How do you avoid interference when bending a sheet metal box with a brake?

I have to admit I rarely use a bending brake and have never made a box before. It would seem problematic to me to do this because the three sides of the box that are already bent might prevent the 4th side from being bent because the other folds will get in the way.

If you watch the following video of an amateur making a box, you can see that he has this exact problem at 7:56 even though he has a pretty sizable brake:

Sheet Metal Box & Pan Brake Tutorial - Grizzly Brake in my home shop - YouTube

In the next video we see a professional making a box but the brake he uses is enormous:

press brake for making metal box - YouTube

So, is it just a question of having a gigantic brake? Is there any easy way to compute how big a brake you need to make boxes of particular dimensions?

The problem he's having is that his brake is an enormous piece of shit. (Grizzly, really?) If you were to try it on, say a Tennsmith, the sides of the box will fit in between the fingers. I notice he didn't cut any tabs into the walls. Think that one though.

jscpm said:

...So, is it just a question of having a gigantic brake? Is there any easy way to compute how big a brake you need to make boxes of particular dimensions?

Click to expand...

I guess the term "gigantic" is relative; depends what you're trying to make. Every brake out there, whether a manual pan break or a "gigantic" hydraulic press
brake, will have its limitations.It's easy to determine the maximum leg length that your machine will handle by doing a couple test bends--after that you design
around those parameters. Some items simply can't be made in one piece; you have to do some welding to put everything together.

Larry Dickman said:

The problem he's having is that his brake is an enormous piece of shit. (Grizzly, really?) If you were to try it on, say a Tennsmith, the sides of the box will fit in between the fingers. I notice he didn't cut any tabs into the walls. Think that one though.

Click to expand...

Give the kid a break, eh. While a Grizzly brake obviously isn't as good as a Tennsmith or a Brown Boggs his problem has nothing to do with the brand of his
machine, it's all about the size. Those legs would fit in a larger machine but that same machine would be limited to what size it would handle. Looks like he's
working in his apartment so using a bigger break (of any brand) isn't exactly an option.

Not sure what you mean about cutting tabs into the walls--the only thing that would have helped with that box would have been to make the legs shorter or
make the pan in 2 pieces...

Yep, its as stuff gets more complex that bending gets expensive fast. Things like this are were magnabends really excell,

Even on a modern $120K 120 ton 3meter wide amada cnc brake press you can soon run out of room - clearance and need extended tooling to do much over even 4" sides on a tray. Even with extended top tools you can really run out of room fast! This kinda stuff has to be considered at the design stage.

magnabend.

also, as with so many things, a machine that seems "large" (like, you need multiple people to move it) may actually have a work envelope or capacity envelope that is pretty small.

The problem is the type of brake. A pan or finger brake as shown is limited because of how the folds are made. The way to handle the problem is a press brake. The only limitation with these is the size of the part you wish to handle. See here.

Press brake - Wikipedia

The tooling is called a punch and a die. In common usage, the punch is goes in the ram (upper), the die in the bottom (lower) so that as the punch moves down, the work is wrapped onto the punch and into the die.

To form a four sided box, the punch is made up of sections to be equal in length to the inside dimensions of the box (minus a little for clearance). The die can be full length. The formed sides of the box straddle the punch. Video of a box being made.

Durma - Press Brake | Deep Box Forming - YouTube

Tom

registro automatico per pressopiegatrice press brake back gauge - YouTube

If your pan has equal sides, you'll use one punch for all sides like the video above.

If the pan has different length and width, you'll use a different punch or a combination of punches together for the longer side.

Wila Tool Changing Video - YouTube

As several others have said, a press brake solves a lot of the issues you run into with a finger brake (or box/pan brake, same thing). There is a lot of tooling designed for long leg-length bending on press brakes, like offset punches, so life gets easier in that way. Also, the forming of radii is more uniform with a press brake compared to a finger brake. With a finger brake, you really need to plan your work around its limitations. In general, pans and boxes can't be very deep, and you need to look carefully at how close bends are to each other, and which direction they bend, and...

You get one of these.

Baileigh magnetic sheet metal brake

https://www.youtube.com/watch?v=zWvMW1Zxoyc

press brake with extended punch or punch and die that are not centered, by that I mean they make some that are maybe 15 deg on one side and 75 on the other. Instead of the traditional 45 on both sides. That way the box doesn't tip upward as much while forming.

So far in my press brake learning curve I have learned that there are things that cannot be bent with out a pile of expensive tooling and most everything with more than a few bends bent in a brake requires alot of thought ahead of time on just how to bend it and what order.