xiehe model

With superb polishing techniques, a SUS becomes a dazzling mirror in minutes.

Polishing of precision parts usually uses whetstone strips, wool wheels, sandpaper, etc. to plastically deform the surface of the material and remove the protrusions on the surface of the workpiece to obtain a smooth surface. Manual operation is generally the primary method. The ultra-fine polishing method can be used for those with high surface quality requirements. The ultra-fine polishing adopts a special abrasive tool, which presses the machined surface of the workpiece in a polishing liquid containing abrasives for high-speed rotation. Polishing can reach a surface roughness of Ra0.008 μm.

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, and rotary grinder.

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

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

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

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, long conical shape.

8) Bamboo: Various shapes are suitable for the shape of the operator and the mold. The function is to press the sandpaper on the workpiece for grinding to achieve the desired 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 by a rotary surface polishing machine with a speed of 35000-40000r/min. Then comes 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-precision polishing mainly uses sandpaper and kerosene. The amount of sandpaper is as follows:

400#→600#→800#→1000#→1200#→1500#.

In fact, #1500 sandpaper only uses die steel that is suitable for hardening (above 52HRC), not pre-hardened steel, as this may damage the surface of the pre-hardened steel part and fail to achieve the desired finish.

(3) Fine polishing

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

9μm(1800#)→6μm(3000#)→3μm(8000#)

1. The hairy scratches 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) 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 is completed 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 with precision requirements 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-proof. When the polishing process is stopped, all abrasives and lubricants should be carefully removed to ensure that the surface of the workpiece is clean, and then a mold anti-rust coating should be applied to the surface of the workpiece.

(4) Polishing technology

Since polishing is mainly done manually, human skills are still the main reason for the quality of polishing.

It is generally believed that the polishing technique affects the surface roughness. In fact, good polishing techniques must be combined with high-quality steel and the correct heat treatment process to obtain satisfactory polishing results. Conversely, if the polishing technique is not good, even if the steel is good, the mirror effect will not be achieved.

(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 types of over-polishing: "orange peel" and "pitting." Overpolishing mainly occurs in mechanical polishing.

(2) The reason for the "orange peel" of the workpiece

The irregular 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. The main cause of "orange peel". 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 softer 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 polished well, many people will increase the polishing pressure and prolong the polishing time. This practice usually results in poor surface quality.

You can use the following methods to remedy:

1) Remove the defective surface, the particle size is slightly coarser than the last time, and then grind, and the polishing force is lower than the last time.

2) Stress relief is carried out 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

Due to the presence of some non-metallic impurities in the steel, usually hard and brittle oxides, they are pulled from the steel surface during the polishing process to form pitting 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 Pitting Corrosion of Workpieces

1) Carefully re-sand the surface. The grit size is one grade coarser than previously used. Use a soft, sharp whetstone 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.