No need for a flat anvil, if you want to use the wheel to put a single curvature into a panel (non compound curve) you can use a low crown anvil on the bottom and put a rubber band (section of inner tube) on the top wheel.
yes you do to de stress a panel after working it.
After Ive wheeled a panel it can be pingy trying to twist etc so you give it a wash over with a flat wheel(obviously not if highly crowned)
I don't get how a flat anvil can be used on a panel with any crown. Shape of the panel dictates the min anvil radius needed to be physically able to wheel it and 90+% of that radius is only there for clearance/support. When i produce a panel that isn't stable i try to identify the problem area- the missed edge or under/over stretched transition etc that's 'locking' the instability in. About the only time i've used the rubber band trick is when i've failed to put enough curviture in the blank beforehand and only realised once it was too late to put it back through the slip rolls. Usually (i always get that bit wrong) i just tweak it over my knee etc but that obviously doesn't work so well when you're almost done and still need more curve in one plane only
Hotrodder, sometimes you can take a panel with crown and force it out of arrangement to to a flatter state to wash over it, but you have to be careful with the anvil edges digging in.
Why? I get that there are many ways to use a wheeling machine but i can't my head around this at all. A wheeling maching is about stretching metal, unless one of the wheels is covered/replaced with something compliant like rubber that's all it can do. If the panel being made fits the buck/pattern/whatever in some spots but not others and all you've got is the wheel then the bits that touch need raising to effectively 'lower' the bits that have too much crown. The other (direct) way would be shrink the high spots using something other than the wheel
John Glover explains it much better than me towards the end of Pt 1... http://www.youtube.com/watch?v=ENQptq1UiRk and into pt 2 unless i've gotten completely the wrong end of the stick in which case my appologies. To me a 'washing' pass with the wheel is about improving the surface by refining tiny discrepancies in tracking patterns i.e. same radius anvil that was used to shape it but with the pressure backed right off and even more attention paid to tracking
I,m sure Mr Glover advocates always using the lowest crown anvil whenever possible as most do. Panel shape will dictate that. Obviously the lower crown anvils have a larger contact area(assuming non full radius anvils are used). I describe a wash over to people as using a long board on body filler, it's a smoothing, levelling,relieving the panel type operation at low pressure.
I have small lowers so maybe I am just being lucky with the shapes.
Also, by lifting the panel up as you wheel you can actually shrink highs. (never done that myself but seen it documented by Nawrocki)
A wheel is also to stabilise metal, dont just think stretching
ps John Glover did his apprenticeship at dagenham, I did mine there 45 years or so later!
Stretch was perhaps a bad choice of word on my part. Compress works better... virtually always that results in the metal stretching as it's locally compressed, stabilise/stress relief = not enough pressure to stretch things macroscopically (noticably) but the mechanism is the same- microscopically the crystal structure is being rearranged in the exactly same way but with no real pressure instead of forcing a change of shape we're supplying just enough energy for things to relax. If the geometry is correct can set up a situation where compression (or hammering) results in shrinking instead of the more usual stretching, the metal needs to be trapped or restrained in the direction it wants to move leaving it no choice except to shrink into itself for this to work. I'm picturing the 'lift as you wheel' thing as 'flowing' the metal sideways which can sometimes have a shrinking effect?
How's that work for everyone? Still don't get the flat anvil thing lowest crown that fits the geometry of the panel, yep that's a given, but it's never gonna be actually flat
I,ll try and find the vid/pics of Cass Nawrocki demonstarting the lifting approach.
I still use the rolling dough analogy to explain wheeling techniques. I think it's possible to "chase" a bump off the panel by the lifting method.
I don't have the vocabulary to properly explain it so I,ll try and find the relative link.