Machine for polishing and/or grinding

The prior art discloses machines which, for cooling the surface to be machined, have a gas supply line which opens into a gas outlet which is in the direct vicinity of the polishing disk. When CO.sub.2 is used as cooling gas, stoppages, which can be traced back to snow formation of the CO.sub.2, may occur in the line and in the gas outlet. In order to avoid blockages, the machine is assigned means for the supply of the liquid or supercritical CO.sub.2 which comprise a valve for regulating the throughflow quantity of the CO.sub.2 with an expansion nozzle, the valve being arranged inside the machine.

BACKGROUND OF THE INVENTION 
DE-C 39 10 590 discloses a machine from the applicant which comprises a 
polishing disk and a cold gas supply for cooling the surface to be 
polished or to be ground, the cold gas supply preferably opening into a 
gas outlet which is arranged in the axis of rotation of the polishing 
disk. The appliance is preferably operated using N.sub.2 as cold gas, 
which is stored in liquid form, evaporated and fed to the gas outlet via 
the cold gas supply. In order to ensure an optimum operating temperature 
of the cold gas, which temperature essentially depends on the material 
properties of the surface to be treated, the nitrogen has to be 
temperature-controlled in a complicated manner. It is also known to 
operate such a machine using CO.sub.2 as cold gas, which in the case of 
the evaporation of liquid CO.sub.2 has a temperature of -78.degree. C. 
This temperature is advantageous for the operation of such machines, since 
it generally satisfies the requirements for the cold needed. However, the 
machine employed according to the prior art is preferably operated with 
N.sub.2, since this gas can be fed in lines without problems. For this 
purpose, it is accepted that, the evaporated nitrogen is brought to the 
desired temperature by means of the supply of energy. Although CO.sub.2 
comes substantially closer to the actual requirements of the operating 
temperature, and the production of the raw material liquid CO.sub.2 
consumes substantially less energy, N.sub.2 has been preferred until now, 
since CO.sub.2 has the property of forming CO.sub.2 snow during its phase 
transition from liquid to gas, said snow blocking the lines and the gas 
outlet. The operation of the machine is considerably hampered thereby and 
the operating duration is restricted. 
SUMMARY OF THE INVENTION 
The invention is therefore based on the object of providing a machine for 
polishing and grinding with which treatment of surfaces with CO.sub.2 as 
cold gas is possible in a fault free manner. 
Using the machine, it is now possible to polish and/or to grind highly 
sensitive materials such as paint surfaces or plastics, using CO.sub.2 
cooling, without blockages of the gas supply or of the gas outlet with 
CO.sub.2 snow leading to disturbances in the working process or to a 
premature termination of the treatment.

DETAILED DESCRIPTIONS 
In the machine 1 shown in FIG. 1, a polishing disk 2 as machining tool is 
set rotating by means of a gearbox 3 which is driven by a compressed air 
motor 4. Into the axis of rotation there leads a gas supply 5, which opens 
into a valve which essentially comprises an adjusting device 11, a pipe 6, 
valve spindle 12 arranged inside the pipe 6 and an expansion nozzle 7. 
The gas supply 5 and the valve are shown more closely in FIG. 2. The gas 
supply 5 opens into the pipe 6, whose upper end is closed via a 
compression washer 9 and a stuffing box 10 with the aid of the adjusting 
device 11, which is designed as a union nut, using which the valve spindle 
12 can be moved axially inside the pipe 6 by means of the thread 8. The 
expansion nozzle 7 has an opening 13 tapering conically at the outlet, an 
orifice plate 14 and a compression screw 15. 
Shown in FIG. 3 are the opening 13, tapering conically at the outlet, the 
orifice plate 14, the compression screw 15, the pipe 6 and the gas supply 
5. 
During operation, the liquid or supercritical CO.sub.2 flows via the gas 
supply 5 into the machine 1 from a rising-tube flask, a high pressure, 
medium pressure, low pressure tank or a pipeline. In the process, it 
passes through the valve, which regulates the throughflow quantity of 
liquid or supercritical CO.sub.2. The liquid or supercritical CO.sub.2 
passes into the pipe 6 and emerges via the expansion nozzle 7. As a result 
of the abrupt expansion, the liquid or supercritical CO.sub.2 evaporates 
and changes into the gas phase. At this point it has a temperature of 
-78.degree. C., which is very well suited for the cooling of sensitive 
surfaces. In this arrangement it is characteristic that no impermissible 
pressure losses occur in the gas supply 5 and the pipe 6. As a result, the 
disturbing formation of CO.sub.2 snow in these lines cannot occur, said 
snow formation blocking the supply of CO.sub.2 and causing a blockage of 
the gas paths. The expansion takes place only in the area of the nozzle. 
The expansion nozzle 7 is arranged directly in the vicinity of the working 
area of the machining tool. It is preferably in the axis of rotation of 
the polishing disk 2. However, arrangements are also conceivable in which 
the expansion nozzle 7 is fitted in the area of the polishing disk 2 in 
such a way that it carries out a rotational movement with the polishing 
disk 2. It is of course also possible for a plurality of expansion nozzles 
7 to be fitted. Control cones or exchangeable openings tapering conically 
at the outlet are considered for the opening of the expansion nozzle 7. 
Perforated diaphragms can also be used. In the case where a control cone 
is used, the gas throughput can be adapted to the polishing or grinding 
task to be accomplished at that time. 
According to the invention, what is important in any case is that the 
diameter of the valve spindle 12 and the clear width of the pipe 6 are 
dimensioned such that the liquid or supercritical CO.sub.2 does not expand 
in the interior of the pipe 6. The dimensions of the valve spindle 12 and 
of the pipe 6 are preferably selected such that the annular gap between 
the inner surface of the pipe 6 and the outer surface of the valve spindle 
12 is about 0.2 to 2 mm. In this case, a pressure drop can occur only at 
the emergence of the still liquid or supercritical CO.sub.2 from the pipe 
6, and the flow paths of the liquid or supercritical CO.sub.2 are thus 
reliably kept free. 
By means of the use of CO.sub.2, it is also possible in the configuration 
of the gas supply 5 to do without highly-insulated materials, such as are 
used when liquid nitrogen is employed. For liquid or supercritical 
CO.sub.2, flexible plastic lines can be used, which enable easier handling 
and greater mobility of the machine 1. Complicated thermal insulation is 
not necessary. 
For the machining of surfaces which place special requirements on the 
machining temperature, it is possible to equip the expansion nozzle 7 with 
a temperature sensor which measures the temperature currently present and 
controls a heating device, preferably a small heating coil, which is 
arranged in the area of the expansion nozzle 7. By this means, temperature 
control of the environment of the expansion nozzle 7 can be carried out. 
However, such an embodiment would represent a variant for special cases, 
since the advantage of the use, now improved, of CO.sub.2 is precisely 
that it is possible, in terms of apparatus, to work using CO.sub.2, which 
makes temperature regulation largely superfluous, since the freshly 
evaporated CO.sub.2 has a temperature which, according to experience, 
leads to particularly ideal operating conditions.