Hi Joost,
While you will get good responses here, you may also try rec.boats.building.
For the job you are describing, you neednt be too accurate with the rolling.
For a job like this, what we would usually do is just put it through a
roller to 'break its back' as the technique is known. Once you have put
even the slightest roll in the plate, it will flex more easily. Once you
have rolled it, it doesnt matter too much if you went slightly under or
over; this is only 4mm plate. tack the 'straight' side on, then put one
tack on either curved side. Now use dogs and wedges to progressively pull
the pate and tack it in place an inch at a time. You may even find you can
just tack some pipe or whatever to one end of the plate and use that to
leverage the plate instead of dogging.

Shaun



"joost" wrote in message
...
Hi everyone,

Related the rusty boat deck described in my previous question "Cutting
bolt with minimal heat" I encountered a new problem.

Most of the steel of my 4 mm mild steel deck is fortunately still
there, but there is one place where a section of about 20x40 cm is
completely eaten away by rust. I´m planning to cut out this area as
far until the surrounding steel is again 3-4 mm and weld a new piece
of metal in there. Only complication is that the deck is slightly
curved. Not much, but enough to make a flat piece of steel plate look
ugly there.

What I would like to ask is what the normal procedure is to fit a new
4 mm mild steel piece in the hole following the curve of the
surrounding area. The curve is mainly in one direction and in the
middle approx 1 cm away from the "straight line". Perhaps there is
also some (much smaller) slight curvature in the other direction.

I thought about preforming the metal on a roller, but measurement is
not easy for that. I´d rather take some in-situ approach. Only the
material is too thick to deform with simple tools. Perhaps welding
some sort of long lever on the plate and use that to curve it while
tacking every spot that is on the right position? And cutting the
lever away afterwards? Or is it possible to use a propane burner to
heat the material and have it bend itself?

Related to this I´m also not sure if any problems might occur after
the piece is tack welded in the perfect position. Can it deform while
make the final welding all around? I´ve heard that in bad repair jobs
like this, the curving "flips" inward.

I´d be very happy with any comments on this.

Joost