Sheet metal brake

Your thinking is right, IF the clamping leaf is adjustable. You need to be able to move the clamping leaf backward so that there is enough space between the nose of the clamping leaf and the edge of the bending leaf when you bend a peice of 16 ga. material to 90 degrees or past. If the clamping leaf has a fixed position for 20 ga. and you try to bend 16ga. when you get to the point were the bend is close to 90 degrees there isn t going to be enough clearance between the parts of the brake to allow for the difference in material thickness and either the bending leaf or clamping leaf is going to flex. If the machine has adjustability for varies thicknesses I would think that the owners manual would give you its maximum capability at different thicknesses.
I hope you can make some sense out of this.

If your considering a 4ft solid leaf brake there not as handy as they sound. Now a 20ga box brake (one that has a sloted nose bar) you can use to make alot of different projects. A new Tensmith 4ft box brake is probably around $600 new. Not the best for a commerical shop but is nice for a home shop.

To check a used brake before buying:
Look at the pivot points of the clamping nose bar for ware.

Flip up the bending leaf 90 and observe the gap between the leaf and the bed. This gap should be equal all the way accross. If not then look for ware. Not too much ware then it can be adjusted.

Take a scrape piece of 20ga black iron and bend a 90. If the ends of the 90 are under/over broke then the nose bar is warped. Most nose bars can be adjusted with the center nut a small amount.

T_Bone