JP7840038B2 - Barrel polishing method - Google Patents

Description

This invention relates to a barrel polishing method .

Patent Document 1 discloses a barrel polishing method in which a workpiece and polishing stones are placed in a barrel tank to polish the workpiece. In barrel polishing, rotational motion or vibration is applied to the barrel tank to create a relative motion difference between the workpiece and the polishing stones, thereby polishing the workpiece with the stones. Polishing with the stones allows for desired finishing processes such as chamfering, deburring, surface preparation, and gloss enhancement of the workpiece.

Japanese Patent Publication No. 2003-225854

The more the abrasive stone polishes the workpiece, the more it wears down itself. As the abrasive stone wears down, its weight decreases, reducing its pressure on the workpiece and thus decreasing its polishing power. Furthermore, the wastewater after polishing contains abrasive stone particles, requiring wastewater treatment with chemicals and filters for environmental protection. However, if the abrasive stone wears down significantly, the amount of chemicals used and the frequency of filter replacements will increase. Additionally, if abrasive stone particles adhere to the workpiece after polishing, a cleaning process such as ultrasonic cleaning is necessary. However, if the abrasive stone wears down significantly, the time required for this cleaning process will increase.

One way to reduce the wear of the abrasive stones is to reduce the relative motion difference between the workpiece and the abrasive stones by lowering the rotation speed of the barrel tank. However, this method is not practical because it weakens the pressure of the abrasive stones on the workpiece, reducing the abrasive power and making it impossible to obtain the desired chamfering or deburring amount. A way to reduce abrasive stone wear without reducing abrasive power is to significantly revise the abrasive stone formulation design. However, adjusting the formulation of multiple types of abrasive stones with different sizes and hardnesses requires not only a huge number of prototypes and a long preparation period, but also changes to the production process, resulting in significant costs.

This invention was completed based on the circumstances described above, and aims to reduce wear on abrasive stones without reducing the abrasive power of the stones themselves or increasing costs.

The present invention
A barrel polishing method comprising placing a workpiece, polishing stones, water, and a buffer material into a barrel tank, and polishing the workpiece with the polishing stones,
The median diameter (D50) of the aforementioned cushioning material is set to 1.2 μm to 2000 μm.
The amount of the buffer material added is set to 2 wt% to 30 wt% relative to the water.

The cushioning material provides a buffering effect during collisions between the workpiece and the abrasive stone, as well as between abrasive stones themselves, thus reducing wear on the abrasive stones. Since adjustments to the abrasive stone's composition or significant process changes are unnecessary, costs do not increase. This allows for reduced wear on the abrasive stones without increasing costs.

A schematic diagram showing the process of placing a workpiece, polishing stone, water, buffer material, and compound into the barrel tank of Embodiment 1. Enlarged perspective view of the workpiece Magnified perspective view of a polishing stone A schematic diagram showing, in magnified view, how the buffer material is interposed between the abrasive stones.

<Embodiment 1>
Hereinafter, Embodiment 1 of the present invention will be described with reference to Figures 1 to 4. The barrel polishing method of Embodiment 1 is performed using a barrel tank 10 which constitutes a centrifugal barrel polishing machine (not shown). During polishing, as shown in Figure 1, a plurality of workpieces 11, a plurality of polishing stones 12, a predetermined amount of water 13, a buffer material 14 made of powder and granules, and a compound 15 are put into the barrel tank 10. The buffer material 14 and the compound 15 may be put into the barrel tank 10 separately, or the buffer material 14 and the compound 15 may be put into the barrel tank 10 together as a package. The workpieces 11 may have a shape having at least one plane (a surface with curvature of 0), or they may have a shape without a plane.

The workpiece 11, abrasive stones 12, water 13, buffer material 14, and compound 15 placed in the barrel tank 10 are defined as a mass. Once the mass is placed in the barrel tank 10, the barrel tank 10 is rotated and revolved to cause the mass to flow within it. This flow creates a relative difference in motion between the abrasive stones 12 and the workpiece 11, causing the abrasive stones 12 to collide with the surface of the workpiece 11, thereby polishing the surface of the workpiece 11. Polishing is primarily performed to smooth the flat surfaces 11S and curved surfaces with small curvatures of the workpiece 11, as well as to chamfer corners 11E and deburr.

During polishing, as shown in Figure 4, the buffer material 14 fills the microscopic depressions on the surfaces of the workpiece 11 and the polishing stone 12. The presence of this buffer material 14 makes it difficult for the protrusions of the polishing stone 12 to penetrate the depressions of the workpiece 11 when the polishing stone 12 collides with the workpiece 11, and also makes it difficult for the protrusions of the workpiece 11 to penetrate the depressions of the polishing stone 12. This suppresses excessive wear of the protrusions of the polishing stone 12 due to collision with the inner surface of the depressions of the workpiece 11, and prevents excessive wear of the inner surface of the depressions of the polishing stone 12 due to collision with the protrusions of the workpiece 11, thus reducing wear on the polishing stone 12. Similarly, when two polishing stones 12 collide, it becomes difficult for the protrusions of one polishing stone 12 to penetrate the depressions of the other polishing stone 12, thus reducing wear on the polishing stone 12. If the particle size of the cushioning material 14 is larger than that shown in Figure 4, the protrusions of the abrasive stone 12 and the protrusions of the workpiece 11 are less likely to collide, and excessive wear of the abrasive stone 12 by the protrusions of the workpiece 11 is suppressed, thus reducing wear on the abrasive stone 12. Collisions between the protrusions of the abrasive stones 12 are also less likely to occur, further reducing wear on the abrasive stone 12. In this way, the cushioning material 14 exhibits a cushioning effect during collisions between the workpiece 11 and the abrasive stone 12, and during collisions between the abrasive stones 12 themselves, thereby suppressing wear on the abrasive stone 12.

The cushioning material 14 can be made from either organic fibrous material or synthetic resin material. Examples of organic fibrous material include wood powder, corn cob, walnut, and peach pit. Examples of synthetic resin material include polystyrene, urethane resin, and acrylic resin. Regarding the size of the cushioning material 14, powders with a median diameter of 1.2 μm to 0.1 mm and granules with a median diameter of 0.1 mm to 2 mm can be used. Cushioning material 14 made of granules with a relatively large median diameter can be recovered and reused after polishing. In particular, cushioning material 14 made of highly wear-resistant urethane resin is preferred because it can be repeatedly recovered and reused. These materials are commercially available and relatively inexpensive. The cushioning material 14 has a lower hardness than the polishing stone 12 and a lower hardness than the workpiece 11. The Mohs hardness of the cushioning material 14 is preferably 5 or less.

The material of the cushioning material 14 is preferably elastic. Elastic materials include both materials that can be elastically deformed in their standalone state without mixing with water 13, and materials that do not elastically deform in their standalone state without mixing with water 13, but can be elastically deformed when mixed with water 13 to form a slurry. By using an elastic cushioning material 14, the cushioning material 14 and the abrasive stone 12 collide elastically, mitigating the impact during collision, providing a sufficient cushioning effect, and suppressing wear of the abrasive stone 12.

In barrel polishing, the surface of the workpiece 11 is polished fairly evenly across its entire surface. Therefore, when polishing is primarily performed to chamfer or deburr the corners 11E of the workpiece 11, areas other than the corners 11E and burrs (flat surfaces 11S and curved surfaces with small curvature) are unnecessarily removed. However, using a barrel polishing method with a cushioning material 14, the amount of polishing in these non-polished areas can be reduced, as described below.

In the flat areas 11S and curved areas with small curvature of the workpiece 11, which are not to be polished, the cushioning material 14 is densely arranged over a wide area, covering the non-polished areas. Therefore, when the polishing stone 12 collides with the cushioning material 14 from an oblique direction, the impacted cushioning material 14 is less likely to shift position due to the presence of adjacent cushioning materials 14, and the non-polished areas are easily kept covered by the cushioning material 14. In this way, the non-polished areas are easily kept covered by the cushioning material 14, making it difficult for the polishing stone 12 to proceed. Furthermore, because the cushioning effect of the cushioning material 14 is high, wear of the polishing stone 12 is also suppressed.

On the other hand, the corners 11E and burrs of the workpiece 11, which are to be polished, are also covered with the cushioning material 14. However, at the corners 11E and burrs, when the polishing stone 12 collides with the cushioning material 14, the cushioning material 14 is ejected from the tip of the corner 11E or burr, exposing the corners 11E and burrs. Therefore, the polishing stone 12 directly contacts and grinds the corners 11E and burrs. Consequently, polishing progresses more at the corners 11E and burrs than at the non-polishing areas.

According to the polishing method using the cushioning material 14 of this embodiment, when polishing the corners 11E and burrs of the workpiece 11, the corners 11E and burrs can be effectively polished by the polishing stone 12. Furthermore, for flat surfaces 11S and curved surfaces with small curvature that are not to be polished, the cushioning material 14 suppresses unnecessary polishing, minimizing material loss from the workpiece 11 and wasted consumption of the polishing stone 12.

The inventors of this application experimentally verified the extent to which the type, size, and amount of cushioning material 14 affect the wear rate of the abrasive stone 12, and the experimental results are shown in Table 1. The experiment used a centrifugal barrel polishing machine HS-1-4V manufactured by Chipton Co., Ltd. The capacity of the barrel tank 10 used was 1 L. The rotation speed of the barrel tank 10 was 260 rpm, and the polishing time was 30 minutes.

Workpiece 11 is a rectangular prism measuring 15 mm x 15 mm x 20 mm (see Figure 2), and the material of workpiece 11 is SS400. The abrasive stone 12 is GT-4 manufactured by Chipton Co., Ltd. The abrasive stone 12 is a triangular prism with sides of 10 mm and a height of 8 mm (see Figure 3), and the material of the abrasive stone 12 is ceramic. The amount of water 13 added to the barrel tank 10 is 0.25 L. The compound 15 is powder compound CO-56 manufactured by Chipton Co., Ltd. The amount of compound 15 added is 1 wt% relative to the water 13. That is, the weight ratio of water 13 to compound 15 in the barrel tank 10 (weight of compound 15 / weight of water 13) is 0.01.

The buffer material 14 used in the experiment consisted of wood powder, polystyrene resin powder, corn cob (corn), and acrylic resin powder. In the following explanation, the amount of buffer material 14 added represents the ratio of the amount of buffer material 14 to the amount of water 13 added to the barrel tank 10. The amount of buffer material 14 added = (weight of buffer material 14 / weight of water 13) × 100. The amount of buffer material 14 added in the experiment ranged from 0 to 30 wt%.

In Table 1, the wear rate is the ratio of the amount of wear after polishing to the weight of the polishing stone 12 before polishing (difference between the weight of the polishing stone 12 before and after polishing / weight of the polishing stone 12 before polishing). The wear rate ratio is the ratio of the wear rate of each example or comparative example (wear rate of each example or comparative example / wear rate of Comparative Example A) when the wear rate under the condition without the cushioning material 14 (Comparative Example A) is set to 1. The wear rate evaluation was rated as "◎ (excellent)" for a wear rate ratio of 0.8 or less, and "〇 (good)" for a wear rate ratio of 0.9 or less. Since the variation in the wear rate ratio without the cushioning material 14 is approximately ±10%, examples with a wear rate ratio greater than 0.9 were considered within the normal variation range and were rated as "× (poor)". Examples with a wear rate evaluation of "◎" or "○" are labeled as examples, and examples with a wear rate evaluation of "×" are labeled as comparative examples.

The radius (R) is the measured value of the radius of the quarter-circular arc portion at the corner 11E of the workpiece 11. A Tokyo Seimitsu Contour Record 1600GR was used as the measuring instrument. The R ratio is the ratio of the R amounts in each example (R amount in each example / R amount in Comparative Example A) when the R amount under the condition without cushioning material 14 (Comparative Example A) is set to 1. Since the variation in the R ratio when cushioning material 14 is absent is approximately ±5%, the R evaluation was such that an R ratio greater than 0.95 was considered within the normal variation range and rated as "○ (Good)". An R ratio of 0.95 or less was rated as "× (Poor)".

When the cushioning material 14 is one of wood powder, polystyrene, or cone, and the median diameter (D50) of the cushioning material 14 is 250 μm, the abrasion rate evaluation for Comparative Examples B, C, and D, where the amount of cushioning material 14 added is 1 wt%, is "×". The abrasion rate evaluation for Examples 1, 9, and 16, where the amount of cushioning material 14 added is 2 wt%, is "〇". The abrasion rate evaluation for Examples 2-8, 10-15, and 17-23, where the amount of cushioning material 14 added is 3 wt% to 30 wt%, is "◎".

When the cushioning material 14 is acrylic and the amount of cushioning material 14 added is 3 wt%, the wear rate evaluation for Comparative Example E, where the median diameter of the cushioning material 14 is 0.15 μm, is "×". The wear rate evaluation for Examples 24 to 28, where the median diameter of the cushioning material 14 is 1.20 μm to 10.00 μm, is "〇". The wear rate evaluation for Example 29, where the median diameter of the cushioning material 14 is 50.00 μm, is "◎".

When the cushioning material 14 is acrylic and the amount of cushioning material 14 added is 3 wt%, the wear rate evaluation for Example 30, where the median diameter of the cushioning material 14 is 2000.00 μm, is "○". The wear rate evaluation for Comparative Example F, where the median diameter of the cushioning material 14 is 3000.00 μm, is "×".

When the cushioning material 14 is acrylic and its median diameter is 1.20 μm, the wear rate evaluation for Example 31, where the amount of cushioning material 14 used was 5 wt%, and for Example 32, where the amount of cushioning material 14 used was 9 wt%, is "○". The wear rate evaluation for Examples 33 to 36, where the amount of cushioning material 14 used was 20 wt% or more, is "◎".

When the cushioning material 14 is acrylic and its median diameter is 2000.00 μm, the wear rate evaluation for Example 37, where the amount of cushioning material 14 used is 7 wt%, is "○". The wear rate evaluations for Examples 38-41, where the amount of cushioning material 14 used is 8 wt% to 20 wt%, are "◎".

There was no significant difference in wear rate between Example 7, where the amount of cushioning material 14 was 20 wt%, and Example 8, where the amount of cushioning material 14 was 30 wt%. Similarly, there was no significant difference in wear rate between Example 22, where the amount of cushioning material 14 was 20 wt%, and Example 23, where the amount of cushioning material 14 was 30 wt%. Therefore, from the viewpoint of the material cost of cushioning material 14, it is preferable to use 20 wt% or less of cushioning material 14.

When the cushioning material 14 is wood powder, the R-value evaluation for Example 4, where the amount of cushioning material 14 added was 5 wt%, and Example 5, where the amount of cushioning material 14 added was 6 wt%, is "○". When the cushioning material 14 is polystyrene, the R-value evaluation for Example 11, where the amount of cushioning material 14 added was 4 wt%, and Example 12, where the amount of cushioning material 14 added was 5 wt%, is "○". When the cushioning material 14 is cone, the R-value evaluation for Example 19, where the amount of cushioning material 14 added was 5 wt%, and Example 20, where the amount of cushioning material 14 added was 6 wt%, is "○". When the cushioning material 14 is acrylic, the R-value evaluation for Example 33, where the amount of cushioning material 14 added was 20 wt%, and Example 34, where the amount of cushioning material 14 added was 28 wt%, is "○". When the cushioning material 14 is acrylic, the R-value evaluation for Example 37 (7 wt%), Example 38 (8 wt%), and Example 39 (9 wt%) is "○".

Regarding the median diameter of the cushioning material 14, the wear rate evaluation for Comparative Example E, where the median diameter of the cushioning material 14 was 0.15 μm, and for Comparative Example F, where the median diameter was 3000 μm, was "×". The wear rate evaluation for Examples 1 to 41, where the median diameter of the cushioning material 14 was 1.2 μm to 2000 μm, was "〇" or "◎". If the median diameter of the cushioning material 14 is smaller than 1.2 μm, sufficient cushioning effect cannot be achieved. If the median diameter of the cushioning material 14 is larger than 2000 μm, the cushioning material 14 has difficulty interposing between the workpiece 11 and the abrasive stone 12, or between the abrasive stones 12 themselves. From these results, a median diameter of 1.2 μm to 2000 μm is preferable for the cushioning material 14.

After polishing, the polished workpiece 11 and polishing stone 12 are separated, and sludge such as polishing powder from the workpiece 11 and wear particles from the polishing stone 12 are drained to clean the barrel tank 10. If the median diameter of the buffer material 14 is greater than 1000 μm, only the polishing stone 12 can be separated and recovered during the cleaning process using a sieve or filter. By reusing the recovered buffer material 14, running costs can be reduced. Therefore, a median diameter of 1000 μm to 2000 μm is more preferable for the buffer material 14.

The amount of buffer material 14 to be added to the barrel tank 10 is preferably between 2 wt% and 30 wt%. If the amount of buffer material 14 is less than 2 wt%, sufficient buffering effect will not be achieved. If the amount of buffer material 14 is more than 30 wt%, the buffer material 14 is likely to remain as dirt on the workpiece 11 after polishing.

In this embodiment 1, the barrel polishing method involves placing a workpiece 11, polishing stones 12, water 13, and a buffer material 14 into a barrel tank 10, and polishing the workpiece 11 with the polishing stones 12. The median diameter (D50) of the buffer material 14 is set to 1.2 μm to 2000 μm. The amount of buffer material 14 added is set to 2 to 30 wt% relative to the water 13. That is, the weight ratio of the buffer material 14 to the water 13 in the barrel tank 10 (weight of buffer material 14 / weight of water 13) is 0.02 to 0.30. By using buffer material 14 with the size and amount set as described above, the buffer material 14 exhibits a high cushioning effect during collisions between the workpiece 11 and the polishing stones 12, and during collisions between the polishing stones 12 themselves, thus reducing wear on the polishing stones 12.

By setting the size and amount of the cushioning material 14 within the above range, wear of the abrasive stone 12 can be suppressed without reducing the abrasive power of the abrasive stone 12 itself. Since it is not necessary to adjust the formulation of multiple types of abrasive stones 12 with different sizes and hardnesses to achieve both abrasive power and reduced wear of the abrasive stone 12, a large number of prototypes and a long preparation period are unnecessary, and changes to the production process are not required. Therefore, cost increases due to adjustments in the formulation of the abrasive stone 12 can be avoided. According to this embodiment, wear of the abrasive stone 12 can be reduced without reducing the abrasive power of the abrasive stone 12 itself or increasing costs.

When wood powder or polystyrene is used as an additive in barrel polishing, it is expected to prevent plate-shaped workpieces 11 from sticking to other workpieces 11 or the inner wall of the barrel tank 10. However, the amount of additive used to prevent sticking, relative to the water, is preferably 1 wt% to 2 wt%. Adding more than 2 wt% of the additive to the water is undesirable from a cost standpoint because it results in an excessive anti-sticking effect. The buffer material 14 used in the barrel polishing method of this embodiment differs from the anti-sticking additive in that its amount added to the water 13 is 2 wt% or more.

The greater the amount of cushioning material 14 added, the greater the interference effect of the cushioning material 14. However, in the region where the amount of cushioning material 14 exceeds 20 wt%, the rate of increase in cushioning effect relative to the increase in cushioning material 14 becomes smaller. Therefore, considering the amount of cushioning material 14 used, it is preferable to keep the amount of cushioning material 14 at 20 wt% or less.

If the amount of cushioning material 14 is less than 3 wt%, the effect of reducing wear on the abrasive stone 12 is low. If the amount of cushioning material 14 is 3 wt% or more, the effect of reducing wear on the abrasive stone 12 increases. If the amount of cushioning material 14 is more than 6 wt%, at the corners 11E and burrs of the workpiece 11, the cushioning material 14 is more likely to remain without being repelled when the abrasive stone 12 collides with it, thus reducing the amount of chamfering at the corners 11E and deburring at the burrs. If the amount of cushioning material 14 is 6 wt% or less, at the corners 11E and burrs of the workpiece 11, the cushioning material 14 is more likely to be repelled when the abrasive stone 12 collides with it, thus increasing the wear effect at the corners 11E and burrs. Therefore, the amount of cushioning material 14 is preferably between 3 wt% and 6 wt%.

The compound 15 can be made of a material with lubricating properties. In this case, the polishing stone 12 becomes more slippery on the flat surface 11S of the workpiece 11 due to the compound 15, increasing the contact area between the workpiece 11 and the polishing stone 12. This improves the polishing efficiency on the flat surface 11S of the workpiece 11.

The compound 15 can be made of a material that exhibits etching properties on the workpiece 11. When an etching compound 15 is added, the surface of the workpiece 11 becomes brittle and easily removed by etching, or even melts and is easily removed, thus improving the polishing efficiency on the workpiece 11's surface. Furthermore, if a compound 15 containing abrasive particles is used, the abrasive particles further improve the polishing efficiency on the workpiece 11's surface.

If the buffer material 14 is made of a material containing resin components, such as wood powder, then it is preferable that the compound 15 is made of a material that does not contain fatty acid salts. When the buffer material 14 contains resin components, the pH of the water 13 in the barrel tank 10 decreases due to the influence of the resin components. If the compound 15 contains fatty acid salts, free fatty acids will be generated from the compound 15 due to the decrease in pH. Since free fatty acids are oily, there is a concern that they may adhere to the surfaces of the workpiece 11, abrasive stone 12, and buffer material 14 as dirt. By using a compound 15 that does not contain fatty acid salts, it is possible to prevent free fatty acid dirt from adhering to the surfaces of the workpiece 11, abrasive stone 12, and buffer material 14.

The abrasive stone 12 preferably has a shape with a flat surface 12S, as shown in Figure 3. If the abrasive stone 12 has a shape without a flat surface 12S, such as a sphere, collisions with other abrasive stones 12 will always result in point contact. This makes it easy for the cushioning material 14 to be thrown off, making it difficult to achieve the wear reduction effect of the abrasive stone 12. In contrast, if the abrasive stone 12 has a flat surface 12S, when the abrasive stones 12 collide, the flat surfaces 12S make surface contact with each other. In this case, the cushioning material 14 is more easily interposed between the flat surfaces 12S, thus increasing the wear reduction effect of the abrasive stone 12.

<Other examples>
The present invention is not limited to the embodiments described above and in the drawings, and the following embodiments, for example, are also included in the technical scope of the present invention.
The cushioning material may be an organic fibrous material other than wood flour, corn cob, or walnut.
The cushioning material may be made of synthetic resin materials other than polystyrene resin, urethane resin, or acrylic resin.
The cushioning material may be made of materials other than organic fibrous materials and synthetic resin materials.
The hardness of the cushioning material can be the same as that of an abrasive stone.
The hardness of the cushioning material can be the same as that of the workpiece.
The new Mohs hardness of the cushioning material may be higher than 5.
The cushioning material may be made of a material that does not have elasticity.
You may polish the workpiece without adding any polishing compound.
The compound may be made of materials that do not have lubricating properties.
The compound may be made of materials that do not exhibit etching properties.
The compound does not need to contain abrasive particles.
The compound is a ramen ingredient containing fatty acid salts.
The compound is not limited to a powder; it may also be in liquid form.
The cushioning material may have a shape that does not have a flat surface.

10... Barrel tank 11... Workpiece 12... Polishing stone 12S... Flat surface of polishing stone 13... Water 14... Cushioning material 15... Compound

Claims (11)

A barrel polishing method comprising placing a workpiece, polishing stones, water, and a buffer material into a barrel tank, and polishing the workpiece with the polishing stones,
The median diameter (D50) of the aforementioned cushioning material is set to 1.2 μm to 2000 μm.
A barrel polishing method wherein the amount of the buffer material added is 2 wt% to 30 wt% relative to the water. The barrel polishing method according to claim 1, wherein the amount of the buffer material added is 2 wt% to 20 wt% relative to the water. The barrel polishing method according to claim 1, wherein the amount of the buffer material added is 3 wt% to 6 wt% relative to the water. The barrel polishing method according to any one of claims 1 to 3, wherein the cushioning material is either an organic fibrous material or a synthetic resin material. The barrel polishing method according to claim 4, wherein the cushioning material is one of wood powder, corn cob, walnut, polystyrene resin, urethane, or acrylic resin. The barrel polishing method according to any one of claims 1 to 5, wherein the cushioning material has a lower hardness than the polishing stone. The barrel polishing method according to any one of claims 1 to 6, wherein the cushioning material has a lower hardness than the workpiece. The barrel polishing method according to any one of claims 1 to 6, wherein the new Mohs hardness of the cushioning material is 5 or less. The barrel polishing method according to any one of claims 1 to 8, wherein the median diameter (D50) of the cushioning material is 1000 μm to 2000 μm. The aforementioned cushioning material is made of a material containing a resin component,
The barrel polishing method according to any one of claims 1 to 9, wherein a compound made of a material that does not contain fatty acid salts is introduced into the barrel tank. The barrel polishing method for suppressing wear of the polishing stone according to any one of claims 1 to 10, wherein the polishing stone has a shape having a flat surface.