The first thing to do is to read the articles on this website about
- Fundamental Design Considerations
information given here is of a general nature only. The constructor
need to work out details, such as where the screws go etc, themselves.
I will assume that you have a fairly good workshop otherwise it is
doubtful whether this project should be attempted.
decide what type of magnet cross section that you would like to build.
Unless you have very good milling facilities then I
suggest that you go for a U-type
body. Although this
design is not quite as efficient as an E-type body it is quite a bit
easier to build and in fact can be built without any milling,
especially if you can obtain cold rolled steel sections. (Cold rolled
steel has a nice accurate finish and no mill-scale on the surface).
(If you want to build an
then there is an example model at the end of
drawing shows the basic arrangement of components in a U-type Magnabend.
you need to decide what thickness of sheetmetal you would like to be
able to bend.
will suggest a design which will be able to bend 1.2mm (18 gauge) mild
steel or aluminium. ( If you need more capacity than this then you will
have to use thicker sections than those suggested below).
will also assume that a fairly limited duty cycle will be ok for hobby
use. This will allow a more compact magnet design with a bit
steel and a bit less copper wire.
Magnabend cross section
All dimensions are in millimetres
- Note: The beveled edge
of the clampbar is not essential but the machine will be more versatile
if this can be provided.
Cover Strip over the coil must be a non-magnetic material otherwise it
will short-out part of the magnetic flux and thus cause loss of quite a
bit of clamping force. It is suggested that the cover strip be
commercial Magnabends the cover strip is supported on small rebates
milled into the poles on either side. However to avoid the need for
milling, the cover strip could be supported on a layer of polyester
resin (body-filler) between it and the coil. (Clamp
it flush with the surface until
the resin sets).
dimensions can be substituted if desired eg 3/4" instead of 20mm, 2"
instead of 50mm etc.
- For best performance the
Core Piece should be the
same length as the pole pieces as shown in the perspective
it is tempting, and certainly possible, to make the core piece shorter
than the poles and thereby accommodate the whole coil within the magnet
body, it will result in a very significant fall off in clamping force
near the ends of the machine. (This effect is somewhat worse than might
magnet body can be held together by several bolts, say M8 x 50 long,
SHCS passing thru the rear pole and core piece and tapped into the
front pole. (The length of 50mm assumes that the bolts will be
counter-bored so that their heads are flush with back surface making
their overall length 58mm). These bolts should have generous
clearance holes in the rear pole and in the core piece. This
facilitate getting the surface of the machine flat: The
assembly can be placed upside down on a
flat surface before final
tightening of the bolts.
- No bending beam is
shown in the cross section above. The required dimensions for
bending beam depend markedly on the length between its pivot supports.
Commercial Magnabends use an innovative hinge mechanism which
not restricted to being placed at the ends of the beam and hence the
distance between pivot supports is always small, even on a long machine.
special hinges are really a bit too difficult to make for a hobby
machine and thus it is suggested that more conventional hinges are used
and the length of the machine be kept relatively short. If
length is no more than say 650mm then a bending beam made from the same
section as the front pole (and pivoted at the ends), should be OK.
suggest a coil with 3,800
The best way to think about the design of this is
a wire gauge that would result in a current of 3,800 amps thru 1 turn
(when supplied with the intended voltage). Then if you have 2
turns the current will be (3,800/2) that is 1900 amps, and
turns would reduce the current to 38 amps and so on, but the
ampere-turns will still be 3,800. Thus, once the gauge has been chosen,
and the length of the coil is known, then the ampere-turns has already
Using Ohms Law: V = IR, or R = V/I so:
Resistance per turn = (supply voltage/3,800) ohms per turn.
assuming that the supply voltage will be AC then the effective value of
the rectified voltage is a bit less than the RMS value:
Effective DC voltage of
rectified AC is :
for an AC supply of say 220
volts the effective DC voltage will be 220 x 0.9 = 200 volts (approx.),
So, resistance per turn =
(200/3,800) = 0.053
ohms. (This will apply to all
lengths of machines!)
you now know the essentials of designing a Magnabend coil !
remains to find a wire thickness which "fits the bill", that is a wire
which has a resistance of 0.053 ohms for 1 turn on our particular coil.
average length of a turn:
a core length of say 650mm and a coil depth of 19mm
shown in the cross section above) then from straight forward geometry
we find that the length of an average turn will be : (650 x 2) + (44 x
Pi) = 1440 mm.
So we need a wire that has a resistance of 0.053 ohms per 1440 mm, that
is a resistance of (0.053/1.440) ohms per metre,
that is = 0.0368 ohms
per metre, or 36.8 ohms per Km.
here you could calculate the actual diameter of the copper wire but you
would need to know the conductivity of copper, which is not hard to
find, but in practice what
you now do is just lookup a table of information for various diameters
(It has all
been calculated before).
wire size available in the table is 20 AWG so choose this one.
Now it is well worth noting that the cross-sectional area of 23 AWG is
exactly half of the cross-sectional area of 20
AWG so that 2 strands of 23 gauge will give exactly the same
effect (and occupy the same overall area) as 1 strand of 20 gauge.
means you can use either size, but if you use 23 gauge you will need to
join the strands together at the start (and at the end) and then wind
with 2 strands in
hand. This is called bi-filar
In fact you can use any combination of wire sizes that you
provided they add up to the correct cross-sectional area (0.518 mm2).
If you follow the above procedure for a 110 volt AC supply
then you will find that the size of wire needed will be 17 AWG (or a
bi-filar winding using 20 AWG).
now know what size wire to use and this is the most important parameter
of coil design. But we now have to actually make the coil.
calculation below assumes that you know the area of the winding space,
and you do know this if you are following my suggested cross section
above. However if you were starting from scratch you would perhaps have
to wind a test coil first and then measure how hot it gets when run at
the desired duty cycle in the intended magnet body. If it
too hot then that means that you need more turns and hence a larger
Anyway, the number of turns will just be whatever will fit
in the space
available. This can be calculated from the known packing fraction.
a jumble-wound coil with electrical paper all around it, the effective
packing fraction is around 55%
This means that the space available
will be 55% occupied by copper.
So, number of turns = 0.55 x (area of winding space)/ (area of 1 strand
of wire). =[0.55 x 24 x 19)/0.518] = 484 turns
of the Coil:
resistance can be easily calculated by multiplying the number of turns
by the resistance per turn (which was calculated above).
Thus resistance = 484 x 0.053 ohms = 25.6 ohms
(Note that this will be the resistance at room temperature, 20C. At
higher temperatures the resistance will be higher).
current will just be the effective DC coil voltage (0.9 x AC RMS
voltage) divided by the resistance calculated above:
= 220 x 0.9 ÷ 25.6 = 7.7
of Winding Wire to Use:
magnet winding wire is copper wire coated with a polyester
enamel, eg "PEI". However, for the Magnabend coil, it is
highly desirable to obtain "self-bonding
This wire has an extra coat of a thermo setting material
under heat, will melt and then cure thus binding all the windings in
the coil together. If this wire cannot be obtained then it
be necessary to apply some other form of bonding agent (eg varnish)
during the winding of the coil. (It is important that the
windings are glued together otherwise the wires will vibrate and
eventually wear themselves out).
is usual to make a former into which to wind the coil. The
is later taken out of the former and put into the magnet.
Make up a former looking something like this:
the assembly together with several thru-bolts.
Make a hole in the centre thru which an axle can be inserted.
Line the winding space with electrical
This paper is typically about 0.3mm thick and its function is
insulate the coil from the metal poles of the magnet. (It is not safe
to rely on the enamel insulation on the wire itself for this purpose).
Score the paper to make it fold in the right places.
You will probably need to temporarily tape the paper to the former to
keep it out of the way during winding.
The dimensions of the coil former should be such that the coil which it
forms will have clearance
when it is installed in the magnet body.
core piece of the former could be the same piece that will be used in
the actual magnet. This will save the need to remove the coil from the
core and then re-fit it to another core piece.
Also, have a look at notes about the coil under E-type
Here is a
comparison of electrical papers that you could use:
Resistance (Crossed Wire Test)
|Hyply 10 + 3
I would recommend Nomex
if you can obtain it easily, but any of those shown would be OK.
ALTERNATIVE INSULATION FOR THE COIL:
Electrical paper might be difficult
to obtain in an apprpropriate form, for example a small roll.
In that case you could consider insulating the coil with
high temperature polyimide (Kapton) tape
. This can be
obtained from an electrical supplier such as RS Components or Element14
or even on eBay.
If using this method you would first form the coil and then remove it from the former before wrapping it
with the Kapton tape.
: 33m x 19mm x .07mm, colour
: up to 270 C
(Eg. RS Components number 468-403).
Winding versus Jumble Winding:
winding is where the coil is wound in neat layers, usually with
electrical paper between the layers. This method makes for a
quality coil but it is tedious and it is not really necessary for the
Magnabend coil. The alternative to layer winding is "jumble winding"
where the wires
wind in in a fairly random fashion. It is however worth
up the coil in an even way. In particular you can avoid
too much voltage difference between adjacent windings by making sure
that that outer windings do not come into contact with inner windings.
(This will make it less likely to get insulation breakdown within the
of the Winding Wire and Wire Tension:
wire is normally supplied on a plastic spool. When dispensing
wire do not try to rotate the spool, rather just sit it
vertically on the floor (or in an old drum is better) and withdraw the
some kind of tensioning device mounted vertically above the spool. This
could just be a welding clamp fastened to something (eg a
a bench etc ) with felt wrapped around the jaws to provide some
friction, and therefore tension to the wire. (Don't use anything that
would scratch the varnish insulation on the wire).
It is best to provide the coil with flexible lead wires which are
joined to the winding wire inside
Since part of the lead wire will be inside the coil then it is best to
use wire with teflon (or other high temperature) insulation.
Make the termination by soldering the flexible wire to the winding wire
and then insulating the joint with heatshrink sleeving.
Use say a black wire at the start (inside) of the coil and a red wire
for the end (outside) of the coil.
You will need to thoroughly scrape off the enamel coating from the
winding wire before attempting to solder it.
The terminating wires will be brought out through a pair of holes in
one of the cheek plates.
Winding of the Coil:
a one-off coil it is not worth setting up an electric drive to turn the
coil former. Just mount it on an axle and turn it by hand; it does not
take all that long to put in the required number of turns (484, or even
less if you are making a longer coil). (If you do want to set
an electric drive make it rotate the assembly fairly slowly, say about
50 RPM and control it with a foot-switch). The number of
does not have to be precise. If you don't have an easy way of
counting the turns (or if you loose count) then just monitor
total resistance. When it reaches about 25 ohms you
the Coil to Size:
Because of the long nature of the Magnabend coil the
strands of wire between the ends remain quite loose unless they are
clamped up prior to curing the coil. You need to make up some ?sizing
pieces? as shown below. These can be made out of wood or some other
suitable material. (We used aluminium extrusion sections for production
It is best to have a short pair of pieces for the
ends as well.
You will need to calculate the dimensions
of the rebates in the sizing pieces such that when the
coil gets clamped up it will be the correct size to fit in your magnet
After the coil has been wound and the terminating
wires have been soldered on and brought out thru holes in the coil
former cheek plate, fold the edges of the electrical papers in and
install the sizing pieces. Apply 3 or 4 G-clamps to hold the sizing
pieces in place and screw up the G-clamps until the sizing pieces are
seated on their rebates all the way along.
that you have used self-bonding wire then curing involves heating the
wire up to a temperature recommended by the manufacturer.
Typically this will be something like 190 degrees C. The
way to do this is to connect the coil to a voltage source, via a
and let it self-heat. The temperature within the coil will be
more uniform if the heating happens quickly, so it is preferable to use
a higher than normal voltage for this. But even the standard
supply voltage will still get the coil hot enough because it was
designed for only
cycle so 100% duty will eventually get it pretty hot, especially when
the coil is not installed in a steel magnet body.
how do you know when it is hot enough? Well fortunately the
has a kind of built-in temperature gauge - its resistance increases
with temperature in a very predictable way, so all you have to do is to
monitor the current and when it has fallen to a certain value then the
coil will be hot enough. At 190 C the current will have
fallen to 60% of its
starting value (room temperature value).
If you can't measure the current
then instead you could disconnect the coil and measure its resistance
at intervals. When the resistance has increased to 1.7 times the
initial value then it will be hot enough).
value of 60% mentioned above may have to modified if the wire
manufacturer has recommended a curing temperature much different
to 190 C).
Mathematically the variation of resistance with temperature is
(Where alpha is the coefficient of resistance, which for copper is
You can easily substitute values into this formula.
Note that T must be expressed in degrees C or K, but not
compressing to size, curing and removing from the former, Magnabend
coils will look something like this:
Electrical Circuit for a Hobby Magnabend:
For simplicity you could consider using the minimal circuit:
But practically you should go for a circuit with at least a
2-handed interlock and a light clamping phase such as shown below:
Needed for 2-Handed Interlock Circuit:
bridge rectifier converts the AC mains into DC for the magnet coil. The
rectifier needs to have a current rating equal to or greater than the
magnet coil current and a reverse voltage rating equal to or greater
than the peak mains voltage. However, for improved
it is best to choose a rectifier which exceeds the minimum
requirements by a generous margin.
RS Components part number: 227-8794
Max current: 35 amps continuous,
Max reverse voltage: 1000 Volts,
Terminals: 1/4" quick-connect or 'Faston'
Approx price: $12.00
the bridge rectifier is 'exposed' to the AC mains then it is
susceptible to damage from voltage spikes. (Spikes are
large increases in voltage which sometimes occur on the mains which
relate to sudden events such as truck running into a power pole, a
lightning strike etc). To guard against this possibility it
good idea to fit a MOV ('varistor')
across the ac terminals of the rectifier. The varistor should have a
clamping voltage higher
than the normal peak mains voltage but lower
voltage rating of the rectifier.
Varistor type 250LA20AP, RS part number 543-5063
Clamps at: 650 volts,
Clamping current: 25 amps,
Energy absorption: up to 72 joules,
Approx price: $1.00.
As the Magnabend design is rated for only
operation then the coil could be overheated if somehow the machine was
to be left continuously ON by accident. To guard
this happening it is advisable to fit a bi-metallic switch to sense the
temperature of the magnet body and to cut the current off it it gets
A suitable switch is:
Snap-acting bi-metallic switch,
RS Components part number 339-308
Rated at 240V ac, 10 amps.
Switch goes open circuit above 70°C, auto-resets to closed below 55°C
Terminals: 1/4" quick-connect.
Approx price: $7.00
This switch should be mounted (by 2 screws) somewhere to the base of
the magnet body.
ON/OFF switch can be any switch with suitable ratings.
you want to couple this switch to the motion of the bending beam, ie to
turn ON when the beam starts to move up, then you will probably want to
use a V3
microswitch. These switches have over-center
snap-acting contacts and are highly suitable for this kind of
V3 switch would be:
RS part number: 472-8235
Current rating: 16 amps
Voltage rating: 250 Volts AC
Lever type: long
Operating force: 0.91 Newton,
Terminals: 1/4" quick-connect
Approx price: $5.00
you automate the turn-on with a V3 switch, as above, then it is
strongly advised that you also incorporate the additional circuitry
required to give a 2-handed interlock (otherwise it is all too easy to
accidentally turn the magnet on and possibly cause an accident).
It is possible to do a 2-handed interlock circuit without the need for a relay
but it would require that a button (the Start button) be held in
continuously whilst doing a bend. It is much more convenient if the
Start button only has to be pressed to initiate the interlock and in
that case a relay
will be required.
relay would be:
RS part number: 511-6992
Switching capacity: 10 amp, 240 volt ac,
Coil: 240 volt ac,
Contact arrangement: DPDT,
Approx price: $10.00
relay shown here has 2 poles. This provides for the possibility that
you may also want to implement a demagnetising circuit as well, but if
not then the second pole can be ignored or you can choose a single pole
relay which would be slightly cheaper.
To achieve a 2-handed
interlock circuit with a pre-clamping phase you will need a capacitor
rated for continuous operation on the AC mains. The best type
use are ones designed for power factor correction (eg in fluorescent
lights)., and these may be referred to as 'lighting capacitors'.
RS Components part number: 451-5411
Voltage rating: 250 VAC continuous,
Mounting: M8 stud,
RS Components part number: 123-5095
Capacitance: 10 µF
Voltage rating: 250 VAC continuous,
Dimensions: 41 x 40 x 20 mm
Please also consult the section: Magnabend
Circuit Operation for other ideas and more advanced circuits.
E-Type magnet cross section achieves a higher clamping force from a
given weight of steel and copper wire. If you have good
facilities at your disposal then this is the design to go for.
design assumes that the magnet body will be machined from a
steel blank 100 x 50 mm. The dimensions allow for machining the blank
on all sides to clean up the mill scale and remove a minor amount of
lack of straightness. If you are making a long
it is quite likely that some pre-straightening will be required prior
to machining. (See below).
the blank can be obtained as a
cold-rolled section then clean-up machining may not be
required and hence the above
outer dimensions could be increased to match the size of the blank.
can make the length pretty much anything you like. This same
section has been used for Magnabends up to 3.2 meters long.
However, you should note the relative
between the coil and the magnet body. For instance the coil
than the steel 'island' that it has to fit over but it is narrower than
channels that it has to fit into. It needs to have clearance
it would be very disappointing to have gone to the trouble of making a
nice coil only to find that it will not fit into the space available in
the magnet body!
Note that you will only get full clamping force to within about 50mm of
each end. This is because the flux concentration near the ends gets
considerably reduced by fringing into the (longer) outer poles. This
has a more severe effect than might be expected.
medium carbon steel, say K1045, is a good choice however if you cannot
obtain this then CS1020 would be OK or just mild steel is also OK but
it will damage a bit more easily and does not machine quite as well as
the higher carbon steels.
from a solid steel blank will produce the best result both physically
and magnetically. However it is also possible to
the E-Type body by fabrication. You would probably start with
piece 100 x 16mm (4" x 5/8") for the base and then bolt the poles to it
with SHCS's coming up thru the base plate. You
the dimensions of the poles a little to suit standard sizes. For
example the base could be 100 x 20mm (4" x 3/4") if that size was
easier to obtain. A thicker section will improve the magnetic
performance a little but to get the full benefit of a thicker section
all legs of the magnetic circuit would need to be increased to match
(including the clampbar which is not shown in the above drawing).
"As-supplied" hot-rolled steel blanks are hardly ever straight
enough to be machined without some prior straightening. This
particularly applies to longer lengths.
If a bent piece of steel is pressed until it is straight it
will retain some internal stress and that stress may
to creep over time and relax back into a slightly bent condition again .
To overcome this behaviour a stabilised
can be used.
This is described below.
The general principle of stabilised bending is to “ring” the
bend. That is to successively bend the material to and fro
with each bend progressively reduced in amplitude until no further
"Ringing" is the term often applied to a decaying oscillation such as shown on the left.
When applied to bending this technique will leave the material in the
the bend in a stress-free condition. The material will then
stable and not subject to creep.
In principle the method is a substitute for stress relieving by heating
although for complete stress relief the bar to be straightened would
need to be worked on from end to end.
Typically 3 oscillations of the bend will produce sufficient stability
for our purposes.
Locate the worst hump or hollow in the
workpiece by using a straight-edge.
Using a workshop press of sufficient
tons is sufficient for Magnabend sections) press the hump with sufficient pressure to cause it to be
slightly bent the other way (i.e. over compensated). Check with a
straight edge and if necessary, press again with a slightly increased
force until over-compensation is achieved.
Turn the workpiece over and press the
other way to half the initial force.
Turn the workpiece back to the
original orientation and press to quarter the initial force.
Check the overall straightness and if
it is not straight enough, repeat from step 1.
Maximum deviation from the straight-edge should be
within specifications. (Magnetic Engineering specification was 0.2mm max.)
While testing with the straight-edge on top, the workpiece should be
supported near the ¼ and ¾ points to minimise the effects of
This is particularly important for on-flat testing.
In a production situation a table of pressing forces should be
established to show the forces required to yield different
workpieces (clamp bars, magnet bodies, bending beams, etc) and
according to workpiece support spacings etc.
It is a good idea to drill and tap the magnet body for the various
fasteners required before
installing the coil. That way you minimise the danger of drilling into
the coil which would probably effectively destroy it. You need to think
about fasteners for the hinges,
bars (if required), a utility
some kind of stand
coil will be made much the same way as for the U-type body as described
above except that the terminating wires need to come out on the bottom
face of the coil and would then pass thru a hole in the base of the
body to join up with the electrical circuit.
installing the coil make absolutely sure that there are no sharp burrs
or swarf that might puncture thru the coil insulation. Pay
particular attention to ends of the central pole; file the sharp edges
off and make everything clean. Provided the insulation
sound then the coil should last pretty much forever. (You
to last a long time because it is very difficult to replace later if it
for the Coil:
Magnabends protect the coil with aluminium strips (20 x 3 mm) which are
supported in small rebates milled into the poles. These are peened
after installation to produce a jamming fit and then the surface of the
magnet body is given a final light machining.
for a hobby Magnabend I am going to suggest another method
which involves less machining:
fill up the space above (and at the end of) the coil with a polyester
resin such as body filler. A typical brand being K&H
Australia) or Bondo (a popular brand in the US). This
thixotropic, meaning that it will stay in place and does not run out
(like honey would). Also it has good heat resistance and is not
the resin has cured (about half an hour depending on the temperature
and the amount of catalyst added to the mix), it can be sanded nice and
flush with the steel surface using a belt sander.
| This page last updated 19 September 2015