What are the precautions for drilling stainless steel?

Design of cutting tool parameters related to stainless steel machining technology. There is a big difference when compared to the processing of ordinary steel materials. The specific processing techniques are described below.

1.Drilling

Drilling is not easy due to the low thermal conductivity and elastic modulus of stainless steel materials. The best way to solve the problem of drilling stainless steel is to select the right tool material to determine the geometric parameters of the tool and the amount of cutting of the tool. Kangding Metal’s main products are rotary tops, screws, shafts, CNC lathes, tool handles, tool rods and chuck connectors. When drilling stainless steel, the drill usually has a drill bit made of W6Mo5Cr4V2Al or W2Mo9Cr4Co8. The disadvantage of this type of drill bit is that it is expensive and not very good to buy. However, when using the common W18Cr4V common standard high speed steel drill bit. Due to drawbacks such as small apex angles and oversized shavings, the holes cannot be drained in time and the cutting fluid cannot cool the drill bit in time. In addition, the low thermal conductivity of stainless steel materials concentrates on the cutting edge and raises the cutting temperature. Easily burn and damage the two sides and main edge, shortening the life of the bit.

1>, Tool shape parameters

W18Cr4V When drilling with a normal high speed steel drill bit, the cutting force and cutting temperature are concentrated on the drill tip. In order to improve the durability of the cutting part of the drill bit, the apex angle can be increased appropriately, and the apex angle is generally selected from 135 ° to 140 °. Increasing the apex angle reduces the rake angle of the outer edge, narrows the drill cut and facilitates chip evacuation. However, increasing the top angle widens the chisel edge of the drill bit and increases cutting resistance. Therefore, the chisel chisel edge needs to be polished. After grinding, the bevel of the chisel edge is 47 ° to 55 ° and the rake angle of the chisel edge is 3 ° to 5 °. When grinding the chisel edge, the cutting edge and cylindrical angle should be rounded to increase the strength of the chisel edge.

Since the elastic modulus of stainless steel is small, the elastic recovery of the metal under the chip layer is large, and work hardening is serious during the processing process. If the back angle is too small, the side wear of the drill bit will accelerate, the cutting temperature will rise and the life of the drill bit will be shortened. Therefore, the clearance angle should be increased appropriately, but the relief angle should be 12 ° -15 ° because the rear angle is too large, which makes the main edge of the drill bit thinner and reduces the rigidity of the main edge. is needed. Alternating notches should be opened on the two sides of the bit to make the chips thinner and easier to remove the chips.

2>, selection of cut amount

When drilling, the choice of cutting amount should be based on the basic point of lowering the cutting temperature. The most important part of the cutting amount is the cutting speed, because high-speed cutting raises the cutting temperature and high cutting temperature increases tool wear. In general, the cutting speed is preferably 12-15 m / min. The feed rate has little effect on the life of the tool, but the feed choice is too small to cut the tool with a hardened layer to exacerbate wear. If the feed amount is too large, the surface roughness will deteriorate. In combination with the above two factors, the feed rate is preferably selected from 0.32 to 0.50 mm / r.

3>, selection of cutting fluid

Emulsions can be used as a cooling medium to reduce the cutting temperature during drilling.

2.Reaming process

1> Tool geometry parameters

Most reamer processing of stainless steel materials uses carbide reamers. The structure and geometric parameters of the reamer are different from regular reamers. Reamer teeth are generally small in number to increase the strength of the reamer’s teeth and prevent chip clogging. Reamer rake angles are typically 8 ° to 12 °, but in certain cases 0 ° to 5 ° rake angles can also be used for fast reaming. The back angle is usually 8 ° to 12 °. The choice of lead angle depends on the hole. Generally, through holes are 15 ° to 30 ° and through holes are 45 °.

The edge tilt angle can also be increased appropriately to expel chips forward during reaming. The angle of inclination of the edge is generally 10 ° to 20 °. The width of the edge bandwidth is 0.1 to 0.15 mm. The inverted cone of the reamer should be larger than the normal reamer. Carbide reamers are generally 0.25 to 0.5 mm / 100 mm and high speed steel reamers are 0.1 to 0.25 mm / 100 mm. The length of the reamer correction part is usually 65% ​​to 80% of the normal reamer, and the length of the cylindrical part is 40% to 50% of the normal reamer.

2> Volume selection reaming

The feed rate during reaming is 0.08 to 0.4 mm / r, and the reaming speed is 10 to 20 m / min. Coarse hinges usually have a tolerance of 0.2 to 0.3 mm, and fine hinges have a tolerance of 0.1 to 0.2 mm. Carbide tools should be used for coarse hinges and high speed steel tools should be used for fine hinges.

3>, Reaming fluid selection

When reaming stainless steel materials, total loss system oil or molybdenum disulfide can be used as the cooling medium.

3, stainless steel boring

1>, Tool material selection

Due to the high cutting force and high cutting temperature when machining stainless steel parts, the tool material should be selected as a high quality YC or YG type hard alloy with high thermal conductivity. YT14 and YT15 carbide inserts can also be used for finishing. Due to the properties of such materials, toughness is high and work hardening is serious. The insert produced by cutting these materials into the shape of a unit insert causes the tool to vibrate, easily causing a slight collapse of the blade. Therefore, the first consideration of ceramic cutting tools when cutting such material parts is microscopic toughness.

Currently, Sialon is a better choice, especially for alpha / beta Sialon materials. Due to its excellent resistance to high temperature deformation and diffusion wear, it is applied to cutting nickel-based alloys and has a much longer life than Al2O3-based ceramics. In addition, SiC whiskers reinforced ceramics are also highly effective tool materials for cutting stainless steel or nickel-based alloys.

CBN (cubic boron nitride) inserts can be used to process hardened parts of such materials. The CBN hardness is second only to diamond, and the hardness reaches 7,000 to 8,000 HV, so the wear resistance is very high. Compared to diamond, CBN is much more heat resistant than diamond, up to 1200 ° C, and has the great advantage of being able to withstand high cutting temperatures. In addition, its chemical inertness is so great that it does not play a chemical role with group metals at 1200-1300 ° C. Therefore, it is very suitable for processing stainless steel materials. Its tool life is dozens of times longer than that of carbide or ceramic tools.

2>. Boring tool geometry parameters

The geometric parameters of the boring tool play an important role in cutting performance. To make cutting light and smooth, carbide tools should employ a large front angle to improve tool life. Generally, for rough cutting, the front angle is 10 ° to 20 ° and the semi-finishing is 15 ° to 20 °; the lead angle selection is 20 ° to 30 ° during finishing by the process system. It is based on the fact that it can take 30 ° to 45 ° if it is stiff. If the process system is less rigid, take 60-75 °. If the ratio of length to diameter of the work piece exceeds 10 times, it will be 90 °.

When drilling stainless steel with a ceramic tool, the ceramic tool is most often cut at a negative rake angle. This helps to strengthen the blade and take full advantage of ceramic tools with high compressive strength. The size of the back angle directly affects the wear of the tool and affects the edge strength. Generally, select from 5 ° to 12 °. Changes in lead angle affect the radial and component cutting forces, cutting widths, and cutting thicknesses. The choice of lead angle is beneficial in reducing this vibration, as the vibration of the process system is very unfavorable for ceramic tools. Generally 30 ° to 75 °. When CBN is selected as the tool material, the tool shape parameters are rake angle 0 ° to 10 °, back angle 12 ° to 20 °, and principal angle 45 ° to 90 °.

3>. When sharpening, the rake face roughness value should be small

To avoid tip sticking and to reduce the roughness value, the front and back of the tool should be carefully scraped to reduce chip evacuation resistance and prevent chips from adhering.

4>, keep the sharp edges of the tool

The cutting edge of the tool needs to be sharpened to reduce work hardening. The amount of feed and backing knife should not be small enough to prevent cutting of the tool in the hardened layer and affect the useful life of the tool.

5>, Be careful of grinding of chip breaking groove

Due to the strong and durable properties of stainless steel chips, grinding of chip break grooves on the tool rake face must be appropriate, chip breaking, chip retention, and chip removal. Convenient in the cutting process.

6>, select the cutting amount

Due to the nature of stainless steel materials, it is recommended to use slow and large feeds for cutting.

When using ceramic tools for boring, a reasonable choice of cutting amount is one of the keys to taking full advantage of the performance of ceramic tools. When cutting ceramic tools, the amount of cutting can be selected according to the relationship between the amount of wear and the amount of wear. For intermittent cutting, a reasonable amount of cutting should be determined according to the law of tool damage. Due to the excellent heat resistance and wear resistance of ceramic knives, the effect of cutting amount on tool wear life is smaller than that of carbide tools.

In general, when machining with ceramic tools, feed rates are most sensitive to tool damage. Therefore, depending on the nature of the work material, given the power of the machine, the stiffness of the process system, and the strength of the blades. When drilling holes in stainless steel parts, choose high cutting speeds, large backing knives, and relatively small feeds.