JPH03136758A - Carrier plate type infield polishing device capable of inprocess electrolytic dressing by using ultra-abrasive grain metal bond grind stone - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a carrier plate type grinding wheel using a superabrasive grain (i.e., Tiremond abrasive grain or cubic boron nitride abrasive grain, etc.) metal bonded abrasive wheel and an inorganic abrasive grain. The present invention relates to a feed-type polishing device that enables in-process electrolytic dressing.

[Prior Art] An example of a carrier plate type and infeed type polishing device using a conventional superabrasive metal bonded grindstone is shown in FIG. 7. This conventional device is made of a conductive material. A disc body 3 is formed by bonding a grinding wheel disc 2 made of super-abrasive metal bond to a disc-shaped base disc 1, and these two disc bodies 3
．． The working surfaces of each of the grinding wheels 2.2 are arranged to face each other, and a carrier plate 5 holding a large number of workpieces 4 is arranged between the grinding wheels 2.2. On the other hand, some disc body 3 is pushed in the D direction along the rotation axis, and the workpiece 4 is pressurized by the two disc bodies 3.3, and the two disc bodies 3, 3 are moved, for example, in the same direction. Rotate in directions A and B, respectively, at the same or different speeds. At the same time, the carrier plate 5 is rotated in the direction opposite to the disk body 3.3 (direction C), and the workpiece 4 is polished in an in-feed manner (that is, while holding the workpiece 4 for a predetermined time). .

In addition, as in-feed type polishing equipment that can perform in-process electrolytic dressing using conventional ultra-fine super-abrasive metal bond grinding wheels, for example, Nippon Kogyo Shimbun, ``Ultra-fine grinding machine that can process up to mirror surfaces'', Heisei The one published by Nippon Kogyo Shimbun on August 31, 1998 is known.

As shown in FIG. 8, in this conventional apparatus, a workpiece 8 is fixed on a rotating table 7, and a conductive spindle shaft 9 is connected to the same (conductive base 10 and ultra-fine super-abrasive). A disk body 1 configured by bonding a grindstone disk 11 made of grain metal bond
Fix 2. Then, while pressing a part of the workpiece 8 on the table 7 with a part of the grindstone 11 by the spindle shaft 9, the table 7 and the workpiece 8 are moved in one direction (direction D), and the spindle shaft 9 and the disc body 12 in the opposite direction (
H direction), and further, while supplying cutting fluid through the pipe 13, the workpiece 8 is mirror-finished (ie, polished).

In addition, at a position that does not interfere with the workpiece 8 of the grinding wheel 11,
Electrodes 14 for electrolytic dressing are arranged facing each other at appropriate intervals, and an electrolytic solution is supplied between the grinding wheel 11 and the electrodes 14. Also, the disk body 1 of the spindle shaft 9
The power supply brush 15 is brought into contact with the opposite side of the electrode 1.
4 and the power supply brush 15 to the (-) side of the power supply 16 as shown.
Connect to the terminal and (+) terminal. If the grinding wheel 11 becomes clogged or the amount of protruding abrasive grains becomes small and the sharpness becomes poor, DC current is supplied to the grinding wheel 11 and the electrodes 14, and the electrolytic solution is applied to the grinding wheel. 11
Electrolytic dressing is performed in-process (that is, while polishing is being performed) by melting the metal on the working surface.

[Problem to be solved by the invention]

However, in such conventional carrier plate type and infeed type polishing equipment using the former superabrasive metal bonded grindstone, the amount of protrusion of the abrasive grains of the grinding wheel plate 2.2 becomes small, and If the grinding becomes dull due to clogging, etc., it is necessary to stop the polishing process, separate the grinding wheel 2.2 from the workpiece 4, and then perform electrolytic dressing, which reduces productivity. There were problems in that the process was unfavorable, the cost was high, and the work was time-consuming.

Furthermore, although in-process electrolytic dressing is possible with the latter conventional apparatus, only one workpiece 8 can be produced at a time, which also has the problem of poor productivity.

This invention was made by focusing on the problems of the conventional methods, and enables in-process electrolytic dressing in a carrier plate type/infeed type polishing machine using a super-abrasive metal bonded grinding wheel. It can produce large quantities of workpieces with high accuracy and uniformity in a short time, and has high productivity.
The purpose of this invention is to provide a polishing device that is low in cost and has good workability.

[Means to solve the problem]

Therefore, a carrier plate type/infeed type polishing device capable of in-process electrolytic dressing using a superabrasive metal bond grinding wheel according to the present invention has a disc-shaped base plate made of a conductive material and a superabrasive metal bond grinding wheel. A disc-shaped grindstone disk is bonded to form a disc body, and the working surfaces of the two disc bodies are placed opposite to each other,
At least one of the two disc bodies is rotatable, at least one of the two disc bodies is movable in the direction of the rotation axis, and further, there is a space between the two disc bodies. A carrier plate that holds a plurality of workpieces is arranged, one disk body is moved in the direction of the rotation axis to pressurize the workpieces with a grindstone, and the carrier plate and the two disk bodies are mutually pressed. A carrier plate-type and infeed-type polishing device that uses a super-abrasive metal bond grindstone that is arranged so that it can rotate relative to each other and continuously polishes multiple workpieces held by a carrier plate for a predetermined period of time. In this step, an electrode for electrolytic dressing is mounted on each of the positions on both sides of the carrier plate where the workpiece is not held, and that faces each of the grinding wheels of the two disk bodies at a predetermined distance, and that the electrodes and the grinding wheel are connected to each other. The present invention is characterized in that a current supply means for supplying direct current between the panels is provided.

In addition, the working surface of the grinding wheel of a disk body formed by adhering a disk-shaped base made of a conductive material and a disk-shaped grinding wheel made of super-abrasive metal bond, and a wear-resistant material without abrasive grains. A disc-shaped pressure plate made of In addition to arranging the carrier plate that holds the workpiece,
The pressure plate is moved in the direction of the rotation axis to pressurize the workpiece by the grinding wheel and the pressure plate, and the carrier plate is arranged so that the disk body and the pressure plate can rotate relative to each other, and the plurality of pieces held by the carrier plate are In a carrier plate type/infeed type polishing machine capable of in-process electrolytic dressing using a super-abrasive metal bonded grindstone that continuously polishes a workpiece for a predetermined period of time, the working surface of the carrier plate grinding wheel An electrode for electrolytic dressing facing the working surface of the grinding wheel at a predetermined distance is attached to a position on the surface facing the grinding wheel at a position not holding multiple workpieces, and a direct current is applied between the electrode and the grinding wheel. The present invention is characterized in that it is provided with a current supply means for supplying current.

Furthermore, a groove with a curved cross section is formed on the circumference on the working surface of the grinding wheel of the two opposing disk bodies, and an electrode for electrolytic dressing is placed on the carrier plate facing the curved shape of the groove. It is characterized by being attached at predetermined intervals.

[Effect]

Two opposing disc bodies made by bonding a base made of conductive material and a grinding wheel made of super-abrasive metal bond, and a carrier plate holding a large number of workpieces arranged between the disc bodies. The workpiece is polished by rotating them in appropriate rotational directions and rotational speeds so as to rotate relative to each other. If the amount of abrasive grains protruding from the grinding wheel decreases or the grinding wheel becomes dull due to clogging, the grinding process is continued and the grinding wheel is turned on and off for a predetermined period of time. Direct current is supplied to the electrode for electrolytic dressing from the current supply means, and the working surface of the grindstone facing the electrode is electrolytically dressed.

In addition, when polishing only one side of the workpiece and electrolytically dressing the grinding wheel facing the polished surface,
One side of the two disc bodies is a pressure disc made of abrasive-free wear-resistant material.

Further, a circumferential groove having a semicircular cross section, for example, is formed on the working surfaces of the grinding wheels facing each other, and an electrode for electrolytic dressing is placed on the carrier plate, facing the curved surface of the second groove. By mounting the grooves facing the groove shape at a predetermined interval, it is possible to continuously polish a curved object such as a ball or a spherical roller while performing in-process electrolytic dressing.

〔Example〕 Embodiments of the present invention will be described below with reference to the drawings.

First, the configuration of the first embodiment will be explained.

In Figures 1 and 2, a base plate 1 made of conductive material is shown.
A super abrasive metal bond type grindstone disk 2 is bonded to the disk body 3.
The working surfaces of each grinding wheel 2, 2 of these two disk bodies 3°3 are arranged to face each other, and at least one (six in the illustrated example) is arranged between the two grinding wheels 2.2. ) A carrier plate is arranged to maintain the mutual positional spacing of the workpieces 4. Then, one of the disc bodies 3 is made reciprocating in the axial direction along the rotation axis, and the disc body 3 on one side is pushed in the direction D as shown in FIG. .3 makes it possible to pressurize the workpiece 4, and 2
The disk bodies 3.3 can be rotated in the A and B directions, respectively, in the same direction and at the same speed by a motor (not shown).

At the same time, the carrier plate 18 is made rotatable in the direction opposite to the disk body 3.3 (direction C) by a motor (not shown).

A conductive ring 19 is fixed to the outer peripheral edge of the carrier plate 18, and electrodes 20 and 20 for electrolytic dressing are adhered to both upper and lower surfaces of the portion of the carrier plate 18 that does not hold the workpiece 4, and this Electrode 20.2
0 to the conductive ring 19. This conductive ring 19
While keeping the power supply brush 21 in constant contact with the disk body 3
．． The power supply brushes 22 and 22 are also kept in constant contact with each of the outer surfaces of the base plate 1.1 of No. 3. Then, the power supply brush 21 is connected to the power supply 2 for electrolytic dressing via the common switch 23.
4. Connect to the (-) terminal of 24, and also connect the power supply brush 22
．． 22 are respectively connected to the (+) terminals of the electrolytic electrodes [24, 24].

Further, a conductive cutting fluid/electrolyte is supplied between the working surface of the grinding wheel 2.2 of the disc body 3.3 and the workpiece 4 and the electrode 20.20.

Above conductive ring 19 - switch 23 - electrode 20.20
- Power supply brush 22.22 - Base plate 1. 1 constitutes a current supply means.

Next, the operation of the first embodiment will be explained.

In Fig. 1 and Fig. 2, a base plate 1.1 and a grindstone plate 2,
The disc bodies 3, 3 formed by gluing 2 are rotated in the A and B directions in the same direction and at the same speed, and for example, six workpieces 4 placed between the grinding wheels 2, 2 are held. The carrier plate 18 is rotated in the opposite direction (direction C) so that one machining surface of one grinding wheel 2 and the workpiece 4 and the other machining surface of the other grinding wheel 2 and the workpiece 4 are moved relative to each other. Then, push one disc body 3 toward the other disc body 3 side in the D direction, and grind both sides of the workpiece 4 with the grindstone 2.
, 2. A cutting fluid/electrolyte is supplied between the grinding wheels 2, 2 and the workpiece 4, and the workpiece 4 is polished.

When the amount of abrasive grains protruding from the grinding wheel 2.2 becomes small, or when it becomes clogged and becomes dull, turn on the switch 23 and turn on the power supply brush 22.22 from the power source 24.24. Disc 1.1 - Grinding disc 2, 2 - Electrolyte (also cutting fluid) - Electrode 20.20 - Conductive ring 19 -
The current supply circuit for electrolytic dressing consisting of the power supply brush 21, switch 23, and power supply 24, 24 is closed. In this way, the metal that is the binder on the working surface of the grinding wheel 2.2 is melted by the electrolytic solution, the abrasive grains are appropriately protruded, and the clogging of air bubbles is removed, and the grinding wheel l:! The working surface of 12.2 remains sharp and stable, and polishing can always be performed with an appropriate amount of abrasive grain protrusion.

Therefore, it becomes possible to polish a large number of workpieces with high precision and uniformity, greatly improving productivity, reducing the cost of workpieces, and improving workability.

According to the polishing apparatus of the first embodiment, the workpiece 4 includes, for example, the six faces of the gauge block shown in FIG. 6(a), and the side and end faces of the cylindrical roller shown in FIG. 6(b). , the flat part of the ring shown in FIG. 6(C), or the flat part of the conical roller shown in FIG. 6(d), etc. can be polished.

In the first embodiment, the two disc bodies 3°3 are rotated in the same direction and at the same speed. In order to polish both sides of the workpiece 4 with the same precision, it is usually necessary to consider the weight of the workpiece 4, and therefore the rotational speed of the upper disk body 3 is The rotation speed is set to be slightly faster than the rotation speed of the lower disc body 3. Also,
Even when both planes of the conical roller shown in FIG. 6(d) are polished to the same precision, the rotational speed and direction of rotation are made to be different between the disk bodies 3 on one side and the disk bodies 3 on the other side.

Furthermore, in the polishing of the workpieces shown in FIGS. 6(a), (C), and (b), the machining accuracy on both surfaces may be different, and for this reason, one disk body 3 and the other The rotational speed and direction of rotation may be different from that of the disk body 3. Furthermore, in particular, by fixing the disk body that moves in the direction of the rotation axis to apply pressure, it is possible to prevent excessive load from being applied to the carrier plate 18.

Further, the conductive ring 19 disposed between the electrodes 20, 20 and the power supply brush 21 is formed on the outer periphery of the carrier plate 18, but as shown in the figure, the conductive ring 19 is formed on the inner periphery of the donut-shaped carrier plate 18. conductive ring 25
may be formed and connected to the electrode 20.20,
In addition, the power supply brush 21 is connected to the conductive ring 25 on the inner peripheral edge of the power supply brush 21.
It may be made to come in contact with.

Next, a second embodiment will be described with reference to FIGS. 3 and 4.

First, the configuration will be explained. The same parts as in the first embodiment described above are given the same reference numerals, and the explanation will be omitted.

In this second embodiment, one disk body 3 consisting of a base l and a grindstone disk 2 and a pressure plate 27 made of an abrasive-free wear-resistant material are arranged facing each other, and the disk body 3 is is rotatable in one direction (direction B), the pressure plate 27 is movable back and forth in the direction of the rotation axis, and rotation of the pressure plate 27 can be stopped or rotated together.

The carrier plates are of a planetary gear type, that is, the sun gear 728 is rotatable in one direction (E direction) at the center, and there are, for example, four carrier plates 30a each holding six workpieces 29. , 30b, 3
0b, 30b are meshed with the sun gear 28 to enable rotation and revolution, and their carrier play) 30a, 30
b, 30b.

30b is engaged with an internal gear 31 made of a conductive material, and this internal gear 31 is fixed to a housing of an apparatus (not shown). and 4 career plays) 30a, 30
One carrier plate 30 among b, 30b, 30b
A conductive ring 32 is formed on the outer peripheral edge of the carrier plate 30a, and the conductive ring 32 is engaged with the internal gear 31, and is opposed to the grinding wheel 2 at a position where the workpiece 29 of the carrier play 30a is not held. An electrode 33 for electrolytic dressing is formed in the portion. There may be at least one electrode 33, but each carrier plate 30a has one electrode.

You may provide one each for 30b, 30b, and 30b.

Then, the internal gear 31 is electrically connected to the switch 23, and the power supply brush 22 and power source 24 are connected as shown.

Conductive ring 32 - internal gear 31 - switch 23
The electrode 20, the power supply brush 22, and the base 1 constitute a current supply means.

The operation of the second embodiment is also similar to that of the first embodiment, and by rotating the sun gear 28 in one direction (E direction), the carrier plates 30a, 30b, 30b, 30b rotate in the F direction. , revolves in the C direction, and the workpiece 29
is polished.

Then, if necessary, the switch 23 is turned on to perform in-process electrolytic dressing of the grindstone 2.

This second embodiment is used when polishing only one side of the workpiece 4. Also, according to the second embodiment, a larger number of workpieces 29 can be polished with high precision than in the first embodiment. Can be polished uniformly.

Further, the pressure plate 27 may be fixed to the housing of the device, or may be fixed to the carrier plate 30a, 30b.

It may be arranged to rotate along with the revolution of 30b and 30b. In this way, the carrier plates 30a, 30b, 30b,
It is possible to prevent an excessive load from being applied to 30b.

Further, although the electrode 33 is electrically connected to the internal gear 31 side, it may be electrically connected to the sun gear 28 side.

Next, a third embodiment will be described with reference to FIG. Similarly, parts that are the same as those in the first embodiment described above are given the same reference numerals, and explanations thereof will be omitted.

First, to explain the configuration, two disk bodies 36 each consisting of a base plate 1 and a grindstone disk 35 are arranged with the working surfaces of the grindstone disks 35 and 35 facing each other. On the working surface of the grindstone 35.35,
A semicircular groove 37 whose cross section is an example of a curved shape is formed on the circumference. Similarly to the one shown in FIG. 2, a carrier plate 39 holding at least one ball 38 (for example, six balls), which is an example of a curved object as a workpiece, is placed between the grinding wheels 35 and 35. Place. Then, as in the first embodiment, while pushing the disk body 36 on one side in the direction D, the disk body 36 on one side is pushed in the direction A, the disk body 36 on the other side is pushed in the direction B, and the carrier plate 3
9 can be rotated in the C direction at appropriate speeds. Further, a cutting fluid (electroless fluid) is supplied between the ball 38 and the grinding wheel 35.35.

A conductive ring 19 is fixed to the outer peripheral edge of the carrier plate 39, and a conductive ring 19 is fixed on both sides of the portion of the carrier plate 39 that does not hold the balls 38, and one ring that matches the groove 37 on the circumference of the grinding wheel 35. The electrode 41.4 for electrolytic dressing is inflated into a spherical shape, and across the spherical part 40, an electrode 41.4 for electrolytic dressing is inserted.
1 is glued to the carrier plate 39 at a predetermined distance from the working surface including the groove 37 of the grinding wheel 35.35, and this electrode 41.41 is connected to the conductive ring 19.

As in the first embodiment, the power supply brushes 21, 22.
22. A switch 23 and electric terminals 24 and 24 are provided. Further, an electrolytic solution (non-cutting fluid) is supplied between the working surface of the grinding wheel 35.35 and the electrode 41.41.

The operation of this third embodiment is almost the same as that of the first embodiment, and while pressing one disc body 36 in the D direction, one disc body 36 is pressed in the A direction, and the other disc body 36 is pressed in the B direction. When the carrier plate 39 is rotated in the C direction, the ball 38, which is the workpiece, rolls along the rolling groove 37 formed on the grinding wheel 35.35. Polishing process is performed.

Also in this third embodiment, while polishing a large number of balls 38, if the protruding amount of abrasive grains on the grinding wheel 35 or 35 becomes small or if clogging occurs, the polishing process is interrupted. While continuing without cutting, the switch 23 is closed to supply a direct current for electrolytic dressing between the grinding wheel 35.35 and the electrode 41.41. In this way, the metal on the working surface of the grinding wheel 35.35 is melted into the electrolyte, and the amount of protrusion of the abrasive grains of the grinding wheel 35.35 becomes appropriate, so that a large number of highly accurate and uniform balls 38 can be produced in a short time. It is possible to improve productivity, reduce costs, and improve workability.

Note that an electrode 41.4 is provided on the inner peripheral edge of the carrier plate 39.
A conductive ring 25 connected to 1 may be provided.

In addition, in the third embodiment described above, the ball 38 was used as an example of the workpiece, but it can also be applied to a spherical roller as shown in FIG. 6(f), and the cross-sectional shape of the groove 37 can be changed accordingly Form.

In each of the first to third embodiments described above, a planar polishing device in which disk bodies are vertically opposed is illustrated and explained, but the same applies to a horizontal polishing device in which disk bodies are opposed in the left-right direction. Applicable to

〔Effect of the invention〕

As explained above, according to the present invention, in-process electrolytic dressing is enabled in a carrier plate-type and in-feed type polishing device using a super-abrasive metal bonded grindstone, so that a large number of workpieces can be processed. In addition to holding the grinding wheel with a carrier plate and simultaneously grinding it in the infeed, if the grinding wheel becomes dull due to the protruding amount of the abrasive grains becoming small or clogging, etc., the grinding process should be carried out. Electrolytic dressing can be performed continuously for an appropriate amount of time without interruption, and a large number of highly accurate and uniform workpieces can be produced in a short period of time, improving productivity. , it is possible to reduce the cost of the workpiece and further improve workability.

In addition, in high-precision polishing, especially ramp machining, shape transfer based on the maternal principle is one of the important factors for achieving high precision, and the accuracy of the polishing machine (lap machine) may change as the process progresses. Are known.

Another effect newly discovered with this invention is that it is now possible to control the sharpness without changing the delicate contact state between the workpiece and the polishing machine (lap machine) during machining. However, the accuracy of the above-mentioned lapping machine can be controlled in-process, resulting in a significantly improved product accuracy.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of a carrier plate type/infeed type polishing device capable of in-process electrolytic dressing using a super-abrasive metal bonded grindstone according to the present invention, taken along line 1-1 in FIG. 2 is a cutaway plan view taken along the line ■-■ in FIG. 1; FIG. 3 is a cutaway plan view taken along the line ■-■ in FIG. 4 showing the second embodiment; FIG. FIG. 5 is a cutaway front view taken along line IV-IV in the figure, FIG. 5 is a cutaway front view similar to FIG. 1 showing the third embodiment, and FIG.
a) to (f) are perspective views showing various workpieces, respectively;
FIG. 7 is a cutaway front view showing an example of a conventional carrier plate type/infeed type polishing device, and FIG. 8 is a front view showing an example of a conventional polishing device capable of in-process electrolytic dressing. l...base plate, 2,35...grinding wheel, 3.36...
Disc body, 4.29--Workpiece, 18.30a, 30
b. 39...Carrier plate, 19.25.32...
Conductive ring, 20.33.41... Electrode, 21.22
... Power supply brush, 23... Switch, 24... Power supply, 27... Pressure board, 28... Sun gear, 31...
・Internal tooth gear, 37...Groove, 38...Ball, 40・
... Spherical part.

Claims (3)

[Claims] (1) A disk-shaped base made of a conductive material and a disk-shaped grindstone made of super-abrasive metal bond are bonded together to form a disk, and the functions of the grinding wheels of the two disks are disposed with their surfaces facing each other, at least one of the two disc bodies is rotatable, and at least one of the two disc bodies is movable in the direction of the rotation axis; A carrier plate holding a plurality of workpieces is disposed between the two disk bodies, and the one disk body is moved in the direction of the rotation axis to pressurize the workpiece with the grindstone, and the carrier plate is
A carrier plate using a super-abrasive metal-bonded grindstone that is arranged so as to be able to rotate relative to each other and that is capable of relative rotation with a disc body, and that continuously grinds the plurality of workpieces held by the carrier plate for a predetermined period of time. In the die-cum-infeed type polishing device, an electrolytic dressing plate is provided at each position on both sides of the carrier plate where the workpiece is not held, and faces each of the grinding wheels of the two disk bodies at a predetermined interval. A carrier plate type and in-process electrolytic dressing capable of in-process electrolytic dressing using a super-abrasive metal bonded grindstone, which is equipped with an electrode and is provided with a current supply means for supplying a direct current between the electrode and the grindstone disk. Infeed type polishing equipment. (2) Working surface of the grinding wheel of a disk body formed by bonding a disk-shaped base made of a conductive material and a disk-shaped grinding wheel made of superabrasive metal bond, and the abrasive-free wear resistance disc-shaped pressure plates made of material are arranged to face each other, the disc body is rotatable, and the pressure plate is movable in the direction of the rotation axis; A carrier plate holding a plurality of workpieces is disposed between them, and the pressurizing plate is moved in the direction of the rotation axis to pressurize the workpieces by the grinding wheel and the pressurizing plate, and the carrier plate The disk body and the pressure plate are arranged so as to be able to rotate relative to each other, and a super-abrasive metal bond grindstone is used to continuously grind the plurality of workpieces held by the carrier plate for a predetermined period of time. In a carrier plate type and infeed type polishing apparatus capable of in-process electrolytic dressing, the grinding wheel is placed at a position where the plurality of workpieces are not held on a surface of the carrier plate that faces the working surface of the grinding wheel. A superabrasive metal bonded grinding wheel, characterized in that it is equipped with an electrode for electrolytic dressing that faces the working surface of the wheel at a predetermined distance, and is provided with current supply means for supplying a direct current between the electrode and the grinding wheel. Carrier plate type and infeed type polishing equipment capable of in-process electrolytic dressing using. (3) A groove with a curved cross section is formed on the circumference of the working surface of the grinding wheel of the two opposing disc bodies, and an electrode for electrolytic dressing is provided on the carrier plate facing the curved shape of the groove. 2. A carrier plate type and infeed type polishing apparatus capable of in-process electrolytic dressing using the superabrasive metal bonded grindstone according to claim 1, wherein said superabrasive metal bonded grindstone is mounted at a predetermined interval.