Machine tool

A machine tool is equipped with a machine bed (10) and a carriage (23) of a headstock which is guided linearly on the said bed. To this end, the machine bed is provided with a guide rail (11), while the carriage (23) is equipped with a shoe (20) running on the guide rail (11) in form-locking engagement. The shoe (20) and the guide rail (11) engage each other by flat sliding surfaces (51 to 56) formed by the surfaces (51, 53, 55) of plastic sliding pads (46, 48, 50) and by metallic surfaces (52, 54, 56). The guide rail (11) has a narrower lower portion (15) and a wider upper portion (16) comprising a projection (17) extending laterally beyond the lower portion (15). The shoe (20) is split along a horizontal plane (40) and has its upper part (30) guided by the upper face of the projection (17) and its lower part guided by the lower face of the projection (17) (FIG. 3).

The present invention relates to a machine tool comprising a first machine 
element, for example a machine bed, and a second machine element, for 
example a carriage of a headstock, the machine elements being guided 
linearly along each other in such a way that the first machine element is 
provided with a linear guide rail, while the second machine element is 
equipped with a shoe running on the guide rail in form-locking engagement, 
the shoe and the guide rail engaging each other by flat sliding surfaces 
formed by the surfaces of plastic sliding pads on the one machine part and 
metallic surfaces on the other machine part, and the guide rail having 
further a narrow lower portion and a wider upper portion comprising a 
projection extending laterally beyond the lower portion, and the shoe 
being mounted on the second machine element by detachable mounting means. 
A machine tool of the described type has been known from DE-U-89 03 980. 
The known machine tool comprises a linear guide unit supported on roller 
bearings. A rail of substantially x-shaped cross-section is fixed on the 
machine frame by means of screws. The rail serves as guide for a carriage 
which in its turn is screwed, in detachable relationship, to a worktable 
provided on top of the arrangement. In order to enable the carriage to be 
guided on the rail, the carriage is provided with a matching x-shaped 
recess which is provided with a plastic lining over its full inner 
surface. 
While this arrangement guarantees that the carriage is guided on the 
machine frame so as to prevent it from tilting about the longitudinal axis 
of the rail, the form-locking engagement between the rail and the carriage 
being non-rotationally symmetrical, it is an absolute requirement in the 
case of this machine tool that the carriage be mounted on the rail via the 
latter's ends, as the carriage is designed as a single piece so that the 
only way to remove the carriage from the rail is to pull it off in linear 
direction over the end faces. 
DE-A-35 31 813 describes a machine tool comprising a guide system for the 
carriage where the carriage is guided on a guide rail of trapezoidal 
cross-section, the latter being mounted on the narrow upper face of the 
wall-like bracket. The arrangement is such that the guide rail projects 
laterally beyond the bracket. Consequently, the bottom face of the 
carriage is equipped with a matching trapezoidal recess, and the upper 
face as well as the inclined lateral face of the recess are lined with 
plastic sliding pads. Another vertical lateral surface is in contact with 
the carriage via roller bearings. 
In order to prevent the carriage from tilting about the longitudinal axis 
of the guide rail, the lower face of the carriage is provided with a 
pressing device engaging below the projecting lower face of the guide 
rail. The upper face of the pressing device is also equipped with a roller 
bearing. The resulting overall arrangement is such that the carriage, 
together with the pressing device, are guided on the guide rail via roller 
bearings at the right vertical side and at the horizontal lower face, and 
via a plastic sliding pad at the horizontal upper face and another 
inclined side. 
While this known machine tool provides the possibility to remove the 
pressing device from the carriage by screwing it off so that the carriage 
can then be lifted off the guide rail, the carriage of the known machine 
tool constitutes the whole worktable which means that if the sliding pads 
should get worn, the whole worktable must be disassembled and repaired. 
And the reassembly of the arrangement also has to be carried out with 
extreme care as the roller bearings may easily be damaged at the lateral 
guiding planes of the guide rails during mounting of the very heavy 
worktable, which is designed in this case as carriage. This risk is even 
increased by the fact that the arrangement uses two mirror-symmetrical 
parallel guide rails extending in parallel at a certain distance from each 
other. Due to this configuration, the worktable must be lowered upon the 
two guide rails with extreme precision in order to permit the rolling 
elements of the two roller bearings to slide down along the vertical 
guiding surfaces of the guide rails, in straight alignment and without 
being damaged, until the worktable, with the plastic sliding pads mounted 
thereon, comes to rest upon the guide rails. 
The leaflet "GT" by Gleitbelagtechnik GmbH SKC 3, 1982, page 8, describes 
an arrangement where the guiding surfaces of a machine tool are formed by 
a guide rail which exhibits a substantially rectangular cross-section with 
a lateral projection arranged on one side of its upper half. A guiding 
element running on the guide rail engages the guide rail by its horizontal 
upper face, over one full vertical side and--on the opposite side in the 
area of the projection--by the latter's shorter vertical side. The guiding 
surfaces are formed by plastic sliding pads so that one obtains a 
metal/plastic surface combination. According to the leaflet, the sliding 
pads are either applied with a spatula or produced by molding. Further, it 
has been known from the leaflet that such guides can be used also for 
machine tools. The guide element of the described arrangement is formed 
integrally with the moving unit which means that when repairs or 
maintenance work become necessary, the whole element to be guided has to 
be dismantled and the sliding pad has to be removed from the very large 
guiding element. 
In the case of machining centers, i.e. numerically controlled machine 
tools, where a plurality of successive machining operations, in particular 
milling and drilling operations, are carried out on a workpiece, linear 
guides comprising guide rails and carriages are used, for example, for 
displacing a headstock along two horizontal axes extending perpendicularly 
to each other, and one vertical axis. Conventional machining centers use 
shoes arranged on the carriages, which are guided via a ball bearing guide 
along guide rails of matching prismatic design. This arrangement leads to 
a metal/metal surface combination in the guiding area. 
Although the metal/metal surface combination certainly offers certain 
advantages, as regards the precision of the guide and, thus, the 
dimensional accuracy of the machined workpieces, disadvantages may result 
from the metal/metal combination when the machine tool is exposed to 
vibrations. Such vibrations may occur, for example, during milling when 
large chips are removed by the milling process. The metal/metal surface 
combination may then prove to be a disadvantage as the resulting 
connection between the two machine parts, for example the headstock and 
the machine bed, is extremely hard, and this condition may give rise to 
undesirable vibrations in the machining center. 
On the other hand, such machining operations frequently give rise to the 
problem that in the presence of such high chip removal rates the spindle 
stock is also subjected to considerable forces, i.e. reaction forces. Such 
reaction forces may act in any direction, including the upward direction. 
Now, it is the object of the present invention to improve a machine tool of 
the type described above in such a way that the guiding means by which the 
machine elements are guided along each other provide a certain 
self-damping effect, especially in the case of machining centers intended 
for performing milling operations with high chip removal rates, and that 
in addition the machine elements engage each other in form-locking 
relationship in as many coordinate directions as possible, while the guide 
elements of the machine elements can be repaired with the least possible 
input, without the necessity to handle the entire guided machine element 
to a larger extent. 
The invention achieves this object due to the fact that the shoe is split 
along a horizontal plane and has its upper part guided by the upper face 
of the projection, and its lower part guided by the lower face of the 
projection. 
This solves the object underlying the present invention fully and 
perfectly. On the one hand, the invention makes use of the fact that 
machine tool guides equipped with plastic sliding pads offer a natural 
self-damping effect so that the transmission from one machine element to 
the other of greater vibrations produced by machining operations with high 
chip removal rates is either fully eliminated or at least damped. In 
addition, the invention makes use of a guiding arrangement, where the shoe 
engages the guide rail from below so that the machine elements can be 
guided in form-locking relationship in all coordinate directions 
perpendicular to the longitudinal direction of the guide rail. This is 
true in particular for horizontal guide rails when a vertical, upwardly 
directed force such as a reaction force of the tool is exerted upon the 
guided machine element. Given the fact that the shoe is fastened on the 
second machine element by detachable connection means, the guide elements 
can be easily exchanged if they should be damaged or worn. It is then only 
necessary to release the described connection means, i.e. to untighten the 
screw, in order to remove the shoe from the second machine element and to 
mount another new shoe whereby the downtimes of the machine tool can be 
minimized. This distinguishes the machine tool according to the invention 
from the prior art described at the outset ("GT" leaflet) as in this 
latter case the lining is applied directly on the guiding element, over 
the latter's full surface, with the result that the whole guiding part 
must be handled and worked when the linings are worn. This leads to 
considerable downtimes of the machine which is equipped with such guides. 
According to a preferred improved embodiment of the machine tool according 
to the invention, a plurality of shoes are arranged one behind the other 
on the machine element. In particular, two parallel guide rails are 
arranged on the first machine element for guiding shoes, for example two 
groups of two or two groups of three shoes, arranged on the second machine 
element. 
This feature provides the advantage that the size of the guide elements is 
reduced to the relatively small shoes which leads on the one hand to space 
savings and prevents on the other hand any tilting, the guiding contact 
being established practically by twice two or twice three points. 
According to a preferred embodiment of the invention, the sliding pads are 
retained in the one machine element in form-locking relationship, in a 
manner known as such. 
This feature provides the advantage that the sliding pads are secured 
against displacement on the machine part on which they are mounted. 
According to another particularly preferred embodiment of the invention, 
the sliding pads are formed, in a likewise conventional manner, by 
injection against a mold formed in recesses provided in the one machine 
element, through injection bores arranged in the one machine element. The 
sliding pads end in this case in a sprue extending into the injection 
bores which are designed as threaded bores. 
This feature provides the advantage that the sliding pads are also reliably 
secured in their recesses due to the fact that the sprues sort of dig 
themselves into the threaded bores thus providing a safe mechanical 
support for the sliding pads. 
According to another preferred embodiment of the invention, the recesses 
are provided with a non-level, in particular a toothed surface. 
This feature also provides the advantage that the sliding pads are 
prevented from getting dislodged, in particular in the longitudinal 
direction of the guide rail. 
According to other preferred embodiments of the invention, lubricating 
grooves are provided in the sliding pads. 
This feature provides the advantage to enable lubricants to be transported 
into the area of the plastic/metal surface combination so that the 
frictional forces encountered can be minimized. 
Another particularly preferred embodiment of the invention is further 
characterized by the fact that passage bores intended for fastening the 
shoes to the other machine part are arranged on longitudinal sides of the 
shoe in alignment with the mounting bores intended for screwing the upper 
part to the lower part, the said bores ending in recesses which have been 
worked into the lower part from one side and which are separated by web 
portions of the lower part. 
This feature provides the advantage that the shoe can be assembled, i.e. 
the upper and the lower parts can be mounted on the guide rail, in a 
simple manner, the laterally widening recesses allowing a bent tool, for 
example a hexagon spanner, to be applied for tightening both the screws 
serving to fix the two parts together, and the screws by which the shoe is 
fixed to the other machine part, for example the headstock carriage. By 
providing separate recesses which are separated by web portions of the 
material of the lower part one achieves the considerable advantage that 
the mechanical stability of the shoe is clearly increased as compared to 
an arrangement comprising a recess extending over the whole length of the 
side, across all bores. An additional advantage of the web portions lies 
in the fact that they can be utilized for accommodating lubricant channels 
leading to the lubricating grooves. 
According to another preferred embodiment of the invention it is further 
provided that the sliding pads are arranged on the shoe and that the axial 
end faces of the shoe are equipped with covering frames providing an axial 
cover for at least those sections of the sliding pad which project beyond 
the shoe surfaces. 
This feature provides the advantage that the entire cross-section embraced 
by the shoe, except for the cross-section of the guiding rail, is covered 
reliably which ensures on the one hand that the sliding pad is held in the 
longitudinal direction of the guiding rail, while on the other hand no 
dirt is permitted to enter the area of the guiding rails. 
Finally, it is provided according to another particularly preferred 
embodiment of the invention that the junction plane extends above the 
contact surface of the sliding pad of the bottom element which is in 
contact with the lower face of the projection. 
This feature provides the advantage that the planes of the junction plane 
on the one hand and the supporting surface on the other hand are spaced 
relative to each other so that the bottom part, for example, may be worn 
down in the area of its junction plane, as mentioned before, without the 
contact surface of the sliding pad being damaged itself. 
Other advantages of the invention will appear from the following 
description and the attached drawing. 
It is understood that the features that have been mentioned before and will 
be described hereafter may be used not only in the described combination, 
but also in any other combination or individually, without leaving the 
scope of the present invention.

Regarding FIGS. 1 and 2, reference numeral 10 indicates a bed of a machine 
tool, for example of a machining center intended for carrying out 
numerically controlled milling and drilling operations on work pieces. 
A horizontal surface of the bed 10 carries a guide rail 11. The guide rail 
11 is equipped for this purpose with axially spaced stepped passage bores 
12 through which screws 13 are passed for screwing the guide rail 11 to 
the bed 10. A cover plate 14 is mounted on top of the guide rail and the 
stepped passage bores 12. 
Regarded in a direction perpendicular to the longitudinal direction of the 
guide rail 11, the latter comprises a narrower lower portion 15 and a 
wider upper portion 16 so that the upper portion 16 has one side, in FIG. 
2 the left side, project beyond the lower portion 15 in the form of a 
projection 17. 
The guide rail 11 carries a sliding shoe 20. The sliding shoe 20 is 
provided with passage bores 21 arranged along longitudinal sides of the 
sliding shoe 20. The bores 21 receive screws 22 intended to screw the 
sliding shoe 20 by its upper face to a moving machine part, for example a 
carriage 23 of a headstock of a machine tool. 
Preferably, the machine bed 10 is equipped with two parallel guide rails 
11, and each guide rail 11 carries two or three sliding shoes 20 which are 
arranged one behind the other and which are screwed to the carriage 23. 
The carriage 23 is guided in this manner on the guide rails 11 and, thus, 
on the machine bed 10 by a twice two or a twice three guiding arrangement. 
The axial end faces of the sliding shoe 20 are equipped with covering 
frames 24 which are screwed to the shoe by means of screws 25. The 
covering frames 24 comprise a window the cross-section of which is just 
equal to the radial cross-section of the guide rail 11 so that the 
covering frames 24 move closely along the surfaces of the guide rail 11 
during displacement of the sliding shoe 20. 
FIG. 3 shows, for example, that the sliding shoe 20 comprises an upper part 
30 consisting of an upper horizontal portion 31 and a right vertical 
portion 32. The upper part 30 is followed, in downward direction, by a 
bottom part 35 consisting of a left vertical portion 36 and a lower 
horizontal portion 37. The upper part 30 is separated from the lower part 
35 along a horizontal junction plane 40. 
A lower surface 45 of the upper horizontal portion 31 is provided with a 
first sliding pad 46 of a plastic material which is laterally held, in 
form-locking engagement, in a corresponding recess in the lower surface 
45. The left surface 47 of the right vertical portion 32 is provided with 
a second sliding pad 48 of similar design. 
Finally, an upper surface 49 of the lower horizontal portion 37 is equipped 
with a third sliding pad 50, likewise of similar design. 
As appears very clearly from FIG. 3, the sliding shoe 20 is guided in this 
manner on the guiding rail 11 in form-locking engagement, by a 
plastic/metal surface combination 51, 52 at the upper face of the wide 
upper portion 16 of the guide rail 11, a surface combination 53, 54 at the 
lower face of the projection 17 and a surface combination 55, 56 at the 
continuous right vertical side of the guide rail 11. 
Consequently, the guiding shoe 20 is guided on the guiding rail 11 in such 
a way that the guiding shoe can be neither lifted off the guiding rail 11 
towards the top, nor tilted about an axis parallel to the guide rail 11. 
Further, it can be clearly seen in FIG. 3 that the junction plane 40 is 
located far above the contact surface 53 of the sliding pad 50 by which 
the horizontal portion 37 of the bottom part 25 bears against the lower 
face 54 of the projection 17. 
The upper part 30 of the sliding shoe 20 being detachably connected to the 
bottom part 35, the lower flange-like portion of the sliding shoe 20 in 
the form of the lower horizontal portion 37 can be assembled easily, for 
example when the carriage 23 has been positioned on the guide rail 11 with 
the upper part 30 of the sliding shoe 20 screwed to its bottom face. The 
carriage 23 can then be fixed and secured in form-locking engagement by 
simply screwing the bottom part 35 to the upper part 30 from below. 
In the event the horizontal sliding pads 46 and 50 should get worn, it is 
further possible to remove the bottom part 35 from the upper part 30 by 
untightening the screws, and to grind off the surface of the bottom part 
35, which forms the horizontal junction plane 40, by a small amount, for 
example by 3/100 mm. When the bottom part 35 is then screwed again to the 
upper part 30, the sliding shoe 20 will again be retained on the guide 
rail 11 in form-locking engagement and free from play. 
In order to enable the sliding pads 46, 48, 50 to be placed in position on 
their respective supporting surfaces, there are provided injection bores 
26, 58, 59 extending through the upper part 30 or the bottom part 35, 
perpendicularly to the planes defined by the sliding pads 46, 48, 50. It 
is thus possible to form the sliding pads 46, 48, 50 at the respective 
surfaces of the sliding shoe 20 in the manner which will be described in 
more detail further below, with reference to FIG. 5. 
It can be further seen in FIG. 3 that the right side of the upper part 30 
comprises a recess 60 extending over its full axial length. The axial 
recess 60 is stepped in height in such a way that the recess 20 is wider 
at the upper and lower ends of the sliding shoe 20--as viewed in FIG. 
1--to form the passage bores 21 for the screws 22. In contrast, the recess 
60 is higher in the area illustrated in FIG. 3 so that a horizontal 
lubricant channel 61 can be worked into the part from the side. A side 
channel 62 branching off from the lubricant channel 61 in downward 
direction ends in a bore 63 in the surface 51 of the first sliding pad 56. 
The lubricant channel 61 extends over almost the entire width of the upper 
part 30 and is then followed by a downwardly direction portion 64 which 
terminates, shortly before it reaches the lower face of the bottom part 
35, in an upwardly directed portion 65. The portion 65 finally ends in a 
bore 66 in the surface 53 of the third sliding pad 50. 
Reference numeral 67 indicates a lubricant connection provided at the end 
of the lubricant channel 61 through which the entire channel system can be 
supplied, the single connection being in addition protected mechanically 
by the recess 60. 
FIG. 4 shows a horizontal cross-section through the bottom part 35. The 
illustration shows clearly that--as has been mentioned before--the third 
sliding pad 50 is arranged in a recess in the lower horizontal portion 37, 
i.e. in sort of a trough 70, 71. Further, it can be seen that the 
lubricating groove portions 75, 76, 77, 78 branching off from the bore 66 
of the lubricant channel extend by sections in axial (75, 77) and radial 
(76, 78) direction thus forming a meanderlike arrangement. 
The left lateral face of the bottom part 35 is provided with four spaced 
recesses 80 to 83 which are separated by webs 84 consisting of the 
material of the bottom part. 
The recesses 80 to 83, just as the continuous recess 60 provided at the 
opposite side of the sliding shoe 20, serve the purpose to enable screws 
to be introduced and tightened from the bottom and/or from the side. In 
FIG. 4, for example, the uppermost and the lowermost recesses 80, 83 are 
intended to receive the mounting screws 22, as indicated by a dash-dotted 
bore 21 in the uppermost recess 80. The two central recesses 81, 82 in 
contrast are intended to receive connection screws for screwing together 
the upper part 30 and the bottom part 35. The bottom part 30 is provided 
for this purpose with a passage bore 85 above each of the recesses 81, 82. 
FIG. 5 illustrates by way of the third sliding pad 50 how the latter is 
introduced into the groove 70, 71 (FIG. 4). 
For the purpose of placing the pad into the groove, a mold 87 is positioned 
on the upper surface 49 of the lower horizontal portion 37. The lower face 
of the mold 87 has a shape complementary to the desired shape of the third 
sliding pad 50. Once the mold 87 has been positioned and fixed in place, a 
plastic material is injected through the third injection bore 59. 
The plastic material consists preferably of an epoxy resin containing 
suitable fillers. The injection bores having the form of threaded bores, 
it is on the one hand easily possible to connect an injection tool to the 
injection bores 59, by means of screws. On the other hand, the design as 
threaded bores provides the additional advantage that the sprue sections 
forming in the injection bores, for example the sprue 88 illustrated in 
FIG. 5, dig themselves into the thread so that they constitute additional 
mechanical retention means for the sliding pads 46, 48, 50. 
In order to improve the adhesion of the sliding pads 46, 48, 50 to the 
corresponding surfaces 45, 47, 49 still further, the latter may be 
provided with a toothed or generally irregular surface 90, as illustrated 
in FIG. 6.