Tailstock for rotatably mounting a workpiece in precision machinery

A tailstock for rotatably supporting a workpiece on a machine bed. The tailstock includes a vertical column fixed at the machine bed which has first and second regions. The second region of the column is coaxial with the first region and pivotally mounted to the first region. An offset arm extends laterally from the vertical column and carries a center for engaging the workpiece. The offset arm has an opening for accommodating the regions of the vertical column and is longitudinally displaceable thereon. The offset arm is pivotable when disposed at the second region and non-pivotable when disposed at the first region to dispose same in repeatable precision relation to the machine bed.

BACKGROUND OF THE INVENTION 
The invention relates to a centering rest, as for location of a rotary axis 
of symmetry of a workpiece. So-called "tailstocks" or "center rests" are 
used on lathe and the like machine tools and on measuring devices, to 
provide support for rotatably chucked cylindrical workpieces such as 
gearwheels. The center rest clamps at least one axial end of the workpiece 
at a center (point) in the axis of rotation of the workpiece. In such 
supports, the receiving center (hereinafter sometimes referred to as the 
"opposing center") is axially opposite the rotary drive and is 
displaceable in the direction of the axis of rotation, to permit the 
clamping of workpieces of different axial length. As a rule, the tailstock 
or opposing center is carried by an arm which is longitudinally guided at 
offset from the axis of rotation and parallel thereto. 
Since spaced centers determine location of the rotary axis of the workpiece 
which is to be machined or measured, the work-engaging center, which is 
carried by the displaceable arm of the tailstock, should lie, for all 
clamping distances, in exactly reproducible manner on a predetermined 
straight line. In this connection, permissible tolerances must be within 
the micrometer or submicrometer range. 
However, such a requirement cannot generally be satisfied without special 
measures. First sources of error are attributable to guidance of the arm, 
which must be machined very accurately, since rotation about the 
longitudinal axis of the guide column have an effect, enhanced by the 
length of the arm, on the work-centering position of the tailstock. 
Secondly, thermal effects can cause changes in length of the arm, with 
resultant offsetting displacement of the center point. Another source of 
error resides in the mounting of the center rest to the machine. 
Center rests form a fixed part of machines on which they are developed. If 
they are removably attached, then, after attachment or reattachment, the 
tailstock must be set accurately with its work-engaging center in 
alignment with the rotary axis of the machining or measuring device, and 
guidance of the tailstock arm must be aligned parallel to this axis. Such 
adjustment work requires a relatively great amount of time and shortens 
the useful life of the machine. 
Universal machining and measuring machines are known which are not used 
exclusively for the working of cylindrical workpieces. In this case, a 
center rest (which is required only for workpieces of cylindrical 
symmetry) greatly limits the work area of the machine when it is not 
required, since the tailstock arm interferes with the path of travel of 
movable carriages of the machine. On the other hand, removal and 
reattachment of the center rest makes the above-mentioned adjustment work 
necessary. 
BRIEF STATEMENT OF THE INVENTION 
The object of the present invention is to provide an improved center rest 
whereby it becomes possible to substantially eliminate adjustment work for 
readjusting the position of the tailstock, when making a change in 
workpieces. 
This object can be divided into three specific subordinate objects, namely: 
1. To regenerate or retain the guidance alignment of the arm exactly 
parallel to the axis of rotation, even when the arm is removed from the 
work area of the machine. 
2. To compensate for change in length of the arm as a result of thermal 
effects. 
3. To attain a highly precise arm guidance, insensitive to transverse 
forces. 
The invention achieves the foregoing objects in a machine using a tailstock 
for rotatable support of a chucked workpiece. The offsetting arm which 
mounts the work-engaging center is longitudinally guided by a (main) guide 
column and, in addition is shiftable for (auxiliary) rotation about the 
guide column; the guide column is positioned at the edge of or entirely 
outside the working area of the machine, so that the guide axis is at all 
times fixed in space, maintaining parallelism of the guide axis and the 
work-rotating axis at all times. 
The end region of the guide column is swingable or turnable, and therefore 
the arm itself does not require any articulation which would have an 
effect on the precision of positioning the center. The form-lock between 
arm, guide and machine bed is therefore accurately reproduced each time 
that the arm is shifted out of the auxiliary guide (rotary) and back into 
the main guide (longitudinal). 
For guidance of the arm a plain bearing is provided which utilizes roller 
elements in conjunction with a guide column of polygonal section, such 
that, in the presence of force transverse to the guidance direction, only 
rolling friction occurs, as distinguished from the sliding friction which 
characteristic of a plain bearing in the presence of equivalent transverse 
force. The roller elements are yieldably retained in orientation parallel 
to the guide axis, and the arrangement is such that when transverse force 
reduces to zero, the arm returns to its position of rest with a precision 
which is increased by a factor equal to the ratio of the coefficients of 
friction for rolling and for sliding friction. 
In this connection, there is also a slight movement of the roller elements 
relative to the guided part, i.e. to the bearing portion of the arm. The 
roller elements are therefore preferably mounted to the guided bearing 
portion via an elastic adhesive composition which yields within the 
involved slight displacement regions. Another possibility for mounting the 
rollers is to retain them in yieldable roller cages. 
A lever mechanism is provided to compensate for thermal changes in length 
of the arm; this mechanism contains an elongation member consisting of 
material having a coefficient of thermal elongation which differs from 
that of the arm. The elongation member engages a holder of the 
work-engaging center, and the holder is pivotally mounted to the arm. With 
this arrangement, it is possible, by a suitable selection of distances 
involved in the engagement point of the elongation member, in relation to 
the pivotal mounting of the work-engaging center in the arm, and by 
adapting these dimensional distances to the materials used for the arm for 
the elongation member, to have assurance at all times that the 
work-engaging center will retain its fixed position, even in the 
circumstance of relatively great temperature changes. 
The work-engaging center of the tailstock is advisedly developed as a ball 
center whereby small tilt misalignment of the holder of the center 
(resulting from compensation movements) can remain insignificant as long 
as the center of tilt coincides with the center of the ball. 
Means for compensating for thermal changes in length are, to be sure, in 
themselves known. In this connection, mention may be made of the pendulums 
of regulators, as well as devices on boring machines, such as those 
described in West German Pat. Nos. 1,010,802, 2,450,322, West German Nos. 
OS 2,558,625 and 3,106,701. In the known compensating devices, however, 
bars of materials of different coefficients of expansion are secured to 
one another in the manner of a serial connection, so that the 
position-stabilized end region which generally serves as the reference 
mark is extremely unstable to transverse forces due to the relatively 
great length of the frequently twice-folded arrangement. In 
contradistinction to this, the lever mechanism of the present invention 
consists of bars or strips of different thermal expansion which are 
connected with each other in the manner of a substantially 
parallel-related connection. Even those transverse forces perpendicular to 
the longitudinal axis of the arm which occur when clamping to the 
workpiece are well accommodated by this arrangement. 
It is advantageous for the arm of the center rest to contain an adjustable 
clamping device for applying constant clamping force on the work-engaging 
center, since the work-engaging force of the center reacts in its turn on 
the guidance of the arm, and constant force conditions have a positive 
effect on positional reproducibility of the work-engaging center, i.e. on 
the precision of its positioning. 
It is further advisable for the arm guide to be arcuately deformed in the 
plane of the arm, i.e. in the plane established by the guide and the axis 
of work rotation. The bending stress which causes this deformation can be 
so selected that it is opposite and equal to the bending moment exerted by 
the means for clamping the arm to the guide, the selection being such that 
in the loaded condition, i.e. when the workpiece is clamped, the guide is 
in all cases aligned parallel to the axis of the workpiece. On the other 
hand, bending of the guide in the unloaded condition is completely 
irrelevant to the precise alignment of the workpiece, which is the only 
thing of importance. 
To stabilize the guidance of the arm and to establish the indicated 
deformation (in the unloaded condition), tension or pressure bars may 
suitably be provided within the polygonal section of the guide body. 
Since the bearing for the arm in the direction of the guidance is a plain 
bearing, it can be made self-locking; in other words, upon development of 
clamping force at engagement of the center with the workpiece (by means of 
the above-indicated clamping device), self-locking occurs as a result of 
the relatively great distance between the point of force application and 
the offset location of arm guidance, so that separate clamping of the 
guide engagement is unnecessary. 
Operator-induced forces for arm displacement, induced in the vicinity of 
the guide, do not produce any self-locking. If a handle is provided for 
such manipulation and clamping of the tailstock, i.e. if the handle is 
provided in the vicinity of arm engagement with the guide column, the 
advantage is obtained that a traversing of the arm, a clamping of the 
workpiece, and a clamping of the arm to the guide can all be effected by 
this single operating handle.

In FIGS. 1 and 2, a center rest or tailstock 3 is mounted by its base plate 
12 upon the flat horizontal working surface of a granite table 1 which 
also mounts the traversable portal 2 of a multiple-coordinate measurement 
machine. The useful measurement area of the measurement machine is 
indicated in FIG. 2, within limits of hatching 13. 
The center rest 3 comprises a guide column 9 along which an offsetting arm 
4 is readily displaceable in the vertical direction, by reason of a pulley 
14 and a counterweight 15 which is movable within the square cross-section 
of the walls of guide column 9. Arm 4 carries a work-engaging center 5, 
corresponding and opposed to a center 6 on a turntable 7 which is 
rotatably mounted in the granite bed 1 of the machine. A workpiece 8 is 
clamped between the centers 5 and 6. 
The end region 10 of guide 9 is mounted for rotation about the longitudinal 
axis of the guide column 9. As indicated by arm (4) positions shown in 
phantom lines in FIGS. 1 and 2, once arm 4 has been displaced to the end 
or auxiliary-guide region 10 of column 9, it can be swung together with 
column part 10 and thus can be removed from the working area 13 of the 
measurement machine 2. After each return swing back into the working 
position and down-shifted replacement on guide 9, the work-engaging center 
5 again lies precisely on the axis of workpiece (8) rotation since the 
position of column 9 does not change in the course of swing action; thus, 
parallelism is retained as between guide column 9 and the axis of the 
workpiece 8. 
Upon clamping the workpiece 8, a force F.sub.k acts on the center 5, which 
force bends guide column 9 via arm 4, as shown in exaggerated manner in 
FIG. 3. The bending moment M.sub.k acting on column 9 amounts to 
EQU M.sub.k =a.sub.k .multidot.F.sub.k, 
in which a.sub.k is the offsetting distance between center 5 and guide 
column 9. This bending moment M.sub.k is independent of the vertical 
position of arm 4 along column 9 and may therefore be viewed as constant. 
Now, a similar constant bending moment M can be produced within column 9, 
by the tensile force F.sub.z of a tie-rod 23 clamped between the cover and 
the bottom of the guide column 9 and at a distance a.sub.z from the center 
of column 9 (see FIG. 4). If one so designs the machine that the products 
F.sub.k .multidot.a.sub.k and F.sub.z .multidot.a.sub.z are equal and of 
opposite sign, then the deformation of guide column 9 (due to clamp force 
F.sub.k) can be well compensated by tensile force F.sub.z in the region 
between the base plate 12 and the location of arm (4) engagement (see FIG. 
5). 
FIGS. 6 and 7 illustrate that such compensation within guide column 9 is 
realized using four tie-rods 23a-d, all equally offset from the central 
axis of column 9, and also offset from the respective wall corners within 
column 9. Tie rods 23 are screwed into the base plate 12; they extend 
through corresponding holes in cover plate 22, and their length can be 
adjustably shortened by means of nuts 24a-d. 
It will be seen that by means of these tie-rods, column 9 may be accurately 
oriented in two component directions in space, not only to compensate for 
the above-explained deformation toward the axis of the workpiece, but also 
to correct for such residual defects in the guide path of column 9 as may 
exist in the component direction of the connecting line between rods 23b 
and 23d. 
FIG. 8 shows a preferred arrangement by means of which the constant 
clamping force F.sub.k between the centers 5 and 6 is applied to workpiece 
8. This arrangement comprises a loading spring 19 which is referenced to a 
housing part 20 of arm 4 and which is compressionally loaded against the 
upper end of the work-engaging center 5; for workpiece engagement, the 
force F.sub.k of spring 19 can be released by an operating lever 16, in 
the vicinity of guide column 9. Movement of operating lever 16 about its 
pivot axis 21 is transmitted by a rod 17 to a wedge 18 having cam 
engagement with the rear part of center 5. 
Guidance of arm 4 is effected via a plain bearing which as a bearing body 
has four groups 25 to 28 of three rollers each (FIGS. 9 and 10). These 
rollers are arranged with their longitudinal axes parallel to the 
direction of guidance and are contained under initial preload in the gap 
between guide column 9 and the surrounding bearing portion of arm 4. 
Rollers 25a-c to 28a-c are in each case fixed by an elastic adhesive 
composition 33 to the arm 4 and thus permit practically frictionless--i.e. 
very precise--return of the arm 4 into its position of rest after a 
twisting of the bearing, for example, as a result of transverse forces 
which act on the arm. 
The parallel arrangement of three rollers each has been selected since the 
bearing-contact surface of the involved slide bodies is thereby increased 
as compared with only one roller, thus better integrating the 
microgeometry of the guide. 
As best seen in FIG. 9, the respective pairs of three-roller groups 25/27 
and 26/28 of the front and rear bearing parts are in vertically staggered 
array, as viewed from the center 5, and self-locking of this bearing 
occurs (1) whenever workpiece-engagement forces are applied to the center 
5 and (2) when the point of force application is outside the range a.sub.H 
(see FIG. 8) from the central axis of guide column 9, said range a.sub.H 
extending beyond the point S, which designates the center of gravity of 
arm 4. This condition is satisfied for the workpiece-clamping process, but 
not for an operator's displacement of arm 4 when he is actuating lever 16. 
Such person therefore is able merely to raise or lower lever 16 to 
displace arm 4 freely in the vertical direction and, by pivoted actuation 
of lever 16, to release the workpiece-clamping force, as a result of which 
the guided support of arm 4 is automatically clamped. 
FIGS. 11 and 12 show a system for thermally stabilizing the length of arm 
4; and it will be understood that, for reasons of simplicity of 
illustration, this arrangement has been omitted from FIG. 8 and that, for 
the same reasons, the clamping device of FIG. 8 has been omitted from 
FIGS. 11 and 12. Of course, it is readily possible to equip arm 4 with 
both arrangements at the same time. 
Stabilization of the length of arm 4 can be of importance in particular 
when the material of the machine table 1 of FIG. 1 has a coefficient of 
thermal expansion which is different from that of tailstock arm 4, or when 
the heat capacity of table 1 is so much greater than that of arm 4 that, 
for all practical purposes, variations in temperature have an effect only 
on the length of arm 4. 
In order to maintain the position of center 5 fixed in space, i.e. to hold 
constant its distance a.sub.k from the central axis of column 9, the 
support body or holder 32 of center 5 is pivotally suspended via a 
transverse pin 31 within a recess between bifurcations at the end of arm 
4. Arm 4 is of steel. And a strip 29 of aluminum, arranged substantially 
parallel to arm 4, is mounted thereto via screws 33a and 33b, such that 
its other end is fastened to the upper end of the support or holder 32 for 
center 5. Near its point of contact with the upper end of support 32, 
strip 29 is greatly narrowed by a transverse groove 30. Groove 30 serves 
to effectively provide strip 29 with a pivot point. 
If the length of arm 4 changes by the incremental amount .DELTA.l.sub.1 as 
a result of thermal elongation, then strip 29 will be lengthened by the 
greater increment .DELTA.l.sub.2, due to the greater coefficient of 
expansion .alpha..sub.2 of aluminum as compared with the coefficient of 
expansion .alpha..sub.1 of steel. And if one so provides the distance 
L.sub.1 (between the center point of the ball of center 5 and pin 31) and 
the distance L.sub.2 (between the center point of the ball and the point 
of ball-holder (32) attachment strip 29) as to satisfy the condition: 
EQU L.sub.1 /L.sub.2)=.alpha..sub.1 /.alpha..sub.2, 
then the position of the ball center 5 is invariant with temperature, i.e. 
the distance a.sub.k between center 5 and guide 9 remains constant. In 
this connection, it is presumed that the strip 29 has the same length as 
arm 4, and that the two parts in each case assume the same temperature. 
Naturally, the described arrangement for stabilizing effective arm length 
is valid even if the lengths of the strip 29 and of the arm 4 are 
different. In the latter case, however, the above-indicated simple 
relationship no longer applies as between the pivot-point spacings 
L.sub.1, L.sub.2 and the coefficients of expansion. 
The center 5 is developed as a ball center. In this way, the result is 
obtained that small tilted inclinations of the support 32 of center 5 
(resulting from compensation displacements) remain without substantial 
effect on the axially correct position of the workpiece, even in the case 
of workpieces having large centering bores, since the points of 
application between workpiece and ball surface lie approximately at the 
same elevation as the temperature-invariant ball-center points.