Thread: Tube bending

Hi All,

I'm making a boat canopy out of 25mmOD aluminium tube. Please see attached pdf for dimensions.

My question is how much tube do i need to allow for the radius of the bend. The bending die has an internal radius of 90mm. Based on the 2 * Pi * r theory that means the circumfrence on the internal cricle would be apporx 565mm, divide this by 4 and you get 141.25mm which would be a quarter of a circle. Is this correct or should i be using the outside radius which would be 90mm + 25mm(Tube OD) = 115 Radius. Using the above formula I would end up with approx a 723mm circle and a 1/4 circle approx 181mm.

The frame needs to be 1800mm wide from outside to outside.

Cheers

Michael.

Hi Mike
Whenever I have bent tubing, I have used the center line of the tube for the datum line. In your example, the distance for the radius would be 90 + 12.5 or to round it out, 102mm or 103mm. This should conform to your required dimensions. This computes to a length of +80mm for 102mm rad. and +81mm for a 103mm rad.

Kody

Always a perplexing question.

In a former life, I was involved in tube bending, however it was 25x25x1.6 erw.

I devised a formula after making a few test bends, and feel you may have to do the same.

Tube will bend around it's neutral axis, and this is generally not the centre line. The neutral axis will be somewhere about 1/3rd the way in, and depends on the cross section, type of material, and bend angle. The reason for this is that aluminium will stretch more easily than it will compress.

May I suggest this.

Cut a piece of tube to say 2m long, and record it. Slide your back stop a certain distance away from the centre line of the former, say 500mm, and record that.

Make your bends, both sides to form a U-shape, and note what you finish up with.

With a bit of smoke and mirrors, you should then be able to adjust your blank length and back stop position to get what you need, and only sacrifice one piece of tube.

Your formula for the blank length will take the form of

BL=O/A length of U x 2(for both sides) + O/A width of U - x amount of mm.

Your formula for the back stop will take the form of

BS=O/A length of U - y amount of mm

x and y will become apparent after making your test bend. Confused, I hope not?

Hope this helps.

Ken

Logically, I like the simplicity of Kody's approach but having bent all sorts of material and sections, I know that Ken's approach is best and will ultimately result in least waste.
This is because so much depends on the former profile and the material behaviour. Results will vary a fair bit depending on whether the former has a semicircle profile, or has the profile surounded by side plates to compell the material to maintain a circular c/s instead of partially flattening in the bend. Without side plates, the tube will tend to flatten and take a radius larger than the actual former, producing odd results.

Hi All,

Thanks for the replies. I can follow Kody's logic ok, but i'm struggling a bit with your formula Ken. If my canopy is 1300mm high x 1800mm wide and the tube is 25mm x 1.6mm wall then the following BL should be

BL = 1300 x 2 + 1800 - x How do I calculate x. Is this correct or am i way off the mark.

I'm not real sure what you mean by the Back stop either.

The bender i am using is on of these. see attach photo.

Cheers

Michael.

Hi Michael,

Leave it with me, I will prepare some autocad drawings, to try and illustrate the above.

Ken

Hi Michael,

Attached are 3 pdf files illustrating the procedure I used to used to calculate tube bending.

I'm sure there are rocket scientist formulae around, but this always worked.

This same method can be applied to bending sheet metal.

Just pop your Xmas cheque in the mail for Santa's "medicinal" requirements..............!

Only too pleased to help.

Ken

Ken you amaze me with your cad skills.

Hi Ken,

It's amasing what three drawing does for me. Thank you very much for doing that. I completely understand what you were trying to say in your previous post now. I will try bending up the frame this weekend. Will post results on sunday evening.

I did some research on the web last night and found this site. It has a simple bend radius calculator at the bottom. Maybe useful to someone.

http://www.rorty-design.com/content/tube_work.htm

Cheers

Michael.

Everybody, STOP - STOP
I desperately need Kens HELP

Ken
Can you please, please, tell me how to put the CAD drawings onto the forum? I found a program called "Virtual Printer" that converts my CAD work to a "pdf" file but how do I transfer the file to the forum. I have "Adobe 8.0" on my computer but is this the correct program to use? Your help will be most appreciated and I know Santa will bless you with all the goodies you desire this Christmas. Many thanks in advance for your fabulous help,

Kody / Joe

crawl-crawl, rolling over, waving paws, "Hey Kody, behave yourself!"

Kody, There are numerous programs out there that can convert any printable file to a PDF.

The original was Adobe Acrobat, now in versions 8 or 9. It has always been the best in terms of performance etc but is not cheap.

Alternatives that I use are PDF Creator and Nova PDF. You should be able to google free versions of both, or locate them on computer mag cover disks. Nova PDF free version puts a footer on every page proclaiming that the page was created with Nova PDF, PDF Creator does not have this problem, and is currently my preference. Files from the freebies are always somewhate larger in size than those from Acrobat.

When you install the program of choice, it creates a printer driver on the computer. To create a PDF from a file, simply activate the print function and select the Pdf generator print driver. When you tell the system to print, it will ask where you want to save the file, (folder and filename) then generate the PDF and display it for you in Adobe Reader or your default PDF viewer.

File can then be posted in the forums in the same way as a picture (jpg etc) subject to meeting size constraints. For instructions see here .

Hope this helps
Mal

It's as easy as Malb says.

I use CutePDF Writer, also free.

Google CutePDF Writer, download and install it. Go to Start, Control panel, Printers and Faxes, and you will now have CutePDF Writer listed as one of your printers.

Double click on CutePDF Writer to make this the default printer.

In the normal way, print your Autocad dwg. Instead of printing to paper, you will be asked where you want to print the file, and select a destination, say Desktop for now.

You will need to manually type in the file name, a small drawback with CutePDF Writer. Other programs may do this automatically.

Double clicking on this PDF file will open it with say Acrobat Reader to verify all's well. Ok so far?

When constructing your Thread in the Forum, select "manage attachments", under advanced, browse to your desktop, select your pdf file, and upload it as you would with a photo.

PDF files are generally quite small, so you should not have a problem with size.
Max 393 KB. An A4 size Autocad dwg converted to PDF, is around 50 KB

Wallah, jobs done.

Ken

Gentlemen,

Please note that the notes on sheet 3 have been revised.

When the back stop is moved, this distance also needs to be taken into account when calculating the blank length.

If you have a copy of the original drawing 3, compare it with the revised one to see the difference.

Even a "genius" has his bad days.

Ken

many years ago i lived on an island with no google and i devised a system to work out the offset length on a pipe bender ( each size requires a different value)
take a piece of scrap the size you will be using and measure say one metre from one end and make a mark( this will be the centre of the former) . bend the tube at this point to get 90 degreees . now measure the length from end of tube to outside of the bend the difference in length is the value you deduct to get centre of former mark.... hope this makes sense