Linear bearing arrangement

A bearing arrangement for a wide variety of apparatus, such as machine tool tables, tool slides and carriages, wherein the bearing on one side of a movable element is preloaded by means of a hydraulic device forming a part of or connected to the opposing bearing on the other side of the movable element. A movable element, such as a machine tool table, is received in a support base having a lower guide surface and an upper guide surface. A roller-type bearing is secured to the table and bears against the guide surface which faces it, and, in this case, the last-mentioned guide surface is machined extremely flat and true relative to the tool. A second roller-type bearing is secured to the opposite side of the table and is urged against a guide surface by a hydraulic piston and cylinder. The hydraulic pressure which is developed presses the first roller bearing tightly against the respective guide surface and preloads the bearing so that the table will move with extreme accuracy relative to that surface. Since this surface is flat and true relative to the tool, the table will accordingly move with very high accuracy relative to the tool as well. The piston and cylinder develops a constant force due to the presence of a relief valve in the hydraulic supply line so that a constant preload of the bearing running against the true surface can be achieved. Because of the available stroke of the piston, parallelism of the two guide surfaces is no longer necessary. Only one true surface for each set of bearing pairs is necessary, thereby greatly simplifying machining and setup where extremely accurate linear movement is necessary.

BACKGROUND OF THE INVENTION 
The present invention relates to bearing arrangements, particularly for 
apparatus utilized in machine tool and metal forming environments, such as 
machine tool slides, tables and carriages. 
In machine tools, it is often necessary to traverse either a tool or a 
workpiece over fairly long linear distances during machining of the 
workpiece or between machining steps. For example, a workpiece may be 
rigidly secured to a machine tool table, and then the table traversed 
along a linear distance by means of a ball screw or the like to move the 
workpiece relative to a tool, such as a rotating milling cutter, which may 
cut a groove or chamfer in the workpiece. Conversely, the workpiece would 
be clamped to a stationary support and the tool moved relative to it 
during machining, as by a carriage or slide. In other machining 
operations, such as sequential hole boring, both the workpiece and tool 
carriage may remain stationary during machining, with a rotating bit moved 
into the workpiece by means of an axially movable spindle. In this case, 
the bit is withdrawn and then either the workpiece or tool moved to the 
next location for boring of the subsequent hole. 
In each of the examples outlined above, it is important that the workpiece 
or tool be moved with extreme accuracy so that there is virtually no 
component of movement in directions perpendicular to the primary axis of 
movement. Obviously, a lack of stiffness in directions perpendicular to 
the primary direction of movement would result in non-linear machining in 
the case of the first two embodiments, and would result in improper 
relative placement of the bored holes in the case of the latter 
embodiment. 
In the past, the table or carriage supporting the workpiece or tool was 
held in alignment by means of a V guide groove within which a 
correspondingly shaped ridge would be received, and the table prevented 
from skewing by virtue of the weight of the table forcing the 
complementary surfaces into mating engagement. In other cases, the table 
would be supported for movement by means of hydrostatic bearings mounted 
in either the table or support base, and which developed high pressure oil 
films to reduce the frictional drag between the table and support base. 
Although such a table may have acceptable accuracy under static conditions, 
once a tilting force is applied to the table, as by high cutter force, the 
static weight of the table can be overcome by the moment arm of such 
force, thereby disrupting the accurate mating of the guiding surfaces. 
Although hydrostatic bearings are very effective for reducing frictional 
drag between surfaces moving relative to each other, the effectiveness of 
the bearings are very sensitive to the clearance between the surfaces. 
When the clearance increases, the pressure of the hydraulic fluid 
necessarily decreases unless the overall hydraulic pressure and flow of 
the system is correspondingly increased. Furthermore, each time a 
different workpiece or tool weight is placed on the table or the force 
from a tool acting against the workpiece increases, the thickness of the 
oil film will change. If the oil film does change, then the tolerance is 
degraded by that amount. In other words, the oil film is dependent on a 
known preload under static conditions, and each time the preload changes, 
the thickness of the oil film will also change thereby increasing the 
tolerance of the apparatus. 
Although very accurately machined guiding surfaces can be obtained for 
short traverse distances, the machining tolerance becomes much more 
difficult to obtain for very long traverse distances, such as distances of 
ten feet, for example. In the case where the movable element is sandwiched 
between two guiding surfaces, it is not only necessary for the surfaces 
themselves to be extremely flat, but they must be perfectly parallel to 
each other so that the clearance for the hydrostatic or antifriction 
bearings will be maintained constant. For long traverse distances, such 
parallelism is virtually impossible to obtain. The problem is further 
complicated by the necessity for constraining the movable element, such as 
a workpiece table, in two orthogonal directions. Here, two true, flat and 
parallel pairs of surfaces must be machined along the entire length of 
traverse of the table or carriage. 
It is also known to support a movable element, such as press slide, between 
opposing pairs of hydrostatic bearings. Although this bearing arrangement 
resists, to some degree, lateral deflection of the movable element as it 
moves or reciprocates along its axis of movement, the hydraulic bearing 
clearance is disrupted by thermal expansion. Moreover, the proper 
clearance for the hydraulic film must be maintained on both sides of the 
movable element, and this requires extremely flat, true, and parallel 
opposing surfaces in the cases where high accuracy is required. As 
indicated earlier, maintaining such machining tolerances over long 
traverse distances is very difficult to achieve. 
Preloaded hydrostatic bearings are also known, but the oil film left on the 
exposed ways in long traverse environments, attracts dirt and metal chips, 
thereby interferring with the accuracy of the guide surface. 
Antifriction bearings, such as roller and ball bearings, can be utilized in 
bearing arrangements where very high accuracy is necessary, because they 
will deflect to a known degree with a certain known preload. The 
difficulty, however, is applying a preload to the bearings which is 
generally constant over the entire length of traverse of the table, 
carriage or other movable element. One prior art technique for preloading 
such bearings is to utilize a mechanical spring device, which applies a 
known amount of pressure to the bearing. The problem with such a device is 
that the preload applied by it is only correct for a given clearance, 
which may occur in only one position of the table or carriage. For 
example, if the opposing surfaces between which the table or carriage 
moves are non-parallel so that their separation differs from one position 
of the table or carriage to another, the spring device will be applying a 
different preload to the bearing. This is because of Hook's Law whereby 
the force exerted by a spring is proportional to its deflection. 
One of the problems with hydrostatic bearings and other opposed bearing 
arrangements is the necessity for maintaining the parallelism of the 
surfaces against which the respective bearings bear within a relatively 
narrow range. If the surfaces are not parallel, the clearance for the 
hydrostatic bearing will increase or decrease as the movable element 
traverses thereby requiring a higher or lower hydraulic flow, 
respectively, to maintain the same preload on the other bearing. When the 
preload changes, the deflection of the antifriction bearing changes or the 
oil film of the hydrostatic bearing changes thereby changing the position 
of the movable element relative to the true surface, which defines the 
reference plane for that degree of freedom. The problem of maintaining 
parallelism between the two surfaces increases as the length of traverse 
of a movable element increases. For very long traverse distances, such as 
those of ten feet, for example, it is almost impossible to maintain 
surfaces which are always parallel. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the invention, the above 
problems are overcome by preloading one of the antifriction bearings 
forming the bearing pair with a constant force for a given static load. 
This is done by connecting the other antifriction bearing of the pair to 
an expansible chamber device, such as a piston and cylinder, whereby the 
latter bearing is urged against its guide surface with a predetermined 
degree of pressure. This, in turn, presses the movable element in the 
opposite direction with the same force so as to constantly preload the 
first-mentioned bearing. Parallelism of the guide surfaces is no longer 
necessary, because as the clearance between the bearing which is connected 
to the expansible chamber device increases due to divergence of the 
guiding surfaces or decreases due to convergence of the guiding surfaces, 
the expansible chamber device will expand or retract, but always at the 
same pressure, which is governed by a relief valve in the hydraulic supply 
for the device. 
In the preferred embodiment, the expansible chamber device comprises a 
piston and cylinder, wherein either the piston or the cylinder is the 
movable element connected to the bearing. 
It is preferred that, in the case of a workpiece table or tool carriage, 
that the bearings be secured to the movable element, which is the table or 
carriage, rather than to the stationary supports, which are the ways, 
support bed, etc. In this case, the expansible chamber device would be 
secured to or be part of the movable element, and the true surface would 
be that surface against which the bearing secured to the movable element 
bears. 
The preload for the bearing preferably is selected so that it is 
sufficiently high to withstand the highest static or dynamic force which 
will be exerted against the bearing during use, and this force is 
determined by adjusting a relief valve connected to the hydraulic supply 
for the system. In the case of multiple bearing pairs, the simplest 
arrangement is to connect the expansible chamber devices in parallel with 
a common source of supply and a common relief valve. 
The system is not limited to the movement of workpiece tables or tool 
carriages in machining environments, but is equally applicable to other 
apparatus, both in the machine tool environment and elsewhere, wherein 
extreme accuracy in movement or positioning is desirable. 
Specifically, the preferred embodiment of the present invention 
contemplates a bearing arrangement comprising a first support element, a 
second support element spaced from and generally opposite the first 
element, and an intermediate element interposed between the first and 
second support elements and movable relative to the support elements. 
Either the first support element or the intermediate element has a first 
guide surface which faces the other of the first support element or 
intermediate element, and either the second support element or the 
intermediate element has a second guide surface which faces the other of 
the second support element or intermediate element. A first antifriction 
bearing means is connected to the other of the first support element and 
the intermediate element and is in movable engagement with the first guide 
surface in directions parallel thereto; a second antifriction bearing 
means is on the other of the second support element and the intermediate 
element and is in movable engagement with the second guide surface in 
directions parallel thereto. A preload device is provided for yieldably 
pressing the second bearing means and the second guide surface together, 
and includes an expansible chamber device connected to the second bearing 
means and the other of the second support element and the intermediate 
element such that expansion of the expansible chamber device presses the 
second bearing element and second guide surface together. Means are 
provided for supplying pressurized fluid to the expansible chamber device, 
preferably at the constant pressure. 
In its most basic form, the bearing arrangement according to the present 
invention comprises a single pair of antifriction bearings, preferably 
acting along a line perpendicular to the axis of movement of the movable 
element. In order to support the movable element so that there is only one 
degree of freedom of movement, however, more than one pair of such 
bearings is necessary. For example, to support and guide a workpiece 
table, at least four pairs of bearings, two on each side of the table, and 
acting in the vertical direction, together with at least two pairs of 
bearings acting in the horizontal direction against the edges of the 
table, will be necessary. This arrangement requires three true surfaces, 
two in the horizontal plane, and one in the vertical plane. In order to 
accurately guide the press slide described above, four bearing pairs are 
preferred, two in respective parallel planes parallel to the axis of 
reciprocation of the slide, and two in respective parallel planes also 
parallel to the axis of reciprocation but orthogonal to the 
first-mentioned planes. 
It is an object of the present invention to provide a bearing arrangement 
whereby a movable element, such as a table, slide, carriage, or the like, 
may be constrained in very accurate positions during movement with a 
minimum of accurately machined true and flat surfaces. 
It is a further object of the present invention to provide a linear bearing 
arrangement wherein very accurate parallelism of opposed guide surfaces is 
not necessary, and wherein nonparallelism of such surfaces will not affect 
preloading of the bearings. 
A still further object of the present invention is to provide a bearing 
arrangement wherein preloading of the bearings can be adjusted easily for 
different static and expected dynamic loads so as to minimize the effect 
of such loading on bearing deflection and, therefor, accuracy of movement. 
A still further object of the present invention is to provide a bearing 
arrangement wherein bearings are preloaded by means of hydraulic pressure, 
but wherein the hydraulic fluid is confined in a closed system. Another 
object of the present invention is to provide a bearing arrangement 
wherein a machine tool table can be accurately guided without the 
necessity for relying on the static weight of the table as the sole means 
for alignment. 
Another object of the present invention is to provide a bearing arrangement 
wherein the bearings self-adjust to compensate for thermal growth of the 
relatively movable elements caused by heat. 
Yet another object of the present invention is to provide a bearing 
arrangement wherein a constant preload is maintained on one of the 
bearings without the need for a true and parallel backmounting surface. 
Yet another object of the present invention is to provide a bearing 
arrangement wherein a damping effect opposing dynamic imbalances is 
achieved by means of the fluid in a hydraulic preload system. 
These and other objects of the present invention will become apparent from 
the detailed description taken in conjunction with the drawings.

DETAILED DESCRIPTION 
Referring now to the drawings, FIGS. 1, 2, 3 and 4 illustrate the preferred 
embodiment of the present invention. Although the bearing arrangement 
according to the invention can be utilized with a wide variety of 
apparatus wherein high accuracy is required, for purposes of description, 
it has been shown in conjunction with a work supporting table 12. Table 12 
is customarily made of steel or cast iron and is adapted to have one or 
more workpieces (not shown) mounted to it during machining. If desired, 
table 12 could be an air float table of the type described in U.S. Pat. 
No. 4,174,828, wherein workpieces are supported on a cushion of air as 
they are moved from one location to another on the table and then 
subsequently clamped in place. This patent is incorporated by reference. 
Table 12 is supported on a steel or cast iron base 14, which is in turn 
supported on a plurality of adjustable levelers 15. Levelers 15 are 
rigidly secured to or imbedded in a concrete foundation 16, and can be 
adjusted so as to ensure that base 14 is level and flat with a very high 
degree of accuracy. Levelers 15 are commercially available from Unisorb 
Machinery Installation Systems, for example. Supporting base 14 is 
provided with a pair of lower guide rails 18 having guide surfaces 20, 
which are machined extremely flat and true with very high accuracy. 
Levelers 15 can be adjusted to ensure the flatness of guide surfaces 20 
and to ensure that these surfaces 20 run true to the machine tool spindle 
or cutting head (not shown). 
A pair of upper guide rails 22 are secured to supporting base 14 by screws 
24 such that they overlie the side flange portions 26 of table 12. Upper 
guide rails 22 include downwardly facing guide surfaces 28 which are 
machined flat and generally true to the tool spindle or cutter head as 
well as generally parallel to guide surfaces 20. As will become apparent, 
for the preferred embodiment illustrated in FIGS. 1, 2, 3 and 4, it is not 
essential that guide surfaces 28 be precisely true and parallel to lower 
guide surfaces 20, but it is preferable that they be generally true and 
flat and generally parallel to surfaces 20. 
Six recirculating roller-type bearings 30 are secured to the lower surface 
32 of table 12 immediately opposite the guide surfaces 20 of lower rails 
18. Bearings 30, which are illustrated in detail in FIG. 6, are Bendix 
Scully-Jones Tychoway bearings available from Bendix Corporation. It 
should be noted, however, that the present application is not limited to 
the particular type of antifriction roller bearings described herein, and 
other suitable antifriction bearings, such as ball bearings and other 
types of roller bearings could be utilized equally well provided that they 
are capable of withstanding the preload which is developed. Each of the 
bearings 30 comprises a race 34 on which a plurality of rollers 36 roll in 
recirculating fashion around race 34 much like an endless track. Rollers 
36 are guided by center guide 38 and stabilizer band 40; and end caps 42, 
secured to race 34 by screws 44, serve to contain the rollers 36 as they 
make the transition from one flat surface of the race 34 to the other. 
As illustrated in FIG. 4, bearings 30 are secured to the lower surface 32 
of table 12 by means of cap screws 46 in such a manner that table 12 is 
supported in the vertical direction on the rollers 36 of bearings 30. Due 
to the size of table 12, six such bearings 30 have been provided to evenly 
distribute the static and dynamic loads exerted by table 12. For larger 
tables, more than six bearings 30 may be necessary, whereas for smaller 
tables, only four such bearings 30 may be required to adequately support 
the load. For tables of this type, it is preferred that at least four such 
bearings be utilized. 
In order to preload bearings 30 as taught by the present invention, there 
are provided six additional recirculating roller-type bearings 48, which 
are identical to bearings 30. Rather than being secured directly to table 
12, they are connected to table 12 through an expansible chamber device 
50, which is preferably a piston and cylinder unit. Expansible chamber 
device 50 comprises a cylinder 52 formed directly in the flange portions 
26 of table 12 and provided with a substantially incompressible, hydraulic 
fluid through passages 54 and branch passages 56. Slidably received within 
cylinder 52 is a piston 58 having a seal 60 and protruding slightly above 
the upper surface 62 of flange portions 26. Upper bearings 48 are secured 
directly to pistons 58 by means of screws 64. It will be appreciated that 
upper bearings 48 are directly opposite lower bearings 30 along lines 
perpendicular to the horizontal plane or axis of movement of table 12 
between guide surfaces 20 and 28. 
The purpose of bearing pairs 30 and 48 is to virtually prevent any movement 
of table 12 in the vertical direction as it rolls along support base 14 in 
the directions indicated by arrows 66 (FIG. 1). Since lower guide surfaces 
20 are flat and true relative to the machine tool spindle or cutter with a 
great degree of precision, if table 12 can always be located precisely 
relative to lower guide surfaces 20 then table 12 will always move true to 
the spindle or cutter. This is accomplished by admitting hydraulic fluid 
to cylinder 52 through passages 54 and 56 at a very high pressure 
sufficient to preload lower bearings 30 to a precisely known degree. 
Hydraulic fluid from sump 68 flows through filter 69 and is pumped by pump 
70 through hydraulic line 72 and hydraulic passages 54, and from there 
through branch passages 56 into cylinders 52. As cylinders 52 is 
pressurized, pistons 58 will be driven upwardly so as to press upper 
bearings 48 against upper guide surfaces 28 under very high pressures. 
This, in turn, presses table 12 downwardly so as to press lower bearings 
30 against lower guide surfaces 20, also at high pressures, so as to 
preload lower bearings 30. The amount of preload is maintained constant by 
providing a relief valve 74 connected in parallel with pump 70 so that it 
bypasses hydraulic fluid when its threshold pressure is reached. Thus, the 
hydraulic pressure in lines 72, 54 and 56 and, therefore, in cylinders 52 
will always remain constant at the threshold pressure of relief valve 74. 
If desired, valve 74 could be of the adjustable type so that the preload 
could be varied for different load conditions of table 12. 
A characteristic of bearings 30 is that they will always deflect by a known 
amount for a given degree of preload, so that if the pressure within 
cylinders 52 remains constant, then the preload on bearings 30 will also 
remain constant so that table 12 will always be spaced from lower guide 
surfaces 12 by a constant amount as it traverses along support base 14. It 
will be recalled that lower guide surfaces 20 are machined and adjusted so 
that they are precisely flat and true relative to the machine tool cutting 
element, and this ensures that table 12 will run true to the cutting 
element so long as the preload on lower bearings 30 remains constant. If 
desired, the bearing arrangement of FIG. 4 could be inverted with piston 
58 connected to bearing 48 and surface 28 machined true and flat. In some 
cases, this arrangement is preferred. 
The primary advantage of the described bearing arrangement is that it is 
not necessary for the upper guide surfaces 28 to be either flat or true to 
the machine tool cutting element or even parallel to lower guide surfaces 
20. Assume, for example, that there is a localized low spot in one of the 
upper guide surfaces 28. With prior art bearing arrangements, looseness of 
table 12 would occur at this point because bearings 30 and 48 would not be 
preloaded to the same degree as before, and, in fact, upper bearings 48 
may even move out of contact with the upper guide surface. With the 
present invention, on the other hand, the presence of pressurized 
hydraulic fluid within cylinder 52 will drive piston 58 and the upper 
bearing 48 attached thereto upwardly so that bearing 48 will continue to 
exert pressure against upper guide surface 28 with the same force as 
previously. This, in turn, maintains the preload on the corresponding 
lower bearing 30 at the same level as set. Conversely, if the upper 
bearing 48 meets with a high spot on upper guide surface 28, piston 58 
will be driven downwardly against the constant pressure maintained within 
cylinder 52 and the same preload conditions for lower bearings 30 will be 
maintained. 
For very long traverse distances of table 12 along its support ways, it is 
very difficult to maintain upper and lower guide surfaces 20 and 28 
exactly parallel. With prior art bearings, looseness of the table would 
occur when guide surfaces 20 and 28 diverge thereby reducing the preload 
on lower bearings 30 so that table 12 is no longer the same distance from 
the lower guide surfaces 20, and is, therefore, no longer in the same 
spatial relationship with the cutting tool. With the bearing arrangement 
according to the present invention, however, as guide surfaces 20 and 28 
diverge, pistons 58 will extend upper bearings 48 against the upper guide 
surfaces 28 so that the same degree of preload of lower bearings 30 will 
be maintained. 
In order to prevent lateral movement of table 12, two antifriction bearings 
78, which are identical to lower bearings 30, are secured to the side 
edges 80 of table 12 and bear against guide surface 83 of base 14. 
Directly opposite bearings 78 are mounted two antifriction bearings 82, 
which are identical to bearings 48 and are urged against the side guide 
surface 84 of support base 14 by piston and cylinder devices 85, which are 
identical to piston and cylinder devices 50. Cylinders 85 are connected to 
hydraulic passages 54 so that they are pressurized to the same level as 
cylinders 52. This preloads the opposite bearings 78 to a known constant 
level and ensures that table 12 is always spaced the same distance from 
guide surface 83. Guide surface 83 is machined precisely flat and true to 
the machine tool cutting element, so that table 12 will also run true to 
the element with a great degree of precision. As was the case with upper 
guide surfaces 28, it is not necessary for the other guide surface 84 
opposing piston-mounted bearings 82 to be flat and true to the same degree 
as, nor parallel to, the opposite guide surface 83. The piston driven 
bearings 82 will compensate for high spots, low spots and nonparallelism 
so as to always press table 12 toward guide surface 83 with the same 
force. It is preferred that guide surface 83 be precisely perpendicular to 
lower guide surface 20 so as to maintain a proper orthogonal relationship 
between table 12 and support base 14. 
Two pairs of bearings 78, 82 are provided and located approximately at the 
corners of table 12. Bearing 78 and 82 in each pair are directly opposite 
each other along lines perpendicular to the axis of movement of table 12. 
With the arrangement described above, it will be appreciated that table 12 
will always run at a constant distance from guide surfaces 20 and 83, and 
since these surfaces are true to the machine tool cutting element, table 
12 will also run true to that element. Nonparallelism and unevenness in 
the opposing guide surfaces 84 and 28 is not crucial because the pistons 
58 will always ensure a constant preload of bearings 30 and 78. 
Table 12 can be moved along its rectilinear path by any suitable means, 
such as ball screw 86, which is driven by hydraulic motor 88 connected to 
it through gearing mechanism 90. Moreover, the exact same bearing 
arrangement could be utilized for a carriage (not shown) which carries a 
rotating spindle, boring tool, milling head, or the like. 
The amount of preload which is selected depends to a great degree on the 
static and dynamic loads which the table 12 will exert during normal use. 
With reference to FIG. 7, a specific example of loading conditions for 
table 12 for a given workpiece and machining environment will be 
described. Table 12 is shown as having a workpiece 94 clamped thereto. 
Three sets of bearings 48, 78 and 30, each of which comprises six bearings 
as illustrated in FIGS. 1, 2 and 3 support table 12 within base 14. The 
following conditions are present: 
______________________________________ 
Total weight of table 12 and workpiece 94 
= 20,000 lb. 
Thrust force of tool acting against 
workpiece 94 (F.sub.t) = 12,000 lb. 
Vertical height from center line of table 
to plane of thrust force (h) 
= 72 in. 
Preload (P.sub.1) on bearings R1 and R3 
= W/6 + 6000 = 
9,330 lb. 
Preload (P.sub.1) on R2 and R4 
= 6,000 lb. 
Static load capacity of each bearing 
(Tychoway 21200) = 29,460 lb. 
Deflection of bearing per 1,000 lb. 
= .0001 in. 
______________________________________ 
In order to find the bearing reaction forces on bearing R4 for the given 
thrust force of the tool on workpiece 94, the moments at bearing R1 are 
summed: 
##EQU1## 
This reaction force, which is less than the 6,000 pound preload per 
bearing, will cause a deflection of 0.000066 inches. The reaction forces 
on each bearing R1 are determined by summing the moments at the opposite 
bearing R4: 
##EQU2## 
These reaction forces minus preload cause a table deflection of 0.000467 
inches relative to the tool. The bearing deflection at bearings 30 and 48 
will allow tool runout of 0.0000065 inches per inch. 
Although the specific embodiments of the invention described above are 
directed to arrangements whereby a movable element, such as table 12 (FIG. 
1) or slide 120 (FIG. 7) is received within and moved relative to 
stationary supports, the present invention is equally applicable to the 
converse arrangement wherein the enclosed element is secured to a 
stationary support and the enclosing elements move relative to it. Thus, 
for purposes of the present invention, the term movable refers to relative 
movement between two elements. In the case where the enclosed element is 
secured to a stationary support and the enclosing elements are movable 
relative to the stationary support, the enclosed element is nevertheless 
movable relative to the frame of reference of the enclosing elements. In 
this situation, then, the surface which is machined true and flat would be 
most likely located on the enclosed element, rather than on the enclosing 
elements as in the case of the specific embodiments shown and described. 
While this invention has been described as having a preferred design, it 
will be understood that it is capable of further modification. This 
application is, therefore, intended to cover any variations, uses, or 
adaptations of the invention following the general principles thereof and 
including such departures from the present disclosure as come within known 
or customary practice in the art to which this invention pertains and fall 
within the limits of the appended claims.