wrote:
On Mon, 07 Feb 2011 06:10:17 -0800, Winston
wrote:

wrote:
On Sun, 06 Feb 2011 15:55:54 -0500, Joseph Gwinn
wrote:

Snip
I would be tempted to fabricate the magnetic circuit components by
bolting together four pieces of 1018 steel rectangles: one wide 0.5"
thick piece to form the bottom, two 0.5" thick pieces to form the
outsides, and one 1.0" thick piece to form the center, roughly following
the outline shown in the MagnaBend literature.

This will be a considerable savings on steel material, and on machining
effort.

Bolting-together would be accomplished using hex socket flat head
machine screws through the bottom piece screwed into drilled and tapped
holes in the steel center and side pieces.


Joe Gwinn.


Your fabricated construction should be fine. Although there are
two additional residual air gaps in the magnetic circuit these
are hard bolted gaps dropping to zero near every bolt and would
not add significantly to the series of four main residual gaps
that occur between the pole pieces and the work piece.

A further simplification would be to revert to the U
configuration. but with a single coil on the lower bar of the U
(or on the U leg remote from the bend line). This is slighly less
efficient than a coil on each leg because of the longer mean
turn length but this is more than compensated for by the ability
to locate the full sized pole piece where it matters most -
close to the bend line.

The 'C-E-C' core has another advantage over the 'U' core
aside from reduced reluctance.

http://www.magnabend.com/advantages.html

Notice how the 'C' core features provide clamping force in the
left and right extreme sides of the pole piece.
Those four corners would have much less clamping force in a 'U
core configuration.

--Winston

A correctly proportioned U core has the same reluctance as the
equivalant E core.

True for an electromagnet assembled using factory fixtures
and laminating techniques. I'm trying to envision how to
build a (bottom coil) U - core electromagnet without relying
on an 'L-I' core assembly and it's additional gap, using
'hobbiest' level tools and techniques. I agree that a
U-core electromagnet assembled with coils on the vertical
arms could be built by a hobbyist without any additional
gaps. Making *that* electromagnet in such a way as to not
interfere with the bending surface on the front of the machine
would be an interesting exercise.

The vertical clamping force would be identical.

I agree. And I also agree that the distribution of the
clamping force in the U electromagnet is superior to
that of the E electromagnet.

As discussed in the penultimate paragraph (omitted in your reply)
the bending force depends on the effective lever arm length which
is a free choice determined by the chosen width of the U gap.

For the relatively short distance from pole - to - pole and
given the equal clamping force, the difference might not be
very significant, particularly for ferrous workpieces.

Where did I put that magnetic FEA tool?


With a U core, the flux is delivered by the outer pole pieces.
For the same overall width, the U core delivers a greater bending
moment than an E core because half the E core flux goes to the
centre limb which has a shorter bending moment

These are all second order effects and will make little practical
difference. Ease of manufacture and cost of materials are more
important. The E core construction used by magnabend is
ineresting but I suspect that this was chosen for its mechanical
convenience because it completely surrounds the coil and
protects it from even the most ham fisted mechanic.

Engineering is compromise. The 'C-E-C' core allows one to assemble
the electromagnet (with square sides and bottom) and with
adequate clamping force. The U core has superior flux distribution
due to it's longer magnetic lever arm and might enable a higher
capacity bender.

Another possibility is a U core with a single winding on the back
vertical so that the front pole could define the entire front lip
and corners of the bender without interference.
*That* design has no additional gaps and could be made by a
hobbyist.

--Winston