I am trying to make a 34"dia ring out of hot rolled 8mm dia bar. My method is to make plywood disc (3/4" thick), and then wind the 8mm bar around the disc (about a turn and half or so), and then release it, and end up with just over one turn of 34" dia, then I'll cut off the excess and fit the ring into a circular groove of 34" dia (again the groove will be routed 3/4" thick plywood sheet) and weld the ends together. You've guessed it, I'm using plywood tooling - for cost reasons, and because I want to do everything myself. I made some earlier rings using this method those were 48" dia, again using 8mm dia hot rolled bar. After endless trial and error, I found that for a 48" ring, I needed a 30" dia plywood disc as the former to bend the rod around. I now need to make some 34" rings, but I don't want to have to go through all that trial and error again to find out what dia plywood disc I need as a former. Yet I don't believe I can extrapolate anything from my earlier rings. Is there any rule of thumb or formula to work out what dia disc of plywood (former) I need to make? Don't forget, I need the ring to be 34" dia after I release the force used to wrap the rod around the former (hence it is clear the disc must be significantly smaller dia than the finished ring).
Why don't you buy the correct piece of equipment for the job - see the link below. You might find it a lot cheaper than buying lots of sheets of 3/4" thick plywood - that you cut up - and then throw away - as you try to home in on the correct size of die to make your steel ring. Maybe - you can have a look through the internet - and find a second hand tube roller on Ebay that is cheaper. Hope this helps. http://www.harborfreight.com/tubing-roller-99736.html
More equipment? That roller looks far better and far better than anything I can find on ebay here, and yes I'd love to get one, however I always feel guilty about hoarding tools and machinery which I feel would be underutilised, and I doubt I would make sufficient use of it. My earlier trial and error involved cutting the plywood disc into successively smaller diameters until the ring diameter ended up right. Hence there was only one piece of plywood used, the wastage was mainly in the form if thin wooden rings as I reduced the plywood disc diameter.
O.K. - I did see a cheaper tube roller on Ebay at $130 - a little cheaper than the $170 of the new one I posted earlier. Personally, I would buy the new one - and do a proper job - and then sell it on Ebay once you have finished with it. As you have a welder - you might have a small hydraulic press - maybe?? If you do - you could make your circle from a polygon. This method is sometimes used to bend thick plate into a circular arc. On thick plate this method is sometimes used - see link below. You can see a whole series of straight lines on the plate - where the press brake has pushed the plate round through a few degrees - thereby turning it into a circle. Hope this helps. http://www.wenco.com.au/services/ro...e-press-machines-plate-forming-straightening/
Hmmm.... You are right, I have a welder, pillar drill, and small bandsaw. You can see my plywood disc mandrel looks attractive to me. I was hoping there was a rule of thumb or other formula I could apply to find out what dia I should make it. It has a couple of advantages, in that the ring would be easily repeatable and had a good chance of being quite flat (disc lies flat). It has the added benefit that I can roll the bar into rings (or rather a coil) at the steelyard, which eases my problem of transporting 20foot lengths of 8mm dia black bar home.
The "problem" you are experiencing is what is called spring back. It is possible to model this type of problem with non linear FEA - but to do so - you need to have the full stress strain curve - and know the hardening factor too. If it were a "small bend" - like I was talking about previously - with the polygon - then you could use a spring back calculator - detailed below. However, from what you have described above - with your 30" mandrel - and your final ring of 48" - I suspect that the ring is not "tight" on the 30" ring - because it appears to give a lot more spring back than I would have expected. So - I think you should expand your small workshop - and invest in a tube bender. I don't know where you are in the world - but in the UK - some hydraulic fitters - use the tube bender to bend hydraulic pipe (when the system is hard piped - as opposed to flexible hose - and it is a bit of an art - and it looks very nice if it is well done) - and these guys are very well paid - I have heard of £60K per year Hope this helps. http://www.custompartnet.com/calculator/bending-springback
Spring back it is. Yep, I certainly have springback. My original 48" ring was tight up against the 30" mandrel, so I had 18" of spingback in the diameter. I'm will have a look at the springback calculator, but as it refers to sheet material (which is rectangular cross section) I'm not sure I can apply it to round bar. Looks like I may be back to trial and error.
I agreed with #Lochnagar that the problem you are facing is spring back and you need to full stress stain curve. You should expand your work shop and for the efficient solution invest in a tube bender, for that solution you must contact with that Engineer who have experienced about the tube bender.
how big should this problem be? Well after two attempts my 3/4" thick plywood disk ended up at 21" dia. When I wrap 8mm black round it, when released I end up with my desired 34" dia coil, whixh I trim to end up with a ring of 34" dia, and I can do this repeatably without any difficulty or adjustment. I was hoping to avoid the trial and error by calculating the needed mandrel dia. Little did I know that the trial and error would be far less painful than trying to find out how to calculate it. I prefer my plywood disc mandrel solution because it does no more and no less than is required of it.
Valmiki, I am late to this post, but a simple ratio would get you the answer you need to within an acceptable tolerance. You may want to consider this if you find that you need another sized ring in the future. 48/30=34/x solves out to x=21.25. As long as you are working with similar materials, you should be able to expect proprotionally similar results.
