Basic Magnet Design
- FUNDAMENTAL DESIGN CONSIDERATIONS
The Magnabend machine is designed as a powerful DC magnet with limited
The machine consists of 3 basic parts:-
- The magnet
body which forms the base of the machine and contains the
- The clamp bar
which provides a path for magnetic flux between the poles of the magnet
base, and thereby clamps the sheetmetal workpiece.
- The bending
beam which is pivoted to the front edge of the magnet body
and provides a means for applying bending force to the workpiece.
Various configurations are possible for the magnet body.
Here are 2 that have both been used for Magnabend machines:
The dashed red
in the drawings above represent the magnetic flux
paths. Note that the "U-Type" design has a single flux pathway (1 pair
of poles) whereas the "E-Type" design has 2 flux pathways (2 pairs of
The E-type configuration is more efficient than the the
To understand why this is so consider the two drawings below.
On the left is a cross-section of a U-type
and on the right is an E-type magnet
has been made by combining 2 of the same U-types. If each
magnet configuration is driven by a coil with the same ampere-turns
then clearly the doubled-up magnet (the E-type) will have twice as much
clamping force. It also uses twice as much steel but hardly any more wire
for the coil! (Assuming a long coil design).
(The small amount of extra wire would be needed only because the 2 two
legs of the coil are further apart in the "E" design, but this extra
becomes insignificant in a long coil design such as used for the
To build an even more powerful magnet the "E" concept can be extended
such as this double-E configuration:
Below is a 3-D drawing showing the basic arrangement of
parts in a U-type magnet:
In this design the Front and Rear
poles are separate pieces and are attached by bolts to the Core piece.
Although in principle, it would be possible to machine a U-type magnet
body from a single piece of steel, it would then not be possible to
install the coil and thus the coil would have to be wound in situ (on
the machined magnet body).
a production situation it is highly desirable to be able to wind the
coils separately (on a special former). Thus a U-type design
effectively dictates a fabricated construction.
the other hand the E-type design lends itself well to a magnet body
machined from a single piece of steel because a pre-made coil can
easily be installed after the magnet body has been machined. A
single-piece magnet body also performs better magnetically as it does
not have any construction
gaps which would otherwise reduce the magnetic flux (and
hence the clamping force) a little.
(Most Magnabends made after 1990 employed the E-type design).
Selection of Material for Magnet
magnet body and the clampbar must be made from ferromagnetic
(magnetisable) material. Steel is by far the cheapest
ferromagnetic material and is the obvious choice. However
there are various special steels available which might be considered.
Steel : High resistivity steel which is usually available
laminations and is used in AC transformers, AC magnets, relays etc. Its
properties are not required for the Magnabend which is a DC magnet.
Iron : This material would exhibit lower residual
magnetism which would
be good for a Magnabend machine but it is physically soft which would
mean that it would be easily dented and damaged; it is better to solve
the residual magnetism problem some other way.
Iron : Not as easily magnetised as rolled steel but could
Steel Type 416 : Cannot be magnetised as strongly as
steel and is much
more expensive (but may be useful for a thin protective capping surface
on the magnet body).
Steel Type 316 : This is a non-magnetic alloy
of steel and is
therefore not suitable at all (except as in 4 above).
Carbon Steel, type K1045 : This material is
for the construction of the magnet, (and other parts of the
machine). It is reasonably hard in the as-supplied condition
and it also machines well.
Carbon Steel type CS1020 : This steel is not
quite as hard as
K1045 but it is more readily available and thus may be the
most practical choice for the construction of the Magnabend machine.
that the important properties that are required are:
- High saturation magnetisation. (Most steel alloys saturate
at around 2 Tesla),
- Availability of useful section sizes,
- Resistance to incidental damage,
- Machinability, and
- Reasonable cost.
carbon steel fits all these requirements well. Low carbon
steel could also be used but it is less resistant to incidental
damage. There also exist other special alloys, such as
supermendur, which have higher saturation magnetisation but they are
not to be considered because of their very high cost compared to steel.
carbon steel does however exhibit some residual magnetism which is
enough to be a nuisance. (See section on Residual Magnetism).
coil is what drives the magnetising flux thru
the electromagnet. Its magnetising force is just the
product of the number of turns (N) and the coil current (I). Thus:
number of turns
I = current
in the windings.
appearance of "N" in the above formula leads to a common misconception.
is widely assumed that increasing the number of turns will increase the
magnetising force but generally this does not happen because extra
turns also reduce the current, I.
a coil supplied with a fixed DC voltage. If the number of
turns is doubled then the resistance of the windings will also be
doubled (in a long coil) and thus the current will be halved.
The net effect is no
really determines NI is the resistance
per turn. Thus to increase NI the thickness of
the wire must be increased. The value of extra
turns is that they do reduce current and therefore the power
dissipation in the coil.
designer should be mindful that the wire gauge
is what really determines the magnetising force of the coil.
This is the most important parameter of coil design.
NI product is often referred to as the "ampere turns" of the
Many Ampere Turns are Needed?
exhibits a saturation magnetisation of about 2 Tesla and this sets a
fundamental limit on how much clamping force can be obtained.
From the above graph
we see that the field strength required to get a flux density of 2
Tesla is about 20,000 ampere-turns per metre.
for a typical Magnabend design, the flux path length in the steel is
about 1/5th of a meter and therefore will require (20,000/5) AT to
produce saturation, that is about 4,000 AT.
would be nice to have many more ampere turns than this so that
saturation magnetisation could be maintained even when non-magnetic
gaps (ie non-ferrous workpieces) are introduced into the magnetic
circuit. However extra ampere turns can only be gained at considerable
cost in power dissipation or cost of copper wire, or both. Thus
a compromise is needed.
Magnabend designs have a coil which produces
3,800 ampere turns.
that this figure is not dependent on the length of the machine. If the
same magnetic design is applied over a range of machine lengths then it
dictates that the longer machines will have fewer turns of thicker
wire. They will draw more total current but will have the
same product of amps
x turns and will have the same clamping force (and the
same power dissipation) per unit of length.
concept of duty cycle is a very important aspect of the design of the
electromagnet. If the design provides for more duty cycle than is
needed then it is not optimum. More duty cycle inherently means that
more copper wire will be needed (with consequent higher cost) and/or
there will be less clamping force available.
A higher duty cycle magnet will have less power dissipation which means
that it will use less energy and thus be cheaper to operate.
However, because the magnet is ON for only brief periods then the
energy cost of operation is usually regarded as being of very little
significance. Thus the design approach is to have as much power
dissipation as you can get away with in terms of not overheating the
windings of the coil. (This approach is common to most
Magnabend is designed for a nominal duty cycle of about 25%.
it takes only 2 or 3 seconds to make a bend. The magnet will then be
off for a further 8 to 10 seconds while the workpiece is repositioned
and aligned ready for the next bend. If the 25% duty cycle is
exceeded then eventually the magnet will get too hot and a thermal
overload will trip. The magnet will not be damaged but it
will have to be allowed to cool for about 30 minutes before being used
experience with machines in the field has shown that the 25% duty cycle
is quite adequate for typical users. In fact some users have
requested optional high power versions of the machine which have more
clamping force at the expense of less duty cycle.
cross sectional area available for the coil will determine the maximum
amount of copper wire which can be fitted in. The area
available should not be more than is needed, consistent with required
ampere turns and power dissipation. Providing more space for
the coil will inevitably increase the size of the magnet and result in
a longer flux path length in the steel (which will reduce the total
same argument implies that whatever coil space is provided in the
design it should always be full with copper
wire. If it is not full then it means that the magnet
geometry could have been better.
graph below was obtained by experimental measurements, but it agrees
fairly well with theoretical calculations.
clamping force can be mathematically calculated from this formula:
F = force in Newtons
B = magnetic flux density in Teslas
A = area of poles in m2
µ0 = magnetic permeability constant, (4π
an example we will calculate the clamping force for a flux density of 2
F = ½ (2)2 A/µ0
a force on unit area (pressure) we can drop the "A" in the formula.
Pressure = 2/µ0 = 2/(4π x 10-7)
comes out to 1,590,000 N/m2.
convert this to kilograms force it can be divided by g (9.81).
Pressure = 162,080 kg/m2 = 16.2 kg/cm2.
This agrees rather
well with the measured force for a zero gap shown on the above graph.
figure can easily be converted to a total clamping force for a given
machine by multiplying it by the pole area of the machine.
For the model 1250E the pole area
is 125(1.4+3.0+1.5) =735 cm2.
the total, zero-gap, force would be (735 x 16.2) = 11,900 kg or
11.9 tonnes; about 9.5
tonnes per metre of magnet length .
Flux density and Clamping pressure
are directly related and are shown graphed below:
In practice this high clamping force is only
ever realised when it is not needed(!), that is when bending thin steel
workpieces. When bending non-ferrous workpieces the force
will be less as shown in the graph above, and (a little curiously), it
is also less when bending thick steel workpieces. This is
because the clamping force needed to make a sharp bend is very much
higher than that needed for a radius bend. So what happens is that as
the bend proceeds the front edge of the clampbar lifts slightly thus
allowing the workpiece to form a radius.
small air-gap which is formed causes a slight loss of clamping force
but the force needed to form the radius bend has dropped more sharply
than has the magnet clamping force. Thus a stable situation results and
the clampbar does not let go.
is described above is the mode of bending when the machine
is near its thickness limit. If an even
thicker workpiece is tried then of course the clampbar will
This diagram suggests that if the nose edge of the clampbar was
radiused a little, rather than sharp, then the air gap for thick
bending would be reduced.
this is the case and a properly made Magnabend will have a clampbar
with a radiused edge. (A radiused edge is also much less prone to
accidental damage compared with a sharp edge).
Mode of Bend Failure:
a bend is attempted on a very thick workpiece then the machine will
fail to bend it because the clampbar will simply lift off.
(Fortunately this does not happen in a dramatic way; the
clampbar just lets go quietly).
if the bending load is only slightly greater than the bending capacity
of the magnet then generally what happens is that the bend
will proceed to say about 60 degrees and then the clampbar will start
to slide backwards. In this mode of failure the magnet can
only resist the bending load indirectly by creating friction between
the workpiece and the bed of the magnet.
thickness difference between a failure due to lift-off and a failure
due to sliding is generally not very much.
Lift-off failure is due to the workpiece levering the front edge of the
clampbar upwards. The clamping force at the front edge of the
clampbar is mainly what resists this. Clamping at the rear edge has
little effect because it is close to where the clampbar is
being pivoted. In fact it is only half of
the total clamping force
which resists lift-off.
the other hand sliding is
resisted by the total clamping force but only via friction so the
actual resistance depends on the coefficient of friction between the
workpiece and the surface of the magnet.
clean and dry steel the friction coefficient can be as high as 0.8 but
if lubrication is present then it could be as low as 0.2. Typically it
will be somewhere in between such that the marginal mode of bend
failure is usually due to sliding, but attempts to increase friction on
the surface of the magnet have been found to be not worthwhile.
an E-type magnet body 98mm wide and 48mm deep and with a 3,800
ampere-turn coil, the full length bending capacity is 1.6mm.
This thickness applies to both steel sheet and aluminium
sheet. There will be less clamping on the aluminium sheet but
it requires less torque to bend it so this compensates in such a way as
to give similar gauge capacity for both types of metal.
needs to be some caveats on the stated bending capacity:
The main one being that the yield strength of the sheet metal
can vary widely. The 1.6mm capacity applies to steel with a
yield stress of up to 250 MPa and to aluminium with a yield stress up
to 140 MPa.
thickness capacity in stainless steel is about 1.0mm.
This capacity is significantly less than for most
other metals because stainless steel is usually non-magnetic and yet
has a reasonably high yield stress.
factor is the temperature of the magnet. If the magnet has
been allowed to become hot then the resistance of the coil will be
higher and this in turn will cause it to draw less current with
consequent lower ampere-turns and lower clamping force. (This
effect is usually quite moderate and is unlikely to cause the machine
to not meet its specifications).
thicker capacity Magnabends could be made if the magnet cross section
was made larger.
Alan Magnabend Homepage
last updated: 19 October 2015