Apparatus for compensating for the thermal expansion of two relatively movable machine parts

An apparatus for compensating for the thermal expansion of two machine parts which are relatively movable in longitudinal guideways. The apparatus comprises a rule, a support rod longitudinally disposed on the movable machine part for supporting the rule, and an electric scanning head mounted on the stationary machine part. The support rod is made of a material which exhibits negligible thermal expansion in the concerned temperature range. The forward end of the support rod is rigidly secured to the forward end of the movable machine part while the rear end thereof is movably guided on that machine part. The rear end of the rule is fixed to the rear end of the support rod. The scanning head is disposed on the stationary machine part in such a way that any thermal expansion of the stationary machine part cannot affect the location of the scanning head.

FIELD OF THE INVENTION 
The invention is directed to an apparatus for compensating for the thermal 
expansion of two machine parts relatively movable in longitudinal 
guideways, the apparatus including a support rod which is longitudinally 
disposed on the movable machine part for supporting a rule carrying 
measuring indices and an electro-optical scanning head mounted on the 
stationary machine part. 
BACKGROUND OF THE INVENTION 
In order to satisfy the high precision demands made in respect of modern 
machine systems, in particular program controlled machine tools, it is 
necessary for the employed measuring systems to operate as error-free as 
possible and independently of varying operating conditions. In order to 
prevent contamination of the measuring systems as far as possible, these 
systems are normally disposed quite remote from the operating range of the 
machine. But due to thermal expansion effects of the individual machine 
parts caused by temperature variations, which occur for example during 
start-up and until an approximately constant operating temperature has 
been reached, displacements are thereby caused between the work spindle or 
the machining tool and the measuring system. 
German Pat. No. 2,450,322 discloses an apparatus generally of the specified 
kind for compensating for thermal expansion of a part of a machine tool, 
in which a strain rod carrying the actual rule is mounted in close 
engagement on a side wall of a slide, for example, a headstock, which is 
exposed to the temperature variations. The material of the strain rod is 
selected such that in the respectively occurring temperature ranges the 
thermal expansion of the strain rod is greater by a certain amount than 
that of the slide. The rear end of the strain rod is rigidly mounted on 
the rear end of the slide or headstock, which rear end is heated only 
slightly. Upon heating of the slide or headstock, such as during start-up, 
the increased elongation of the strain rod will compensate for the 
elongation of the machine parts projecting beyond the slide, for example, 
of a vertical milling head attached to the headstock, which elongation 
cannot be directly detected by the measuring system. The rule carrying the 
indices for a scanning head secured to the stationary machine part is 
secured with its forward end to the forward end of the strain rod by use 
of a thermally insulating intermediate layer which insulates the rule 
relative to the heated parts and is intended to prevent thermal expansion 
of the rule. 
But with this prior compensating system problems may arise due to the fact 
that the strain rod is fixedly joined to the machine part only at its rear 
end while its forward end is merely retained in a guide means mounted on 
the headstock. Since the strain rod therefore contacts the machine part so 
as to be relatively movable in longitudinal direction, a continuous 
positive contact along the entire length of the strain rod cannot be 
achieved, thus resulting in a not exactly defined heat transmission 
between the machine part (headstock) and the strain rod. Especially due to 
changes of length of the strain rod, which is made from a mechanically 
relatively flexible material such as aluminum, there is the risk upon 
assembly or during operation that the continuous surface contact with the 
machine part is disturbed, thus resulting in significant changes of the 
heat transmission and therefore of the strain behavior of the rod. 
Moreover, the length dimension of the respective strain rods must also be 
matched with the different machine parts or types of machines. 
For compensating for the thermal expansion of a headstock it is known from 
German Pat. No. 1,010,802 to provide a strain rod in parallel to the 
headstock and to securely fix it on one side to the machine stand and on 
the other side to a machine part carrying the headstock. With that 
structure, upon a change of temperature the entire headstock or the 
machine part carrying the same will be displaced due to the corresponding 
thermal expansion of the strain rod. This compensating system is extremely 
expensive, because the strain rod is integrated in the machine itself. 
Retrofitting of machine tools with this compensating system is not 
reasonably possible. 
SUMMARY OF THE INVENTION 
It is an important object of the invention to provide an apparatus for 
compensating for the thermal expansion of two machine parts which are 
relatively movable in longitudinal guideways, especially of a headstock on 
the stand of a milling machine, which apparatus operates with an improved 
response and higher compensating accuracy while being of simple design. 
The specified object, and others, are achieved in accordance with the 
invention in that the support rod secured to the movable machine part 
consists of a material which exhibits negligible thermal expansion in the 
concerned temperature range. The forward end of the support rod is rigidly 
secured to the forward end of the movable machine part and the rear end of 
the rod is movably guided on the movable machine part. The rear end of the 
rule is fixedly secured to the rear end of the support rod. 
Due to rigid connection between the forward end portions of the movable 
machine part and the support rod, the support rod will be taken along by 
the machine part which expands upon heating in the forward direction. 
Since the support rod itself is resistant to temperature variations in the 
concerned temperature ranges and therefore has substantially constant 
length, the rule fixed to the rear end thereof will be displaced by 
exactly the amount of the thermal elongation of the machine part, thus 
resulting in exact compensation of the thermal expansion of the machine 
part. In order also to take into account the thermal expansion of a 
further machine member attached to the movable machine part, for example, 
a vertical milling head attached to a headstock, wherein the thermal 
expansion of such a member cannot be directly detected, the relatively 
slight elongation of the rule having its rear end fixedly connected to the 
rear end of the support rod is utilized in accordance with the invention. 
It is an essential advantage of this compensating system that the heat 
transmission between the machine part and the support rod has no 
significant influence on the accuracy of compensation, because the length 
of the support rod remains substantially unchanged within the concerned 
temperature range due to the chosen material and because the entire 
support rod is taken along by the expanding end portion of the machine 
part. 
The accuracy of compensation is further substantially and unexpectedly 
improved in respect of the prior art by the feature that the scanning head 
is secured to the free end of a second support rod which, in the concerned 
temperature range, exhibits substantially no thermal expansion and 
therefore has constant length. The other end of the second support rod is 
secured to the stationary machine part in the plane thereof which is 
neutral in respect of expansion--in case of symmetrically designed machine 
parts this plane corresponds to the center plane--so that any thermal 
expansion of the stationary machine part will not affect the position of 
the scanning head.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference now to the drawing, headstock 2 is mounted on stand 1 and is 
guided for motor-driven movement in horizontal guideways 3. Rotatably 
driven horizontal spindle 4 is supported in headstock 2. For performing 
machining operations in the vertical direction, vertical milling head 5 
(indicated in dashed lines) having driven vertical spindle 6 can be 
attached to the front face of headstock 2, as is known to those skilled in 
this art. 
Support rod 8 is secured by bolts or equivalent means to the forward end 
portion of headstock 2 by fixing means 7 such as a mounting plate. In the 
typically occurring temperature range, between normal room temperature and 
about 80.degree. C., the support rod is practically neither extended nor 
subjected to any change of shape. Bunched carbon fibres in a suitable 
matrix have been found suitable as material for the support rod. The rear 
end of support rod 8 is retained for longitudinal movement along slideway 
10 on headstock 2, the support rod being bolted by fixing means 11 to the 
rear end of rule 12. In the illustrated embodiment support rod 8 protrudes 
along its entire length through hollow rule 12, slideway 13 being provided 
between both members at the front end of rule 12. Rule 12 is suspended 
from leaf springs 14, 15 on a sidewall of headstock 2, the leaf springs 
during normal operation exhibiting a certain rearwardly directed spring 
force on rule 12 and serving to dampen vibrations generated in operation. 
Scanning head 20 having optical system 21 and being electrically connected 
to the program control unit (not illustrated) is used for scanning the 
measuring indicia on rule 12. The casing of the scanning head is secured 
by means of two leaf springs 22, 23 to a side wall of stand 1. Moreover, 
scanning head 20 is rigidly connected to one end of a further support rod 
24 which is made of the same material as support rod 8 and exhibits 
substantially no expansion in the concerned temperature range. The other 
end of support rod 24 in the illustrated embodiment is rigidly secured by 
fixing means 25 in the plane of the stand which, because of the 
construction, is neutral in respect of thermally caused extension. 
The operation of the afore-described thermal expansion compensating system 
will now be described. When headstock 2, during start-up, is heated from 
normal ambient temperature to its operating temperature (about 50.degree. 
C.), thermal expansion will normally occur, especially in the front 
portion thereof. The working point of the tool to be used in operation 
with horizontal spindle 4, or the spindle axis upon operation with 
vertical spindle 6, will be displaced due to thermal expansion toward the 
left as viewed in the drawing by an amount of a few tenths of millimeters. 
Since support rod 8, with its forward end in the vicinity of the work 
spindle, is secured to the headstock by fixing means 7, the support rod 
will be taken along by the headstock when elongation occurs. Due to the 
resistance of support rod 8 to length changes caused by temperature 
variations, the rear end of the support rod, together with rule 12 mounted 
thereon, will be displaced in slideway 10 by the same amount of elongation 
so that the measuring indicia on rule 12 will also be moved in the 
direction of elongation of the headstock. 
Compensation of the extremely slight elongation of a tool inserted in the 
horizontal spindle, including the machine taper, or of the vertical head 5 
including the attachments on the spindle head, is not fully taken into 
account by the above-described system. But this very minimal elongation 
will be compensated by the relatively slight thermal expansion of rule 12, 
which is secured with its right-hand end, as viewed in the drawing, to 
support rod 8, resulting in a slight further displacement of the measuring 
indicia in the direction of elongation of the headstock. 
Heating to operating temperature as well as temperature variations during 
operation are not confined to headstock 2 but will also occur in stand 1. 
The thermal expansion of the stand effective in the longitudinal direction 
of the headstock must not be neglected in view of the high demands on the 
accuracy of modern machine tools. This length change is compensated in 
accordance with the embodiment of the invention by mounting scanning head 
20 on further support rod 24 fixed in the center plane of the stand. 
The invention is not limited to the illustrated embodiment. For example, 
rule 12 can either be in direct and large-surface contact with a side wall 
of headstock 2, or intermediate layers may be provided between the rule 
and the side wall of the headstock so as to cause controlled heat 
transmission from the headstock to the rule. The latter feature permits 
the use of rules of identical length for different-length headstocks or 
other machine parts and their disposition in any desired section of the 
headstock. Furthermore, support rod 24 may also be secured at its forward 
end directly to the forward edge of stand 1, whereby due to a suitable 
length of support rod 24 scanning head 20 can be positioned as desired in 
accordance with the selected location of the rule. It is also possible to 
reverse the arrangement of rule and scanning head, in which case the 
scanning head is mounted on the headstock while the rule is mounted on the 
stand. 
In view of the above description it is likely that modifications and 
improvements will occur to those skilled in the applicable art which are 
within the scope of the appended claims.