Starting sheet processing system

Abstract

The invention provides a starting sheet processing system which is characterized in that a starting sheet stripping station and a negative plate detection station are sequentially arranged on a conveying path of a row conveying line, and a starting sheet column conveying line on the starting sheet stripping station is provided with a starting sheet detection station and a starting sheet sorting station; qualified cathode plates B detected by the cathode plate detection station are collected and stored through the cathode plate column conveying line for production of seed plates for standby application, and defective cathode plates B are conveyed to the maintenance station through the waste plate column conveying line. The starting plate stripping station and the negative plate detection station are sequentially arranged on the row conveying line, and stripping, conveying and sorting of the negative plate and the starting plate are completed. The orderly arrangement of all the stations of the treatment system ensures the efficient circulation of stable electrolysis operation of the starting sheet, and the circulation efficiency of electrolytic copper production is improved.

Description

Technical Field

[0001] The invention relates to the technical field of electrolytic copper, and in particular to a plate processing system. Background Art

[0002] At present, in the domestic copper electrolysis process, most of the copper liquid refined by the anode furnace is cast into an anode plate as the anode, and the titanium seed plate is used as the cathode for electrolysis in the seed plate tank. The copper ions dissolve from the anode and enter the electrolyte, and precipitate on the cathode titanium plate. When the copper skin on the cathode plate is 0.5-0.7mm thick, it can be removed from the tank. The middle layer of the assembly plate leaving the tank is the cathode titanium plate. The copper skin generated on both sides of the plate surface of the cathode titanium plate is called the seed plate or the starting plate. After the seed plate is processed, it can continue to be used as a cathode in the electrolytic cell for producing finished copper. In the above copper electrolysis process, after the seed plate is generated on the cathode plate, it needs to be peeled off the cathode plate. In the traditional process, the electrolysis time of crude copper is about 22.5 hours. In order to complete the production cycle within one day, it is generally necessary to peel off the seed plate and return the cathode plate to the slot within 1.5 hours. This involves multiple links such as the transportation, identification, peeling, and sorting of the cathode plate and seed plate. Manual work is time-sensitive and intensive, and has a high error rate. Therefore, a complete and automated seed plate processing system is needed to efficiently complete the above process. Summary of the invention

[0003] The object of the present invention is to provide a seed plate processing system which can efficiently complete the stripping and other processing procedures of seed plates and cathode plates to quickly complete the production cycle.

[0004] In order to achieve the above-mentioned purpose, the technical solution adopted by the present invention is as follows: it includes an assembly plate receiving frame for receiving the assembly plates hoisted by a crane, the assembly plate receiving frame is connected with the first column of conveying lines, the assembly plates on the assembly plate receiving frame and the first column of conveying lines are parallel to the plate surfaces located in the plumb plane and are arranged in rows with spacing, a first transfer mechanism is provided between the upstream end of the row of conveying lines and the downstream end of the first column of conveying lines, and the assembly plates on the row of conveying lines are arranged in rows with the plate surfaces located in the plumb plane being coplanar and the vertical plate edges of the adjacent sides being spaced apart;

[0005] A seed board stripping station is arranged on the conveying path of the row conveying line, a seed board row conveying line is arranged at the seed board stripping station, a seed board detection station is arranged in the middle section of the seed board row conveying line, and a seed board sorting station is arranged at the downstream end of the seed board row conveying line, and the seed board sorting station includes a waste product collection station and a qualified product packing station;

[0006] A cathode plate inspection station is set downstream of the seed plate stripping station. Qualified cathode plates B inspected by the cathode plate inspection station are collected via the cathode plate column conveying line for production of seed plates as spares, while defective cathode plates B are sent to the maintenance station via the waste plate column conveying line.

[0007] The present invention mainly sets up the first column conveying line and the row conveying line, and sequentially sets up the seed plate stripping station and the cathode plate detection station on the conveying path of the row conveying line; the seed plate column conveying line at the seed plate stripping station is sequentially provided with the seed plate detection station and the seed plate sorting station, completing the stripping, transportation and sorting of the seed plate; the cathode plate column conveying line and the waste plate column conveying line are respectively provided at the cathode plate detection station, completing the detection and classified transportation of the cathode plate. The orderly arrangement of each station of the processing system ensures the efficient circulation of the stable electrolysis operation of the seed plate and improves the circulation efficiency of the electrolytic copper production. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 , 2 3 are plan views of three layout schemes of the plate handling system;

[0009] Figure 4 It is a front view of the cathode plate and its edge strip;

[0010] Figure 5 The schematic diagram of the transmission line is shown in FIG.

[0011] Figure 6 , 7 They are the side view and front view of the seed plate stripping station respectively;

[0012] Figure 8 This is an isometric view of the robot arm of the seed plate stripping station;

[0013] Fig. 9 It is a side view of a cathode plate on a cathode plate inspection station;

[0014] Fig.10 This is a schematic diagram of a sorting robot;

[0015] Fig.11 This is an isometric view of the transfer trolley. DETAILED DESCRIPTION

[0016] See also Figures 1 to 11 The seed plate processing system shown includes an assembly plate receiving frame for receiving the assembly plates hoisted by a crane. The seed plate electrolysis workshop generates seed plates A on both sides of the plate surface of the cathode plate B respectively. The present invention refers to the cathode plate B that has not been separated and the seed plates A on both sides of the plate surface as a whole as an assembly plate.

[0017] The assembly plate receiving frame is connected with the first row of conveying lines 1, the assembly plate receiving frame and the assembly plates on the first row of conveying lines 1 are parallel to the plate surfaces located in the plumb plane and are arranged in rows with spacing, a first transfer mechanism is provided between the upstream end of the row conveying lines 2 and the downstream end of the first row conveying lines 1, and the assembly plates on the row conveying lines 2 are coplanar with the plate surfaces located in the plumb plane and are arranged in rows with spacing of vertical plate edges on adjacent sides;

[0018] A seed board stripping station 10 is arranged on the conveying path of the row conveying line 2, a seed board row conveying line 3 is arranged at the seed board stripping station 10, a seed board detection station 31 is arranged in the middle section of the seed board row conveying line 3, and a seed board sorting station 32 is arranged at the downstream end of the seed board row conveying line 3, and the seed board sorting station 32 includes a waste product collection station 322 and a qualified product packing station 321;

[0019] A cathode plate inspection station 20 is arranged downstream of the seed plate stripping station 10. The qualified cathode plates B inspected by the cathode plate inspection station 20 are collected via the cathode plate column conveying line 4 for use in the production of seed plates, while the defective cathode plates B are sent to the maintenance station via the waste plate column conveying line 5.

[0020] In the above scheme, after the crane lifts the assembly plate from the seed plate electrolysis workshop to the seed plate processing system, the assembly plate is first hung on the assembly plate receiving frame, and then transported through the first column of conveying lines 1. The assembly plates on the assembly plate receiving frame and the first column of conveying lines 1 are parallel to the plate surfaces located in the plumb plane and are arranged in rows with spacing, which can facilitate the crane to lift more assembly plates at one time. However, when the seed plate stripping station 10 strips the assembly plate, the mechanical arm used to strip the seed plate A needs to be arranged on both sides of the plate surface of the assembly plate, and it takes up a large space. Therefore, the assembly plates on the first column of conveying lines 1 need to be transferred to the row conveying line 2 through the first transfer mechanism, so that the assembly plates are arranged in rows with the plate surfaces located in the plumb plane being coplanar and the vertical plate edges on the adjacent sides being spaced apart. Only in this way can the space requirements of the seed plate stripping station 10 and the cathode plate detection station 20 be met. In particular, when vacuum adsorption is used to peel the seed plate, both sides of the cathode plate B are simultaneously subjected to the opposite and basically equal separation forces when peeling the seed plates A on both sides. This effectively avoids the peeling failure caused by the swing of the cathode plate B and the torsional deformation between the conductive rod of the cathode plate B and the plate surface, thereby reducing the peeling efficiency and increasing the later maintenance cost of the cathode plate B.

[0021] The assembled plate first passes through the seed plate stripping station 10 on the row conveying line 2. The seed plate A stripped off at the seed plate stripping station 10 is sequentially transported to the seed plate detection station 31 and the seed plate sorting station 32 via the seed plate column conveying line 3; after the seed plates A on both sides are stripped off, the remaining cathode plates B continue to be transported downstream on the row conveying line 2 to the cathode plate detection station 20. The cathode plate B that passes the inspection at the cathode plate detection station 20 is transported via the cathode plate column conveying line 4, and is transferred to the seed plate electrolysis workshop by a vehicle to continue to be used as a cathode production seed plate; the cathode plate B that fails the inspection at the cathode plate detection station 20 is transported via the waste plate column conveying line 5, and is transferred to the repair station, and then transferred to the seed plate electrolysis workshop after passing the repair. The seed plate processing system of the present invention simultaneously completes the detection and classified transportation of seed plates A and cathode plates B. The orderly arrangement of each station of the processing system ensures the high efficiency and stability of seed plate A production, and improves the cycle efficiency of electrolytic copper production.

[0022] For the convenience of description, Figure 1 As shown, in the present invention, the conveying direction of the column conveying line 1 is longitudinal, and the conveying direction of the row conveying line 2 is transverse. In order to improve the seed board processing efficiency, the downstream end of the first column conveying line 1 is connected with the upstream ends of the two row conveying lines 2 respectively, and the two row conveying lines 2 are arranged in parallel and spaced apart. A seed board stripping station 10 composed of a stripping unit is respectively arranged on each row conveying line 2. The seed board stripping station 10 shares a seed board column conveying line 3, or a seed board column conveying line 3 is respectively arranged at each seed board stripping station 10. The two row conveying lines 2 are simultaneously transferred from the first column conveying line 1 and transport the assembled boards, and the stripping and conveying efficiency of the seed boards is doubled. As shown Figure 1 As shown, the seed board stripping stations 10 share a seed board row conveying line 3, which can save the laying cost of the seed board row conveying line 3; Figure 2 As shown, each seed plate stripping station 10 is provided with a seed plate column conveying line 3, so that the seed plate stripping station 10 can be arranged in a staggered manner in the longitudinal direction, which greatly reduces the longitudinal layout space required for the plate stripping station 10. The selection of the above layout scheme needs to consider the restrictive conditions of the enterprise production site. For the transformation and utilization of existing factory buildings, the restricted conditions are strict. For new factory buildings, the overall layout of the electrolysis workshop can be considered to make a selection. Similarly, different cathode plate detection stations 20 can share the cathode plate column conveying line 4 and the waste plate column conveying line 5, or they can set the cathode plate column conveying line 4 and the waste plate column conveying line 5 separately.

[0023] As another embodiment, Figure 3 As shown, at least two seed board stripping stations 10 are arranged in upper and lower positions on the conveying path of the row conveying line 2, and a seed board column conveying line 3 is arranged at each seed board stripping station 10. Compared with the previous two schemes, this scheme further reduces the longitudinal arrangement space required for the stripping station 10, but requires more lateral arrangement space.

[0024] Regarding the stripping device of the seed plate A, the seed plate stripping station 10 is provided with a robotic arm, which includes a mechanical arm 11 with one end hingedly connected to a stripping machine frame 101, and a mechanical arm 12 is hingedly connected to the other end of the mechanical arm 11. The swinging planes of the mechanical arm 11 and the mechanical arm 12 are parallel, and a suction cup frame 102 and a suction cup 103 connected to a vacuum air source are provided at one end of the mechanical arm 12 away from the mechanical arm 11. The seed plate stripping station 10 is provided with a clamping mechanism 104 for clamping a conductive rod B' of the cathode plate B above the cathode plate B.

[0025] A more specific solution is that the cathode plate inspection station 20 is provided with infrared detection sensors, and the number thereof is at least 9. The detection direction of the infrared detection sensor points to the plate surface of the cathode plate B at the station. The infrared detection sensor measures the distance between it and the plate surface of the cathode plate B through light waves to detect the flatness of the plate surface of the cathode plate B. If the cathode plate B is bumped or worn during electrolysis, stripping or other transportation processes, resulting in insufficient plate surface flatness, the quality of the seed plate A generated by the side plate surface will not meet the standard, so the cathode plate B needs to be inspected before it is transported to the seed plate electrolysis workshop.

[0026] The two side edges and the lower edge of the cathode plate B are coated with slender edging strips B10, and the lower ends of the edging strips B10 at the two side edges of the cathode plate B are respectively connected with the two ends of the edging strips B10 at the lower edge of the cathode plate B. The continuous edging strips B10 can ensure that the seed plates A generated by the cathode plate B during electrolysis in the seed plate electrolytic cell are separated from each other. In other words, during the electrolysis process, the copper ions adhere to the cathode plate B, and the edging strips B10 coated at the edge of the cathode plate B surface can prevent the copper ions generated on both sides of the cathode plate B surface from being connected at the edge of the plate surface, so that two independent seed plates A are finally generated on both sides of the cathode plate B surface, rather than a whole copper plate connected to each other; it can also be understood that the edging strips B10 are insulating materials, which can prevent the electrolytically deposited seed plates A from being connected as a whole at the two side edges and the lower edge of the cathode plate B, causing difficulty in peeling and even if peeling is achieved, subsequent trimming and trimming are required.

[0027] Therefore, the integrity of the coating of the edge strip B10 and the accuracy of its position are very important. However, during the process of lifting, conveying or peeling the cathode plate B, the edge strip B10 may be bumped or fall off naturally, resulting in the coating of the cathode plate B not meeting the standards and being unusable.

[0028] In order to inspect the hemming strip B10 of the cathode plate B, a camera detector and a background light projector are provided on the cathode plate inspection station 20, and the lens of the camera detector and the light source of the background light projector are respectively directed toward the plate surface of the cathode plate B. The light source of the background light projector provides background light for the camera detector to avoid insufficient or uneven light brightness in the workshop affecting the measurement of the camera detector.

[0029] In order to improve the detection accuracy of the edging strip B10, brush rollers 201 are symmetrically arranged on both sides of the plate surface of the cathode plate B at the cathode plate detection station 20, and the roller surface of the brush roller 201 is in contact with the plate surface of the cathode plate B, and the wheel core of the brush roller 201 is parallel to the plate surface of the cathode plate B and perpendicular to the displacement and conveying direction of the cathode plate B. In the above scheme, when the cathode plate B at the cathode plate detection station 20 is moved by the chain, the plate surface of the cathode plate B will shake, causing the lens of the camera detector to be unable to focus accurately, seriously affecting the detection accuracy. The roller surface of the brush roller 201 is set in contact with the plate surface of the cathode plate B at the cathode plate detection station 20, which can resist the cathode plate B to prevent it from shaking violently, ensuring the camera accuracy of the camera detector, thereby improving its recognition accuracy.

[0030] Preferably, the contacting part between the brush roller 201 and the cathode plate B on the cathode plate inspection station 20 is located at the lower part of the plate surface of the cathode plate B. Since the cathode plate B is hoisted and transported on the row conveying line 2, the cathode plate B mainly swings around the conductive rod B' on which it is suspended. The arrangement of the brush roller 201 at the lower part of the plate surface of the cathode plate B is more conducive to the stability of the cathode plate B, that is, the force arm between the contacting part of the brush roller 201 on the plate surface of the cathode plate B and the conductive rod B' is about long, and the smaller the pressure that the brush roller 201 needs to apply to the cathode plate B, the better the limiting effect of the brush roller 201 on the cathode plate B.

[0031] In order to facilitate the detection of seed plate A by the seed plate detection station 31 and the processing by the seed plate sorting station 32, the inner plate surface of the seed plate A on the seed plate column conveying line 3 faces downward and contacts with the conveying roller surface, and the outer plate surface of the seed plate A faces upward. The inner plate surface of the seed plate A is the plate surface that is in contact with the cathode plate B.

[0032] A trimming station for trimming the edges of the seed boards A is provided between the seed board detection station 31 and the seed board sorting station 32, so that the size specifications of each seed board A are uniform.

[0033] Regarding the specific detection method of seed plate A, the seed plate detection station 31 is provided with a weighing unit and an optical detection unit for measuring seed plate A. The optical detection unit is located above the seed plate A and its detection direction is toward the outer plate surface of the seed plate A. The weighing unit weighs the weight of the seed plate A to infer whether the thickness of the seed plate A meets the standard; the optical detection unit collects the image of the outer plate surface of the seed plate A and determines whether there are nodules on the outer surface of the seed plate A. If the thickness and surface flatness of the seed plate A meet the standards, it is marked as a qualified product, otherwise it is marked as a defective product.

[0034] The sorting robot on the seed board sorting station 32 includes a gripping end 324 and a sorting mechanical arm 325. The gripping end 324 can rotate around its own center. The gripping end 324 is connected to a vacuum air source to grip the seed board A by vacuum adsorption and drive the displacement through the sorting mechanical arm 325. The sorting robot transfers the seed board A to the waste collection station 322 and the qualified product boxing station 321 respectively according to the detection results of the seed board detection station 31. In the above scheme, a vacuum adsorption structure is provided at the bottom of the gripping end 324. According to the detection results of the seed board detection station 31, the sorting robot places the seed board A marked as qualified products at the qualified product boxing station 321, and places the seed board A marked as defective products at the waste collection station 322. Since the seed plates A peeled off on both sides of the cathode plate B fall onto the seed plate conveyor line 3 in a posture that is roughly symmetrical with respect to the cathode plate B when the assembly plate is peeled at the seed plate peeling station 10, the grasping end 324 of the sorting robot can rotate around its own center to facilitate adjustment of the placement posture of the seed plate A when grasping the seed plate A, thereby ensuring that the stacking posture of each seed plate A is consistent, facilitating subsequent rework or packing processes.

[0035] A more specific solution is that a transfer trolley 323 is respectively provided at the waste collection station and the qualified product packing station. The table surface of the transfer trolley 323 on which the seed board A is placed is surrounded by upright fences 3231. The square area enclosed by the fence 3231 is consistent with the shape of the surface of the seed board A and its enclosed area is slightly larger than the surface area of ​​the seed board A. The bottom of the grabbing end 324 of the sorting robot is a square consistent with the shape of the surface of the seed board A and its horizontal projection is slightly smaller than the table surface enclosed by the fence 3231 of the transfer trolley 323. The shape consistency here does not strictly require to be completely identical in the geometric sense, that is, it only needs to be basically consistent.

[0036] In the above scheme, the bottom of the grabbing end 324 is square and flat, which can facilitate the vacuum adsorption structure inside to adsorb on the four corners of the seed board A near the edge, and can also help the transfer cart 323 to regularize the stacking posture of the seed board A. Since the seed board A is very thin, even if the seed board A is placed horizontally, its board surface will still warp naturally, especially at the corners. Moreover, the flatness and thickness of each piece of seed board A vary slightly. The above factors may cause large gaps between adjacent board surfaces when the seed board A is stacked, which is not conducive to subsequent packaging and other processes. Therefore, every time the sorting robot stacks 3 to 8 seed boards A in the transfer cart 323, the grabbing end 324 moves at a uniform speed for a distance from the top of the transfer cart 323 toward the surface of the seed board A, contacts and presses the surface of the topmost seed board A to eliminate the gap between the surfaces of adjacent seed boards A, reduce the warping amplitude of the stacked seed boards A, and improve the packing efficiency of the seed boards A.

Claims

1. A board processing system, characterized in that: It comprises an assembly plate receiving frame for receiving assembly plates hoisted by a crane, the assembly plate receiving frame being connected to the first row of conveying lines (1), the assembly plates on the assembly plate receiving frame and the first row of conveying lines (1) are arranged in rows with plate surfaces located in a plumb plane in parallel and at intervals, a first transfer mechanism is provided between the upstream end of the row of conveying lines (2) and the downstream end of the first row of conveying lines (1), and the assembly plates on the row of conveying lines (2) are arranged in rows with plate surfaces located in a plumb plane in the same plane and vertical plate edges of adjacent sides at intervals; A seed board stripping station (10) is arranged on the conveying path of the row conveying line (2), a seed board row conveying line (3) is arranged at the seed board stripping station (10), a seed board detection station (31) is arranged in the middle section of the seed board row conveying line (3), and a seed board sorting station (32) is arranged at the downstream end of the seed board row conveying line (3), and the seed board sorting station (32) includes a waste product collection station (322) and a qualified product packing station (321); A cathode plate inspection station (20) is arranged downstream of the seed plate stripping station (10). Qualified cathode plates B inspected by the cathode plate inspection station (20) are collected and stored via a cathode plate column conveying line (4) for use as seed plates for production, while defective cathode plates B are sent to a maintenance station via a waste plate column conveying line (5).

2. The seed board processing system according to claim 1, characterized in that: The downstream end of the first row of conveying lines (1) is connected to the upstream ends of the two rows of conveying lines (2), and the two rows of conveying lines (2) are arranged in parallel and spaced apart. A seed board stripping station (10) composed of a stripping unit is arranged on each row of conveying lines (2), and the seed board stripping stations (10) share a seed board row of conveying lines (3), or a seed board row of conveying lines (3) is arranged at each seed board stripping station (10).

3. The seed board processing system according to claim 1, characterized in that: At least two seed board stripping stations (10) are arranged in upper and lower positions on the conveying path of the row conveying line (2), and a seed board row conveying line (3) is respectively arranged at each seed board stripping station (10).

4. The seed board processing system according to claim 1, 2 or 3, characterized in that: The seed plate stripping station (10) is provided with a mechanical arm, which includes a mechanical arm (11) with one end hingedly connected to a stripping machine frame (101), a mechanical arm (12) hingedly connected to the other end of the mechanical arm (11), the swinging surfaces of the mechanical arm (11) and the mechanical arm (12) are parallel, and a suction cup frame (102) and a suction cup (103) connected to a vacuum air source are provided at one end of the mechanical arm (12) away from the mechanical arm (11). The seed plate stripping station (10) is provided with a clamping mechanism (104) for clamping a conductive rod (B') of the cathode plate (B) above the cathode plate (B).

5. The seed board processing system according to claim 1, 2 or 3, characterized in that: An infrared detection sensor is provided on the cathode plate detection station (20), and the detection direction of the infrared detection sensor points to the plate surface of the cathode plate (B) on the station; The two side edges and the lower edge of the cathode plate (B) are covered with slender edging strips (B10), the lower ends of the edging strips (B10) at the two side edges of the cathode plate (B) are respectively connected with the two ends of the edging strips (B10) at the lower edge of the cathode plate (B), and a camera detector and a background light projector are provided on the cathode plate detection station (20), and the lens of the camera detector and the light source of the background light projector are respectively facing the plate surface of the cathode plate (B).

6. The seed board processing system according to claim 5, characterized in that: The number of infrared detection sensors is at least 9.

7. The seed board processing system according to claim 5, characterized in that: Brush rollers (201) are symmetrically arranged on both sides of the plate surface of the cathode plate (B) at the cathode plate detection station (20), the roller surface of the brush roller (201) is in contact with the plate surface of the cathode plate (B), and the wheel core of the brush roller (201) is parallel to the plate surface of the cathode plate (B) and perpendicular to the displacement and conveying direction of the cathode plate (B).

8. The seed board processing system according to claim 1, characterized in that: At the cathode plate detection station (20), the contacting portion between the brush roller (201) and the cathode plate (B) is located at the lower part of the plate surface of the cathode plate (B).

9. The seed board processing system according to claim 1, characterized in that: The inner plate surface of the seed plate (A) on the seed plate row conveying line (3) faces downward and contacts the conveying roller surface, and the outer plate surface of the seed plate (A) faces upward.

10. The seed board processing system according to claim 9, characterized in that: The seed plate detection station (31) is provided with a weighing unit and an optical detection unit for measuring the seed plate (A); the optical detection unit is located above the seed plate (A) and its detection direction is toward the outer plate surface of the seed plate (A).

11. The seed board processing system according to claim 1, characterized in that: A trimming station for trimming the edges of the seed board (A) is provided between the seed board detection station (31) and the seed board sorting station (32).

12. The seed board processing system according to claim 1, characterized in that: The sorting robot at the seed board sorting station (32) comprises a gripping end (324) and a sorting mechanical arm (325). The gripping end (324) can rotate around its own center. The gripping end (324) is connected to a vacuum air source to grip the seed board (A) by vacuum adsorption and is driven to move by the sorting mechanical arm (325). The sorting robot transfers the seed board (A) to the waste collection station (322) or the qualified product packing station (321) according to the detection result of the seed board detection station (31).

13. The seed board processing system according to claim 12, characterized in that: A transfer trolley (323) is provided at the waste collection station and the qualified product packing station respectively. A vertical fence (3231) is provided around the table surface of the transfer trolley (323) for placing the seed board (A). The square area enclosed by the fence (3231) is consistent with the shape of the seed board (A) surface and its enclosed area is slightly larger than the surface area of ​​the seed board (A). The bottom of the grasping end (324) of the sorting robot is square consistent with the shape of the seed board (A) surface and its horizontal projection is slightly smaller than the table surface enclosed by the fence (3231).

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