When you roll sheet is stretches. If you cut the material to the calculated length it will be too long. I would roll it over length and then get a strip of paper cut to the circumference required and wrap it around the metal and trim to length. Silver solder the ends and re roll to true it up.

When I rolled steel for traction engine wheels 12" dia the metal stretched 1/4" or more.

When I made the rims for my traction engine front wheels I calculated as follows. The OD of the completed wheel is 15", and the thickness of the steel tyres is ¼", so subtract ½" to get 14½", the OD of the rim itself. The rim material is 3/16" so subtract another 3/16" to get the diameter of the neutral line, assuming that the neutral line is at 50% of the material thickness. The neutral line diameter is 14.3125" giving a circumference of 44.964". That's the length I cut the metal to; well an approximation as I used a tape measure to check the length. Once rolled and welded the rim was pretty much spot on the right diameter. The T-ring, which was welded from 4 laser cut arcs, is a tight push fit:

Andrew

True for pyramid rolls, but not so for initial pinch rolls if you reverse the direction of the material at each pass. For rolling my wheel rims I made a set of initial pinch rolls:

Andrew

I think it is rather more complex than that. Out of curiosity I measured the OD of my wheel rim in four places with a micrometer. These are the values I got (nominal size 14.5" ):

14.490" 14.480" 14. 491" 14.494"

I assumed that the neutral line was 50% of material thickness with a sequence of cut, roll and weld, ie, no adjustments. I used initial pinch rolls to bend the rims, so there were no flats that needed to be removed or re-rolled.

Intuitively the location of the neutral line will depend upon the relative compressive and tensile forces. Many formulae are quoted for bend allowances. The key here is that the bend radius is often equal to, or not much more than, the material thickness. Again intuitively for a small bend radius the compressive forces on the inside will be higher than the tensile forces on the outside, the result being that the neutral line moves towards the OD.

However, for rolling wheel rims, and smokeboxes, the bend radius is much larger than the material thickness, so it is likely that the compressive and tensile forces are nearly equal and hence the neutral line will be around 50%.

Andrew

Edited By Andrew Johnston on 02/06/2015 12:05:30

Andrew,

If you need a more accurate calculation to find the neutral axis for bending plate check the web site which I listed earlier.

Paul.

I have the Expert version of Alibre, including the sheet metal function. One can specify the metal thickness, minimum bend radius and the K value, default value is 0.330. Once a part is designed by adding bends and folds then it is a simple button push to get a flat pattern to cut to, taking into account the metal used in the bends. I've used the facility quite a lot and it does seem to work. However, the facility does have some limitations and ironically, given the original post, one thing it does seem to fall down on is large radius bends. It fell over completely when I tried to model the brake band for my traction engines.

Andrew

I've never done this, but my instinct would be to cut slightly over length and file the ends to get it spot on. Hand filed won't be an issue if you are brazing the joint.

Neil

The rolls in the above link are interesting but so was the adaptation of a sander to vibrate the cement! The steel ring made with the rolls is for part of a furnace that is filled with refractory cement and a bar was bolted to a hand held sander and used to make sure the cement was evenly distributed. I looked at the video just to see the rolls but found my self watching the whole thing.

John

I have an interest in casting come up – parts for a telescope that I can't turn up on my lathe. It will save a lot of waste too as it will be mostly a hole. He's the best source of info I have come across and watching him work generally I get the impression he knows what he is doing all round.

He was also cutting thick steel with a circular hand saw commercially made for just doing that. Might be in the same video series. Not sure.

John

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Edited By John W1 on 09/07/2015 23:39:42