Re: Bender question (long reply)

Hello,

I made a CNC backguage for my 3-in-1 (Enco, Mfg. Feb '94) some years
ago. It works for both the shear and the brake.

I threw away the original backgauge the day I brought the machine
home. IMO it is junk.

The new gauge uses 16mm printer rods which are 19-3/8" long. they
are held by thick fender washers and 10-32 screws going through
bronze bushings inserted into two existing holes in the blade/lower
die. At their rear is a 1-1/4 x 2-1/2" rect tube--alum--which has
the mentioned sprockets and chain to effect movement of two 1/4-20
leadscrews. It also mounte the stepper at one end and a sherline
handwheel at the other. The leadscrews act upon two large blocks of
UHMW which serve as both bearings and attachment points for the two
different fences. The press brake fence is a u-shaped extrusion 1-
12/ wide, with 1/2" legs, that is scred directly to the UHMW blocks.

The shear fence is in two parts, also fastened to the UHMW blocks.
Two parts makes it possible to adjust its relationship to the press
brake fence. Right now a note on the rear crossbar reminds me that
the press brake fence plus .828 equals the shear position. (This is
stored as an offset when using the stepper to move the gauge.)

The minimum flange is 3/8 plus metal thickness, but I just use .5 as
my minimum. The maximum distance for my fence from the bend point is
16-3/4". (I've cheated and used the ability of the metal to flex to
bend longer parts by attching an aux fence to the existing one.

Minimum shear is just less than 1-1/8 (obvious from previous
description!)

The whole back gauge moves up and down with the blade/die.

I added a second handle because the torque through the slip rolls to
the other link arm is part of the reason for poor results with large
bends.

I also re-machined the vee groove when first purchased.

The machine itself can do a pretty good job with bends up to 20
inches in 062 aluminum. That's a "staple" for us. You HAVE TO take
the time to set it up and adjust it well to get reliable repeatable
results, IMO/E.

The lack of a sectional lower die can be a problem. I've made some
parts for mine which "reverse" the bend to get around this
limitation (vee on top, inverted V on bottom: IOW a square bar in
the existing vee, and a self made "die" or is it a punch<G> with the
vee opening downwards.)

We've also made other than sharp bends--like those seen on the
430MCU at cncresource-- using a delrin rod with a corner cut out to
fit over the punches. We just hold it on with masking tape at its
ends.

As for your questions:

the "depth" of most large press brakes is set by trial and error.
There is at least one new and expensive model which DOES accurately
calc the bending force adaptively, and thereby create repeatable
bends over differeing material thicknesses. And ALL sheet material
is variable in thickness at best... It ALSO varies acoording
to "grain" --rolling direction-- and local material differences.

Short answer is that precision sheet metal bending is still as much
an art as a science.

The punches(ocassionally) and dies(always) of commercial machines
have screw adjustments for crowning to offset the distortions caused
by the large forces involved.
You can do this with your cheap bender by using shims under the
punches. (BTW, forgot to mention that I re-m,achined all the punches
when forst purchased as well. needed them to be the same size and
added punch marks on the face so they always install the same
direction (face forward).

I've also used shims under the material--on the die--for the same
purpose. You will ALWAYS have to adjust things for the particular
job and material, IMO/E.

As someone else mentioned, do as much of your work at the ends of
the machine.

You can get pretty good at empirically determining when the bend
is "complete" during the running of a job. It's a feel as much as a
position.

Hope this helps,

Ballendo

P.S. It's funny; I just posted a reply about this machine at CNCzone
last night. Strange tides in the minds of men...

P.P.S. Side note to the previous reply poster: So you added a steel
bar to the ram? I was gonna add a strap like the one the mfr has on
the rear of the die/bladeguide, to thetop of the ram. I'll have to
think about the steel bar instead. (It might get in the way of some
of our more "technical" bends...)

>In CCED, John Dammeyer <johnd@...> wrote:
>On Thursday one of those 3 in 1 30" sheet metal tools followed me
>home. I didn't have the heart to shoo it away so it now resides in
>my shop. The<snip>