TECHNICAL FIELD

The present invention relates to a spherical lens surface processing method and a spherical lens surface processing apparatus in which a spherical lens surface is ground with a cup-shaped grinding stone.

BACKGROUND ART

Glass lenses are commonly manufactured through the steps of rough grinding (crude rubbing), precision grinding, polishing, and centering, and the rough grinding and precision grinding involve the use of different processing apparatuses and different grinding stones. For example, in the rough grinding of the processing of a spherical lens surface, curved surface processing is performed on the lens surface of a lens material by a curve generator (CG machine), using a diamond wheel or another cup-shaped grinding stone. In the subsequent precision grinding, the processing is performed by a sphere-center-type processing apparatus using a diamond pellet plate or another plate-shaped grinding stone, and the lens material is finished to a lens having the necessary surface accuracy and center thickness.

To minimize changes in plate-shaped grinding stones for precision grinding and reduce the amount of processing done with plate-shaped grinding stones according to recent requirements to improve lens processing precision, to reduce processing time, etc., the shape after rough grinding with a CG machine has needed to be closer to a perfect sphere, surface roughness has needed to be lessened, thickness (of a center part after both lens surfaces have been processed) has needed to be kept fixed, the optical axes of both lens surfaces have needed to be in alignment, etc.

However, it is extremely difficult to make a processed curved surface into a perfect sphere with a CG machine. This matter is described with reference toFIGS. 5A and 5B, which show the principle of processing by a prior-art CG machine.

A lens105A (105B) is fixed to and held in a rotating chuck104, and with the lens tilted at an incline angle θa (θb) in relation to a lens rotation axis113, the lens moves in the direction A of a rotating cup-shaped grinding stone109A (109B), and cut-processing is performed. The incline angle θa (θb) is determined with the following formula, involving the spherical surface radius R of the lens105A (105B) being processed, and the contact diameter φT of the cup-shaped grinding stone109A (109B) and the lens105A (105B).
sin θa=φT½R
sin θb=φT½R

The point at which the lens processed surface105a(105b) becomes a perfect sphere is the point at which the point where the cup-shaped grinding stone109A (109B) makes contact with the lens105A (105B), perfectly aligns with the lens center P2. When the center is offset even slightly, depressions and protrusions are produced in the center of the processed lens105A (105B), and the lens will not be a perfect sphere. Consequently, a mechanism is provided for moving the cup-shaped grinding stone109A (109B) back and forth so as to align with this point, and this mechanism is used to make adjustments.

However, a high level of technology and experience are needed to correct positional offsetting caused by wear of the cup-shaped grinding stone109A (109B). This is because the effects of wear in the cup-shaped grinding stone109A (109B) show in both the radius and the shape of the created lens surface. Additionally, the worn tip end shape of the cup-shaped grinding stone109A (109B) cannot be specified, and because the curved surface shape created in the lens surface is not a spherical surface, it is not viable to calculate the contact diameter φT of the lens105A (105B) and the cup-shaped grinding stone109A (109B) for calculating the incline angle θa (θb) anew. Consequently, the incline angle θa (θb) and the longitudinal position of the cup-shaped grinding stone109A (109B) must continue to be adeptly adjusted in accordance with the wear of the cup-shaped grinding stone109A (109B), on the basis of the experience of a skilled worker.

The surface roughness is affected by the lens material and the grinding stone material, but is primarily determined by the apparatus mechanisms. The lens is pushed at a fixed speed against the rotating cup-shaped grinding stone while being held by a chuck and forcibly rotated. When a rotational speed or pushing speed exceeding the cutting performance of the cup-shaped grinding stone is reached, slight positional offsetting occurs due to flexure of the apparatus or the chuck. The amount by which the cup-shaped grinding stone digs into the lens thereby changes, and a chrysanthemum pattern referred to as tool marks is therefore produced in the lens surface as a result. There is also displacement of the lens during forced rotation, and undulation occurs in the processed surface. Grinding with zero depth of cut, an action referred to as sparking-out, is performed at the moving end of the lens in order to reduce tool marks and undulation, but the cup-shaped grinding stone digs in and deeply cut portions cannot be eliminated.

It is also further difficult to keep the thickness fixed or to bring the optical axes into alignment. Because the lens is held by a chuck, the lens outer periphery is the reference for chucking. Because the chucking position changes when there is strain in the lens outer periphery, the rotational center in a chucked state does not align between the already processed surface and the yet-to-be-processed surface, and the lens cannot be held at a right angle to the rotational axis of the chuck.

There is also a restriction on the contact diameter between the cup-shaped grinding stone and lens used. Referring toFIGS. 5A and 5B, the mechanisms of the apparatus are also a factor, but generally, the maximum angle of the incline angle θa (θb) of the cup-shaped grinding stone109A (109B) is approximately 45°. Therefore, for the cup-shaped grinding stone109A (109B) that can be used, the contact diameter φT with the lens105A,105B is limited to the range of the following formula. L1in this formula represents the chord length of an arc from the lens center P2to the outer peripheral end edge in the lens processed surface105a(105b) that is being processed.
1.4×processing radius>contact diameter φT>L1

To avoid the undesirable effects described above, processing the lens material (cut material, pressed material) by means of a sphere-center-type processing apparatus using a plate-shaped grinding stone from the beginning has been considered. However, in this case, the lens material partially comes into contact with the plate-shaped grinding stone at the start of processing. As a result, the periphery of the lens material will get chipped, the plate-shaped grinding stone will wear in parts, the plate-shaped grinding will not have a stable shape, and the precision with which the spherical lens surface is processed will be unstable.

The purposes of processing using a prior-art plate-shaped grinding stone are to improve the precision of curvature in the lens surface, to establish the thickness of the lens center, and to improve surface roughness. Therefore, the plate-shaped grinding stone used is finely textured, and the cut amount per unit time is lessened. When such a finely textured plate-shaped grinding stone is used for processing starting with a lens material, the cut amount is greater and the processing time is therefore longer, which is impractical.

Various structures of sphere-center-type processing apparatuses are known. Patent Document 1 proposes a lens processing apparatus that can move a grinding stone in various configurations including sphere center oscillation, without the use of a cam mechanism.

PRIOR ART LITERATURE

Patent Documents

Patent Document 1: JP-A 2009-178834

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

Thus, in the prior art, spherical lens surfaces are processed using different processing machines and different grinding stones. To achieve the necessary surface precision and center thickness, processing machines are also adjusted according to the experience and intuition of a skilled practitioner.

An object of the present invention is to provide a spherical lens surface processing method and a spherical lens surface processing apparatus with which a spherical lens surface can be processed with a high degree of precision using one processing machine and one type of grinding stone.

Means of Solving the Problems

To solve the above problems, a spherical lens surface processing method of the present invention including the steps of:

forming a contact state in which a rotating cup-shaped grinding stone is brought into contact at a predetermined pressure with a lens surface of a glass lens to be processed;

while maintaining the contact state, forming a state of sphere center oscillation in which the cup-shaped grinding stone oscillates along the lens surface centered on a sphere center as an oscillation center, and grinding the lens surface to a spherical surface having a predetermined surface precision and center thickness;

wherein, in the sphere center oscillation,

a distance from the oscillation center to a point where the cup-shaped grinding stone makes contact with the lens surface is set to be equal to the radius of the sphere surface; and

an oscillation width of the sphere center oscillation is set so that the point where the cup-shaped grinding stone makes contact with the lens surface moves past the lens center on the lens surface, from one outer peripheral edge side to the other outer peripheral edge side of the lens surface.

According to the present invention, the lens surface is processed to a spherical surface while the cup-shaped grinding stone is caused to undergo sphere center oscillation and the point where the cup-shaped grinding stone makes contact with the lens surface is moved reciprocatingly past the lens center along the lens surface. Due to this configuration, it is possible to eliminate depressions, protrusions, etc., produced in the lens center when spherical surface processing is performed by a CG machine using the cup-shaped grinding stone, and to process the lens surface to a perfectly spherical state. There is also no need to perform rough grinding with a CG machine in advance, as in the case of using a plate-shaped grinding stone.

According to the present invention, grinding time can be reduced to a much greater extent than with processing a spherical surface in a lens from the start using a plate-shaped grinding stone. Furthermore, when a plate-shaped grinding stone is used, a problem arises in that the lens material will come into partial contact with the plate-shaped grinding stone at the start of processing, the periphery of the lens material will get chipped, the plate-shaped grinding stone will be worn in parts, the shape of the plate-shaped grinding stone will be unstable, and the precision with which the spherical lens surface is processed will be unstable. Such problems can be resolved.

Thus, in the method of the present invention, a spherical lens surface is processed with novel use made of a combination of a cup-shaped grinding stone and sphere center oscillation, on which there had been no focus in the prior art. Processing a spherical lens surface in the prior art has been performed in two steps: rough grinding and precision grinding. Additionally, rough grinding has been performed by a curve generator (CG machine) using a cup-shaped grinding stone, and the subsequent precision grinding has been performed by a sphere-center-type processing apparatus using a plate-shaped grinding stone, to obtain a spherical lens surface having the necessary surface precision and center thickness. According to experimentation by the present inventors, it has been confirmed that a spherical lens surface can be processed with a precision equal to or greater than that of spherical lens surface processing in the prior art, by one sphere center oscillation-type processing apparatus using one type of grinding stone (a cup-shaped grinding stone).

Furthermore, according to the present invention, the oscillation width of the sphere center oscillation is set so that the point where the cup-shaped grinding stone makes contact with the lens surface moves from one outer peripheral edge to the other outer peripheral edge of the lens surface, past the lens center on the lens surface. In other words, the oscillation width of the cup-shaped grinding stone is changed in accordance with the size of the cup-shaped grinding stone, and the point where the cup-shaped grinding stone makes contact with the lens surface can be moved from the outer periphery of the lens surface, along the lens surface, to a position past the lens center. It is thereby possible to use cup-shaped grinding stones of various sizes.

In the spherical lens surface processing method of the present invention:

the lens is forcibly rotated at a lesser speed than the cup-shaped grinding stone; and

the forced rotation state is ceased when the torque exerted on the lens by the frictional force between the lens surface and the sphere-center-oscillating cup-shaped grinding stone creates a passively rotatable state in which the lens can rotate passively following the cup-shaped grinding stone at a speed greater than the forced rotation speed.

For example, in cases such as when the lens surface is processed from a flat surface to a concave spherical surface, there are cases in which the torque needed for dependent rotation may not be achieved due to the state in which the cup-shaped grinding stone is in contact with the lens at the start of grinding. In the present invention, the lens is forcibly rotated, and is switched to dependent rotation at the point in time when the torque needed for dependent rotation is achieved. The cup-shaped grinding stone can thereby be reliably prevented from digging into the lens; therefore, the processing roughness of the lens surface can be improved, and undulation in the lens surface can be prevented.

In the spherical lens surface processing method of the present invention, preferably:

the lens, having been brought into contact with the cup-shaped grinding stone, is supported by an elastic stretching member; and

the cup-shaped grinding stone and the lens are brought into contact by the elastic force produced by the stretching of the elastic stretching member.

To eliminate tool marks caused by the cup-shaped grinding stone digging into the lens, the lens is preferably held so that excessive pushing force is not generated between the lens surface and the cup-shaped grinding stone. I n the present invention, the lens is supported using the elastic stretching member, and excessive force generated between the lens and the cup-shaped grinding stone can be released by the elastic deformation of the elastic stretching member. It is thereby possible to prevent tool marks from being produced.

Next, in the spherical lens surface processing method of the present invention:

the lens is preferably held with vacuum suction by a lens holder in order to stabilize the lens thickness and align the optical axes of spherical surfaces processed on both surfaces of the lens.

It is thereby possible to obtain a spherical lens surface in which, after one lens surface has been processed to a spherical surface, the processing standard is already processed in the processing of the other lens surface. Consequently, it is possible to accurately detect the centers of both lens surfaces, and also the distance from the center of one lens surface to the center of the other lens surface, and it is therefore possible to align the optical axes and to stabilize the thickness.

Next, the spherical lens surface processing apparatus of the present invention for performing spherical lens surface processing according to the above-described method, the spherical lens surface processing apparatus including:

a cup-shaped grinding stone;

a grinding stone rotation mechanism that rotates the cup-shaped grinding stone about a central axis;

a lens holder that holds a lens to be processed;

a lens movement mechanism that moves a lens held in the lens holder so that a lens surface of the lens moves in directions towards and away from the cup-shaped grinding stone;

a sphere center oscillation mechanism that causes the cup-shaped grinding stone to oscillate centered on a spherical center as an oscillation center along the lens surface of the lens held in the lens holder; and

a controller that controls the grinding stone rotation mechanism, the lens movement mechanism, and the sphere center oscillation mechanism.

The controller is characterized by:

forming a contact state in which the rotating cup-shaped grinding stone is brought into contact with the lens surface at a predetermined pressure; and

while the contact state is maintained, forming a sphere center oscillation state in which the cup-shaped grinding stone oscillates along the lens surface centered on a sphere center as an oscillation center, to grind the lens surface to a spherical surface having a predetermined surface precision and center thickness,

wherein, in the sphere center oscillation state, a distance from the oscillation center of the sphere center oscillation to the point where the cup-shaped grinding stone makes contact with the lens surface is set to be equal to the radius of the spherical surface; and

an oscillation width of the sphere center oscillation is set so that the point where the cup-shaped grinding stone makes contact with the lens surface moves past a lens center on the lens surface, from one outer peripheral edge to the other outer peripheral edge of the lens surface.

The spherical lens surface processing apparatus of the present invention preferably has, in addition to the configuration described above, a forced rotation mechanism that forcibly rotates the lens holder about a central axis thereof, and a one-way clutch capable of ceasing the forced rotation caused by the forced rotation mechanism. In this case, the controller forcibly causes the lens to rotate at a lesser speed than the cup-shaped grinding stone, and the one-way clutch is set so as to cease the forced rotation state when the torque exerted on the lens by the frictional force between the lens surface and the sphere-center-oscillating cup-shaped grinding stone creates a passively rotatable state in which the lens can rotate passively following the cup-shaped grinding stone at a speed greater than the forced rotation speed.

The spherical lens surface processing apparatus of the present invention preferably has, in addition to the configuration described above, an elastic stretching member that supports the lens holder from the direction along the holder central axis, and brings the lens surface of the lens held in the lens holder into contact with the cup-shaped grinding stone at a predetermined force. The elastic force produced by the stretching of the elastic stretching member becomes a contact force with which the cup-shaped grinding stone is brought into contact with the lens surface.

The spherical lens surface processing apparatus of the present invention preferably has a vacuum suction-holding mechanism in addition to the configuration described above, and the lens holder is preferably designed to hold the lens through vacuum suction-holding force provided by the vacuum suction-holding mechanism.

