Machine tools abstract
A collet chuck which can be centred relative to an axis of the
workpiece to be machined and suitable for machine tools in general
and in particular machine tools having a number of successive positions.
An inner prehensile member is capable of gripping the workpiece,
and an intermediate member coaxial to the first has a conical seating
which slides axially causing the prehensile member to close on the
workpiece to be machined. An outer casing is rigidly attached to
the prehensile member by means of a pin, and an outer ring is fitted
with radial play around the periphery of outer casing. Spring members
of variable tension compress the outer casing and outer ring against
each other to lock them together by friction, and pusher means operable
axially by a thrust member unlock the outer casing and the outer
Machine tools claims
1. A collet chuck which can be centred relative to an axis of the
workpiece to be machined and suitable for machine tools in general
and in particular machine tools having a number of successive positions
characterised by an inner prehensile member (1) capable of gripping
the workpiece, an intermediate member (2) coaxial to the first with
a conical seating which slides axially causing the prehensile member
(1) to close on the workpiece to be machined (5); an outer casing
(3) rigidly attached to the prehensile member (1) by means of a
pin (4), an outer ring (7) fitted with radial play around the periphery
of outer casing (3), spring members (8) of variable tension to compress
the outer casing (3) and outer ring (7) against each other to lock
them together by friction, and pusher means (9) operable axially
by a thrust member to unlock said outer casing and said outer ring.
2. A collet chuck as in claim 1 characterised in that the aforementioned
outer casing (3) rotatably supported on a lower table (14) by means
of parts placed between the two, in particular balls (13) which
allow rotation with minimal effort, i.e. without friction on the
bearing cage (13) which is centred.
3. A collet chuck as in claim 1 characterised in that the aforementioned
pushers (9) are three pins set at 120.degree. intervals with their
axes parallel to that of the workpiece (5) and sliding without play
in tubes (11) containing balls (10); the aforesaid spring members
comprising, for each pin, a cup spring (8) between pin (9) and tube
(11); the tension of each spring (8) being regulated by means of
at least one threaded bush (20) screwing onto the tube (11); and
pressing against the above mentioned three threaded plugs (21) the
thrust member comprising a plate (12) movable whenever it is necessary
to unlock the outer casing (3) relative to the ring (7).
4. A collet chuck as in claim 1 characterised by a transverse
pin (4) which makes the outer casing (3) secured to the prehensile
member (1); the intermediate member (2) being capable of being shifted
axially by means of a hook (6) to effect closure of the prehensile
member (1) onto the workpiece to be machined (5).
5. A collet chuck as in claim 1 characterised in that the fit
between the outer casing (3) and the outer ring (7) is achieved
by means of an upper projection (7') around the ring and a corresponding
groove (3') on the casing; there being between the two sufficient
radial play to allow the outer casing (3) to be shifted radially
so as to center the collet relative to the axis of the workpiece,
centring being achieved by a cone (15) which may be fitted axially
into the workpiece.
6. A collet chuck as in claim 5 characterised in that in the space
between the outer casing (3) and the ring (7) is placed a corrugated
psring which keeps the bearing cage (13) centred when the aforementioned
casing (3) is slightly raised.
7. A collet chuck as in claim 1 characterised in that the aforesaid
inner prehensile member (1) which is capable of gripping the piece
to be worked has an extension at the lower end with a seating (24)
which contributes to accurate centring, where the upper cavity or
hole (5') corresponds to the axis of centring of the workpiece to
be machined (5).
Machine tools description
The present invention consists of a collet chuck which can be centred
relative to one or more axes of the work piece to be machined, and
suitable for machine tools in general and in particular machine
tools with a number of successive machine positions (transfer).
It is well known that with machine tools with a number of successive
machine positions there frequently arises the problem of ensuring
quickly and accurately, to a tolerance of a few thousandths of a
millimeter, the centring of the work piece to be machined relative
not to the outer cylindrical surface, but for example to the axis
of a hole drilled in it.
Given the high degree of accuracy required the problem is not easy
to solve except by the use of special devices.
The collet chuck the subject of this invention seeks to solve precisely
It is characterised by a prehensile inner member capable of gripping
the piece; an intermediate member coaxial to the first with a conical
seating which slides axially causing the prehensile member to close
on the piece to be machined; an outer casing integral with the prehesile
member and fitted around its periphery, with radial play, with an
outer ring. This outer casing and ring can be compressed against
each other and thus locked by friction by means of spring members
of variable tension and unlocked by means of pushers operated axially
by a thrust member.
In one preferred form of the invention this inner prehensile member,
which is capable of gripping the work piece to be machined, has
an extension at the lower end with a seating which contributes to
perfect centring, where the upper cavity or hole corresponds to
the axis of centring of the work piece to be machined.
The present invention also comprises a method for centring the
collet chuck as in the above form of the invention. The characteristic
of this method is the use of a centring apparatus consisting of
two opposed coaxial pistons which can move towards each other, of
which one is inserted into the aforementioned lower seating and
the other into the upper hole or cavity in the work piece to be
Embodiments of the invention will now be described, by way of example,
with reference to the accompanying drawings, wherein
FIG. 1 shows, in partial cross section, one embodiment of the invention;
FIG. 2 shows a plan of the embodiment of the invention drawn in
FIG. 3 shows the embodiment of the invention shown in FIG. 1 in
identical view to that of FIG. 1 but also in operative position
in relation to the machine tool;
FIG. 4 shows in a cross section a second embodiment of the invention;
FIG. 5 shows the embodiment of FIG. 4 when in operative position
in relation to the machine.
The collet chuck in question (FIGS. 1 2 and 3) comprises: an inner
prehensile member 1 (FIG. 1) which is capable of gripping the work
piece to be machined 5 in which the central hole 5', owing to defects
in previous machining, is not coaxial with the outer cylindrical
surface 5", or whose interior is purposely eccentric relative
to the outer surface.
Outside the prehensile member 1 and coaxial to it is the intermediate
member 2 of which the outer surface 2' is conical and fits the
corresponding conical surface of the casing 3.
At the lower end of this intermediate member 2 is a hooked section
which is pulled by the connecting rod 6 so as to close the prehensile
member 1 onto the work piece 5.
Outside the intermediate member 2 is the outer casing 3 which is
secured to the prehensile member 1 by means of a transverse pin
4 so as to eliminate relative movement radially or axially between
the prehensile member 1 and casing 3.
The outer casing 3 is fitted, around its periphery, with ample
radial play greater than the expected maximum eccentricity of workpiece
hole 5' relative to surface 5", with the outer ring 7. The
two are coupled by means of the upper projection 7' on the ring
and the corresponding groove 3' on the casing 3.
The outer ring 7 has three pushers 9 consisting of three axial
pins placed at 120.degree. intervals, sliding without radial play
on balls contained within three tubes 11.
Coaxially with each pin 9 are cup springs 8 the pressure of which
can be regulated by turning the threaded bush 20 screwed onto the
The lower plate 12 of the machine (FIG. 3) presses against the
three threaded plugs 21.
The outer casing 3 is supported by the lower table 14 and between
the two are placed parts to allow it to roll, in particular the
balls 13 which allow the casing itself to rotate without friction.
Between the outer casing 3 and the outer ring 7 is placed a weak
corrugated spring 16 (FIG. 2) which centres the bearing cage 13
when the ring 7 is raised.
The operation of the described arrangement is as follows. The lower
plate 12 presses in the direction of the arrow 22 (FIG. 3) and the
three pins 9 raise the ring 7 relative to the outer casing 3 allowing
the latter to rotate and move laterally on the balls 13 within the
limits of the radial play between ring 7 and casing 3 so that by
placing the guide cone 15 into the hole 5' in the workpiece, the
outer casing 3 is coaxial to the axis of the workpiece 5.
As is shown by the explanation given here, owing to the presence
of the balls 10 which eliminate radial play, perfect centring of
the collet chuck relative to the workpiece 5 is obtained; a contributing
factor is that the balls 13 allow it to be positioned with minimum
Given the wide diameter of the casing 3 corresponding to its periphery
3', that is where it meets the ring 7 the torque friction resistance
between the two surfaces 7' and 3' when in contact is considerable
and sufficient to ensure the immobility of the piece 5 in the collet
It is obvious that in the case of a machine with a number of successive
machine positions, once the workpiece has been centred relative
to the collet chuck at the start of a machining operation, the position
of the workpiece will remain unchanged, thereby eliminating all
tolerances not only of the workpiece but also those relative to
the eccentricity of the collet chuck itself.
The centring of the collet chuck on the axis of the piece to be
machined is moreover immediate and requires very little time. Hence
the collet chuck in question is extremely useful in mass production
machining where the time taken is of particular importance.
Finally, it may be noted that when positioned, it is possible to
rotate the collet casing 3 after raising ring 7 by means of the
positioner 15A in order to shift the point of contact of the balls
relative to the casing itself.
In the second embodiment, shown in FIGS. 4 and 5 the locking system
and the operation of the collet is identical to that described above.
In this case, the outer casing 3 has a special feature, namely
an extension corresponding at the lower end to a truncated cone
seating 24 perfectly centred relative to the axis of the collet
The procedure for centring is carried out by means of the apparatus
shown in FIG. 5.
The apparatus consists of two pistons 22-23 of which the upper
one 22 is centred on the axis of the workpiece 5 and the lower one
23 is centred on the axis of the truncated cone seating 24 of the
inner prehensile member 1 of the collet chuck.
The collet casing 3 is unlocked in exactly the same way as described
with reference to FIGS. 1 to 3 thereby allowing the two pistons
22-23 to align the axis of the workpiece to be machined 5 and the
inner casing of the collet relative to the axis 26.
Moreover, piston 23 is controlled by a slightly higher pressure
than piston 22 so that the casing 3 is slightly detached from the
surface of the plate 27 whilst still remaining disengaged from
the locking ring 7.
At this point, it is possible to rotate through any angle, the
whole collet casing system by means of the mechanism which consists
of the hydraulic motor 28 and the geared transmission 30. The advantage
of this is that it is possible to vary the points of contact between
the plate 27 and the collet casing surface 3'.
Piston 23 is then retracted to its original position while pressure
is maintained on piston 22 so as to re-establish contact between
the collet casing 3 and the plate 27; the collet casing 3 is then
locked by the ring 7 as described.
This variation is particularly suited for workpiece similar in
shape to that shown in the drawing in FIG. 4.