JP2002018675A - Feed screw device of machine tool - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a feed screw device that keeps constant a force axially applied to a feed screw shaft, and has no change of mechanical accuracy such as straightness or perpendicularity and no damage of a bearing group. SOLUTION: The both end support mode feed screw device of a machine tool has a feeding device having a cooling liquid flowing hole 14b for passing cooling medium around the screw shaft 14 in the axial direction. The feeding device detects the force axially applied to the screw shaft 14 with load cells 21 and 24 disposed on the outer rings of the bearing groups 22 and 23 of the screw shaft 14, and controls flow rate and flow-in temperature of the cooling medium to be passed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a feed screw device used as a feed device in a machine tool or the like.

[0002]

2. Description of the Related Art Conventionally, a ball screw feeder has been employed as a feeder for a spindle head, a column, a table, and the like in a machine tool such as a machining center. A so-called double anchor type feed screw device having bearing groups at both ends is often used.

In order to prevent changes in mechanical accuracy such as straightness and squareness due to thermal displacement due to frictional heat generated by rotation of a screw and damage to a bearing group, a feed for cooling a screw shaft with a cooling medium, for example, a cooling liquid. Screw devices are widely used. FIG. 3 shows an example of such a feed screw device with a cooling function. The feed screw device has a screw shaft 14 rotatably supported between two brackets 2 and 3 provided on a bed 1 of a machine tool, a column (not shown) or the like, and a nut 15.
Is fixed to the table 4 or the like to be driven, and is driven in the axial direction of the screw shaft 14 according to the rotation of the screw shaft 14. A coolant flow hole 14b is provided in the screw shaft 14 in the axial direction.
Are formed. The screw shaft 14 is connected to the driving servomotor 11 via the coupling 12.
Further, a coolant recovery jacket 16 is provided on the screw shaft 14 at a shaft end 14 n opposite to the servomotor 11. Also, a shaft end 14 of the screw shaft 14 on the servo motor 11 side is used.
A coolant supply jacket 13 is provided near m.

The supply flow rate of the coolant is adjusted by sending a signal corresponding to the rotation speed of the screw from the NC device 20 to the flow rate control valve 17 provided in the supply pipe 18a of the coolant supply unit 18. I have.

[0005]

However, in spite of the fact that the coolant flows through the coolant flow hole 14b of the screw shaft 14, the temperature of the screw shaft 14 increases in the conventional double anchor system. Screw shaft 14 due to thermal expansion
In this case, a bending force acts on the bed 1 or the column (not shown) that supports the screw shaft 14 to deform the bed 1 or the column. And upset the right angle. In addition, the bearing load may increase due to the thermal expansion of the screw shaft 14, and the bearing group may be damaged.

Although the ball screw feeder has a relatively high efficiency, if the required feed power is W and the transmission efficiency is η, W ×
The energy of (1- [eta]) is converted into heat, which causes the screw shaft 14 and the nut 15 to rise in temperature. As the rotation speed of the screw increases, the temperature rise value of the screw shaft 14 increases. Therefore, a method of adjusting the flow rate of the coolant to be supplied according to the rotation amount has been conventionally used. However, by observing the relationship between the rotation speed of the screw and the temperature rise value of the screw shaft 14 in detail, it can be seen that in this method, the adjustment of the flow rate of the cooling fluid accurately follows the change in the axial force acting on the screw shaft 14. It turned out to be difficult.

According to the theoretical formula, the temperature rise value is ΔT [° C.]
Then, for the simplified model, ΔT = Q / β (1−exp (−β / CM × t)). Here, Q: the amount of heat given to the screw shaft 14 per unit time [J /
h] β: Heat release amount per unit time / unit temperature difference from outer diameter and inner diameter of screw shaft 14 [J / (h · ° C)] CM: Heat capacity of screw shaft 14 [J / ° C] t: Elapsed time [h ]

From this equation, the temperature rise of the screw shaft 14 becomes a predicted value after a sufficient time has elapsed since the start of the operation at a certain feed speed by Q.

When the feed rate is increased and the coolant flow rate is immediately changed to β so that the temperature rise value ΔT becomes constant in accordance with the rotation speed of the screw, the screw shaft 14 is rotated until a sufficient time elapses. Is in a state of “too cold”, the force applied in the axial direction of the screw shaft 14 decreases, and the “bed 1”
Or, "bending deformation of the column" is caused.

In general, the feed speed of a machine tool frequently changes with the progress of machining of a workpiece.
Cannot be expected to be steady. That is, in the conventional method of adjusting the flow rate of the supplied coolant according to the rotation speed of the screw shaft 14, a time delay of the temperature change of the screw shaft 14 occurs, and the force acting on the screw shaft 14 in the axial direction is reduced. Since it cannot be kept constant, there is a problem that changes in machine accuracy such as straightness and squareness and damage to the bearing group are caused.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, to keep the axial force acting on the screw shaft 14 constant, and to improve the machine accuracy such as the sharpness, straightness and squareness of the machine. Of the present invention is to provide a feed screw device which is free from changes of the bearings and damage to the bearing groups 22 and 23.

[0012]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a feed screw device in which a cooling medium is formed in the center of a screw shaft through which a cooling medium flows along an axial direction by a feed screw device of a two-end supporting type of a machine tool. The apparatus is characterized in that the axial force acting on the screw shaft is detected to control the flow rate and the inflow temperature of the cooling medium to be passed.

According to a second aspect of the present invention, as the force detecting means applied in the axial direction, a force acting on an outer ring of a bearing group of a screw shaft at both ends is used.

According to a third aspect of the present invention, as means for detecting the force acting on the outer ring of the bearing group, a load cell is provided on both the shaft end outer ring and the screw outer ring of the both end bearing group, and the load cell of each load cell is provided. The present invention is characterized in that an axial force acting on a screw shaft is detected using a difference between the measured forces.

According to a fourth aspect of the present invention, in the feed screw device, a strain gauge provided on a bearing bracket of a screw shaft at both ends is used as force detecting means for applying the force in the axial direction.

[0016]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a sectional view of a feed screw device of a machine tool according to the present invention. FIG. 2 shows another embodiment of the feed screw device for a machine tool according to the present invention. In the description, the present invention differs from the conventional example only in the detection of the axial force acting on the screw shaft 14. Therefore, the same members as those in the conventional example are denoted by the same reference numerals, and the description thereof will be omitted and newly added. Only the members that have been described are given new numbers and will be described.

A load cell 21 is provided between the shoulder 22a of the outer ring of the bearing group 22 provided on the screw shaft 14 and the bearing retainer 16a, and a load cell 24 is provided between the shoulder 23a of the outer ring of the bearing group 23 and the bearing retainer 13a. The load cells 21 and 24 detect a force applied in the axial direction of the screw shaft 14 generated by thermal expansion due to a temperature rise of the screw shaft 14 and the nut 15. External forces acting on the table 4 such as cutting force are nut 15, screw shaft 1.
4 to the load cells 21 and 24. For the external force, the values detected by the load cells 21 and 24 are represented by F21 and F21, respectively.
24, it appears as the difference | F21−F24 |. On the other hand, the force applied in the axial direction of the screw shaft 14 generated by the temperature rise appears as an average (F21 + F24) / 2 of the values detected by the load cells 21 and 24.

When the force applied in the axial direction of the screw shaft 14 detected by the load cells 21 and 24 increases, the NC device 20 increases the opening of the flow control valve 17 to increase the flow rate of the coolant from the cooling supply unit 18. Conversely, when the force applied in the axial direction of the screw shaft 14 decreases, the opening of the flow control valve 17 is reduced to reduce the flow rate of the coolant. As described above, the axial force acting on the screw shaft 14 is detected, and the flow rate of the coolant is controlled based on the detected value.

If the initial tension is applied to the screw shaft 14, the load cells 21 and 24 are
3 may be provided on the screw side.

On the other hand, under the condition that the heat generated by the operation of the bearing groups 22 and 23 is large, the temperature of the outer ring may be considerably higher than the temperature of the brackets 2 and 3. At this time, the bearing outer ring thermally expands more than the brackets 2 and 3 and generates a large force on the load cells 21 and 24 by the increased internal force, even though the force applied in the axial direction of the screw shaft 14 does not increase. As a method for coping with this phenomenon, FIG. 2 shows an embodiment in which load cells 21a and 21b are provided on both sides of the outer ring of the bearing group 22 and load cells 24a and 24b are provided on both sides of the outer ring of the bearing group 23. The increase in the internal force due to the thermal expansion of the bearing groups 22, 23 appears as an average value of the load cells 21a, 21b and 24a, 24b. The difference between the detected values of the load cells 21a and 21b and the detected values of the load cells 24a and 24b indicates the force applied in the axial direction of the screw shaft 14.

Each of the values detected by the load cell is F
21a, F21b, F24a, and F24b, that is, ｛(F21b-F21a) + (F24b-F24
a)｝ / 2 is a force generated by thermal expansion of the screw shaft 14, and this value may be used for adjusting the flow rate of the coolant.

Since the flow rate of the coolant is controlled by directly detecting the axial force generated on the screw shaft 14 as described above, the time delay effect of the temperature change of the screw shaft 14 can be eliminated. The force applied in the axial direction of the shaft 14 can be controlled more constantly.

Strain gauges are provided on the side surfaces of the brackets 2 and 3, and the brackets 2 and 3 are provided via outer rings of the bearing groups 22 and 23.
It is also possible to control the flow rate of the coolant in the same manner as described above by detecting the force applied to the screw shaft 14 in the axial direction.

[0024]

As described above, in the present invention,
Since the effects of the time delay of the temperature change of the screw shaft and the change in the heat generation of the bearing can be eliminated, the force acting on the screw shaft due to thermal expansion can be removed, and the force applied in the axial direction can be kept constant. The feed screw device does not cause changes in mechanical precision such as degree or squareness and damage to the bearing group.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a feed screw device showing one embodiment of the present invention.

FIG. 2 is an explanatory view of a feed screw device showing another embodiment of the present invention.

FIG. 3 is an explanatory view of a conventional feed screw device.

[Explanation of symbols]

 1 Bed 2, 3 Bracket 4 Table 11 Servomotor 12 Coupling 13 Coolant Supply Jacket 14 Screw Shaft 14b Coolant Flow Hole 15 Nut 16 Coolant Recovery Jacket 17 Flow Rate Control Valve 18 Coolant Supply Unit 18a Supply Pipe 20 NC Device 21 , 21a, 21b Load cell 24, 24a, 24b Load cell 22, 23 Bearing group

Claims (4)

[Claims] In a feed device in which a cooling medium flows along the axial direction at the center of a screw shaft in a feed screw device of a two-end supporting type of a machine tool, an axial force acting on the screw shaft is detected. A feed screw device for a machine tool, wherein a flow rate and an inflow temperature of a cooling medium to be passed are controlled. 2. The feed screw of a machine tool according to claim 1, wherein a force acting on an outer ring of a bearing group of both end screw shafts is used as said axial force detecting means. Screw device. 3. The feed screw device according to claim 2, wherein load cells are provided on both the shaft end outer ring and the screw side outer ring of the both end bearing group as means for detecting a force acting on the outer ring of the bearing group. A feed screw device for a machine tool, wherein a force acting on a screw shaft in an axial direction is detected by using a difference between measured forces of the load cells. 4. The feed screw device for a machine tool according to claim 1, wherein a strain gauge provided on a bearing bracket of the screw shaft at both ends is used as a force detecting means applied in the axial direction. .