[0001] The present invention relates to an apparatus for use in forming articles from rods
or bars of metal by rotation of an element of the apparatus. More specifically, the
present invention relates to an apparatus for forming from a plurality of metal bars
secured together at each end a twisted and enlarged basket of bulbous shape, as illustrated
in Figure 2 of the accompanying drawings. These basket shaped articles are commonly
used as decorative features in wrought iron designs.
[0002] It is known to employ electro-hydraulically operated machines to twist and compress
metal bars into the required shape. However, these machines are relatively expensive
and require an appropriate power source to operate. Whilst they are well suited to
mass production of the basket shaped articles, they are unsuited to small scale operations.
In this regard, it must be understood that many craftsman working in wrought iron
do so for a hobby and on a semi-professional basis. The use of complex and expensive
machines is for them neither economically nor practicably justifiable.
[0003] Hand operated versions of the electro-hydraulic machines referred to above are known
for use by craftsman. Generally, these comprise a fixed gripper ring or clamp for
securing one end of the plurality of metal bars and a rotatable gripper ring or clamp
for securing the other end of the plurality of metal bars. The rotatable end of clamp
is caused to rotate by a large handle and power thread drives. The plurality of metal
bars (usually 4 in number) is twisted through 360° and then the twist is reversed
through 180° to create a slender basket. In more complex machines, during the reverse
twist, the distance between the fixed and rotating clamps is reduced, thereby compressing
the metal bars, causing them to bow out further and thereby increasing the size of
the basket.
[0004] It is an object of the present invention to provide a method for forming metal bars
which is capable of achieving the increased size of basket associated with shortening
of the distance between the two clamped ends of the bars and increased compression
thereof, without it actually being necessary to shorten the distance between the two
clamped ends of the bars.
[0005] It is another object of the present invention to provide an improved design of gripper
ring for securing the ends of a plurality of metal bars in a metal forming apparatus.
[0006] According to a first aspect of the present invention there is provided a method of
forming a basket shaped article from a plurality of metal bars secured together in
a parallel relationship wherein one end of the said plurality of metal bars is held
in a fixed position in a gripper ring, whilst the other end is held in a rotatable
gripper ring and is rotated in one direction to impart a twist thereto, and then the
said other end is rotated in the opposite direction to partially untwist the bars,
characterised in that the initial twist is greater than 360°.
[0007] Preferably the initial twist is between 540° to 720° and the partial untwist is of
approximately 180°.
[0008] By imparting more twist to the said plurality of bars than is normally the case (no
more than 360° of twist is applied in the conventional method) the bars are required
to undergo more severe distortion or stretching. Surprisingly, the result of this
is that when the plurality of bars are partially untwisted a larger basket is formed.
This is so even when the partial untwist remains at 180° as it is in the conventional
method.
[0009] The increased twist imparted to the plurality of bars in the method according to
the first aspect of the present invention results in partial shearing of the outer
corners of the bars at the point where they exit the gripper rings. This shearing
is illustrated in Figure 7 of the drawings. This shearing results from the design
of the known gripper rings.
[0010] According to a second aspect of the present invention there is provided an apparatus
for forming a basket shaped article from a plurality of metal bars secured together
in a parallel relationship, the apparatus comprising a fixed end for securing one
end of the said plurality of metal bars and a rotatable end which is adapted to be
connected to the other end of the said plurality of metal bars, wherein both the fixed
end and the rotatable end each comprise a gripper ring defining a through hole of
square section which is so dimensioned that it receives and tightly accommodates between
the walls thereof an end of the said plurality of bars, characterised in that each
comer of square sectioned through hole is partially relieved.
[0011] Preferably the relief created at each comer of the square section through hole in
each gripping ring is that of a 90° cone whose centre is set in from the internal
comer towards the theoretical centre of the through hole.
[0012] The effect of the relieved area at each comer of the through hole is to prevent distortion
of the said plurality of bars at the outer comers thereof. Such distortion, which
is, of course, more likely as the amount of twist imparted to the bars is increased,
reduces the integrity and strength of the bars and can lead to shearing as they are
partially untwisted.
[0013] An embodiment of the present invention will now be described, by way of example,
with reference to the accompanying drawings, in which:
Figure 1 shows a cluster of four metal bars prior to twisting and untwisting to form
a basket shaped article;
Figure 2 shows a typical basket shaped article formed from the cluster of four metal
bars shown in Figure 1;
Figure 3 shows an end view of a fixed end clamp in accordance with the present invention
for use in a metal forming apparatus;
Figure 4 is a sectional view of the fixed end clamp shown in Figure 3 along lines
A-A;
Figure 5 is a sectional view of the fixed end clamp shown in Figure 3 along lines
B-B;
Figure 6 is a view of the end of a cluster of metal bars held in a fixed ring in accordance
with the present invention after they have been twisted through 540° to 720°; and
Figure 7 is a view of the end of a cluster of metal bars held in a conventional fixed
ring after they have been twisted through 540° to 720°.
[0014] Referring firstly to Figure 1 there is shown four lengths of metal bar 4 which are
held together in parallel positions with the ends joined together by welding or other
means. By twisting the four metal bars 4 about their central longitudinal axis and
then partially untwisting them a basket shaped article is formed as previously discussed
and as shown in Figure 2.
[0015] In accordance with the method of the present invention significantly more twist is
imported to the metal bars 4 than the 360° which is the absolute maximum in the conventional
method. Typically, the initial twist is between 540° to 720°. The effect of this is
that the bars undergo more severe stretching. When the bars are then partially untwisted
a larger basket is formed than is normally the case. Indeed, the size of the basket
is comparable to that achieved with apparatus which simultaneously with the partial
untwisting of the bars impart a degree of compression on the bars by shortening the
distance between the fixed and rotating ends of the apparatus.
[0016] One consequence of the greatly increased twist imparted to the metal bars in the
method according to the first aspect of the present invention is that the bars are
subject to shearing forces at the point where they enter the fixed and rotatable gripping
rings. The severity of this can lead to partial shearing of the outer corners of the
bars thus preventing them from being able to deform correctly during the partial untwisting
step. This partial shearing of the end of the metal bars at the point where they enter
the gripping ring is shown in Figure 7.
[0017] Referring now to Figures 3, 4 and 5 there is shown a gripping ring in accordance
with the present invention for use in a metal forming apparatus. This apparatus may
by an electro-hydraulically operated machine, but more typically will be a hand operated
apparatus.
[0018] The gripper ring in accordance with the present invention is used as a direct replacement
of the conventional gripper rings of the metal forming apparatus. It is identical
to the conventional gripper rings to the extent that it comprises a through hole of
square section which is so dimensioned that it receives and tightly accommodates between
its inner walls the ends of the metal bars. The gripper ring in accordance with the
present invention differs from the conventional gripper ring by virtue of the enhanced
relief 8 provided at each internal comer of the through hole adjacent to the point
where the bars exit the gripper ring.
[0019] The relief allows the outer corners of the overstretched bars to move by a limited
amount without being subjected to a shear action thus avoiding damage and subsequent
failure to support themselves during the reverse rotation as can be seen in Figure
6. (In a conventional gripper ring the actual edge of the aperture causes the bars
to shear.)
[0020] The actual geometry of the relief created at each corner of the internal square profile
of the gripping ring is that generally of a 90° cone whose centre is set in from the
internal comer towards the theoretical centre of the square hole.
[0021] This relief will prevent the comers of the bars held within the gripping ring from
being subjected to highly concentrated shear forces thus preserving the integrity
and strength of these bars when they are being subjected at the same time to high
tensile forces as they stretch due to the twisting action of the equipment within
which they are the work pieces. This protection of the bars from shearing is essential
in the forming of baskets by the method according to the first aspect of the invention
because the secondary rotational action of this process needs the bars to have stretched
in length by a greater degree than in any other process during the first rotation
to guarantee a more bulbous effect of the basket.
1. A method of forming a basket shaped article from a plurality of metal bars secured
together in a parallel relationship wherein one end of the said plurality of metal
bars is held in a fixed position in a gripper ring, whilst the other end is held in
a rotatable gripper ring and is rotated in one direction to impart a twist thereto,
and then the said other end is rotated in the opposite direction to partially untwist
the bars, characterised in that the initial twist is greater than 360°.
2. A method according to claim 1, characterised in that the initial twist is between
540° to 720° and the partial untwist is of approximately 180°.
3. Apparatus for forming a basket shaped article from a plurality of metal bars (4) secured
together in a parallel relationship, the apparatus comprising a fixed end for securing
one end of the said plurality of metal bars (4) and a rotatable end which is adapted
to be connected to the other end of the said plurality of metal bars, wherein both
the fixed end and the rotatable end each comprise a gripper ring defining a through
hole of square section which is so dimensioned that it receives and tightly accommodates
between the walls thereof an end of the said plurality of bars (4), characterised
in that each corner (8) of square sectioned through hole is partially relieved.
4. Apparatus according to claim 3, characterised in that the relief created at each corner
of the square section through hole in each gripping ring is that of a 90° cone whose
centre is set in from the internal corner towards the theoretical centre of the through
hole.