Machine tools abstract
A method for improving the transient thermal properties of hydrostatic
air main spindles for precision machine tools comprises the steps
of installing an entire precision machine tool that has a main spindle
with an air-lubricated hydrostatic bearing in a temperature-controlled
constant-temperature environment, and raising the temperature of
the lubricating air supplied to the air-lubricated hydrostatic bearing
above the temperature of the constant-temperature environment until
the thermal displacement of the main spindle reaches a target value,
and thereafter lowering it below the temperature of the constant-temperature
environment.
Machine tools claims
What is claimed is:
1. A method for improving the transient thermal properties of air-lubricated
hydrostatic bearings for the main spindles of precision machine
tools, comprising the steps of:
installing an entire precision machine tool that has a main spindle
with an air-lubricated hydrostatic bearing in a temperature-controlled
constant-temperature environment, and
supplying the air-lubricated hydrostatic bearing of said main spindle
with lubricating air of a temperature higher than the temperature
of said constant-temperature environment from the start of operation
of said main spindle until the thermal displacement of said main
spindle reaches a previously-determined target value, and
once the thermal displacement of said main spindle reaches the
target value, supplying the air-lubricated hydrostatic bearing of
said main spindle with lubricating air of a temperature lower than
the temperature of said constant-temperature environment.
2. The method for improving transient thermal properties of claim
1 in which the temperature of the lubricating air from the start
of operation of said main spindle until the thermal displacement
of said main spindle reaches a previously-determined target value
is approximately 10.degree. C. higher than said constant-temperature
environment, and the temperature of the lubricating air after the
thermal displacement of said main spindle reaches the target value
is approximately 10.degree. C. lower than said constant-temperature
environment.
Machine tools description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for improving the transient
thermal properties of air-lubricated hydrostatic bearings for the
main spindles of lathes, machining centers, turning centers, and
other precision machine tools.
2. Prior Art Statement
In the prior art, when precision machining is performed with various
types of precision machine tools, air-lubricated hydrostatic bearings
are used for the main spindle bearings. The entire machine tool
is installed within a constant-temperature chamber, and the air
supplied to the spindle bearings is supplied at the same temperature
as the temperature within the constant-temperature chamber. This
method is used to homogenize the temperature distribution of the
machine tool.
A main spindle lubricated by air in this manner innately has low
friction, and the lubricating air will absorb and exhaust any heat
generated, so very little thermal deformation would occur. However,
when extremely high precision machining is performed, even thermal
deformation on the order of several .mu.m can adversely affect precision,
thus requiring measures to reduce such thermal deformation.
In order to reduce this thermal deformation, the entire machine
is externally cooled using oil, water, air or the like.
However, the method of external cooling of the entire machine tends
to cool even unnecessary areas, requiring the addition of an inordinate
amount of energy in comparison to the magnitude of the decrease
in thermal displacement, thus degrading efficiency.
To solve this problem, the inventor of the present invention has
proposed a method of reducing displacement by supplying the air
spindle bearings with air of a temperature lower than the temperature
within the constant-temperature chamber as lubricating air (Japanese
Patent Application Public Disclosure No. 3(1991)-274737).
This newly proposed method can effectively reduce the absolute
value of the thermal displacement in the steady state. However,
this method requires a warm-up time of roughly 6-7 hours until the
thermal displacement reaches the steady state, so a reduction in
this warm-up time is desirable.
The present invention came about in light of the above, and its
purpose is to provide a method for improving the transient thermal
properties that can effectively reduce the absolute value of the
thermal displacement in the steady state, and moreover, can shorten
the warm-up time until the thermal displacement reaches a steady
state.
SUMMARY OF THE INVENTION
The method of the present invention for improving the transient
thermal properties of air-lubricated hydrostatic bearings for the
main spindles of precision machine tools, intended to achieve this
purpose, comprises the steps of installing an entire precision machine
tool that has a main spindle with an air-lubricated hydrostatic
bearing in a temperature-controlled constant-temperature environment,
and raising the temperature of the lubricating air supplied to the
air-lubricated hydrostatic bearing above the temperature of the
constant-temperature environment until the thermal displacement
of the main spindle reaches a previously-determined target value,
and thereafter lowering it below the temperature of the constant-temperature
environment.
As described above, by supplying the spindle bearing with lubricating
air of a temperature higher than the constant-temperature environment,
displacement in the spindle bearing increases rapidly. When this
displacement reaches the target value, the temperature of the lubricating
air supplied is reduced to below the temperature of the constant-temperature
environment. This target value is a fixed temperature lower than
room temperature, previously determined to be the ultimate value
of the thermal displacement reached when air is supplied. By changing
the temperature of the lubricating air as described above, the change
in thermal displacement in the spindle bearing over time assumes
an ideal shape, allowing the warm-up time to be shortened, simplifying
the process of compensation by NC or the like.
These and other objects and features of the present invention will
become more apparent from the following detailed description taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional diagram that shows the bearing structure
of a main spindle with an air-lubricated hydrostatic bearing.
FIG. 2 is a graph showing the history of the thermal displacement
of the bearing when the present invention is used.
FIG. 3 is a graph showing the history of the thermal displacement
of the bearing when the prior art is used.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an overview of the bearing structure of an air-lubricated
hydrostatic bearing for the main spindles of precision machine tools
that is subject to reduction of thermal deformation by means of
the method of the present invention. A precision machine tool equipped
with a main spindle incorporating this air-lubricated hydrostatic
bearing is installed in a constant-temperature chamber or other
constant-temperature environment maintained at a specific temperature
(not shown).
The air-lubricated hydrostatic bearing main spindle in this precision
machine tool is configured as a thrust-type high-speed main spindle,
arranged such that a stator 1 supports a rotor 2 via a film of lubricating
air 3. An air-supply channel is provided upon this stator 1 in order
to supply lubricating air to the bearing section.
When the rotor 2 rotates, heat is generated due to the viscous
friction of the lubricating air 3 within the bearing accompanying
this rotation. In this case, if lubricating air of the same temperature
as the set temperature within the constant-temperature chamber is
supplied, as in conventional air lubrication methods, this heat
conducts to the upper surface of the stator 1 and the lower surface
of the rotor. Temperature differences arise with respect to other
portions of the stator 1 and rotor 2 that are exposed to external
air, and these temperature differences are the cause of thermal
deformation of the main spindle.
For example, during long-term operation, thermal deformation on
the order of several .mu.m will normally occur.
According to the present invention, first, the steady-state thermal
displacement of a bearing provided with lubricating air of a temperature
5.degree. C.-10.degree. C. below the setting temperature of the
constant-temperature environment (hereafter called the steady-state
temperature) is found experimentally or heuristically. This thermal
displacement becomes the target value.
Next, lubricating air is supplied to the bearing together with
the start of operation of the bearing. The temperature of the air
supplied at this time is higher than the temperature within the
constant-temperature environment. Since the purpose of supplying
this higher-temperature air is to heat the bearing rapidly, its
temperature is roughly 10.degree. C. higher than the temperature
of the constant-temperature environment. Accordingly, the bearing
is heated by the supplied lubricating air, and its thermal displacement
increases rapidly.
Once the thermal displacement reaches the target value set previously,
the lubricating air is then supplied with its temperature at the
steady-state temperature. Thereby, the thermal displacement of the
bearing reaches the steady state.
Once the steady state is reached, lubricating air of a fixed temperature
5.degree. C.-10.degree. C. below the setting temperature of the
constant-temperature chamber is supplied. Thereby, as heat is generated
within the lubricating air and the temperature of the lubricating
air rises, since it was originally of a lower temperature, this
retards the development of temperature differences with other parts
of the stator 1 and rotor 2 that are exposed to external air. This
suppresses thermal deformation of the air-lubricated hydrostatic
bearing main spindle, allowing the axial elongation of the main
spindle due to heat to be greatly reduced in comparison to the normal
case.
A preferred embodiment of this invention is given below.
A precision machine tool equipped with an air-lubricated hydrostatic
bearing main spindle whose rotor rotates at a speed of 400 rpm is
installed within a constant-temperature chamber with a temperature
of 25.degree. C..+-.0.3.degree. C. Air is supplied to the air-lubricated
hydrostatic bearing at a pressure of 4 kgf/cm.sup.2.
First, 35.degree. C. lubricating air is supplied for roughly 60
minutes after the start of operation of the main spindle, at which
time the thermal displacement reaches the target value for thermal
displacement of 2.0 and then 15.degree. C. lubricating air is supplied.
The state of the thermal displacement of the main spindle is as
shown by curve a in FIG. 2 showing that it took roughly 80 minutes
to halt the increase in thermal displacement and reach the steady
state.
If the target value for thermal displacement is set to 1.5 35.degree.
C. lubricating air is supplied for 40 minutes after the start of
operation of the main spindle, at which time the thermal displacement
reaches the target value for thermal displacement, and then 15.degree.
C. lubricating air is supplied. In this case, the thermal displacement
of the main spindle is as shown by curve a in FIG. 2 showing that
it took roughly 60 minutes to halt the increase in thermal displacement
and reach the steady state.
For comparison, when lubricating air of a temperature the same
or lower than the temperature of the constant-temperature environment
is supplied as described in the prior art, the results of measuring
the state of thermal displacement of the main spindle in this case
are shown in the graph of FIG. 3. Curve c shows the case of when
25.degree. C. lubricating air is supplied continuously from the
start of operation of the main spindle. Curve d shows the case of
when 20.degree. C. lubricating air is supplied continuously from
the start of operation of the main spindle. Curve e shows the case
of when 15.degree. C. lubricating air is supplied continuously from
the start of operation of the main spindle.
As is evident from FIGS. 2 and 3 in contrast to the case of the
present invention illustrated by curves a and b on FIG. 2 in which
the increase in the thermal displacement of the main spindle was
halted and the steady state was reached in roughly one hour, in
the case of the prior art illustrated by curves c, d, and e on FIG.
3 this took roughly six hours.
It is thus evident that the present invention can greatly reduce
the warm-up time.
Thus, by means of the present invention, a method for improving
transient thermal properties can be implemented by which one can
effectively reduce the absolute value of the thermal displacement
of air-lubricated hydrostatic bearings in the steady state, and
moreover, can markedly shorten the warm-up time until the thermal
displacement reaches a steady state.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is therefore
to be understood that within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
herein. |