Introduction to mirror polishing techniques for CNC processing of stainless steel parts
With exquisite polishing technology, SUS304 parts turn into dazzling mirrors in minutes.
Polishing of precision parts usually uses oilstone strips, wool wheels, sandpaper, etc. to cause plastic deformation on the surface of the material, remove protrusions on the surface of the workpiece, and obtain a smooth surface. Manual operation is generally the main method. Ultra-finish polishing methods can be used for high surface quality requirements. Super fine polishing uses a special abrasive tool that is pressed against the machined surface of the workpiece in a polishing fluid containing abrasives and rotates at high speed. Polishing can achieve a surface roughness of Ra0.008μm.
Common tools and specifications for polishing
1) Commonly used tools for mold polishing include: sandpaper, oilstone, tape felt wheel, abrasive paste, alloy file, diamond grinding needle, bamboo chip, fiber oilstone, circular grinder, etc.
2) Sandpaper: 150#, 180#, 320#, 400#, 600#, 800#, 1000#, 1200#, 1500#
3) Whetstone: 120#, 220#, 400#, 600#
4) 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 needles: Generally 3/32 shank or 1/8 shank, available in round corrugated, cylindrical, long straight cylindrical, long conical shapes.
8) Bamboo: Various shapes to suit the shape of the operator and the mold. Its function is to press the sandpaper on the workpiece and grind it to achieve the required surface roughness.
9) Fiber whetstone: 200# black, 400# blue, 600# white, 800# red
Stainless steel parts processing
Polishing Process
(1) Rough polishing
The surface after fine 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, and kerosene is added to the strip whetstone as a lubricant or coolant. The order of use is:
180#→240#→320#→400#→600#→800#→1000#.
(2) Semi-precision polishing
Semi-finishing polishing mainly uses sandpaper and kerosene. The sandpaper numbers are as follows:
400#→600#→800#→1000#→1200#→1500#.
In fact, #1500 sandpaper only uses mold steel suitable for quenching (above 52HRC), not pre-hardened steel, because this may damage the surface of the pre-hardened steel parts and fail to achieve the expected polishing effect.
(3) Fine polishing
Diamond polishing paste is mainly used for fine polishing. If the polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste for grinding, the usual grinding sequence is:
9μm(1800#)→6μm(3000#)→3μm(8000#)
1 The hair-like wear marks left by 200# and 150 0# sandpaper can be removed with 9μm diamond abrasive paste and polishing cloth wheel. Then use sticky felt and diamond abrasive paste for polishing, the sequence is:
1μm(14000#)→1/2μm(60000#)→1/4μm(100000#)
(4) Polished working environment
The polishing process should be carried out in two working positions, that is, the rough grinding processing position and the fine polishing processing position are separated, and attention should be paid to cleaning up the sand particles remaining on the surface of the workpiece from the previous process.
Generally, after rough polishing with oilstone 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 mold surface. The polishing process that requires an accuracy of 1 μm or above (including 1 μm) can be carried out in a clean polishing room. For more precise polishing, it must be in an absolutely clean space, because dust, smoke, dandruff and saliva may scratch the high-precision polishing surface.
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 surface of the workpiece is clean, and then a layer of mold anti-rust coating is sprayed on the surface of the workpiece.
Factors affecting surface polishing
(1) Workpiece surface condition
During the processing of materials, the surface layer will be damaged due to heat, internal stress or other factors, and improper cutting parameters will affect the polishing effect. EDM-machined surfaces are more difficult to grind than machined or heat-treated surfaces. Therefore, EDM should be carried out before the end of EDM, otherwise a thin hardened layer will form on the surface. If the EDM fine-tuning rules are improperly selected, the depth of the heat-affected layer can reach 0.4mm. The hardened thin layer is harder than the base material 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 prerequisite for good polishing quality. 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 polished surface roughness at the beginning of processing. When a workpiece is determined to require mirror polishing, it must be steel with good polishing performance and undergo heat treatment, otherwise the expected effect will not be achieved. Influence.
(3)Heat treatment process
If the heat treatment is improper, the surface hardness of the steel is uneven 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 polishing quality.
It is generally believed that polishing technology affects surface roughness. In fact, good polishing technology must be paired with high-quality steel and the correct heat treatment process to achieve satisfactory polishing results. On the contrary, if the polishing technology is not good, no matter how good the steel is, it will not achieve the mirror effect.
Stainless steel processing
How to solve common polishing problems
(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 surface quality of the mold. There are two symptoms of over-polishing: "orange peel" and "pitting." Over-polishing often occurs in mechanical polishing.
(2) Causes of “orange peel” on workpieces
Irregular and rough surfaces are called "orange peel." There are many causes of “orange peel.” The most common cause is overheating or carburization of the mold surface. The polishing pressure is too high 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 over-polishing. Research shows that the time required for over-polishing varies depending on the hardness of the steel.
(3) Measures to eliminate “orange peel” on cultural relics
When they find that the surface quality is not good for polishing, many people will increase the polishing pressure and extend the polishing time. This practice often results in poorer surface quality.
This can be remedied using:
1) Remove the defective surface, the abrasive grains are slightly coarser than the previous ones, and then grind, the polishing force is lower than before.
2) Perform stress relief at a temperature 25°C lower than the tempering temperature. Before polishing, polish with the finest sand grade until a satisfactory effect is achieved, and finally polish with a lighter force.
(4) Causes of “pitting corrosion” on the workpiece surface
Because steel contains some non-metallic impurities, usually hard and brittle oxides, they are pulled out of the steel surface during the polishing process, forming pits or pitting corrosion. The main factors that produce "pitting corrosion" 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 rusty.
4) The black leather was not removed.
(5) Measures to eliminate pitting corrosion of workpieces
1) Carefully re-grind the surface. The particle size of the sand is one level coarser than that previously used. A soft yet sharp pumice stone is used for the final grinding step before proceeding with the polishing process.
2) Avoid using the softest polishing tools when the grit size is less than 1mm.
3) Use the shortest polishing time and minimum polishing intensity possible.
