Tool presetting device

A tool presetting device comprises a carrier device for rotatably holding a tool. The tool is received in a bearing bushing which defines a bearing axis and provides a rotationally symmetrical bearing surface wherein said tool is enabled to be rotated in a desired measuring and setting position. An air suspension can be built up for enabling rotation of the tool. To this end, a source of pressurized air is provided which enables a certain throughput of air. At least one air outlet opens at the bearing surface, while receiving pressurized air from the said source. A switching device either admits or prohibits access of pressurized air to the air outlet. A throttle valve is connected to the source of pressurized air for limiting the throughput of air.

FIELD OF THE INVENTION 
This invention relates to a tool presetting device comprising a tool 
carrier for holding a tool which is rotatable for measuring or setting 
purposes about a substantially vertical bearing axis into a desired 
position. The tool carrier comprises further a bearing bushing having a 
bearing surface, which is rotationally symmetric about the bearing axis, 
as well as an arrangement for building up an air suspension which renders 
the tool rotatable. The arrangement for building up an air suspension 
includes at least one distribution channel for pressurized air extending 
about the bearing axis and being situated in the hearing surface, and a 
supply of pressurized air which may be switched on or off by means of a 
switching device and includes a source of pressurized air. 
BACKGROUND OF THE INVENTION 
Presetting devices are used in machining centers where various exchangeable 
tools can be inserted. The tools are measured and/or set in a presetting 
device before use. To this end, the connecting zone of the tools, in 
particular a tapered trunnion, is put into a corresponding, and in 
particular also tapered, receiving recess. In doing this, the tool's axis 
has to be oriented in a defined direction and has to be enabled to be 
rotated about the tool's axis for measuring purposes. 
From U.S. Pat. No. 4,532,716, a presetting device having an air bearing is 
known. While rotatability of the tools is ensured by an air bearing, 
rotating may be facilitated or made more difficult by building up or 
reducing the air suspension. Orientation in rotation is effected when the 
air suspension is built up. Having attained the desired rotatational 
orientation, the supply of pressurized air to the air suspension is 
switched off. In this way, the tool assumes a supported position in which 
it is rotatable only with high forces applied, and in which it is 
supported within its carrier at a precise level which can be defined and 
determined. In this supported position, the tool can be measured precisely 
and may be set without the risk of undesired rotation while being 
continuously measured and monitored. 
In order to build up the air suspension, a source of pressurized air is 
connected to an annular recess in a conical bearing surface via an On/Off 
valve and a pressure regulator. In this way, pressurized air may be led 
between the bearing surface and a supporting surface of the tool assigned 
to each other which are rotationally symmetric about the bearing and the 
tool's axis. Without supply of air under pressure, these supporting 
surfaces are pressed together by the weight of the tool, thus ensuring a 
desirable braking friction against rotation. Since the surfaces of the 
bearing surface of the carrier and tlie supporting surface of the tool, 
which are effective by friction and engage each other in a state without 
pressurized air, are tapered or conical, they engage each other in a 
centered manner, thus ensuring that the tool's axis extends always the 
same way. 
When pressurized air is introduced under sufficient pressure, an air 
cushion is created between the two supporting surfaces which renders the 
frictional force neglectibly small. Since the required pressure depends on 
the weight of the carrier and the tool, a pressure regulator is provided. 
Thus, the pressure can be adjusted by the regulator in such a manner that 
the desired characteristics of rotation are attained. If the pressure is 
too high, undesirable vibrations could occur; if it is too low, the 
resistance against rotation is too high. 
Now, it has been found that the tool carrier and the tools are often not 
supported by an appropriate pressure so that measuring and setting is done 
under bad supporting conditions which, in turn, involve a higher 
expenditure of energy and, particularly, lower precision as well as, in 
some cases, even damages caused by vibrations. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a presetting 
device the air bearing of which enables both rotating the tool with low 
forces and an efficient adjustment thereof without the need of repeatedly 
and laboriously setting the pressure regulator. 
This object is achieved in that the supply of pressurized air comprises a 
throttle valve connected to the source of pressurized air for limiting the 
throughput of air. 
When an air cushion is built up, the upper supporting surface, and thus the 
tool, has to be raised by a small amount. In order to be able to generate 
the force necessary, the supporting surfaces should have a minimum 
dimension and a certain surface proportion having generatrices transversal 
to the tool's axis. In principle, a substantially horizontal orientation 
or an orientation along a plane normal to the tool's axis would also be 
possible, but in this case at least two other supporting surfaces assigned 
to each other would be needed for guiding rotational movement, a, 
preferably cylindrical, air bearing being enabled to build up between the 
two supporting surfaces. 
In correspondence to the supporting area and the weight of the tool, at 
least a minimum air pressure has to be provided. Too high a pressure 
together with too high a throughput of air could result in vibrations 
between the two supporting surfaces. In contrast, by providing a throttled 
throughput of air, a further pressure range is created which enables 
operation on a higher pressure level. Since, however, a higher throughput 
of air is needed for a short time to build up the air cushion, the air 
supply comprises preferably a pressure accumulator or a stored volume of 
pressurized air from which air under pressure can flow between the 
supporting surfaces substantially without throttling it. With precisely 
matching supporting surfaces, only a very small throughput of air is 
needed to maintain an air cushion already built up. Consequently, the 
annular gap between the supporting surfaces which is occupied by the air 
cushion is correspondingly small. Due to the small thickness of this gap, 
the tool will be raised by a very little amount. 
The air supply is preferably provided in such a manner that the air 
cushion(s) extend(s) in a rotationally symmetrical fashion around the 
bearing axis. To this end, a channel for distributing pressurized air is 
preferably arranged around the bearing axis in a respective supporting 
surface, particularly in the stationary or lower or outer one. This 
channel is open towards the opposite supporting surface and may be fed 
with pressurized air by a supply conduit. 
According to the invention, the air supply may be switched on or off by a 
switching device, preferably a switching valve, and comprises a source of 
pressurized air as well as a throttle valve connected thereto for limiting 
the throughput of air. In particular, a pressure accumulator is interposed 
between the throttle valve and the switching valve. In some cases, 
however, the switching device may only consist of an on/off switch for the 
source of pressurized air, e.g. for switching the electric motor of a fan.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows a presetting device 1 comprising a vertical and a horizontal 
adjusting and measuring assembly 2a and 2b adapted and connected to each 
other in such a way that a projector 3 and its projected beam may be moved 
into any position of a vertical partial plane. For moving the projector 3, 
coarse and fine adjustment knobs 4 and 5 are provided. In a display zone 
3a provided with a graticule, the position of a tool 6 and its machining 
portion 6a is shown when the machining portion 6a is within the path of 
the projection beam of the projector 3. The tool 6 is held in a carrier 
device 7 and extends upwards along the axis 8 of the tool and its bearing, 
respectively, substantially in vertical direction. The position of the 
projector 3 is indicated on a display 9a, preferably showing both portions 
which extend along the tool's axis 8 and an axis of the vertical partial 
plane perpendicular thereto. In order to simplify measuring operations and 
memorizing of values determined, an input panel 9b, a measuring control 
and a memory are preferably provided. 
In the embodiment shown, the carrier device 7 comprises merely a bearing 
bushing 10 having a supporting bearing surface 11 which is rotationally 
symmetrical, preferably conical, around the bearing axis 8. As a second 
supporting surface, a tapered outer surface of a bearing trunnion 12 of 
the tool 6 is assigned to the bearing surface 11. In order to be able to 
build up an air suspension between the two supporting surfaces, an air 
supply 13 is provided which comprises at least one source 14 of 
pressurized air, a throttle valve 15 connected thereto for limiting the 
throughput of air, and, in particular, a pressure accumulator 17 arranged 
between the throttle valve 15 and switching valve 16, the pressure 
accumulator 17 providing the amount of pressurized air necessary for 
building tip the air suspension even with a small throughput of air during 
normal operation. The switching valve is preferably designed and arranged 
in such a manner that it may be actuated by foot or, optionally, by hand 
or finger. When the air suspension is switched off, the tool is held by 
one or both hands. 
FIG. 2 shows the carrier device according to FIG. 1 including the bearing 
bushing 10 in the conical bearing surface 11 of which a groove-like 
distribution channel 18 is formed and is open towards the bearing axis 8. 
The distribution channel 18 extends annularly around the bearing axis 8 
and has a cross-section which ensures that the pressure is substantially 
uniform over the whole circumference when pressurized air is supplied. As 
a second supporting surface, the tapered outer surface 12a of the bearing 
trunnion 12 of the tool 6 is assigned to the bearing surface 11. In order 
to be able to build up an air suspension between the two supporting 
surfaces 11 and 12a, the air supply is connected to the distribution 
channel 18 through a supply conduit 19, a joint coupling 19b and a supply 
bore 19a in the bearing bushing 10. Air from the distribution channel 18 
supplied with air under pressure flows through a thin annular gap which 
will establish between the two supporting surfaces 11 and 12a towards the 
upper and lower end surfaces of the bearing bushing 10. 
Instead of a distribution channel 18, optionally a plurality of outlet 
openings connected to supply bores 19a could be provided in the lower 
bearing surface 11 which are, preferably, distributed in equal annular 
distances around the bearing axis 8. If desired, it would also be possible 
to have a bearing bushing 10 which is closed at its lower end and to 
provide there only one central outlet opening from which air may flow 
between the supporting surfaces towards to upper, open end of the bearing 
bushing 10. With such an air supply, even if the bearing trunnion 12 were 
not oriented precisely parallel to or aligned with the bearing axis 8 
after rising, it could be displaced into the desired rotational 
orientation by a small torque only. When measuring, the supply of air 
under pressure is interrupted so that the supporting surfaces 11 and 12a 
engage each other and the tool's axis coincides with the bearing axis 8. 
The shape of the tapered bearing surface 11 of the bearing bushing 10 
corresponds preferably to the conicalness of a standard bearing trunnion, 
such as a steep-angle taper according to the ISO Standard, so that bearing 
trunnions 12 can be directly, and without any adapter, inserted into the 
bearing bushing 10. In order to be able to support other bearing 
trunnions, especially other standard trunnions, an adapter sleeve can be 
inserted into the bearing bushing 10, the outer surface of such an adapter 
sleeve being adapted to the inner surface of the bearing bushing 10, while 
the inner surface of the adapter sleeve is adapted to the outer surface of 
a respective bearing trunnion 12. Thus, the adapter sleeve would form part 
of the carrier device 7 and the bearing bushing 10, and the air bearing 
would be formed between the bearing bushing 10 and the adapter sleeve. 
FIG. 3 shows an embodiment of a low-cost carrier device for a variety of 
connecting portions of tools. In this embodiment, only a small axial 
portion of the inner surface of the bearing bushing 10' is formed as a 
tapered bearing surface 11a, whereas a larger axial portion forms a 
cylindrical bearing surface 11b. One distribution channel 18a and 18b is 
assigned to each of the bearing surfaces 11a and 11b. Supply of air to 
these distribution channels 18a and 18b is effected through an axial bore 
19a and a respective radial bore 19c, each radial bore 19c being tightly 
closed at its radially outer side by a sealing element 19d. In order to be 
able to support standard bearing trunnions of different conicalness, 
bearing sleeves 20 are provided, the outer surfaces 21a and 21b of which 
forming supporting surfaces which are adapted to fit into the tapered 
bearing surface 11a or the cylindrical bearing surface 11b of the bearing 
bushing 10'. The inner surface 22 of each bearing sleeve, in turn, matches 
the respective shapes of a standard bearing trunnion. 
The conical supporting surfaces 11a and 21a of the bearing bushing 10' and 
the bearing sleeve 20, which are assigned to each other, ensure the 
desirable stucking under an air-less condition, but a friction-less 
rotation of the bearing sleeve 20 and any tool inserted therein when an 
air cushion is established. In order to ensure a good truth of rotation, a 
separate air cushion is preferably built up also between the cylindrical 
supporting surfaces 11b and 21b for rotation. The tapered bearing region 
is preferably located directly at the upper edge of the bearing bushing 
10' or immediately below a radial flange forming said edge, as may be seen 
from FIG. 3. If desired, however, it can be provided in a lower or 
intermediate portion of the length of the bearing bushing 10'. The 
advantage of the arrangement described, which has a bearing bushing 10' 
and a bearing sleeve 20, consists in that a cylindrical bearing surface or 
portion can be manufactured in a simpler and more favorable way than a 
tapered one. Moreover, the conicalness and the size of the tapered bearing 
portion can be chosen independently from the shape and size of a 
respective bearing trunnion in such a way that optimum stucking and 
supporting by air is possible. When an air bearing is built up between the 
bearing bushing 10', and a bearing sleeve 20, the risk of damaging the 
supporting surfaces 11a, 11b, 21a and 21b, thus affecting the air 
suspension, is very small, because these supporting surfaces are not 
stressed very much. 
FIG. 4 illustrates another embodiment of the carrier device in which an 
adapter 23 is inserted into the bearing sleeve 20. As in the embodiment of 
FIG. 3, the bearing sleeve 20 is rotatably supported within the bearing 
bushing 10'. It will be understood that the adapter 23 could equally be 
inserted into a bearing bushing 10 which matches the outer shape of the 
adapter 23. The adapter 23 has an outer shape corresponding to standards, 
preferably to a steep-angle taper according to the ISO standard, and 
inside any other standardized receiving hole having a connection surface 
25 for receiving a special connecting portion of a tool, such as a conical 
hollow shaft. 
It will be understood by those skilled in the art that the air suspension 
described above can also be applied in connection with other known 
presetting devices, in particular in connection with those employing a 
measuring pressure foot.