In article ,
David R Brooks wrote:
My project involves folding 1.5mm and 2mm aluminium sheet (think tube
amplifieers

CAD packages such as Solid Edge will compute sheetmetal bending
allowance, etc., for a specified "bend radius". This of course works
fine if you are bending in a press, with V-dies formed to the radius
in question.

I am bending using a brake (the machine with a hinged "wing" that
swings up to make the bend.) The only adjustments I have a
1. Set the scribed bend-line forward of the edge of the clamping bar.
2. Set the "wing" to swing a little in front of that bend line.

Who made the brake you have? My DiAcro finger brake has two
settings -- but different from your description of yours:

1) Adjustment for thickness of metal to be clamped -- moves the
"fingers" up and down so the clamping is just over-center for
maximum grip.
2) At the back -- an adjustment to move the edge of the fingers
back relative to the joint between the "wing" (apron is more
what I would have called it) and the stationary part.

3) Actually -- there is also a third setting -- the stop pins
which determine just how far you can bend before the apron comes
to a stop. These pins can be moved to various holes appropriate
for different desired angles.

Is there a convenient rule for converting thickness & bend-radius into
those settings? The design has 2 U-folded pieces fitting within each
other (simplifying somewhat): I want them to fit with minimal
clearance (obviously, not negative clearance!), so my bending needs to
be accurate.

I would consider the bend radius to be a function of how far
back the edge of the fingers is behind the line joining the apron and
the base. I think that you will have to try with some scrap of the same
type and thickness of metal to get it really right.

But -- for the most simple state -- when the fingers are exactly
one metal thickness behind the join line between the apron and the base,
you will get a zero radius at the inside of the bend, and a radius equal
to the metal thickness at the outside of the bend. (But if it is too
hard an alloy, you will also get a crack along the bend.)

If you are measuring the bend radius at the inside of the bend,
I think that a reasonable approximation would be the setback of the
fingers minus the thickness of the metal.

There are three reasonable places to make the radius measurement
-- the inside of the bend, the outside of the bend, and the center line
between those -- and I don't know which is the standard one to which
your software is working. I just set up with some scrap to get the feel
of the settings -- but I also don't use a CAD program for doing the
layout.

Perhaps someone else will respond with more experience in this
field. But at the moment, I see no other followups.

Good Luck,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

David R Brooks wrote:
My project involves folding 1.5mm and 2mm aluminium sheet (think tube
amplifieers


Cool, Iv'e built a few of these myself.

snip

I am bending using a brake (the machine with a hinged "wing" that
swings up to make the bend.) The only adjustments I have a
1. Set the scribed bend-line forward of the edge of the clamping bar.
2. Set the "wing" to swing a little in front of that bend line.

Is there a convenient rule for converting thickness & bend-radius into
those settings? The design has 2 U-folded pieces fitting within each
other (simplifying somewhat): I want them to fit with minimal
clearance (obviously, not negative clearance!), so my bending needs to
be accurate.


Look he

http://www.engineersedge.com/dist_bend_line.htm
http://www.engineersedge.com/bend_allow_calc.htm

I haven't done this in 40 years, but these look correct.

--
Gary A. Gorgen | "From ideas to PRODUCTS"
| Tunxis Design Inc.
| Cupertino, Ca. 95014