Abstract:
The present invention relates to process and apparatus for automatically engraving stone memorial markers comprised of granite or marble. The markers have their polished facing surfaces covered with a cut-out blast-resistant stencil having prescribed lettering, numerals, decorative patterns and the like. The markers are conveyed through a sand blast chamber at a uniform rate and plural direct-air-feed nozzles are moved uniformly transversely thereover to effect precisely uniform engraving of the facing surfaces. The engraving is more precisely defined to a prescribed uniform depth with finer detail than heretofore possible by manual or other automated single-nozzle methods. The plural nozzles are essentially continuously fed with pressurized air and blast cutting media to obtain faster and more accurate engraving.

Description:
This application is a continuation of application Ser. No. 07/936,622, filed Aug. 28, 1992, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to process and apparatus for automatic sand blasting of stone memorial markers, tiles, tablets of stone and other materials having a surface to be acted upon by a finely-divided abrasive material blast to provide grooved lettering, decorative indicia, satin or frosted surfaces, and the like. 
     BACKGROUND INFORMATION 
     Normally, the surface to be acted upon is covered by a protective stencil made of flexible rubber or plastic which is adhesively attached to the surface to be forcefully sand blasted by the abrasive material. The stencil has a cut-out configuration desired to be acted upon by the forceful blast of abrasive material. 
     The finely-divided abrasive material blast is produced by a sandblasting jet or jets of pressurized air carrying the abrasive material, directed against the exposed cut-out portions of the stencil-covered surface. The cut-out portions of the stencil are acted upon by the blast to engrave the surfaces therebeneath. The protected surfaces underneath the solid or non-cut-out portions of the stencil are not acted upon by the blast. 
     Prior art sand blasting processes have previously been performed in a suitable enclosure to receive the workpieces to be acted upon. Such enclosures have commonly had an access opening on one side. Usually a blast-resistant curtain having a horizontal slot is hung over the access opening and is movable vertically with respect to the enclosure. A single sandblast jet is inserted through this slot and moved back and forth horizontally as the curtain is moved vertically. In the majority of such operations the jet is manually operated and the workpiece is mounted vertically. 
     It has been found that manual operation of the jet by most skillful operators has been unable to produce sandblasting effects of uniform quality or depth of penetration of precise magnitude. Since the jet cannot be manually operated with precisely uniform motion, the workpieces are frequently cut too deeply in some areas and not sufficiently deep in other areas. 
     In addition, such manual operations are harmful to the operators as the sand, or other harmful substances in the abrasive material, can be inhaled by the operators who in time can and do contract lung diseases. 
     As stated, various types of automatic sand blasting machines have been developed which include a horizontally-movable carriage and a vertically-movable curtain having a slot-type opening therein, the sand blast jet being located on or through the carriage and projecting through the slot in the curtain. Such machines have inherent difficiencies in the combinations of carriages and curtains to permit the blasting jet to operate therethrough. Also such machines only employ a single jet and are often modified to permit manual or semi-manual operation resulting in inefficient operation. Further, the resulting end products are frequently of poor quality engraving primarily depending on the skill of the operator. 
     In the case of stone memorial markers, over substantial periods of time, the markers do not wear evenly and the engraved indicia tend to become illegible in the shallower engraved areas. Thus, precisely uniform engraving is highly desirable in high-quality workpieces which can withstand the tests of time in outdoor exposure. 
     U.S. Pat. No. 3,436,866 to Nye discloses an automatic sandblasting machine in which an upright rectangular frame is mounted parallel to and spaced from the front of the curtain. A horizontally-mounted elongated carriage is vertically movably mounted in the frame and means is provided attaching the carriage to the curtain. The carriage is moved up and down in the frame and the curtain is moved therewith. A second movable carriage is also provided. 
     In this type of sandblast machine, a single blast jet is employed to move with the curtain and within the curtain slot to direct the single jet against the work surface. The curtain-type machine is impractical in modern-day use wherein the waste products of sand blasting, usually air-borne dust, must be fully contained and are not permitted to pass into the atmosphere causing health and contamination problems. In addition, the single jet cannot engrave any substantial surface of a workpiece to a uniform depth of high quality. 
     U.S. Pat. No. 2,617,225 to O&#39;Brien discloses a method of sandblasting employing a single venturi-type nozzle and not a direct-air-type nozzle of the present invention. The nozzle of this prior art method is manually controlled and cannot provide high-quality engraving of uniform depth. 
     U. S. Pat. No. 2,450,401 to Thompson also utilizes a single venturi-type nozzle and has the same deficiencies of the other prior art. 
     SUMMARY OF THE INVENTION 
     A primary object of the present invention is to provide an automatic sandblast machine and process of engraving stone memorial markers which are highly efficient and can provide precisely uniform engraving having various types of lettering, numerals, decorative patterns, and other indicia of high quality. 
     Another object of the present invention is to provide method and apparatus for engraving of stone markers and the like which are automatically engraved in series in an enclosed sand blast chamber with both lateral movement of the marker and transverse movement of plural sand blast nozzles, both being precisely controlled by a programmed source to ensure uniform engraving to a prescribed uniform depth with finer detail than heretofore possible. 
     Another object of the present invention is to provide method and apparatus for stone engraving employing a plurality of direct-air operated nozzles utilized in series of two or more in overlapping relation with respect to the surface to be acted upon, the engraving being precisely and uniformly formed throughout with more precisely defined configurations than obtainable previously. 
     Still another object of the present invention is to provide highly-efficient method and apparatus for stone engraving of multiple workpieces delivered serially to an enclosed sandblast chamber, the operation being computer-controlled with respect to relative movements of the workpieces and the direct-air-fed nozzles of the apparatus, the operation being carried out with complete capture of the abrasive media and waste products of the blasting with the media being reused in a most efficient and dust-free environment. 
     In addition, the method and apparatus are essentially fully automated to obtain precisely duplicatable results in stone engraving with a single cyclone-type separating chamber combined with a dual-blast chamber system for recovery of the abrasive media and waste products for reuse of the media on substantially continuous basis. The dual-blast chamber system is designed for alternate pressurizing and depressurizing of two similar loading chambers to provide essentially continuous delivery of a particulate abrasive blast media to the plural blasting nozzles. 
     The present invention also permits the simultaneous engraving of multiple markers disposed in side-by-side horizontal relation depending upon their relative dimensions for most efficient and time saving operation. 
     Other objects and advantages of the present invention are the efficient time-saving operation of the apparatus on a substantially continuous basis with relative movements of the workpieces and the unique multiple sand-blast nozzles being precisely controlled eliminating the vagaries and inefficiencies of manual operation. 
     With the aforesaid objects in mind as well as additional advantages which will be apparent from the following specification, applicant hereby discloses in detail his invention for automatic stone marker engraving which is a substantial advance in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Several preferred embodiments of the invention, illustrative of the best mode in which the applicant has contemplated applying the principles, are set forth in the following description and shown in the drawings, and are particularly and distinctly pointed out and set forth in the appended claims. 
     FIG. 1 is top plan view of the sand blasting apparatus and workpieces in accordance with the present invention; 
     FIG. 2 is a side elevational view of the sand blasting apparatus and workpieces shown in FIG. 1; 
     FIG. 3 is a schematic view in the form of a block diagram of the collection, separation and reuse of the abrasive media used in the apparatus of FIGS. 1 and 2. 
     FIG. 4 is a perspective view of an individual memorial marker having a cut-out stencil adhered to its planar surface prior to sand blasting. 
     FIG. 5 is a fragmentary vertical sectional view taken on the line 5--5 of FIG. 4 prior to sand blasting. 
     FIG. 6 is a view similar to FIG. 5 after sand blasting. 
     FIG. 7 is an exploded view of the marker after sand blasting with the cut-out stencil removed from the marker. 
     FIG. 8 is an enlarged front elevational view of the enclosed sand blasting chamber of the apparatus shown in FIGS. 1 and 2. 
     FIG. 9 is an enlarged end view of the enclosed sand blasting chamber shown in FIG. 8. 
     FIG. 10 is a perspective view of the sand blasting chamber shown in FIGS. 8 and 9 and the associated apparatus for collecting, separating and reusing the abrasive media. 
     FIG. 11 is a further enlarged vertical sectional view of a direct-air fed nozzle for use in the sand blast apparatus of the present invention. 
     FIG. 12 is a horizontal Sectional view of the nozzle taken along the line 12--12 of FIG. 11. 
     Similar numerals refer to similar parts throughout the drawings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a general description of the overall apparatus for practicing the present invention, the combined apparatus shown in FIGS. 1 and 2 is representative of the preferred combined components for engraving the workpieces, preferably memorial markers. The markers 10 are generally rectangular in shape each having a highly-polished planar surface which is placed on the apparatus facing upwardly in all cases. The markers are comprised of durable solid stone such as granite and marble having roughened side surfaces and dimensions ranging from about 1 to 4 feet in length and about 1 to 2 feet in width, with a thickness of about 4 to 6 inches. The markers 10 shown in FIGS. 1 and 2 have similar dimensions for a single operational campaign of the apparatus. 
     The apparatus 12 is comprised of a combined series of horizontal conveyors which are located in a generally lineal alignment for moving the markers through the sand blast chamber a on powered conveyor. The prescribed conveyors aligned in end-to-end relation are described hereinafter in greater detail. The apparatus 12 has a pair of side conveyors leading onto an accumulating table at each end to facilitate loading of markers to be engraved and unloading of engraved markers. The accumulating table 13 at the left side of FIG. 1 has a pair of conveyors 14 and 15 disposed perpendicularly to the main lineal conveyor for loading markers to be engraved into a serial end-to-end alignment. The markers have the engraving stencils applied to their facing surfaces while retained on conveyors 14 and/or 15. The accumulating table 16 at the right side of FIG. 1 has a pair of conveyors 18 and 19 disposed perpendicularly to the main conveyor for packaging an d  unloading engraved finished markers for shipping. 
     A type of conveyors which are particularly useful for conveying the heavy stones are the &#34;Unibolt&#34; conveyors of the Webb-Norfolk Conveyor Division of the J. B. Webb Company. Their equipment is comprised of modular bolted-together lines of belt and gravity roller accumulation conveyor components called &#34;Unibolt&#34; systems. They offer variable speeds to move the work-pieces uniformly at a prescribed rate of travel. 
     The preparation of the markers for engraving is conducted as follows. As stated, the markers 10 are preformed to the desired dimensions and normally in rectangular shape with the surface to be engraved being highly-polished and planar. Preferred sizes are set forth hereinbelow. Such facing surface is cleaned to receive a cut-out stencil bearing the desired indicia for engraving. The stencil 20 as shown in FIG. 4 has rectangular dimensions to cover the full polished surface of an individual marker 10 and may extend slightly beyond its face-dimensions. Each of the markers 10 has an individual stencil 20 affixed thereto bearing the information and/or decorations to be engraved in the desired pattern. 
     The stencil material may be varied as desired. A preferred material is Product Style No. 123 blue liner plastic material manufactured and sold by 3-M Co. This stencil is multi-layered having an overall thickness of about 1/16 inch and is very durable, heat-resistant and possesses high wearability features when subjected to sand blasting operations. The stencil has a pressure-sensitive adhesive backing for ready and durable attachment to the marker surface. The indicia patterns 21 may be cut-out of the stencil through its full thickness by computer-controlled apparatus such as the CAD stencil cutting machine. The cut-out portions 21 are removed prior to blasting by manual selection. The preferred stencil mat is a laminated plastic sheet bearing an exterior adhesive backing to provide a stable mat application positioned on the marker throughout the abrasive blasting operation. The stencil mat has favorable characteristics to produce a high quality product having stencil engraved images and indicia which are consistent and uniform throughout the entire marker surface, even in extremely fine detail having circular or irregular dimensions as small as 1/16 inch. 
     Stencil Style No. 123 is designed for machine cutting with less wear of the cutting dies. This material offers excellent blast resistance and readily-recognizable blue liner. The plastic sheet laminated to the stencil eliminates taping during panel transfer to stone, yet allows easy blast-through and fast clean-up. 
     The lead-in area of the apparatus carries the markers 10 bearing their adhered stencils 20 over the facing surface of each marker. The loading area 13 of the apparatus from conveyors 14 and 15 has a plurality of freely-rotatable close-spaced rollers to permit gravity loading or manual pushing of the markers into aligned end-to-end relation in an essentially straight-line operational pattern. The markers may be aligned side-by-side when permitted by lesser lateral dimensions which will fit within the side rails of the plural conveyors or singly when wider to preclude such alignment. The markers have sizes such as follows to meet all standardized requirements of such markers including those of governmental agencies: 
     MEMORIAL MARKER SIZES (in inches) 
     5&#34;×10&#34;×13&#34; 
     41/2&#34;×12&#34;×24&#34; 
     41/2&#34;×15&#34;×40&#34; 
     41/2&#34;×12&#34;×48&#34; 
     41/2&#34;×22&#34;×48&#34; 
     ENGRAVING APPARATUS 
     FIGS. 1 and 2 show in considerable detail a preferred arrangement of interconnecting horizontal conveyors for automatic handling of the markers as they are moved through the engraving process. As stated, the stencil-bearing markers 10 are moved sequentially from the loading table 13 either by gravity or manually into adjacent end-to-end alignment onto a series of powered conveyors designated by the numeral 12. The markers are aligned such as by suitable side rails and preferably moved horizontally by a series of three 5-foot sections of first-powered conveyors designated by the numerals 21, 22 and 23. The powered conveyors connect to an elongated power belt of another powered conveyor 24 of comparable width which extends through the sand blast chamber 26 and beyond both ends thereof. The engraving is performed by direct air blasting within the enclosed sand blast chamber 26 as set forth below under process conditions. The discharge end of the elongated power belt connects with two or more shorter power belts of conveyors 27 and 28 which carry the engraved markers from the blast chamber. The said shorter power belts connect with another gravity conveyor 29 which has freely-rotatable rollers to carry the markers to the existing accumulating table 16 and to the two juxtaposed inspection and packaging conveyors 18 and 19 disposed perpendicularly to the main line of conveyors. The powered belts of the aligned conveyors are arranged to move in synchronism at the desired operating speed to automatically convey the markers through the engraving operation. During any given campaign of the process, the apparatus controls are set to operate with optimum efficiency for a continuous operation. The powered conveyors may be varied in speed and operated collectively in unison for a given sand blasting operation. Thus, the markers are delivered serially on a continuous basis to the blast chamber 26, the markers each having their own individual information and decorations to be engraved. The powered conveyors may be of similar or dissimilar construction, however, it if preferred that they be operable in synchronism such as by a computer-controlled programmed source so that the markers can be transported at a prescribed uniform rate of speed throughout their lineal movement, and especially through the sand blast chamber 26. Obviously, other types of stencil materials may be used for the marker facing stencil for the blasting operation; preferably ones which are wear, heat and blast resistant to only permit engraving in the cut-out portions 21. FIG. 5 shows in a vertical sectional view a marker 10 having the cut-out stencil 20 affixed to its face, the cut-out portions being designated by the numeral 21. FIG. 6 shows in a similar view the marker after sand blasting with the engraved indicia 10a cut into the stone face by the forceful blasting operation. FIG. 7 is a perspective view of the marker 10b with the stencil removed, and the desired indicia 10a cut into the stone face to the desired depth. 
     SAND BLAST CHAMBER 
     The main lineal power belt 24 extends through the sand blast chamber 26 and beyond both sides thereof to carry the markers therethrough for engraving. The chamber 26 is rectangularly shaped and fully enclosed having vertically-movable doors 29 and 30 at each end extending over the power belt 24, both being capable of sealing thereto when the doors are closed. The doors are capable of power-assisted opening and closing in synchronism to permit entry and exiting of the markers. FIGS. 1 and 2 show the relative location of the blast chamber in a medial region of the power belt 24. The blast chamber is fully sealed when its juxtaposed doors are closed and may vary from about 3 to 12 feet in overall size. FIG. 8 shows in a front view the chamber 26 with the markers being conveyed from right-to-left. The various components for delivery of the abrasive media to the plural nozzles 36 of the blasting chamber, and collection and separation of the waste products of the blasting are also shown in FIGS. 1 and 2. FIG. 8 shows in greater detail a pair of hydraulic cylinders 31 and 32 having attached piston rods which are employed to raise and lower the entry and exit doors of the chamber. 
     A plural series of direct-air feed blasting nozzles 36 is mounted over the markers as they are carried on power belt 24 through the blasting chamber. The total number of nozzles is preferably six (6) mounted adjacent to each other in lineal alignment parallel to the direction of travel of the markers. The nozzles are all similar in construction being direct-air-feed type rather than venturi type having substantially straight open passages therethrough as shown in FIGS. 11 and 12 for improved sand blasting. The nozzles are preferably operated in series of 2 to 6 depending upon the desired engraving depth required for a given size, lettering and numerals. The nozzles have a blasting media pattern overlap ranging from about 1 to 2 inches depending upon the spray patterns desired. The multiple nozzles provide a greater cutting action on the markers than where a single nozzle is employed. The nozzles are capable of delivering about 11/2 pounds of media per square inch where 5/16 inch nozzles are utilized. Such nozzles have a delivery angle of about 13° 40&#34;, the plural nozzles providing about 6 times coverage over the spray area. Thus, for the smaller letters and numerals requiring lesser engraving depth about 2 to 4 nozzles are operated together in synchronism, while for the larger letters and numerals requiring deeper engraving depth, the maximum number of about 5 to 6 nozzles is operated together for optimum efficiency. The individual nozzles have separate operating valves (not shown) which are operated individually, and are turned on or off as programmed as required for the given depth of engraving desired. The nozzles are mounted with their delivery ends positioned about 101/2 inches from the stencil-carrying upper surfaces of the markers and at right-angles thereto. The precise distance from nozzle ends to the workpiece surface can be varied as required. 
     The nozzles are each connected to pressurized air lines 40 as shown in FIG. 10 which carry the prescribed amount of abrasive media along with the pressurized air which varies from about 70 to 100 psi. The air lines extend from the inlet ends of the nozzles to a manifold 41 as shown in FIG. 10, the individual lines being grouped together and extending in a collective bundle of durable blast resistant hoses. The grouped nozzles within the blast chamber 26 comprise a unique multi-head air blasting nozzle assembly which can be adjusted vertically within the chamber to provide proper vertical spacing from nozzle delivery ends to the workpiece upper surface. The air lines 40 which terminate at the manifold 41 have individual on-off valves thereat to permit individual operation of each of the nozzles as required, normally in a series of two or more. The nozzles are fixedly mounted on a common movable carriage 42 for proper vertical positioning. The carriage can be raised and lowered as desired on mounting column 43 as shown in FIG. 8. The nozzles are direct pressure nozzles mounted on a horizontal traverse mechanism preferably having 12 cycle strokes per minute. 
     The blasting chamber has been referred to as a sand-blast chamber, although sand and other abrasive media may be used as the cutting instrumentality. The fully-enclosed blast chamber permits recovery of dust and air-borne particles from the blasting operation. Also the chamber is insulated both interiorly and exteriorly for noise reduction to be substantially sound-proofed when in operation. 
     SAND BLAST MEDIA 
     The blast media may be varied as required, from 36 to 60 mesh Alox material being particularly useful for the engraving. Alox 36 U.S. mesh with a working mix of reclaimed finer particles of 36 mesh media is desirable to obtain faster cutting rates. The engraving using air pressure of 80 psi has provided consistent depth of penetration among all like characters of a given stencil pattern. Air and media are held in at least one pressurized loading chamber and are dispensed to the direct-air blast nozzles 36 through individual supply lines 40. The present process provides automatic substantially continuous operation using a mat-type stencil and direct-air nozzles for fine engraving of high quality. In the aforesaid operation using the 36 U,S. mesh Alox a series of 5 nozzles were used at the sand blasting station employing a belt speed of 6 inches per minute and at a nozzle oscillation rate of 12 cycles per minute. Alox is a trade name for finely-divided alumina (Al 2O   3 ) manufactured by a number of manufacturers. Obviously other types of blast media such as fine sand may be used in the blast engraving. 
     The plural hoses 40 leading to the nozzles 36 are grouped together as shown in FIGS. 8 and 9, the latter showing the traversing mechanism 44 powered by an electric motor 45 which moves the nozzles attached to horizontal slide bar 46 through their horizontal stroke. The stroke may be varied as desired but must be maintained at a uniform rate of travel while the markers are simultaneously moved. FIG. 9 shows the length of travel of the nozzles designated by the letter &#34;X&#34; for one pass of the nozzles. 
     FIG. 10 shows in a perspective view the overall apparatus leading to and from blast chamber 26. The air line hoses 40 extend from the chamber 26 to a pair of blast storage chambers 46 mounted in side-by-side relation preferably in upright horizontal relation. The chambers 46 are used to store reclaimed blast media recovered from the blasting operation. The chambers 46 are each connected to a switching valve (not shown) which is capable of pressurizing each of the chambers individually when the media is being delivered into the media carrying pressurized hoses and depressurizing each chamber when the media is being loaded therein from the media recovery system. 
     The media recovery and reuse system as shown in FIG. 10 utilizes a vacuum line 50 extending from an upper region of blast chamber 26 to a vacuum generating mechanism 51 powered by an electric motor. The vacuum line 50 has a cyclone type separation chamber 52 located in an intermediate area for separation of the waste products generated by the blasting operation. The used media, particles of stone and eroded particles of the stencil are collected by vacuum from the sealed blast chamber and conveyed by negative pressure as dust and debris particles into separation chamber 52. Such unit effects a centrifugal force on the collected particles for their separation into individual fractions primarily by density. The reclaimed media flows downwardly by gravity into a holding bin 58 and thereafter into the depressurized blast chamber during the engraving cycle at which time the pressurized blast chamber is actively supplying direct air and media to sand blast chamber 26. As stated, these chambers are alternately pressurized and depressurized to permit delivery of the air and media into the blast hoses 40 when pressurized and to permit accumulative storage of the media in the other chamber when depressurized. The collection of virtually all by-products of the blasting operation provides a dust-free environment around the blast chamber and one which meets all requirements of environmental protection agencies and their regulations. The waste products from the operation are collected at the filter basket 57 and at the vacuum generating mechanism 51 for suitable disposal. Reused finer particles of 36 mesh Alox are obtained through adjustment downwardly of vacuum line 50b inside of cyclone chamber 52. FIG. 10 shows in simplified detail a power source 55 for powered movement of the conveyor 24 through the blast chamber. FIGS. 11 and 12 show the twin-tapered straight line opening 35a through an individual nozzle 35 for forceful delivery of the media and pressurized air for the engraving. 
     PROCESS DETAILS 
     The present invention employs a plurality of direct-air fed nozzles in a multi-head nozzle assembly. The preferred six-head assembly is designed to switch air and media feed between two similar blast machines for loading the media and air combinedly to the individual delivery hoses and their nozzles. The first pressurized blast chamber 46a becomes the active unit through a special selector switching valve which at the same time blocks air/media flow at the discharge point of the second blast chamber 46b which is then inactive and depressurized for media loading. The change from active and inactive blast machines or chambers takes place during the unloading of finished memorial stones or markers and loading of stones or markers to be engraved. 
     The single cyclone reclaim component of the apparatus is used to deliver reclaimed media to the twin blast machines having dual storage chambers on an alternating basis. The nozzles are then kept ready for substantially continuous automatic blasting without undue delay. 
     The present process employs a programmable controller (PC) program for controlling the combined apparatus. The PC unit contains a memory bank to automatically initiate operation and control the various steps of the process for most efficient quality and quantity of production. The program selects the requisite number of active air nozzles required to produce the engraved character depth specification for a given marker or markers. The program initiates and controls (1) the beginning and end of each production cycle (2) the marker unloading and loading phases of the operation, and (3) the beginning of each subsequent operating cycle. In addition, the program controls the safety warning indicators just prior to automatically opening and closing the exit and entrance doors of the blast chamber. In essence, virtually all phases of the process operation are automatically controlled for most efficient and optimum production. 
     FIG. 3 shows in a schematic flow diagram, how the dust collected in blast chamber 26 is transported by vacuum to the reclaim cyclone chamber 52, such transport to various areas being designated by the numerals 1, 2 and 3. The cyclone separates the dust waste products, the finest material passing into a dust collector 56a and a dust drum 56b, designated by the number 1 and the oversize debris passing into a filter basket 57 designated by the numeral 2. The reclaimed blast media is passed into a media storage hopper 58 designated by the numeral 3. The media from the storage hopper 58 is alternately loaded into either No. 1 blast machine 46a or No. 2 blast machine 46b, such passage being designated by the numerals 3. The re-usable media is then delivered to the nozzles 36 as aforesaid, also designated by the numerals 3. The collected fine dust and oversized filtered material are properly disposed of at intervals, while the media is re-used in an essentially closed cycle. 
     Various modifications may be resorted to within the spirit and scope of the appended claims.