickness would be 1/8". Is it annealed or normalized 4130?

 

 

[facthunter]

All metals that are ductile will spring back a certain amount The others won't bend. They break. If you are making a spring you have to make a former that is smaller to allow for this If the bend is very even it will open out all along it. You can't reduce the angle without increasing the radius at the same time. Nev

 

 

[old man emu]

Then there's the question of heat.

 

If you cold bend, then the metal will tend to "spring" back a bit, but if you heat it cherry red, the metal will align itself as it melts and will not spring back.

 

OME

 

 

[facthunter]

Quite right, but the heated part will be annealed and not as strong as It could be. Some aircraft parts (like the undercarriage of a Bonanza) are heat treated alloy steel and any time heat is applied (as in a welding repair, or hot bend) it has to be re heat treated in a furnace to the proper specs for that material.. Nev

 

 

[Methusala]

If folding sheet metal in a bending break the way to increase the radius of the bend is to clamp an additional piece of metal above the one to be bent. The mandrel (or clamp holding the metal) is then in a raised position which forces a larger radius when the fold is made. Experimenting with this will satisfy you that the radius will be as specified. About 3-4 degrees should be ample allowance for spring back but this also can be tested by experimenting with a small test piece. The radius 3mm equals a diameter of 6mm. Don