On Thu, 19 Nov 2009 11:52:19 -0800 (PST), Drawfiler
1 I do not intend making one 48" x 12" 100mm x 50 would be enough for
a first off.
2 High power Magnets are not expensive and easily available.
3 It is a project that would stretch my brain cells.
4 I need (would like) something for the grinder that I am getting
On Thu, 19 Nov 2009 19:17:43 +0000, Richard Edwards
I don't know of published information but the magnetic
design is not dificult and the construction is straightforward
engineering. It's become much easier since rare earth magnets
have became readily available. These have the enormous advantage
that their coercive force is so high that they don't need
magnetising in situ.
shows two I have made. The foot rule shows the size. The right
hand one is a magnetic chuck, mechanically switched, using
ferroxdure magnets. The left hand chuck is a magnetic sine table
using Alcomax magnets which are pulsed on or pulsed off by a
separate control unit. To forestall the immediate question -
there are NO drawings! - I rarely draw anything and much of the
design tends to be dominated by the size and shape of bits I
happen to find in my junk box '
However this is one of the rare cases
where the old irish proverb is directly applicable* because both
chucks were made before rare earth magnets were available and
this is now be the preferred magnetic material..
The right hand chuck shows the basic mechanically switched
design. The top surface is. alternate bars of light alloy and
mild steel (the darker grey). Below this there is a movable
sandwich of ferroxdure and mild steel bars alternating at the
same pitch as the top surface. The top set of bars are roughly
square section. The lower sandwiched bars are rectangular section
- much deeper - about 4:1 ratio to accomodate large area magnets.
The magnets are magnetised in the long direction of the chuck.
When the bottom steel bars are lined up with the top steel
bars the steel at each side of the ferroxdure magnet behaves like
a horshoe magnet conducting the flux directly through the top
steel bars to the work piece. When the sandwich is moved one half
pole pitch in either direction each top steel bar magnetically
short circuits the corresponding lower pair of steel bars
diverting the flux away from the workpiece.
Large force is needed to move the sandwich as it has to
slide against the friction generated by the combined force of all
the magnets in the stack. A largish spanner on the hex nut is
needed to rotate the eccentric that provides the motion .
Magnetic attraction depends on the square of the flux
density so the top steel bars should be working near their
saturation flux density. The working flux density of rare earth
magnets varies quite a lot but this is not critical.. As long as
the pole area of your magnet(s) is at least 2 or 3 times the
top area of its corresponding top bar it will be OK.
What used to be a problem is the minimum magnetic length
(thickness) of magnet needed to ensure that it doesn't become
partially demagnetised when the chuck is switched on without a
workpiece. That is one of the reasons for the thick light alloy
bars on the ferroxdure chuck- to accommodate the magnetic length
(ie thickness) of the necessary ferroxdure magnets. With rare
earth magnets this is no longer a problem - even the thinnest
magnets commonly available are more than thick enough.
It's best to find a suitable set of magnets and design the
chuck around them. It's only the pole piece area that counts so
you can use more than one magnet per pole piece area. Old hard
disk magnets are useful for odd experiments.. First separate them
from the hardware them by bending away the soft iron mounts.
Then break them in half. This is because they are normally
magnetised as a two section NS SN magnet. You want single NS or
Hope this helps
*"If you want to go there I wouldn't start from here!"
On Fri, 20 Nov 2009 00:27:55 +0000, email@example.com wrote:
I am currently looking at some patent information to get some ideas.
It sounds pretty much the way that I work! The accumulated material
from the scrappy etc is analysed then the design accomodates what is
available. I do drawings for finicky bits where I need to determine
clearances, angles etc but generally I work as you do.
Thanks for the pictures they inspire me <G>
I am looking at www.supermagnete.de for magnets.
This sort of "number" information helps a great deal.
I have multiple sketches on my desk considering the options!
My thoughts exactly
A collection of ex Hard Drive magnets lies on the bench at the moment.
These together with some 1/8th inch thick parallels are giving me more
insight into the requirements. Your comments on being magnetised as
two sections is interesting, I will check this out.
Would you object to answering some queries off list once I get my
Thanks again for the post.
I have a powered magnetic chuck / table (presumably electromagnetic though it
might be electrically operated) in my 'interesting junk' pile at the moment.
It was made by Humphreys. Can anyone tell me something about it ?
I have had a play with some designs and decided that I needed to know
what sort of attractive force is appropriate in a mag chuck.
I took a look on the web and found a
Vertex chuck from Chronos 175 x 100 mm attractive force 18Kgf
which works out at (18 x 10)/(17.5 x 10) = 1.03 N/cm^2
Eclipse quote their Standard at 80N/cm^2
Bit of a difference but maybe my calcs are wrong?
My initial 50 x 100mm design seemed to work out at about 11.3 N/cm^2
And that is with 26 Euros worth of magnets!
Or maybe I am wrong!
The voice over is not in English but there are subtitles. The device
he is making is for Sherline size mills but the principle is
scaleable. Some of his other videos are worth watching too.
On Fri, 20 Nov 2009 05:54:14 -0800 (PST), Ned Ludd
This is a simple design, very low cost and easy
to build but it can only produce a fraction of the holdown force
that is available in sliding pole piece designs. This may not
matter for light duty work particularly as Tryall has recognised
this limitation and provided a set of adjustable stop pieces to
prevent sideways movement.
The reason for the reduced force is that, even
with optimum design, the maximum flux density available at the
surface of a rare earth magnet is not much more than half the
flux density that is possible when the flux is guided with soft
iron/mild steel. pole pieces.
The magnet arrangement used can't achieve this because it
doesn't take into account the magnetisation pattern used for
hard disk magnets.
shows the problem. This is a hard disk magnet overlaid with a
piece of magnetic film. This clearly shows the way in which it is
magnetised as a pair of magnets - the left hand dark area is NS
thickness magnetised S pole uppermost. - the right hand area is
reversed polarity with N pole uppermost.
A magnet can only achieve it's maximum flux density in a
closed magnetic circuit . This means that the N poles must
connect to S poles through a continuous ferromagnetic (soft
iron/mild steel) path interrupted by the minimum practicable air
When used in this sort of magnetic chuck the workpiece
forms part of the ferromagnetic path but, as in the originall HDD
application, the reverse side of the magnet needs to be covered
with a mild steel plate to connect the rear N to the rear S
poles. This plate doesn't appear to be present in the Tryall
design. If this plate is present (by simply using magnets still
attached to their original backing piece) the total airgap is so
much smaller that there is little to be gained by series stacking
more than one magnet. The key thing is to minimise the thickness
of the non magnetic top surface of the chuck.
On Sun, 22 Nov 2009 09:30:36 -0800 (PST), Ned Ludd
For a mickey mouse magnetic chuck for small undemanding work
it's a pretty fair effort. He hasn't made best use of the magnets
- the changes discussed in my last paragraph would simplify the
construction and slightly improve the performance.
However, there's no way that it could get anywhere near the
performance of a professional magnetic chuck. That needs a
properly dimensioned polepiece arrangement to both concentrate
the flux and permit a near zero airgap closed magnetic circuit.
On Sun, 22 Nov 2009 22:09:26 +0100, "Uffe B?rentsen"
This is a most unusual and intriguing configuration.
The pictorial flux diagram shows that, for the same total magnet
volume, it would produce greater single sided flux than that
produced by the equivalent unbacked (no rear plate) magnet pair
of the ex HDD magnets.
However this useful flux is mainly generated by segments 2 and
4. The orthoganol fields of segments 1,3 and 5 are mainly
concerned with cancelling and re-enforcing the return fields of
2 and 4 and so are only partially useful because they do not
wholly add to the total useful flux.
In contrast, when a low reluctance backing plate is added
to to a HDD magnet pair, because the return flux is short
circuited, the total volume of both magnet segments produces
useful single sided flux.
This is an opinion based on looking at the pictorial flux
diagram. I have not used one of these arrays - there may be
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