Forgive me if don't do a good job painting this picture in words:

Picture a piece of 18- or 16-gauge aluminum, bent in a square "U" shape.
The arms of the "U" have holes in them that need to be at fairly equal
height up from the base of the "U" (20 mil is good, 50 mil is acceptable,
100 mil is definitely not). Dimensions are about 8 inches deep (i.e., we
start from a sheet 8" wide) by six inches tall by four or five inches
wide.

Assuming that we can reference the holes with perfect accuracy to the
edge of the sheet that will be at the top of the "U", how closely can we
get the hole heights, if we bend the part in a pan brake?

I'm thinking that if the break has a stop that registers that "top" edge
of the U the same for both sides, that we should be able to do better
than 20 mil most of the time, and never exceed 50 mils unless there's
just something wrong with the equipment.

Does this sound reasonable to you, or am I all wet? Is this something
that a normal sheet metal shop should be able to do, if they were
provided with suitably cut sheets?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

On 2/8/2013 9:39 AM, Tim Wescott wrote:

Does this sound reasonable to you, or am I all wet? Is this something
that a normal sheet metal shop should be able to do, if they were
provided with suitably cut sheets?

If you make each bend referencing the edge used to reference the holes,
you should have no trouble holding your tolerance, assuming the holes
themselves are well within said tolerance. If not, there's something
wrong with your setup or technique.

Your ultimate accuracy will depend on the accuracy and consistency with
which the holes are punched. If this varies, might see if there's some
way you can set up to register off one of the holes.


Jon

Bend first, punch holes later?

Christopher A. Young
Learn more about Jesus
www.lds.org
..

"Tim Wescott" wrote in message
...
Forgive me if don't do a good job painting this picture in words:

Picture a piece of 18- or 16-gauge aluminum, bent in a square "U" shape.
The arms of the "U" have holes in them that need to be at fairly equal
height up from the base of the "U" (20 mil is good, 50 mil is acceptable,
100 mil is definitely not). Dimensions are about 8 inches deep (i.e., we
start from a sheet 8" wide) by six inches tall by four or five inches
wide.

Assuming that we can reference the holes with perfect accuracy to the
edge of the sheet that will be at the top of the "U", how closely can we
get the hole heights, if we bend the part in a pan brake?

I'm thinking that if the break has a stop that registers that "top" edge
of the U the same for both sides, that we should be able to do better
than 20 mil most of the time, and never exceed 50 mils unless there's
just something wrong with the equipment.

Does this sound reasonable to you, or am I all wet? Is this something
that a normal sheet metal shop should be able to do, if they were
provided with suitably cut sheets?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Stormin Mormon wrote:

Bend first, punch holes later?
Where accuracy of hole placement on the final, bent piece is
tight, then this is the only way to do it. If .05" is acceptable,
then you CAN do it by milling/punching it flat and bending after.
You may have to make a test article and bend, then adjust the work
stop for additional parts. You may need to do this "bend calibration"
step for each batch of aluminum. You will also need to make sure EACH
piece of aluminum is oriented with the rolling "grain" in the same
direction. Once set up and calibrated like that, you can probably hold
..020" tolerance pretty consistently, if the brake is in good shape.

I do this sort of stuff in a home shop with well-used equipment, and
it works out pretty well. But, if the hole location relative to the
bend is critical, I bend first, then machine.

Jon

On Fri, 08 Feb 2013 14:20:55 -0600, Jon Elson wrote:

Stormin Mormon wrote:

Bend first, punch holes later?
Where accuracy of hole placement on the final, bent piece is tight, then
this is the only way to do it. If .05" is acceptable, then you CAN do
it by milling/punching it flat and bending after. You may have to make a
test article and bend, then adjust the work stop for additional parts.
You may need to do this "bend calibration" step for each batch of
aluminum. You will also need to make sure EACH piece of aluminum is
oriented with the rolling "grain" in the same direction. Once set up
and calibrated like that, you can probably hold .020" tolerance pretty
consistently, if the brake is in good shape.

I do this sort of stuff in a home shop with well-used equipment, and it
works out pretty well. But, if the hole location relative to the bend
is critical, I bend first, then machine.

Actually, I can easily design the piece so that the absolute height of
the holes can vary by 1/4 inch up or down -- it's just the relative
height that's important, so that a shaft stuck through them is more or
less level to the bottom.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

"Tim Wescott" wrote in message

I'm thinking that if the break has a stop that registers that "top"
edge
of the U the same for both sides, that we should be able to do
better
than 20 mil most of the time, and never exceed 50 mils unless
there's
just something wrong with the equipment.

When I became a draftsman the sheet metal foreman told me to assume a
tolerance of 1/32" (0.031") on bend locations, though they usually
could and did do better. I think the presence or absence of shearing
burrs on the edge made a substantial contribution. The Strippit that
punched the holes located on a small area near the corner rather than
the full width like the press brake's back gauge.

Does this sound reasonable to you, or am I all wet? Is this
something
that a normal sheet metal shop should be able to do, if they were
provided with suitably cut sheets?

Ask the ones you plan to send the quotes to. When I was designing
circuit boards I invited the sales reps to stop by to discuss their
normal and extra-cost tolerances in considerable detail. I suspect
they saw MITRE as a big defense contractor and didn't realize my
12-board prototype orders were all they would ever get.
jsw

Tim Wescott wrote:


Actually, I can easily design the piece so that the absolute height of
the holes can vary by 1/4 inch up or down -- it's just the relative
height that's important, so that a shaft stuck through them is more or
less level to the bottom.

OK, then you only need the two sides to bend the same way. If your
brake is working properly, that should happen with a pretty small
variation.

Jon

"Tim Wescott" wrote in message

Actually, I can easily design the piece so that the absolute height
of
the holes can vary by 1/4 inch up or down -- it's just the relative
height that's important, so that a shaft stuck through them is more
or
less level to the bottom.

Is the quantity big enough to justify making a drill jig plate with
edge locating pins that protrude from both sides?
jsw