Optical basis: fine grinding (sand hanging)

① Reduce the depth of the uneven layer on the surface of the part and improve its uniformity;

② Further improve the surface shape accuracy and related dimensional accuracy of the part surface to meet the requirements of polishing for the surface structure and shape of the part.

method:

Fine grinding of loose abrasives and fixed abrasives.

1. Fine grinding of loose abrasives

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The fine grinding of optical parts generally uses two sands, that is, two abrasives with different particle sizes.

If the abrasive grain size of the first sand is W28, the abrasive grain size of the second sand is W14.

Some special parts require three or four sands, and the abrasive grain size can be W28, W14, W10, W7, etc. The fine grinding process factors mainly include the following points:

①The relative size of fine grinding mold and mirror plate;

②The swing range of the thimble;

③The front and back extension of the thimble;

④ Spindle revolution and thimble swing speed.

2. Diamond high speed fine grinding

1. Diamond high-speed precision grinding tools

Diamond high-speed fine grinding abrasives are made of diamond fine grinding discs, which are glued to the base of the abrasives in a certain arrangement and made by grinding.

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(1) The form of spherical diamond fine grinding mold is either convex fine grinding mold or concave fine grinding mold. All are composed of a fine grinding sheet, a binder and an abrasive base.

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(2) The radius of curvature of the spherical diamond fine grinding mold is the nominal size of the radius of curvature of the processed spherical surface after polishing. And according to the aperture after fine grinding a few apertures lower than the aperture after polishing to trim the radius of curvature.

Generally, 3~5N is reserved for high-speed fine grinding, and 5-10N is reserved for fine grinding of loose abrasives.

Convex spherical surface: Rcm>Rtm>Rjm1>Rjm2>Rpm

Concave spherical surface: Rcm<><><><>

(3) The relative position and size of the fine grinding mold and the mirror plate

1. The relative position of the fine grinding mold and the mirror plate is finely ground with loose abrasives, and the convex surface is installed on the spindle of the machine tool. Diamond high-speed fine grinding, the diamond grinding tool is installed on the spindle of the machine tool.

2. The relative size of the fine grinding mold and the mirror plate

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3. The base curvature radius of the fine grinding mold

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In the formula, the punch takes the "-" sign, and the concave die takes the "+" sign.

Second, the selection of the characteristic parameters of the fine grinding disc

(1) The choice of particle size In high-speed precision grinding, diamond abrasive tools are required to have both higher grinding efficiency and better surface roughness.

For the first fine grinding, the appropriate diamond grinding disc size is W28 or W20;

For the second fine grinding, W10 or W7 is appropriate.

If only one fine grinding is used, W14 is appropriate.

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(2) The choice of concentration. Too low or too high concentration is unfavorable for the efficiency and quality of the fine grinding. The most suitable concentration is not uniform at home and abroad. Domestically, it is 30-50%; internationally, it is 45%.

(3) Bonding agent for grinding discs In high-speed precision grinding, metal bonding agents are widely used at home and abroad, including bronze bonding agents and steel bonding agents. Bronze bonding agent is mainly used in China.

(4) The shape and size of the fine grinding sheet

The shape of the polished disc: generally round, but also rectangular abroad (Soviet Union). The circle has three forms: flat, convex and concave.

The size of the grinding disc: mainly refers to the diameter and thickness of the grinding disc. In addition, there are diamond layer thickness and radius of curvature.

The diameter of the fine grinding plate: depends on the radius of curvature, the diameter of the fine grinding mold and the diameter of the mirror disc.

The thickness of the fine grinding disc: mainly depends on the diameter of the fine grinding disc.

(5) Choice of coverage ratio

Coverage ratio: refers to the ratio of the sum of the surface area of the fine grinding plate arranged on the abrasive tool to the surface area of the entire ball of the abrasive tool.

Z—the number of fine grinding pieces; Ap—the area of each fine grinding piece; AW—the surface area of the grinding tool ball.

The choice of coverage ratio mainly depends on the radius of curvature of the abrasive tool.

(6) The arrangement of the fine grinding discs should meet the following requirements: to ensure the shape stability of the grinding tools; no dead spots should be produced, and the coolant should be unimpeded to facilitate heat dissipation and chip removal.

 1. Arranged in concentric circles

There are two situations: one is that the two adjacent fine grinding discs are arranged staggered on each concentric circle, that is, there is overlap in the radial direction; the other is that the clearances of the fine grinding discs arranged on different circles are not equal.

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2. Spiral arrangement

3. The ideal abrasion law of spherical diamond mold. Wear ratio: the ratio of mold wear to glass and the ratio of mold wear to glass removal. For cast iron and glass, the abrasion ratio is 1:100; for diamond grinding discs, it is 1:3000.

1. The relationship between the amount of wear of diamond abrasive discs and friction work The wear of diamond abrasive discs is mainly the result of mechanical action, that is, the wear, breakage, shedding of diamond particles and the wear of the bonding agent.

Experiments show that:

In a certain pressure range, the wear of the grinding disc is proportional to the pressure;

In a certain speed range, the wear is proportional to the relative grinding speed;

When the original surface roughness of the glass is the same, the wear amount of the grinding disc is proportional to the effective grinding time.

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In the ideal wear situation, the cosine relationship between the wear of each point on the surface of the abrasive tool, so we call the ideal wear law as the cosine wear law.

How to achieve ideal wear? There are two main ways:

(1) Design the abrasive tool under the given speed and pressure distribution, so that the arrangement and distribution of the abrasive discs meet the law of cosine wear;

(2) When the abrasive tool has been set, select an appropriate speed and pressure distribution to match the abrasive tool, so as to achieve the purpose of cosine wear.

4. Coolant in diamond high-speed fine grinding

Diamond high-speed precision grinding coolant has four major functions: cooling, lubrication, cleaning and chemical action.

One, cooling effect

Cooling effect: refers to the ability of the coolant to absorb and take away heat when it flows through the surface of the abrasive tool and the workpiece.

Among all liquids, the specific heat of water is the largest, which is 2 to 3 times that of oils; when boiling at standard atmospheric pressure, the heat of evaporation of water is 7 to 15 times that of oils; the thermal conductivity of water is that of oils 3 to 5 times.

2. Lubrication

Lubrication: refers to its ability to reduce the friction between the diamond abrasive tool and the contact surface of the workpiece, that is, the ability to reduce the grinding resistance and abrasive wear.

Three, cleaning effect

Cleaning function: mainly refers to its ability to remove glass chips and abrasive chips produced in processing in time.

Four, chemical action

At present, the coolant widely used in high-speed fine grinding at home and abroad is mainly water, and then a small amount of triethanolamine, glycerin, nickel sulfate, etc. are added to form triethanolamine-water, glycerin-water, triethanolamine-glycerin-water, etc. Water-soluble coolant.

1. The chemical self-sharpening effect of coolant on diamond abrasive tools

The bonding agent copper in the diamond fine grinding disc can form two basic copper carbonates with carbon dioxide, oxygen in the air and water in the coolant: one is called patina [CuCO3·Cu(OH)2·H2O], and the other The species is called blue azurite [2CuCO3·Cu(OH)2]. ,

2. The absorption of carbon dioxide by coolant

After the glass is hydrolyzed, in addition to the silicic acid gel film, sodium hydroxide is also generated. It reacts with carbon dioxide in the air to generate sodium carbonate. This kind of salt will corrode the glass surface.

5. The upper plate of the optical parts 1. Parallel plane upper plate

(1) Rosin beeswax glue loading method

method one:

First wipe the bonding mold clean, apply a layer of bonding glue after heating, and then put on the preheated work piece, add a certain pressure to squeeze out the excess glue.

Features: This method is often used in the rough grinding process of the upper plate and the fine grinding of the upper plate for parts that do not require high aperture parallelism.

Method Two:

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First wipe the bonding mold clean, then put the cleaned work piece, sprinkle the powdery glue on the gap of the part, heat the bonding mold, so that the bonding glue melts and penetrates from the edge of the part.

Advantages: easy to operate.

Disadvantages: The parallelism is not high, and it is not suitable for thin slices with high surface accuracy.

(2) Dispensing and placing method

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Heat the workpiece to a temperature that can stick the adhesive strip, and arrange the adhesive strips evenly according to the size of the workpiece. After cooling, stick it on the placement mold, then put the heated bonding mold, let it fall naturally on the positioning strips around the placement mold, and after natural cooling, pull down the mirror plate in the horizontal direction.

Advantages: better surface shape (N=0.5) and parallelism (θ=30') can be obtained

Disadvantages: Can not withstand high-pressure and high-speed processing conditions, fear of vibration, and require higher indoor temperature gradients.

(3) Floating glue loading method

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Clean the bonding pad first, and then put the cleaned parts on. If it is a polished surface, you can see a clear aperture, and then drop the adhesive into the gap between the parts. After cooling, scrape off the glue on the surface of the part to be processed.

Advantages: The workpiece and the bonding plate do not need to be heated, and the workpiece can achieve higher parallelism and surface accuracy (N≤0.5, θ≤10').

Disadvantages: Can not withstand high-speed and high-pressure processing conditions, easy to move around.

(4) Photo-plastic loading method

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After wiping the glossy rubber surface of the part, place it on the glossy rubber backing plate to show a clear aperture, and apply a little pressure from one side of the part. If there are bubbles or white spots on the surface of the glue, use an alcohol lamp to locally heat the work piece, remove the glue and clean it again, and then finish the glue again.

Advantages: Parallelism and flatness can reach very high precision (N<>, θ<>).

Disadvantages: It has a certain impact on the defect level of the photoresist surface; afraid of severe vibration, sudden cold and hot.

Second, the lens on the plate

(1) Flexible listing

First make the fire lacquer ball according to the lens shape, put the fire lacquer ball on the preheated lens, take it out after cooling, and then wipe the surface to be processed, according to the determined arrangement, paste the lens to the paste that has been coated with a thin layer of oil Put in the mold, put the heated bonding mold and wait for it to sink to the positioning parts on the surrounding edges, and take it out after cooling.

Features: Based on the processed surface, the bonding mold is simple and versatile.

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(2) Rigid hanging plate

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First heat the bonding mold to a temperature that can melt the bonding glue, stick the bonding paper or use the bonding glue stick to coat the fixed groove, and then stick the preheated workpiece on the bonding mold, and cool it. If the polished surface is bonded, it should be protected.

Features: Taking the bonding surface as the reference surface, the process of loading the disk is simple, the rough grinding can be processed into a disk, and it can withstand higher speed and pressure. The bonding mold is highly specific and the processing mold cost is high.

Three, prism hanging plate

(1) The elasticity method of clamping mold hanging plate:

After heating the parts, stick a fire paint strip on the bonding surface; after it cools, arrange it on the flat mold according to the position of the clamping groove, and place a limit strip with a thickness of about 2mm around the clamping mold. After melting the temperature of the lacquer, place it accurately on the arranged parts until the limit bar. Then let it cool naturally.

Rigidity method:

Just put the parts in the clamping mold groove and press it with screws; after heating the clamping mold, apply adhesive glue in the bonding groove, and put the preheated blank prism to cool.

Advantages: Simple operation on the plate.

Disadvantages: It is only suitable for parts with medium and low precision, and has certain limitations for parts with complex shapes.

(2) Plaster hanging plate

Wipe the surface to be processed on the placement mold coated with a thin layer of oil, and sprinkle about 2mm of paraffin wax on the gap between the prisms. Put a rubber band around the placement mold, put on the support mold, pour the adjusted gypsum slurry, and submerge the prism about 30mm. After the gypsum slurry has solidified, remove the rubber ring and pull it down from the placement mold in the horizontal direction , Melt off the surface paraffin, and then apply a layer of paraffin on the surface of the exposed plaster mold to prevent erosion.

Features: Strong versatility, low angle accuracy (θ>3'), anti-corrosion protection is required for the polished surface.

(3) Putting on the plate

Clean the photoresist tool and place it on the backing plate, and then stick the cleaned prism photoresist surface to the photoresist surface of the tool. At this time, clear interference fringes appear. Gently squeeze the air on the photoresist surface to remove the stripe It disappears and forms a photoresist. Then put the light plastic tool on the light plastic backing board, and finally coat all the gaps of the light plastic surface with protective paint.

Advantages: higher angle accuracy can be obtained.

Disadvantages: high precision of the photo-glue tool is required. For prisms with high requirements for surface defects, the photoresist surface has certain limitations; photoresist discs should not be subjected to severe vibration and sudden temperature changes.

(4) Hanging by the body

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Rely on the angle accuracy and parallelism of the body to ensure the accuracy of the workpiece. In fact, the photoresist hanging plate is also a kind of body hanging plate. Glue the workpiece to the backing body, and then fix the backing body to the flat mold.

Advantages: high processing accuracy; not easy to corrode the polished surface; no need to manually trim the angle of a single piece; after some workpieces are glued on the body, multiple surfaces can be processed continuously by turning the body.

Disadvantages: It requires high body precision and difficult manufacturing.

6. Process factors affecting fine grinding

1. Bulk abrasive processing

(1) The swing amplitude is the swing amplitude of the upper die. The larger the swing, the more grinding the middle of the upper mold and the edge of the lower mold. Therefore, the swing amplitude should be appropriate.

For flat molds, the swing distance of the upper mold is within the range of 0.45~0.65 of the diameter of the lower mold;

For the spherical mold, the upper mold swing angle is within the range of 0.4~0.55 of the lower mold opening angle 2γ.

(2) The deviation between the center of the upper mold and the center of the lower mold before and after the thimble; the greater the deviation, the more the center of the upper mold and the edge of the lower mold are ground.

For the plane, the deviation can be 0~0.1 of the swing amplitude,

For spherical surface, it is 0~0.4.

(3) The ratio of spindle speed to upper die swing speed is the ratio of spindle speed to eccentric wheel speed. The greater the spindle speed, the greater the grinding of the edge of the lower mold; the faster the eccentric wheel rotates, the faster the center part of the upper mold and the lower mold will be ground.

For the plane, the spindle speed is 0.4~0.8 times the swing amplitude;

For the spherical surface, the spindle speed is 1~2.5 times the swing amplitude.

(4) Matters needing attention

①The particle size of the abrasive should be uniform, which is one of the keys to precision and quality;

②Each time when adding abrasive, it should be evenly dispersed, and it is not suitable to add in a group;

③The abrasive suspension should not be too thin or too thick, otherwise it is easy to scratch;

④ When changing the abrasive, the workpiece and mirror disc should be scrubbed clean, and the machine table should also be wiped clean;

⑤After finishing the fine grinding, use a magnifying glass to check whether there are rough sand holes or scratches on the surface;

⑥ The radius of curvature or flatness of the polished surface should be checked frequently, and the mold should be trimmed in time when the change is found to be large.

(5) Common defects and their causes

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2. High-speed diamond abrasive processing

(1) Spindle speed of the machine tool The grinding amount of glass is proportional to the speed of the machine tool spindle.

(2) The grinding amount of the pressure glass of the mirror disc is proportional to the pressure of the mirror disc.

(3) The effect of processing time The grinding amount of glass is directly proportional to the processing time.

(4) The initial surface roughness of the workpiece The initial surface roughness of the workpiece affects the accuracy of high-speed fine grinding. The greater the roughness, the greater the amount of glass grinding, and the greater the roughness after fine grinding.

(5) Aperture matching Aperture matching refers to the difference between the radius of curvature of the processed lens disk and the final completed radius of curvature; in general, the lens disk is 3 to 4 apertures lower than the finished aperture.

(6) The coverage ratio of the diamond fine grinding plate is inversely proportional to the grinding volume. For large spherical surfaces, the coverage ratio is smaller, and for small spherical surfaces, the coverage ratio is larger.