General rule of thumb is about 1 gauge "increment" thicker -- 12 guage mild steel to roughly 10 gauge aluminum (about 3/32"), or other non-ferrous metal. However, this depends on the aluminum alloy and temper. There are some aluminum alloys that are quite hard and brittle.
Also the bend radius should be no tighter than the thickness of the metal. Otherwise the metal gets real weak at the bend.
Finally, I think the "max" values given by the less expensive tools, like those from Harbor Freight, are to be taken with some scepticism.
Which of these metals fall under the 1 gauge thicker rule?
2024-T3 BARE ALUMINUM SHEET/PLATE
5052-H32 ALUMINUM SHEET
6061-T6 ALUMINUM SHEET / PLATE
7075-T6 BARE ALUMINUM SHEET/PLATE
This was actually going to be my next question. What exactly am I measuring (how do I make the measurement)?
Yeah, I was worrying about that. Based on your comments and that the part I'm bending is only 1/3 the rated capacity in width, I think this might actually work OK. Some of the sites I read about various brakes would list their max values when the material was max width and would say larger thickness can be done if using smaller width pieces.
These are alloys and tempers. No thickness is given. Most aluminum is stated in "inch" sizes, not gauge, so you'd look up the gauge for the machine (say 12 gauge) and use a conversion table to look up the nearest inch size. You can find these on the Internet or they'll be in any metal working book.
There will be a maximum absolute thickness that the tool will be able to accept. The "one gauge thicker" is just a rough approximation, given the softness of aluminum over steel, but it doesn't take into consideration the maximum thickness the tool will accept. So you might take in various small pieces of the aluminum you're planning on using to see if any are too thick.
The better brakes have a radius former, or there will be a radius set adjustment. If yours doesn't, from experience you'll find what kinds of bends work best. The bending roller already in the machine will probably suffice for the maximum thickness (12 gauge or so) metal you plan on using.