BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a device for preparing metallo¬ graphic samples, in particular devices provided with at least one grinding and/or polishing wheel having vertical axis, being accommodated in the body and driven by an electric motor.

2. Description of the Related Art

Known appliances for the preparation of metallographic samples, and namely devices used for cutting, stamping, grinding and polishing, are usually small bench-type circular saws, presses, grinders or polishing devices that are electric motor driven and provided with speed and pressure control, as a rule. They are designed for being plugged to public mains 380/220 V.

The possibility of direct connection to the mains may appear to be an advantage for the user, however, it represents a drawback for the manufacturer: Taking into consideration the rather low demand for such devices and the small number of their producers, the manufacturing companies organize their sales in many countries. If the device works under current mains voltage, or is at least intended for direct connection to the public networks, the manufacturer is obliged to ensure the respective approval of the authorized testing laboratory in each country of sales and for each

product to be marketed. Naturally, the cost related to the approval procedure is reflected in the price of the equipment.

Anyway, the users have at their disposal equipment for safe voltage supply, as a rule, or they may not find it difficult to procure such devices.

The bodies of known grinding and polishing devices provided with a grinding and/or polishing wheel pivoting around vertical axis that are accommodated in the body of the equipment and driven with an electric motor, such as, e.g., DE 39 17 786 Al, are executed as frame structures or, possibly, as rugged castings, which has an unfavourable impact upon their weight as well as the manufacturing cost.

Known devices for grinding and polishing metallographic samples have, as a rule, a sample holder fastened to an electric motor driven shaft protruding from the head in downward direction. At the same time parts of the head are means bringing the head and the samples from the upper position to the bottom position where the samples held in the holder mesh with the grinding wheel. Such means can be a mechanism a component part of which is a motion screw.

DE 39 17 786 Al introduces a polishing device the head of which is provided with a vertical motion screw with a nut, said screw being suspended on a bending type tensometric sensor of weight. The motion screw is servo-motor controlled. The nut pivots at the shaft carrying the sample holder at its bottom end. The servo-motor drives the motion screw and serves for lowering the shaft with holder. As soon as the samples fixed in the holder contact the surface of the grinding wheel, the weight sensor registers a drop of weight. The difference between the actual weight of all vertically movable parts of the motion mechanism and the value that has

been measured determines the size of the pressure force of the samples on the grinding wheel. The instantaneous value measured by the weight sensor is an input value for controlling both the pressure force and the grinding wheel speed.

A drawback of the described solution resides in the considerable complexity of both the mechanical section and the whole regulating system, resulting in high manufacturing cost of the polishing system as a whole. Further, the design of the mechanical system of such equipment and the way of regulating the same require relatively high power input of the electric engines driving the polishing wheel and the shaft with the samples. The electric motor driving the motion screw shall be powered throughout the operation interval of time.

The fastening of the sample holder on the vertical drive shaft can be implemented by way of various connecting means. In known devices of the described type, e.g., DE 33 33 029 has a cylindrical pin with a recess at its upper end carrying a sleeve, the upper surface of which is equipped with an opening for the vertical shaft provided with a pin. The fastening is achieved by inserting the pin into the recess and its locking by turning the sleeve. The disadvantage of this known fixing mechanism resides in the poor attachment rigidity between the holder and the shaft, as well as the complexity of the fixing means

OBJECT AND SUMMARY OF THE INVENTION

The aim of the present invention consists in improving a device for preparing metallographic samples, with the objective of substantially reducing the manufacturing cost and the marketing expenses, and improving the security of the tender.

Such aim can be solved by the device for preparing metallo¬ graphic samples the substance of which resides in that its power input is rated so as to allow only safe voltage power supply not exceeding 50 V AC and 150 V DC.

The device can be equipped with the indication of instantaneous power off-take. Thus the operator can, if case be, overload the electric motors in a controlled way for a short period of time.

In an embodiment of the device with at least one grinding and/or polishing wheel on vertical axis, accommodated in the body and electric motor driven, the grinding and/or polishing wheel can be preferably driven by a DC motor that is equipped with impulse current transformer for speed control with feed-back maintaining constant speed under variable load.

In a preferable embodiment of the device a substantial part of the body is a metallic casting or a plastics moulding forming a self-supporting shell carrying the grinding and/or polishing wheel and its drive, said shell forming a receptacle for collecting used coolant under the wheel.

In a device provided with at least one grinding and/or polishing wheel having vertical axis, being accommodated in the body of the device and electric motor driven, and having at least one head out of which a shaft is protruding that is vertically adjustable by a motion screw and is driven by a second electric motor the bottom end of which is provided with means for carrying a holder of the samples to be ground, it is preferable to have a vertical movable screw passing through a vertically adjustable head and pivoting on a bracket attached to the body, said screw carrying a nut that is vertically slidable in the head, while at least one spring, that can be fastened to the nut and to the head, is accommodated between the nut and the head.

The vertically movable head with the samples is not supported directly by the nut ensuring its vertical shifting, but engages with the nut by means of a spring. Accordingly, when the samples are lowered to the grinding/polishing wheel, the whole weight of the head does not apply at once at the moment of contact, but grows gradually in the course of lowering the nut and, due to that, through relieving the spring. It is an advantage that each position of the nut, or each angle of the motion screw, corresponds very precisely with a certain thrust force.

In a particularly advantageous embodiment one pressure spring is inserted between the upper side of the nut and the head and a second pressure spring is inserted between the bottom side of the nut and the head. The purpose of the first spring is to act against the thrust after the sample has been lowered to the position where it contacts the grinding wheel, whereas the purpose of the second spring is to act in the direction of the exerted thrust after the nut has slid downward to the position where the whole weight of the head is transferred onto the samples contacting the grinding/ polishing wheel,the user being interested in further increas¬ ing the thrust. Accordingly, the second spring has a loading function.

The motion screw can be suitably driven by an electric step motor allowing very fine remote setting of the thrust force.

In order to achieve an easy installation of the sample holder onto the shaft the bottom part of shaft can be preferably provided with an axial bore for receiving the pin of the sample holder with two lateral notches, and further with entraining means, while the bottom part of the shaft carries a sleeve with two inclined notches formed in its wall, the notches accommodating movable locking rollers and the pin of the holder being provided with a recess for securing the pin

against axial shift by way of the locking rollers. A pres¬ sure spring can be accommodated between the sleeve and the stop.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained by means of drawings, of which:

FIG.l is a schematic side view of a preferable embodiment of the device for grinding/polishing metallographic samples, the body being represented in partial section;

FIG.2 is a top view of an embodiment according to Fig.l;

FIG.3 is a detail of the adjusting mechanism of the head in side view, the head being in its bottom position;

FIG.4 is a top view of the mechanism according to Fig. 3;

FIG.5 is a vertical section representing means for attaching the sample holders on the shaft; and

FIG.6 is a detail of the shaft end including the sleeve, also in section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The basic element of an example embodiment of the device, see Fig.l and Fig.2, is the body 1 formed by bottom 2 and shell 3 made of light alloy integrating a receptacle 4 for collecting used coolant, its outlet being designated as 5. The inlet of coolant is marked as 6. The bearings 9 of the shell 3 accommodate the grinding wheel 8 with its vertical axis 7 and pulley 10 that is powered with an electric motor not represented in the drawing. The upper side of the shell

accommodates a display 11. The head 13, arranged in a casing together with other parts of the device, is mounted on a vertical column 12 enabling vertical shifting of the head.

The mechanism for vertical adjustment of head 13 and means for setting the thrust force on sample holder 30 are represented in detail in Figs. 3 and 4. The column 12 carries a bracket 14 created substantially by U-profile in its vertical section. The vertical guide bars 15 attached to bracket 14 serve for slidable accommodation of head 13 by means of sliding sleeves 16, the electric motor 17 with gear case 18 being part of said head. The vertical motion screw 19, pivoting at its both ends in bracket 14 and driven by the electric step motor 20, passes through bead 13 created, substantially, by U-profile in its section. Obviously, another embodiment can be envisaged in which the motion screw 19 does not pass directly through the head 13, but cooperates with embossments, brackets etc. connected with the head 13. The motion screw 19 accommodates a moving nut 21 through which the bar 22 passes, preventing the nut 21 from rotating. Accordingly, the nut 21 can move in vertical direction against the head 13, however, it can not turn. The first pressure spring 23 is inserted between the upper side of nut 21 and the upper side of head 13, whereas the second pressure spring 24 is inserted between the bottom side of nut 21 and the bottom side of head 13. The bottom end of shaft 27 protruding out of the gearbox 18 carries the holder 30 of samples to be ground with the aid of means 35 for fastening the holder 30.

The bottom end of shaft 27 is provided with means for attaching the sample holder 30: The bottom part of shaft 27 is provided with an abore 31 for receiving the pin 32 of the holder 30, and further with two side notches 33 and entraining teeth 34 at its bottom end. The bottom part of shaft 27 is provided with a sleeve 35 with two inclined slots

milled in the upper front end of the lateral wall of sleeve 35. The notches 33, 36 accommodate the movable locking rollers 37 passing through the whole section of sleeve 35. The pin 32 is provided with a recess 38 for securing the pin 32 against axial shifting by means of locking rollers 36 and pin 39 for meshing with the entraining teeth 34. The recess in shaft 27 is provided with a stop 40 engaging with the pressure spring 41 pushing the sleeve 35 into its bottom locking position. The sleeve is provided with a protection casing 42 preventing the locking rollers 37 from falling out and further with the upper bushing 43 limiting the movement of the casing 42 in downward direction by contacting the stop 40.

The power input of the device is rated for electric current supply of safe voltage not exceeding 24 V AC and 120 V DC. The first electric motor is a DC motor with pulse speed control by means of a current transformer with feedback ensuring constant speed under varying load.

The device is equipped with indication of instantaneous output of each electric motor, which is purposeful for informing the operator, in particular in case of making use of short time overloading of the motors.

The sample holder 30 should be installed on the shaft 27 when the head 13 is in its upper position. Then the electric step motor 20 turns the motion screw 19 and the movable nut 21 is being lowered. Along with it also the head 13, supported by the nut 21 over the spring 23, is going down. At the moment the bottom surface of samples has contacted the surface of the grinding/polishing wheel 8, there is no thrust between the samples and the wheel 8. In the course of further lowering of nut 21 the thrust force is increasing, as the spring 23, acting against the weight of the head 13, is gradually relieved. At the moment of complete load relief in

the pressure spring 23 the thrust force equals the weight of head 13. Upon further lowering, the nut 21 starts depressing the second spring 24 thus affecting the bottom side of head 13. The pressure of spring 24 and, accordingly, also the thrust of samples at the wheel 8 gradually increase, exceeding the thrust determined by the weight of the head 13.

It is obvious that the described mechanism ensures, on the one hand, the lowering of the head 13 with the samples to the position in which the samples contact the grinding wheel 8, and on the other hand, the growth and the control of the thrust force in a considerable range, starting from zero value. It is natural that after the samples have reached the surface of wheel 8, each angle position of the motion screw 19 corresponds, quite unambiguously, with a certain thrust force. This fact can be used with advantage, if the motion screw is driven by electric step motor 20 provided with automatic control or connected with a control computer. If the grinding/polishing device according to the invention is controlled in a comprehensive way, the same computer can serve for controlling the speed of the electric motor 17 and for driving the grinding wheel 8.

The described embodiment of the adjustment mechanism uses two pressure springs 23, 24 inserted between the nut 21 and the head 13. However, also one of both springs 23, 24 can serve the same purpose, if it is attached to the nut 21 and the head 13 and subject to pressure load in one part of course of nut 21 and to pull in the other part of the course. It is thinkable to design also other resilient connections between nut 21 and head 13, e.g. by means of a leaf spring or a torsion spring.

The samples intended for grinding or polishing should be fixed in holder 30 removed from the device. The head 30 being upheld, the sleeve 35 on the shaft 27 is pushed upwards by

hand. The inclined notches 36 push the locking rollers 37 out of the axial bore 31 in the side notches 33, thus releasing its profile for receiving the pin 32 of holder 30. After the pin 32 has been inserted into the bore 31, the recess 38 bets onto the level of side notches 33 and the pin 39 enters inserted between the carrying teeth 34.

As soon as the sleeve 35 is released, it is lowered into its bottom position by its own weight and by the effect of spring 41, while the inclined notches 36 push the locking rollers 37 into the recess 38, thus preventing the holder 30 from moving vertically. The entraining of the holder 30 in the sense of rotation of the shaft 27 is ensured by pin 39.