xiehe model

Introduction to mirror polishing skills

With superb polishing techniques, SUS304 parts turn into dazzling mirrors in minutes.

The polishing of precision parts usually uses whetstone strips, wool wheels, sandpaper, etc., to plastically deform the surface of the material, remove the protrusions on the surface of the workpiece, and obtain a smooth surface. Manual operation is generally the primary method. Ultra-fine polishing methods are available for high surface quality requirements. Ultra-fine polishing uses a special abrasive tool that is pressed against the machined surface of the workpiece in a polishing fluid containing abrasives for high-speed rotation. Polishing can reach a surface roughness of Ra0.008μm.

Polishing tools and specifications

1) The commonly used tools for mold polishing are: sandpaper, whetstone, tape felt wheel, grinding paste, alloy file, diamond grinding needle, bamboo chip, fiber whetstone, circular grinder, etc.

2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#, 1200#, 1500#

3) Whetstone: 120#, 220#, 400#, 600#

4) With felt wheel: cylindrical, conical, square tip

5) Grinding paste: 1# white, 3# yellow, 6# orange, 9# green, 15# blue, 25# brown, 35# red, 60# purple

6) File: square, round, flat, triangular and other shapes

7) Diamond grinding needle: generally 3/32 shank or 1/8 shank, with round wave shape, cylindrical shape, long straight cylindrical shape and long conical shape.

8) Bamboo: Various shapes are suitable for the operator and the shape of the mold. The function is to grind the sandpaper on the workpiece to achieve the required surface roughness.

9) Fiber whetstone: 200# black, 400# blue, 600# white, 800# red

Polishing Process

(1) Rough polishing

The surface after finishing milling, EDM, grinding, etc. can be polished with a rotating surface polishing machine with a speed of 35000-40000r/min. Then there is manual whetstone grinding, in which kerosene is added as a lubricant or coolant. The order of use is:

180#→240#→320#→400#→600#→800#→1000#.

(2) Semi-precision polishing

Semi-fine polishing mainly uses sandpaper and kerosene. The sandpaper numbers are as follows:

400#→600#→800#→1000#→1200#→1500#.

In fact, #1500 sandpaper only uses die steel suitable for quenching (above 52HRC), not pre-hardened steel, because this may damage the surface of the pre-hardened steel and not achieve the desired polishing effect.

(3) Fine polishing

Diamond polishing paste is mainly used for fine polishing. If the polishing pad is mixed with diamond powder or paste, the usual grinding sequence is:

9μm(1800#)→6μm(3000#)→3μm(8000#)

1. Hair-like wear marks left by 200# and 1500# sandpaper can be removed with 9μm diamond abrasive paste and polishing cloth wheel. Then polish with sticky felt and diamond paste in the following order:

1μm(14000#)→1/2μm(60000#)→1/4μm(100000#)

(4) Polished working environment

The polishing process should be carried out separately in two working positions, that is, the rough grinding position and the fine polishing position are separated, and attention should be paid to cleaning the sand particles remaining on the surface of the workpiece in the previous process.

Generally, after rough polishing with whetstone to 1200# sandpaper, the workpiece needs to be transferred to a clean room for polishing to ensure that no dust particles in the air adhere to the surface of the mold. The polishing process whose accuracy is required to be above 1μm (including 1μm) can be carried out in a clean polishing chamber. For more precise polishing, it must be in an absolutely clean space, as dust, smoke, dandruff and saliva may scratch the surface of the high-precision polishing.

After the polishing process is completed, the surface of the workpiece must be dust-proofed. When the polishing process is stopped, all abrasives and lubricants should be carefully removed to ensure that the workpiece surface is clean, and then a mold anti-rust coating should be applied to the workpiece surface.

Factors Affecting Surface Polishing

(1) Surface condition of workpiece

During the processing of the material, the surface layer will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after machining or heat treatment. Therefore, EDM should be carried out before the end of EDM, otherwise a hardened thin layer will form on the surface. If the EDM fine-tuning rules are not selected properly, the depth of the heat-affected layer can reach 0.4mm. The hardened thin layer is harder than the substrate and must be removed. Therefore, it is best to add a rough grinding process to provide a good foundation for polishing.

(2) Steel quality

High-quality steel is the premise of good polishing quality, and various inclusions and pores in the steel will affect the polishing effect. In order to achieve a good polishing effect, the workpiece must be marked with the polished surface roughness at the beginning of processing. When a workpiece is determined to need mirror polishing, it must be steel with good polishing performance and undergo heat treatment, otherwise the expected effect will not be achieved.

(3) Heat treatment process

If the heat treatment is improper, the surface hardness of the steel is not uniform or the characteristics are different, which will cause polishing difficulties.

(4) Polishing technology

Since polishing is mainly done by hand, human skills are still the main factor affecting the quality of polishing.

It is generally believed that the polishing technique affects the surface roughness. In fact, good polishing technology must be matched with high-quality steel and correct heat treatment process to obtain satisfactory polishing effect. On the contrary, if polishing technology is not good, no matter how good the steel is, it will not achieve the mirror effect.

How to solve common problems in polishing

(1) Excessive polishing

The biggest problem encountered in the daily polishing process is "over-polishing", that is, the longer the polishing time, the worse the mold surface quality. There are two phenomena of over-polishing: "orange peel" and "pitting". Over-polishing mostly occurs in mechanical polishing.

(2) Reasons for "orange peel" in the workpiece

An irregular and rough surface is called "orange peel". There are many reasons for "orange peel". The most common cause is overheating or carburizing of the mold surface. The polishing pressure is too large and the polishing time is too long. For example: polishing with a polishing wheel, the heat generated by the polishing wheel can easily cause "orange peel". Harder steels can withstand higher polishing pressures, while relatively soft steels are prone to overpolishing. Studies have shown that the time to overpolish varies with the hardness of the steel.

(3) Measures to eliminate the "orange peel" of cultural relics

When it is found that the surface quality is not good for polishing, many people will increase the polishing pressure and prolong the polishing time. This practice tends to deteriorate the surface quality.

You can use the following methods to remedy:

1) Remove the defective surface, the abrasive grain is slightly coarser than the previous one, and then grind, the polishing force is lower than the previous one.

2) Relieve stress at a temperature 25°C lower than the tempering temperature. Grind with the finest grit size before polishing until a satisfactory result is achieved, and finally polish with a lighter force.

(4) Causes of "pitting" on the surface of the workpiece

Because the steel contains some non-metallic impurities, usually hard and brittle oxides, they are pulled from the steel surface during the polishing process, forming pits or pitting corrosion. The main factors that produce "pitting" are as follows:

1) The polishing pressure is too high and the polishing time is too long.

2) The purity of the steel is not enough, and the content of hard impurities is high.

3) The mold surface is rusted.

4) The black leather is not removed.

(5) Measures to eliminate the pitting corrosion of the workpiece.

1) Carefully regrind the surface. The grain size of the sand is one grade coarser than that used previously. Use a soft, sharp grindstone for the final grinding step before the polishing process.

2) When the grit size is less than 1mm, avoid using the softest polishing tools.

3) Use the shortest polishing time and minimum polishing intensity as possible.