阅读说明：本技术 一种三元整流体带冠叶轮电火花加工工艺 (Electric spark machining process for crown-contained impeller of ternary rectifier ) 是由 吴江 曹春晓 严小琳 于 2021-07-20 设计创作，主要内容包括：本发明公开了一种三元整流体带冠叶轮电火花加工工艺,涉及电火花加工技术领域,包括如下步骤：安装工件、孔位对准、锥形孔加工、圆柱形孔加工以及加工完毕。本发明通过在升降机构上设置工具电极组件,由驱动电机的输出端经主动轮、齿条以及转环,来带动电极二在电极一上滑移至最下端,带出加工过程产生的电蚀产物,驱动电机的输出端反向转动后,能够有效排出位于较深的圆柱形孔内电蚀产物,进而避免出现拉弧或短路现象,保证气膜孔内的孔壁质量；通过设置机械手臂及升降机构,可以实现避开工件的斜向干涉部位,保证工件叶身和叶冠相接处的待加工位置顺利进行气膜孔的加工。(The invention discloses an electric spark machining process for a three-element rectifier shrouded impeller, which relates to the technical field of electric spark machining and comprises the following steps: installing a workpiece, aligning hole positions, processing a conical hole, processing a cylindrical hole and finishing the processing. According to the invention, the tool electrode assembly is arranged on the lifting mechanism, the output end of the driving motor drives the electrode II to slide on the electrode I to the lowest end through the driving wheel, the rack and the rotating ring, so that an electric corrosion product generated in the machining process is taken out, and after the output end of the driving motor rotates reversely, the electric corrosion product in a deeper cylindrical hole can be effectively discharged, thereby avoiding arc discharge or short circuit and ensuring the quality of the hole wall in the gas film hole; through setting up robotic arm and elevating system, can realize keeping away from the slant of work piece and interfere the position, guarantee that the processing position of waiting of work piece blade body and blade crown department of meeting carries out the processing in air film hole smoothly.)

1. An electric spark machining process for a three-element commutator shrouded impeller is characterized by comprising the following steps:

s1: installing a workpiece: mounting a workpiece to be machined on a workbench of an electric spark machine tool, and aligning and fixing;

s2: hole position alignment: controlling a lifting mechanism (3) to perform five-axis motion by a mechanical arm (2) arranged on a base (1), so that the lower end of a tool electrode assembly (4) arranged at the end part of the lifting mechanism (3) is arranged at a hole position to be processed between a blade crown and a blade root on a workpiece;

s3: processing a taper hole: pumping working fluid for electric spark machining into the lifting mechanism (3) through a fluid supply pipeline (6) by a fluid supply pump (5), driving the tool electrode assembly (4) by the lifting mechanism (3) to complete the modeling and machining of the conical hole on the workpiece, simultaneously flushing the working fluid from the lower end of the tool electrode assembly (4) through two sides of the conical hole, and discharging an electroerosion product;

s4: machining a cylindrical hole: the lifting mechanism (3) drives the tool electrode assembly (4) to finish the modeling processing of the cylindrical hole on the workpiece, and meanwhile, the working solution is obliquely and downwards flushed into one side of the cylindrical hole from the tool electrode assembly (4), is flushed out from the other side of the cylindrical hole after passing through the lower part of the tool electrode assembly (4), and discharges a deeper electrolytic corrosion product in the cylindrical hole;

s5: and (3) finishing the processing: and after all the machining holes on the workpiece are finished in sequence, closing the liquid supply pump (5) and unloading the machined workpiece.

2. The electric spark machining process for the three-way rectifier crowned impeller according to claim 1, characterized in that: the lifting mechanism (3) comprises a driving motor (31), an installation part (32) and a driving wheel (33), the installation part (32) is installed at the end part of the mechanical arm (2), the upper end of the installation part (32) is communicated with the liquid supply pipeline (6), a T-shaped isolation plate (34) is arranged in the installation part (32), the driving motor (31) is installed at the upper end of the installation part (32), the driving wheel (33) is installed at the output end of the driving motor (31), a rack (35) is meshed on the driving wheel (33), the rack (35) is arranged on the side end of the installation part (32) in a sliding mode, a rotating ring (36) is fixedly arranged at the lower end of the rack (35), the rotating ring (36) is rotatably arranged on the tool electrode assembly (4), a liquid spraying head (37) is installed above the isolation plate (34) in the installation part (32), and a first bevel gear (38) is coaxially and fixedly installed on the driving wheel (33), a second bevel gear (39) is meshed with the first bevel gear (38), and the lower end of the second bevel gear (39) is connected with the tool electrode assembly (4) through a perforated connecting plate (310).

3. The electric spark machining process for the three-way rectifier crowned impeller according to claim 2, characterized in that: and a cavity above the partition plate (34) in the mounting part (32) is communicated with the liquid supply pipeline (6).

4. The electric spark machining process for the three-way rectifier crowned impeller according to claim 2, characterized in that: the liquid spraying head (37) is obliquely installed in the installation part (32), and the outlet of the liquid spraying head (37) corresponds to the perforated connecting plate (310).

5. The electric spark machining process for the three-way rectifier crowned impeller according to claim 1, characterized in that: the tool electrode assembly (4) comprises a first electrode (41), a second electrode (42) and a plugging ball (43), wherein the first electrode (41) is hollow, the upper end of the first electrode is fixed with the same shaft of the perforated connecting plate (310), the first electrode (41) is provided with two symmetrically-arranged sliding grooves (44), the first electrode (41) is further provided with a plurality of uniformly-distributed oblique flow holes (45) in the vertical direction, a perforated fixing plate (46) is arranged inside the first electrode (41), the lower end of the first electrode (41) is provided with a vertical flow hole (47), the plugging ball (43) is tightly abutted against the lower end of the perforated fixing plate (46) through a spring (48), the lower end of the plugging ball (43) is tightly abutted against the vertical flow hole (47), the lower part of the first electrode (41) is further provided with two symmetrically-arranged pushing balls (49) in a sliding manner on two sides of the plugging ball (43), and the second electrode (42) is arranged in the sliding manner in the sliding grooves (44), the upper end of the second electrode (42) is connected with the rotating ring (36) in a rotating way in the same axis.

6. The electric spark machining process for the three-way rectifier crowned impeller according to claim 5, characterized in that: the two push balls (49) are matched with the second electrode (42) in the vertical direction.

7. The electric spark machining process for the three-way rectifier crowned impeller according to claim 5, characterized in that: the diameter of the blocking ball (43) is larger than the aperture of the vertical flow hole (47) and smaller than the inner diameter of the first electrode (41).

8. The electric spark machining process for the three-way rectifier crowned impeller according to claim 2, characterized in that: the lower end of the rack (35) is in a shape of a missing tooth smooth rod, and when the rack (35) is located at the lowest point in the vertical direction, the height of the lower end face of the second electrode (42) is not higher than that of the lower end face of the first electrode (41).

Technical Field

The invention relates to the technical field of electric spark machining, in particular to an electric spark machining process for a crown-contained impeller of a ternary rectifier.

Background

The internal turbine blade of aeroengine operating condition is very abominable, it is high-speed rotatory under the impact of high pressure corrosivity gas, it cools off mainly to rely on the cooling air film hole that the terminal surface was seted up in the structure, prior art adopts the electric spark machining technique to carry out the micropore processing in cooling air film hole to the blade usually, be about to the work piece submergence is positioned in the electric spark machining working solution, utilize rotatory integral type electrode direct processing bell mouth and cylindrical hole on the machined surface, simultaneously through the ultrasonic vibration auxiliary assembly of integration setting, discharge the electroerosion product that produces in the electric spark machining process.

However, I find that the prior art still has certain disadvantages in the long-term modeling and processing process of the three-way rectifier shrouded impeller blade: firstly, the gas film hole is tiny in structure and deep in depth, before the gas film hole on the end face is completely opened, ultrasonic vibration can only be matched with working liquid to bring out an electric corrosion product on the side of the integrated electrode close to the hole opening, and the electric corrosion product on the lower part of the integrated electrode with a large height-diameter ratio in the hole cannot be effectively removed, so that the phenomena of arc discharge or short circuit occur, and the quality of the hole wall of the lower part in the gas film hole is influenced; second, because the blade body extension length of the blade shroud relative blade of ternary fairing shrouded impeller is longer, and the two department of meeting possesses certain structure angle, leads to the integral type electrode to have the interference phenomenon when processing this department, and the interference air film hole that processes out has the air flow direction interaction with other cooling air film holes, leads to the cooling effect of blade to remain to promote.

Disclosure of Invention

The invention aims to provide an electric spark machining process for a crown impeller of a ternary rectifier, which aims to overcome the defects caused in the prior art.

An electric spark machining process for a three-element commutator shrouded impeller comprises the following steps:

s1: installing a workpiece: mounting a workpiece to be machined on a workbench of an electric spark machine tool, and aligning and fixing;

s2: hole position alignment: the machining tool is arranged at a hole position to be machined between a blade crown and a blade root which are arranged on a workpiece;

s3: processing a taper hole: pumping working fluid for electric spark machining into the lifting mechanism through a fluid supply pipeline by a fluid supply pump, driving the tool electrode assembly 4 by the lifting mechanism to complete the shaping machining of the conical hole on the workpiece, simultaneously flushing the working fluid from the lower end of the tool electrode assembly through two sides of the conical hole, and discharging an electroerosion product;

s4: machining a cylindrical hole: the lifting mechanism drives the tool electrode assembly to finish the modeling processing of the cylindrical hole on the workpiece, and meanwhile, the working solution obliquely downwards rushes into one side of the cylindrical hole from the tool electrode assembly, then rushes out from the other side of the cylindrical hole after passing through the lower part of the tool electrode assembly, and discharges the deep electrolytic corrosion product in the cylindrical hole;

s5: and (3) finishing the processing: and after all the machining holes on the workpiece are finished in sequence, closing the liquid supply pump 5 and unloading the machined workpiece.

Preferably, elevating system includes driving motor, installation department and action wheel, the installation department is installed in robotic arm's tip, and the upper end and the liquid supply pipe of installation department are linked together, are provided with the division board of "T" shape in the installation department, driving motor installs in the upper end of installation department, the action wheel is installed on driving motor's output, and the meshing has the rack on the action wheel, the rack slides and sets up on the side of installation department, and the lower extreme of rack is fixed and is provided with the change, the change rotates and sets up on tool electrode subassembly, installs the hydrojet head in the top of division board in the installation department, and coaxial fixed mounting has bevel gear one on the action wheel, the meshing has bevel gear two on the bevel gear one, the lower extreme of bevel gear two links to each other with tool electrode subassembly through foraminiferous fishplate.

Preferably, the cavity in the mounting part above the partition plate is communicated with the liquid supply pipeline.

Preferably, the liquid spraying head is obliquely installed in the installation part, and an outlet of the liquid spraying head corresponds to the perforated connection plate.

Preferably, the tool electrode assembly comprises a first electrode, a second electrode and a plugging ball, the first electrode is hollow, the upper end of the first electrode is coaxially fixed with the perforated connecting plate, two symmetrically-arranged sliding grooves are formed in the first electrode, a plurality of uniformly-distributed oblique flow holes are formed in the first electrode in the vertical direction, a perforated fixing plate is arranged inside the first electrode, a vertical flow hole is formed in the lower end of the first electrode, the plugging ball is tightly abutted to the lower end of the perforated fixing plate through a spring, the lower end of the plugging ball is tightly abutted to the vertical flow hole, two symmetrically-arranged pushing balls are further slidably arranged on the lower portion of the first electrode on two sides of the plugging ball, the second electrode is slidably arranged in the sliding grooves, and the upper end of the second electrode is rotatably connected with the same shaft of the rotating ring.

Preferably, two of the pushing balls are matched with the electrode in the vertical direction.

Preferably, the diameter of the plugging ball is larger than the aperture of the vertical flow hole and smaller than the inner diameter of the first electrode.

Preferably, the lower end of the rack is in the shape of a missing tooth smooth rod, and when the rack is located at the lowest point in the vertical direction, the height of the lower end face of the second electrode is not higher than that of the lower end face of the first electrode.

The invention has the advantages that: (1) the tool electrode assembly is arranged on the lifting mechanism, the output end of the driving motor drives the electrode II to slide on the electrode I to the lowest end through the driving wheel, the rack and the rotating ring, two pushing balls are pushed to open the vertical flow hole arranged at the lower end of the electrode I, high-pressure working fluid is punched out from two sides of the tool electrode assembly in the conical hole after being pushed out through the vertical flow hole, and then an electric corrosion product generated in the machining process is brought out, after the output end of the driving motor rotates reversely, the vertical flow hole formed at the lower end of the electrode I is blocked by the blocking ball again, the flow direction of the working fluid is changed into that the working fluid is punched out from the inclined flow hole on the electrode I obliquely downwards, and is punched into the single-side direction of the cylindrical hole and is punched out from the other side of the electrode I after passing through the lower area of the electrode I in the hole, and the flow direction can effectively discharge the electric corrosion product in the deeper cylindrical hole, so that the phenomenon of arc drawing or short circuit is avoided, the quality of the hole wall in the air film hole is ensured;

(2) through setting up robotic arm and elevating system, drive electrode two earlier and remove to the lower extreme of electrode one, the robotic arm of five-axis motion is cooperated, the slant that can realize avoiding the work piece is interfered the position, take first to carry out the bell mouth processing to blade body and blade shroud junction on the work piece, cylindrical hole of bell mouth lower extreme is processed by the less electrode one of cross-section relative dimension again, electrode two that are located the top this moment can not cause the cylinder hole molding to interfere, guarantee that the processing position of treating of work piece blade body and blade shroud junction carries out the processing in gas film hole smoothly.

Drawings

FIG. 1 is a flow chart of the present invention.

Fig. 2 is a schematic view of the present invention in a state of machining a tapered hole.

Fig. 3 is an enlarged view of a structure at a in fig. 2.

Fig. 4 is a schematic view of the state of the present invention when a cylindrical hole is machined.

Fig. 5 is an enlarged view of the structure at B in fig. 4.

Fig. 6 is a schematic view showing a state in which the working fluid is obliquely and downwardly flushed out from the tool electrode assembly in the present invention.

Fig. 7 is a schematic structural view of the swivel of the present invention.

Fig. 8 is an interference representation of a prior art integral electrode in machining an interference hole.

FIG. 9 is a schematic view of the process of machining interference holes according to the present invention.

Fig. 10 is a view showing a structure of a work in the present invention.

The device comprises a base 1, a mechanical arm 2, a lifting mechanism 3, a tool electrode assembly 4, a liquid supply pump 5, a liquid supply pipeline 6, a driving motor 31, a mounting part 32, a driving wheel 33, a separating plate 34, a rack 35, a rotating ring 36, a liquid spray head 37, a bevel gear I38, a bevel gear II 39, a perforated connecting plate 310, a first electrode 41, a second electrode 42, a sealing ball 43, a chute 44, an inclined flow hole 45, a perforated fixing plate 46, a vertical flow hole 47, a spring 48 and a pushing ball 49.

Detailed Description

In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further described with the specific embodiments.

As shown in fig. 1 to 10, an electric discharge machining process for a three-way rectifier crowned impeller comprises the following steps:

s1: installing a workpiece: mounting a workpiece to be machined on a workbench of an electric spark machine tool, and aligning and fixing;

s2: hole position alignment: controlling a lifting mechanism 3 to perform five-axis motion by a mechanical arm 2 arranged on a base 1, so that the lower end of a tool electrode assembly 4 arranged at the end part of the lifting mechanism 3 is arranged at a hole position to be processed between a blade crown and a blade root on a workpiece;

s3: processing a taper hole: pumping working fluid for electric spark machining into the lifting mechanism 3 through a fluid supply pipeline 6 by a fluid supply pump 5, driving the tool electrode assembly 4 by the lifting mechanism 3 to complete the shaping machining of the conical hole on the workpiece, and simultaneously flushing the working fluid from the lower end of the tool electrode assembly 4 through two sides of the conical hole and discharging an electroerosion product;

s4: machining a cylindrical hole: the lifting mechanism 3 drives the tool electrode assembly 4 to finish the modeling processing of the cylindrical hole on the workpiece, and meanwhile, the working solution obliquely downwards rushes into one side of the cylindrical hole from the tool electrode assembly 4, rushes out from the other side of the cylindrical hole after passing through the lower part of the tool electrode assembly 4, and discharges a deep electrolytic corrosion product in the cylindrical hole;

s5: and (3) finishing the processing: and after all the machining holes on the workpiece are finished in sequence, closing the liquid supply pump 5 and unloading the machined workpiece.

In this embodiment, the lifting mechanism 3 includes a driving motor 31, a mounting portion 32, and a driving wheel 33, the mounting portion 32 is mounted on the end portion of the robot arm 2, the upper end of the mounting portion 32 is communicated with the liquid supply duct 6, a T-shaped partition plate 34 is provided in the mounting portion 32, the driving motor 31 is arranged at the upper end of the mounting part 32, the driving wheel 33 is arranged at the output end of the driving motor 31, a rack 35 is meshed on the driving wheel 33, the rack 35 is slidably disposed on the side end of the mounting portion 32, a rotary ring 36 is fixedly disposed at the lower end of the rack 35, the rotating ring 36 is rotatably arranged on the tool electrode assembly 4, a liquid spraying head 37 is arranged above the separation plate 34 in the mounting part 32, a first bevel gear 38 is coaxially and fixedly arranged on the driving wheel 33, the first bevel gear 38 is engaged with the second bevel gear 39, and the lower end of the second bevel gear 39 is connected with the tool electrode assembly 4 through the perforated connection plate 310.

In this embodiment, the chamber in the mounting portion 32 above the partition 34 communicates with the liquid supply duct 6.

In this embodiment, the liquid ejecting head 37 is obliquely installed in the installation portion 32, and an outlet of the liquid ejecting head 37 corresponds to the perforated receiving plate 310.

In this embodiment, the tool electrode assembly 4 includes a first electrode 41, a second electrode 42, and a sealing ball 43, the first electrode 41 is hollow, and the upper end of the first electrode is fixed to the same shaft as the perforated connection plate 310, the first electrode 41 is provided with two symmetrically arranged sliding grooves 44, the first electrode 41 is further provided with a plurality of uniformly distributed diagonal flow holes 45 in the vertical direction, the first electrode 41 is provided with a perforated fixing plate 46 inside, the lower end of the first electrode 41 is provided with a vertical flow hole 47, the sealing ball 43 is abutted against the lower end of the perforated fixing plate 46 through a spring 48, the lower end of the sealing ball 43 is abutted against the vertical flow hole 47, the lower portion of the first electrode 41 is further provided with two symmetrically arranged pushing balls 49 in a sliding manner on two sides of the sealing ball 43, the second electrode 42 is slidably arranged in the sliding grooves 44, and the upper end of the second electrode 42 is rotatably connected to the same shaft as the swivel 36.

It should be noted that the end surface of the outer edge of the first electrode 41 in the vertical direction is circular.

In this embodiment, two of the pushing balls 49 are vertically matched with the second electrode 42.

In this embodiment, the diameter of the plugging ball 43 is larger than the diameter of the vertical flow hole 47 and smaller than the inner diameter of the first electrode 41.

The lower end of the rack 35 is in a shape of a missing tooth smooth rod, and when the rack 35 is located at the lowest point in the vertical direction, the height of the lower end surface of the electrode II 42 is not higher than that of the lower end surface of the electrode I41.

The working process and principle are as follows: when the invention is used, the work piece installation, hole position alignment, taper hole processing, cylindrical hole processing and discharging operation after processing are carried out in sequence according to the working procedures, in the hole shaping processing process after the hole position alignment, the work piece is firstly connected with the anode of an electric spark processing power supply, a tool electrode assembly 4 is connected with the cathode of the electric spark processing power supply, then a driving motor 31 is started to lead the output end of the driving motor to drive a driving wheel 33 and a bevel gear 38 to synchronously rotate, the driving wheel 33 drives an electrode II 42 to slide on the electrode I41 to the lowest end through a rack 35 and a rotating ring 36 in the rotating process, at the moment, the electrode II 42 pushes two pushing balls 49 which are horizontally arranged at the lower end of the electrode I41 in a sliding chute 44, under the action of the two symmetrically arranged pushing balls 49, a sealing ball 43 moves upwards in the electrode I41, at the moment, a spring 48 is stressed and compressed, and opens a vertical flow hole 47 which is arranged at the lower end of the electrode I41, the first bevel gear 38 drives the second bevel gear 39 to rotate along with the first bevel gear in the rotating process, so as to drive the tool electrode assembly 4 at the lower end to synchronously and continuously rotate;

then the liquid supply pump 5 is started, working liquid for electric spark machining is supplied to a cavity on the isolation plate 34 in the installation part 32 at high pressure through the liquid supply pipeline 6, the working liquid passes through the obliquely arranged liquid spraying head 37 and then is flushed into the rotating tool electrode assembly 4 through the perforated connecting plate 310, the rotating tool electrode assembly 4 can enable the end face of the rotating tool electrode assembly 4 to be uniformly worn, deflection caused by reverse acting force of the high-pressure and high-speed working liquid is avoided, most of the working liquid flowing into the electrode I41 through the perforated connecting plate 38 is flushed out from the vertical flow hole 47 with the larger aperture, after the electric spark machining power supply is started, electric spark machining of a conical hole is started, the working liquid is flushed out from two sides of the tool electrode assembly 4 in the conical hole to further bring out an electric corrosion product generated in the machining process, in the process, because the electrode II 42 moves to the lowest end of the electrode I41, the mechanical arm 2 which moves in cooperation with the workpiece can realize that the oblique interference part of the five shafts is avoided, processing an air film hole at a position to be processed at the joint of the blade body and the blade shroud of the workpiece;

after the machining of the tapered hole is finished, the driving motor 31 is started again immediately to enable the output end of the driving motor to rotate reversely, the second electrode 42 slides on the first electrode 41 to the highest position under the pulling of the rack 35 and the rotating ring 36, at the moment, two pushing balls 49 which are separated from the pushing of the second electrode 42 reset under the pushing action of the spring 48, the blocking balls 43 block vertical flow holes 47 formed in the lower end of the first electrode 41 again, the flow direction of the high-pressure working fluid is changed into that the high-pressure working fluid flows downwards from inclined flow holes 45 on the first electrode 41 in an inclined mode, when the inner cylindrical hole is machined, the working fluid which flows downwards from the rotating first electrode 41 in the inclined mode can flow into one side direction of the cylindrical hole and then flows out from the other side of the first electrode 41 through the lower area of the first electrode 41 in the hole, the flow direction can effectively discharge electric corrosion products in the deeper cylindrical hole, further, the phenomenon of arc drawing or short circuit is avoided, and the hole wall quality in the air film hole is ensured, meanwhile, the first electrode 41 with the smaller cross section can process a cylindrical hole at the lower end of the conical hole, the second electrode 42 which is positioned at the uppermost end cannot cause the interference of the cylindrical hole shape, and the smooth processing of the air film hole is ensured at the position to be processed at the joint of the blade body and the blade shroud of the workpiece.

Based on the above, the invention sets the tool electrode assembly 4 on the lifting mechanism 3, the output end of the driving motor 31 drives the electrode two 42 to slide on the electrode one 41 to the lowest end through the driving wheel 33, the rack 35 and the rotating ring 36, the vertical flow hole 47 set at the lower end of the electrode one 41 is opened by pushing the two pushing balls 49, the high-pressure working fluid is flushed out from the two sides of the tool electrode assembly 4 in the tapered hole after being flushed out through the vertical flow hole 47, and then carries out the electric corrosion product generated in the processing process, after the output end of the driving motor 31 rotates reversely, the vertical flow hole 47 set at the lower end of the electrode one 41 is blocked again by the blocking ball 43, the flow direction of the working fluid is changed to be flushed out from the inclined flow hole 45 on the electrode one 41 obliquely downwards, and flushed into the single side direction in the cylindrical hole, and flushed out from the other side of the electrode one 41 after passing through the lower area of the electrode one 41 in the hole, and the flow direction can effectively discharge the electric corrosion product in the deeper cylindrical hole, thereby avoiding the phenomena of arc discharge or short circuit and ensuring the quality of the hole wall in the gas film hole;

through setting up robotic arm 2 and elevating system 3, drive electrode two 42 earlier and remove to the lower extreme of electrode one 41, cooperation five-axis motion robotic arm 2 can realize avoiding the slant of work piece and interfering the position, take first to carry out the bell mouth processing to blade body and blade shroud junction on the work piece, cylindrical hole of bell mouth lower extreme is processed by electrode one 41 that the relative size in cross-section is less again, electrode two 42 that are located the top this moment can not cause the interference of cylinder hole molding, guarantee that the position of treating of work piece blade body and blade shroud junction carries out the processing in gas film hole smoothly.

It will be appreciated by those skilled in the art that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed above are therefore to be considered in all respects as illustrative and not restrictive. All changes which come within the scope of or equivalence to the invention are intended to be embraced therein.