U.S. patent number 3,818,399 [Application Number 05/326,532] was granted by the patent office on 1974-06-18 for permanent magnet devices.
This patent grant is currently assigned to James Neill Holdings Limited. Invention is credited to Alun Edwards.
United States Patent |
3,818,399 |
Edwards |
June 18, 1974 |
PERMANENT MAGNET DEVICES
Abstract
A permanent magnet device comprising at least two pairs of
aligned plate-like polepieces of ferromagnetic material such as
mild steel, and at least two permanent magnets in upper and lower
rows magnetised in opposite directions through their thickness,
associated with and lying between the pairs of polepieces, at least
one of the magnets being capable of movement relative to at least
one other and relative to the pairs of polepieces from a first
position where the magnets and the polepieces co-operate to provide
a maximum external magnetic field strength (the ON position) to a
position where the external magnetic field is reduced to
substantially zero (the OFF position).
Inventors: |
Edwards; Alun (Sheffield,
EN) |
Assignee: |
James Neill Holdings Limited
(N/A)
|
Family
ID: |
9783756 |
Appl.
No.: |
05/326,532 |
Filed: |
January 24, 1973 |
Foreign Application Priority Data
Current U.S.
Class: |
335/295;
335/306 |
Current CPC
Class: |
B23Q
3/1546 (20130101) |
Current International
Class: |
B23Q
3/154 (20060101); B23Q 3/15 (20060101); H01f
007/04 () |
Field of
Search: |
;335/285,295,306 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Harris; George
Attorney, Agent or Firm: Lowe, King & Price
Claims
What I claim is:
1. A permanent magnet device comprising at least two pairs of
aligned plate-like polepieces of ferromagnetic material such as
mild steel, and at least two permanet magnets in upper and lower
rows magnetised in opposite directions through their thickness,
associated with and lying between the pairs of polepieces, at least
one of the magnets being capable of movement relative to the other
and relative to the pairs of polepieces from a first position where
the magnets and the polepieces co-coperate to provide a maximum
external magnetic field strength an on position, to a position
where the external magnetic field is reduced to substantially zero
an off position.
2. A permanent magnet device as in claim 1, wherein the relative
movement is such that the magnets can be moved to a third position
where an external magnetic field is produced opposite in direction
to that provided by the first position.
3. A permanet magnet device as in claim 1, wherein the magnets are
of slab form and magnetised through the thicknesses of the slabs
such that opposite polar faces are of opposite polarity.
4. A permanent magnet device as in claim 1, wherein corresponding
polar faces of the magnet associated with adjacent pairs of
polepieces in the on position are of opposite polarity.
5. A permanent magnet device as in claim 1, wherein the upper and
lower magnets are moved, relatively to each other by sliding
between the polepieces until each bridges the gap between the pairs
of polepieces.
6. A permanent magnet device as in claim 1, wherein adjacent pairs
of polepieces are provided with at least one magnet permanently
held between one pair of polepieces and a second magnet magnetised
through its thickness in the reverse direction movable from a
position between the other pair of polepieces, the ON position, to
a position where both magnets are associated with one pair of
polepieces, the OFF position.
7. A permanent magnet device as in claim 1, wherein the magnetic
attractions between each polar surface of each magnet and the
neighbouring polepieces are substantially equal to one another but
of opposite direction, whereby there is substantial balancing of
the magnetic forces acting on the polar faces of the magnets.
8. A permanent magnet device as in claim 1, wherein the device is a
work-holding device.
9. A permanent magnet device as in claim 8, wherein the magnets and
the polepieces are so proportioned that the polar surface area of
permanent magnet in contact with a polepiece is greater than the
area of the same polepiece in contact with any ferromagnetic
article held by the device.
10. A permanent magnet device as in claim 1, wherein the movable
magnets are mechanically supported to hold the magnets in their
positions between the polepieces and to move them as required.
11. A permanent magnet device as in claim 10, wherein the movable
magnets are positioned in appropriately shaped holes in a support
bar of non-magnetic material.
12. A permanent magnet device as in claim 11, wherein the bar is
moved longitudinally between the polepieces and as it moves it
carries the magnets with it.
13. A permanent magnet device as in claim 1, wherein all the
movable magnets are moved simultaneously.
14. A permanent magnet device as in claim 1, wherein some of the
movable magnets are movable separately from the remainder of the
movable magnets whereby the device can be switched on and off in
sections.
15. A permanent magnet device as in claim 1, wherein for maximum
utilisation of the magnets their width substantially equals that of
the polepieces.
16. A permanent magnet device as in claim 1, wherein the magnets
are produced from high coercivity ferrite material, when thin
magnets may be employed with a consequent narrow spacing of the
polepieces.
17. A permanent magnet device as in claim 8, comprising a base
plate of non-magnetic material, side plates and end plates of
ferromagnetic material, the base plate having a series of
rectangular recesses running longitudinally of the base plate into
each of which is secured a number of spaced polepieces, the
longitudinal gaps between adjacent rows of polepieces being loaded
with upper and lower rows of magnets, the gaps between the upper
faces of the polepieces being filled with non-magnetic
material.
18. A permanent magnet device as in claim 17, wherein the movable
magnets are located in support bars provided with recesses in which
the magnets are a close fit, and the ends of the support bars
extend to operating means for simultaneous movement of the upper
and lower support bars in opposite directions.
19. A permanent magnet device as in claim 18, wherein the operating
means is a pin passing through a vertical slot in each support bar,
each pin being located in and secured to rotatable plates, to one
of which is secured an operating handle.
20. A permanent magnet device as in claim 19, wherein the length of
the slot is dictated by the degree of movement required of the
support bar.
21. A permanent magnet device as in claim 17, wherein the spacing
between adjacent polepieces in the transverse direction is
maintained by rods bridging the side plates on which distance
pieces are provided, the distance pieces lying in the longitudinal
gaps between the polepieces.
22. A permanent magnet device as in claim 8, comprising a base
plate of non-magnetic material, side plates and end plates of
ferromagnetic material, the inside faces of the side plates being
provided with spaced location pegs to locate transversely disposed
packs of polepieces in alternation with magnets whereby adjacent
packs provide the longitudinal rows of polepieces and lower fixed
magnets, the gaps between the polepieces above the fixed magnets
being loaded with the upper rows of magnets, and the gaps between
the polepieces above the upper rows of magnets being filled with
non-magnetic material.
23. A permanent magnet device as in claim 22, wherein the movable
magnets are located in support bars provided with recesses in which
the magnets are a close fit, the ends of the support bars extending
to operating means whereby the support bars can be moved
simultaneously.
24. A permanent magnet device as in claim 23, wherein the operating
means is a rack and pinion, the racks being formed on one edge of
the support bars and engaged by the pinion passing across the
support bars, the pinion extending to epicyclic gearing to which is
secured an operating handle whereby all the supporting bars are
moved simultaneously.
25. A permanent magnet device as in claim 1, wherein additional
polepieces and associated magnets are disposed transversely
adjacent said two permanent magnets, transversely adjacent magnets
being oppositely magnetised.
Description
This invention relates to permanent magnet devices and in
particular relates to such devices when provided with a means to
modify the external magnetic field of the permanent magnets by
reduction, reversal, or complete elimination.
Such devices are already known, and in particular there are known
work holding devices, e.g., permanent magnet chucks but such
devices as are in present commercial use are known to have certain
serious disadvantages. Hitherto, one well-known design of magnetic
chuck has been constituted by two "packs" of permanent magnets and
ferro-magnetic pole pieces, the relative position of the two packs
determining whether or not there is an external magnetic field. The
upper pack of such a chuck is held stationary and its upper surface
is the work holding surface; the lower pack is movable in relation
to the upper pack. Such a construction has two inherent and serious
drawbacks. Firstly the attractive force between the packs is so
great as to make the lower pack difficult to move unless mechanical
means are provided to separate the packs slightly and make relative
movement practicable. Secondly, the upper pack is unsupported
except at its edges and therefore when pressure is applied to work
pieces during milling, grinding etc., the work-holding surface is
deflected, thereby limiting the flatness and accuracy of machining
and in extreme cases causing chatter marks on the finished
surface.
The object of the present invention is to provide a permanent
magnet device which is easily switched from the "on" to the "off"
position and is free from the disadvantages described above when
used as a work-holding device.
According to the present invention, a permanent magnet device
comprises at least two pairs of aligned plate-like polepieces of
ferromagnetic material such as mild steel, and at least two
permanent magnets magnetised in opposite directions through their
thickness, associated with and lying between the pairs of
polepieces, at least one of the magnets being capable of movement
relative to at least one other and relative to the pairs of
polepieces from first position where the magnets and the polepieces
co-operate to provide maximum external magnetic field strength (the
ON position) to a position where the external magnetic field is
reduced to substantially zero (the OFF position). The relative
movement may be such that the magnets can be returned to their
original position or moved to a third position where an external
magnetic field is produced, opposite in direction to that provided
by the first position.
Thus, in its simplest form, the device may have two pairs of
polepieces, with one magnet positioned between the lower portions
of one pair and a second magnet positioned between the upper
portions of the other pair. The two magnets may be of slab form and
magnetised through the thicknesses of the slabs such that
corresponding polar surfaces (lying adjacent to the parallel with
the inside faces of the polepieces) are of opposite polarity. In
this ON position, the magnets and the polepieces provide external
fields of maximum strength. The two magnets may be moved,
simultaneously or separately, relatively to each other by sliding
between the polepieces until each bridges the gap between the pairs
of polepieces, the OFF position. As the magnets approach this
position the external field is reduced in strength until a critical
position is reached at which the external fields become
substantially zero, by virtue of the short circuit paths provided
by the polepieces. The magnets may be returned to their original
position when the external field returns to its maximum value, or
the movement may be continued so that each magnet becomes
associated with the other pair of polepieces when the external
fields are increased to a maximum but of reversed directions.
Alternatively, instead of moving the two magnets in opposite
directions such as to bridge the gap between the pairs of
polepieces, one magnet may be permanently held between one pair of
polepieces and the second magnetised through its thickness in the
reverse direction, moved from its position between the other pair
of polepieces to a position where both magnets are associated with
one pair of polepieces. Thus, as the magnet is moved, the external
magnetic field is gradually reduced until such time as both magnets
lie between the same pair of polepieces, when again complete short
circuiting is provided to eliminate the external fields. The magnet
that has been moved may be returned to its original position to
increase the magnetic fields back to a maximum or alternatively the
second magnet may be held and the first magnet moved into
association with the other pair of polepieces, when the external
fields are increased to maximum but of reversed directions. In
certain usages of the device of the invention, it is desirable that
in the OFF position, there is in fact a low strength external field
of reverse direction to that in the ON position. This is especially
advantageous when the device is a work holder, when demagnetisation
of a hardened steel workpiece can be effected.
Each polar surface of each magnet attracts, and is attracted by,
the neighbouring ferromagnetic polepieces, and is, therefore, acted
on by forces at its two polar surfaces which are substantially
equal to one another but are in opposite directions. This
substantial balancing of magnetic forces makes it particularly easy
to slide the magnet between the polepieces in a direction
transverse to its direction of magnetisation.
In accordance with the ultimate use to which the permanent magnet
device is to be put so may the arrangement of polepieces and
magnets be modified, thus any number of aligned pairs of polepieces
may be provided and there may be a magnet associated with each pair
of polepieces in the ON position, i.e., the position where an
external magnetic force of maximum strength is provided.
Alternatively, two magnets may be provided in association with each
pair of polepieces in the ON position, the two magnets being
magnetised through their thickness in the same direction so that
the corresponding face of each magnet associated with one of the
pair of polepices is of the same polarity, the corresponding face
of the magnets associated with the adjacent pair of polepieces
being of opposite polarity in that position. Such an arrangement
can be designed to provide a heavy concentration of flux at the
exposed edges of the polepieces thus making the device eminently
suitable as a work-holding device. However, work-holding devices of
large surface area are often required. Therefore, in addition to
there being any number of pairs of aligned polepieces and
associated magnets in what for ease will be referred to as the
longitudinal direction, additional polepieces and associated
magnets may be provided in what can be referred to as the
transverse direction with corresponding magnets in the transverse
direction magnetised in the reverse direction. Thus, a "sandwich"
construction is provided of any length and breadth as may be
required to suit any application.
Moreover, the magnets and the polepieces are readily so
proportioned that the polar surface area of permanent magnet in
contact with a polepiece is greater than the area of the same
polepiece in contact with any ferromagnetic article held by the
device. According to his conception, the polepiece collects
magnetic flux at low density from a large area of permanent magnet
surface, and concentrates it into the limited area of polepiece
surface in contact with the ferromagnetic article being held. Under
such circumstances as is well established both practically and
theoretically, the total attraction between a polar surface of a
magnet and its adjoining polepiece is much less than the total
attraction between the same polepiece and the ferromagnetic article
held, although substantially the same total amount of magnetic flux
passes across both boundaries as parts of the same magnetic
circuit.
Examining the previously-known construction from this point of view
the reasons for its unsatisfactory mechanical operation are
understood. The polepieces move with the magnets and the relative
movement between the packs is at surfaces of high flux density. The
force on the movable pack is all in one direction, there is no
balancing force. By contrast the invention has fixed polepieces
between which the magnets can easily slide, partly because the
attraction between the magnets and the polepieces is small in any
case, and partly because these forces are so arranged that they
practically cancel one another.
From its simplest form where but two magnets are provided up to its
most complex form where a large number of pairs of magnets are
provided, it is necessary to provide mechanical support to hold the
magnets in their positions between the polepieces and to move them
as required. Thus, when both magnets are intended to be moved, it
is preferred that the magnets be positioned in appropriately shaped
holes in a support bar of non-magnetic material (e.g., brass of
synthetic plastics material), there being separate support bars for
the upper and lower rows of magnets. The bar as a whole may be
moved longitudinally between the polepieces by known and
appropriate arrangements of levers, eccentrics, racks, pinions
etc., and as it moves it carries the magnets with it. The magnets
may be either a tight or a loose fit in the support bar as may be
appropriate for the method of manufacture, the magnetisation may
take place either before or after the magnets have been fitted into
the bar or the bar has been moulded around them. A very small
mechanical clearance is allowed to permit the bars and magnets to
slide freely between the rows of polepieces. Alternatively, when
only one row of magnets are to be moved, the other row may be
secured directly between the polepieces.
Because there is a cancellation of attractive forces between the
magnets and the polepieces and because magnetic flux is fed between
magnets and polepieces at low density, the mechanical forces
required to move all the upper magnets and the lower magnets
simultaneously is still small. Whilst the various upper and lower
bars holding magnets are normally coupled together for simultaneous
movement this is not essential, and the magnetic device may be
switched ON and OFF in sections if desired.
For maximum utilisation of the magnets their width should equal to
slightly exceed that of the polepieces. A wider magnet is larger
and produces more flux but excessive width increases the proportion
of flux lost by leakage between neighbouring magnets.
It is preferred that the magnets should be produced from so-called
ferrite material, when thin magnets may be employed with a
consequent narrow spacing of the polepieces, a distinct advantage
particularly when the device is used as a magnetic chuck. In normal
constructions the thickness of the polepieces is somewhat greater
than the thickness of the magnets between them.
When the device has two magnets associated with each pair of
polepieces in the ON position, each polepiece is normally in
contact with four magnet polar surfaces corresponding to the upper
and lower bars on each side, and each of these surfaces is of the
same polarity. Exceptions are the four corner polepieces of the
construction which are in contact with one magnet polar surface
only, and the other polepieces in the outermost rows and at one or
both ends of each row, which are each in contact with two magnet
polar surfaces.
If the device is switched to the OFF position by moving the upper
and lower bars with their associated magnets in opposite directions
by approximately half a longitudinal pole pitch to bridge the
polepieces, most of the magnets become associated with no less than
four internal flux diversion paths. Therefore the flux diversion is
particularly complete and the external magnetic field of the device
can be controlled such as to substantially eliminate it or provide
slight reversal, giving complete release of workpieces when the
device is used as a workholder.
To explain the invention further, it will now be described by way
of example only with reference to the accompanying drawings, in
which:
FIG. 1 is a diagrammatic representation of a flux switching device
according to the invention in which both magnets associated with
the pairs of polepieces move, the magnets being shown in the ON
position;
FIG. 2 corresponds to FIG. 1, but shows the magnets in the OFF
position;
FIG. 3, 4 and 5 are sections on the lines III--III, IV--IV and V--V
respectively of FIG. 2 showing the internal flux diversion
paths;
FIG. 6 corresponds to FIG. 1, but shows an arrangement where only
the upper magnets move, the magnets being shown in the ON
position;
FIG. 7 corresponds to FIG. 6, but shows the magnets in the OFF
position;
FIG. 8 is a section on the line VIII VIII of FIG. 7 showing the
internal flux diversion path;
FIG. 9 is a plan view of a magnetic chuck in which the magnets are
moved in accordance with FIG. 1;
FIG. 10 is a side elevation of FIG. 9;
FIG. 11 is a section on the line XI--XI of FIG. 9;
FIG. 12 corresponds to FIG. 9, but shows a magnetic chuck in which
the magnets are moved in accordance with FIG. 6;
FIG. 13 is a side elevation of FIG. 12; and,
FIG. 14 is a section on the line XIV--XIV of FIG. 12.
In FIG. 1, a flux switching device is shown as having a number of
pairs of ferro-magnetic polepieces 1, between which lie upper and
low rows of magnets 2. Each magnet is magnetised through its
thickness so that opposite polar faces of the magnets are of
opposite polarity, and are positioned between the polepieces such
that the corresponding polar faces of adjacent magnets in each row
are of opposite polarity. Thus, with the upper and lower rows of
magnets moved to a position where all the magnets lie between
respective pairs of polepieces, and with the corresponding polar
face of an upper and lower magnet associated with one pair of
polepieces being of the same polarity, the magnets and the
polepieces combine to provide an external magnetic field of maximum
strength, above and below the polepieces. To reduce these external
fields to substantially zero, the magnets 2 of the upper and lower
rows are moved by half a pole pitch to the position shown in FIG.
2, when each pair of polepieces, apart from the outermost pairs,
are contacted by a mouth pole and a south pole of the magnets of
the upper and lower rows, a north pole being adjacent a south pole
in both the horizontal and vertical directions. The effect of this
is shown by FIGS. 3, 4 and 5 which illustrate the flux diversion
paths between the magnets through the polepieces, which effectively
reduce the external field to substantially zero. The upper and
lower rows of magnets can be returned to the position shown in FIG.
1, when the external magnetic field is returned to maximum
strength, but the magnets could be moved by half a pole pitch in
the opposite direction, when an external magnetic field of maximum
strength would be created but of reverse direction to that of FIG.
1. That is, while viewing FIG. 1 and using the end magnets 2 as a
reference point, assume the top row is shifted all the way to the
left and the lower row is shifted all the way to the right, then
the direction of the magnet field when considering the flux paths
running from the north pole to the south pole of the adjacent
magnet is opposite to that shown in the original position of FIG.
1.
As an alternative to moving both rows of magnets, the same effect
can be obtained by moving either the upper or lower row. Thus, as
is shown by FIGS. 6 and 7, a number of pairs of polepieces 1 are
provided between which lie upper and lower rows of magnets, each
magnet again being magnetised through its thickness. The lower row
of magnets are secured between the polepieces with corresponding
polar faces on adjacent magnets of opposite polarity. With the
magnets of the upper row positioned as shown by FIG. 6, the magnets
of both rows combine with the polepieces to provide an external
magnetic field of maximum strength. To reduce the external field to
substantially zero, the magnets 2 of the upper row are moved by a
full pole pitch to the position shown in FIG. 7, when the
corresponding polar faces of the magnets of the upper and lower
rows associated with each pair of polepieces are of opposite
polarity. The effect of this is to provide a flux diversion path
through the polepieces as is shown by FIG. 8. To return the
external magnetic field to full strength, the magnets 2 of the
upper row may be returned to the position shown in FIG. 6, or they
may be moved by a full pole pitch in the opposite direction (when a
further pair of polepieces 1A would be provided) again to provide
an external magnetic field of maximum strength Using another
reference point from that used in describing FIG. 1, the magnet 2
that appears at the particular polepiece 1 first from the left of
added polepiece 1A in FIG. 7, it can be seen that the resultant
external magnetic field may be considered to remain in the same
direction between the shifted positions described, i.e. the
magnetic field is toward the reference magnet after the top row is
shifted in either direction as assumed above. Only when both the
upper and lower rows are shifted by half a pole pitch apiece as in
FIG. 1, would the field be of reversed direction to that of FIG. 6.
Obviously, similar analysis of field directions with respect to
other reference magnets 2 and other reference points in either FIG.
1 or FIG. 6 may be made as desired.
The concept as defined above may be used in an instance where an
external magnetic field is required, which field is required to be
reduced, substantially eliminated, partially reversed or completely
reversed. Because heavy concentration of flux can be arranged at
the exposed edges of the polepieces and because the polepieces are
held stationary as the magnets are moved (with relative ease by
virtue of the self-balancing of magnetic attraction forces between
the magnets and the polepieces) the flux switching device is
eminently suitable as a work holding device (a magnetic chuck).
Thus, as is shown in FIG. 9, a magnetic chuck comprises a base
plate 3 of non-magnetic material to which are secured end plates 4
and side plates 5 preferably of ferromagnetic material. The base
plate 3 has a series of rectangular recesses 6 (FIG. 11) running
longitudinally of the base plate into each of which is secured a
number of polepieces 1 (FIG. 10), the polepieces being secured in
place by bolts 7. It will be appreciated that any number of
polepieces can be provided in both the longitudinal and transverse
directions of the base plate 3) to suit a particular
application.
The longitudinal gaps between adjacent rows of polepieces are
maintained by transverse rods 5A bridging the side plates 5, with
distance pieces 5B located between the polepieces. The gaps are
loaded with upper and lower rows of magnets 2 of high coercivity
ferrite material in slab form, each row of magnets being located in
a support bar 8 provided with recesses 9 in which the magnets are a
close fit. The lower support bar rests directly on the base plate 3
between the polepieces and the upper support bar rests on the top
face of the lower support bar. The gaps between the polepieces
above the top face of the upper support bars are filled with
non-magnetic material (preferably a suitable form of resin filler)
prior to the application of the support bars to the gaps, the top
face being machined after being filled to ensure that the upper
surface of the polepieces are left exposed. After the support bars
have been loaded, a further end plate 10 is secured to the side
plates.
Inwardly of the end plate 10, the mechanism to provide simultaneous
movement of all the upper and all the lower support bars and thus
magnets in opposite directions is provided. Thus, as shown
particularly by FIG. 10 the corresponding end of the upper and
lower support bars are each provided with vertical slots 11 through
which pass pins 12, the pins being located in and secured to
rotatable locking plates 13, to one of which is secured an
operating handle 14. The length of the slots 11 is dictated by the
degree of movement required of each support bar. In this embodiment
the upper and lower support bars are each intended to move by
approximately half a pole pitch.
Thus, with magnets, magnetised through their thickness either
before or after being loaded in the support bars to provide
opposite polar face, and with the magnets being disposed in each
support bar in the manner depicted in FIG. 1 with corresponding
polar faces of adjacent magnets in each support bar of opposite
polarity and with corresponding magnets in adjacent support bars
being magnetised in the reverse direction, the handle 14 is
operated to cause the pins 12 to move the upper and lower support
bars to the position shown in FIG. 10, the ON position, when all
the magnets and all the polepieces co-operate to provide an
external magnetic field of maximum strength in the manner described
with respect to FIG. 1. To reduce the external magnetic field to
substantially zero, the handle 14 is rotated in the opposite
direction within the limits imposed by the lengths of the slots 11
in the support bars, to bring the upper and lower rows of magnets
to the position corresponding to FIG. 2, when the magnets and the
polepieces co-operate to provide internal flux diversion paths
after the manner described with reference to FIGS. 3 to 5. Thus,
any ferromagnetic workpieces resting on the upper surface of the
chuck can then be removed. With certain workpieces, the application
of the external field can cause permanent magnetisation of the
workpiece and it is therefore advisable that the lengths of the
slots 11 are such as to allow a movement of the magnets to a point
slightly beyond the position where the external field has been
substantially eliminated, to provide a low strength external field
of reverse direction thereby demagnetising the workpiece.
Because the upper surfaces of the polepieces actually form the
work-supporting surface, and the polepieces extend directly to the
base plate 3 which is in turn secured in the machine, a chuck of
considerable mechanical strength is provided allowing great
accuracy in any machining operation performed on the workpiece by
virtue of the elimination of any tendency of the work-supporting
surface to be deflected. Also, with the base plate, side plates and
end plates formed from a suitable non-magnetic material, and with
the efficiency of elimination of the external magnetic field by the
flux diversion paths in the OFF position, stray external magnetic
fields are substantially eliminated thereby preventing
magnetisation of the machine itself with consequent prevention of
magnetisation of the cutting means, (e.g., a milling cutter). This
also considerably improves the surface finish capable of being
given the workpiece.
FIG. 12, 13 and 14 show a flux switching device again in the form
of a magnetic chuck, but in this case embodying the principle
depicted in FIGS. 6, 7 and 8, where only the row of magnets is
moved by a full pole width.
Thus, a magnetic chuck has a non-magnetic base plate 15 provided
with side plates 16 and end plates 17, 18, preferably of
ferromagnetic material. On the inside faces of each side plate 16
are provided spaced locating pegs 19. To provide the longitudinal
rows of polepieces 1 and lower fixed magnets 2 (again of ferrite
material in slab form), transverse rows of polepieces and magnets
are separately formed as "packs." Polepieces in alternation with
magnets are placed in a jig, there being adhesive applied to both
polar faces of the magnets. Distance pieces are placed between the
polepieces at their upper ends, and the whole assembly compressed
to provide the required transverse spacing between adjacent
polepieces and the pack held to allow the adhesive to eat. The pack
is then located between the side plates 16 on the pegs 19 by virtue
of vertical slots 20 in the outside face of each outermost
polepiece 1 and the pack secured from below, e.g., by bolts. With
the area between the side plates loaded with the requisite number
of packs, and before application of the end plate 18, a series of
spacers are placed in the longitudinal gaps between adjacent pairs
of polepieces, the spacers being lubricated with, e.g., a silicone
base lubricant, to effect sealing of the gaps between the spacers
and the polepieces. Resin is then run in to fill the gaps between
the polepieces and the top surface finally ground to leave the
polepieces with an exposed upper face. The spacers are then removed
and replaced by support bars having recesses into which magnets are
fitted, the magnets again being thin ferrite magnets magnetised
through their thickness to provide polar faces of opposite
polarity.
Thus, with adjacent polar faces of the lower fixed rows of magnets
alternating in polarity both longitudinally and transversely and
with the magnets in the support bars also alternating in polarity
both longitudinally and transversely, the support bars are moved to
a position such that the magnets assume a position equivalent to
that shown in FIG. 6 whereby an external magnetic field is provided
of maximum strength, and to reduce the magnetic field to
substantially zero the rows of support bars moved until the magnets
assume a position equivalent to that shown in FIG. 7 when internal
flux diversion paths, as depicted in FIG. 8, are created with the
resultant elimination of the external magnetic field. To move all
the support bars simultaneously, the lower edge of each support bar
21, at its end adjacent the end plate 18 is provided with a rack 22
all the racks being engaged by a pinion 23 extending across the
full width of the chuck through notches provided in the endmost
polepieces. The pinion is secured to a gear wheel 24 lying
externally of one side plate 16, the gear wheel 24 engaging an
internally toothed gear wheel 25 having a drive shaft 26 on which
is mounted a handle 27. A cover plate 28 encloses the epicyclic
gearing 24, 25, the cover plate being provided with stops 29 (which
may be adjustable) to limit movement of the handle 27. To reduce
the overall length of the chuck, as is distinctly advantageous, the
operating pinion is situated inwardly of the end plate and as a
consequent, the endmost magnets must be shorter than the rest. This
slightly reduces the maximum external field available at that oint
but this does not affect the overall performance of the chuck. By
utilising an epicyclic gear transmission, a very convenient handle
movement of approximately 180.degree. is needed to switch the
device ON and OFF.
Thus, rotation of the handle through approximately 180.degree.
simultaneously moves all the support bars to bring the magnets in
the support bars to a position corresponding to that shown in FIG.
6, reverse movement of the handle bringing the support bars and
thus the magnets to a position corresponding to that shown in FIG.
7. The position of the stops 29 is such that the movable magnets
can be brought to a position where the external magnetic field has
been reduced to substantially zero, or to a position where a low
strength external magnetic field is created of reverse direction,
to effect demagnetisation of a workpiece applied to the working
surface.
As with the construction shown in FIGS. 9 to 11, the chuck of FIG.
12 to 14 again has polepieces extending from the work-holding
surface directly to the base plate of the chuck with its effect on
the rigidity of the workholding surface, and the effectiveness of
the flux diversion paths in the OFF position is such that again no
stray external field exists which could otherwise magnetise the
machine to which the chuck is applied.
It will be understood that the drive mechanism shown in FIGS. 9 and
11 can be applied to the work-holding device of FIGS. 12 and 14 and
vice versa.
With both constructions shown, it is advisable to have magnets
slightly greater in length than the length of the polepieces and to
have the thickness of the magnets slightly less than the thickness
of the polepieces, whereby the maximum possible concentration of
flux through the polepice is obtained.
* * * * *