someone please explain bend allowance of fabricating sheet metal flush patches.?

Answer 1

Bend Allowance is the amount of material you need to add or subtract from a length of flat sheet to ensure the finished dimensions after bending are correct.

Material stretches on bending, at different amounts - this is dependent on the material type, material thickness, bend radius and angle through which it needs to be bent. (linked to neutral axis location)

This has nothing to do with overbending to allow for springback.

Do an internet search and there are plenty of links that can explain it further.

Answer 2

if you push on a piece of sheet steel, it flexes and returns to it's previous shape. this is the elastic deformation.
if you want to make a 90 degree bend in the sheet then you push it into the plastic deformation range. but if you only bend it 90 degrees, when you let go it will spring back a bit, maybe 2~4 degrees. so you make a 'bend allowance'. That is you bend it 92~94 degrees, and when you let go it springs back and ends up with a 90 degree bend.
the exact amount you have to over bend (the allowance), is dependant mainly on the steel. annealed steel like spreing steel usually requires more. cold worked steel usually requires less. if you want to calculate it properly you'll want to get a text book that discusses the process more thouroghly.

as for flush patches it sounds like you're doing body work. the same theory applies. if the body panel has a curve, you will have to work the sheet into a tighter curve inorder for it to spring back to the desired curve. doing that is more of an art that separates good body shop guys from amateurs.