How To Mill Different Materials? (Summary Of Experience)

01 Steel milling

The machinability of steel varies with alloying elements, heat treatment and manufacturing processes (forging and casting, etc).

When processing softer low-carbon steel, the main problem is the formation of a built-up edge and burrs on the workpiece.

When machining harder steel, in order to avoid tipping, the relative position of the milling cutter and the workpiece will become more important.

Suggestions:

When milling steel parts, to optimize the position of the milling cutter to avoid thick chips when the tool is retracted.

Make sure to consider dry cutting without the use of cutting fluid, especially in the rough machining process.

Steel milling

02 Stainless steel milling

Stainless steel can be classified into ferritic/martensitic stainless steel, austenitic stainless steel and duplex (austenitic/ferritic) stainless steel.

Each type has its own milling recommendations.

1) Milling of ferritic/martensitic stainless steel

Material classification: P5.x

The machinability of ferritic stainless steel is similar to that of low-alloy steel, so steel milling recommendations can be used.

Martensitic stainless steel has higher work hardening performance and it requires relatively high cutting force when cutting.

It needs to use the correct tool path and arc cutting method to obtain the best results, and a higher cutting speed Vc to overcome the work hardening effect.

Higher cutting speeds, tougher materials and enhanced cutting edges ensure greater safety

2) Milling of austenitic and duplex stainless steel

Material classification: M1.x, M2.x and M3.x

The main wear criteria for milling austenitic stainless steel and duplex stainless steel are cutting edge chipping due to hot cracks, groove wear and built-up edge/bonding.

For parts, burr formation and surface quality issues are the main problems.

Hot crack

Hot crack

Blade cutting edge tipping

Blade cutting edge tipping

Burr formation and poor surface quality

Burr formation and poor surface quality

Rough machining recommendations:

Use high cutting speed (Vc=150-250m/min) to avoid built-up edge

Use dry cutting without cutting fluid to minimize hot cracking problems.

Finish machining recommendations:

  • In order to improve the surface quality, sometimes it is necessary to use cutting fluid or preferably oil mist lubrication/minimum lubrication. There are fewer hot cracking problems during finishing because the heat generated in the cutting area is lower.
  • When using cermet materials, a sufficiently good surface quality can be obtained without using cutting fluid.
  • If the feed fz is too low, the cutting edge may cut in the deformation hardened zone and cause more serious wear of the insert.

03 Cast iron milling

1) Gray cast iron

Material classification: K2.x

The main wear criteria for gray cast iron milling are abrasive/flank wear and hot cracking.

For parts, tipping of the workpiece and surface quality are the main problems.

Typical blade wearing

Typical blade wearing

Workpiece tipping

Workpiece tipping

Rough machining recommendations:

1) It is best to cut dry without using cutting fluid to minimize the problem of hot cracks.

To use thick-coated carbide blades.

2) If there is a problem of workpiece tipping:

  • Check flank wear
  • Reduce feed fz to reduce chip thickness
  • Use a groove with a larger positive rake angle
  • Best to use 65°/60°/45° milling cutter

3) If cutting fluid must be used to avoid dust and so on, wet milling material needs to be chosen.

4) Coated cemented carbide is always the first choice, but ceramic materials can also be used.

Please note that the cutting speed Vc should be rather high at the speed of 800-1000m/min.

The formation of burrs on the workpiece limits the cutting speed.

Do not use cutting fluid.

Finish machining recommendations:

1) Use thin-coated carbide blades or uncoated carbide blades.

2) CBN material can be used for high-speed finishing.

Do not use cutting fluid.

2) Ductile iron

Material classification: K3.x

1) The machinability of ferritic ductile iron and ferritic/pearlite ductile iron is very similar to that of low alloy steel.

Therefore, the milling recommendations provided for steel materials should be used when selecting tools, insert geometries and materials.

2) Pearlitic ductile iron is more abrasive, so it is recommended to use cast iron materials.

3) The use of PVD coating material and wet cutting can ensure the best processing ability.

3) Compacted Graphite Cast Iron (CGI)

Material classification: K4.x

The pearlite content is less than 90%.

This type of CGI, which is most commonly used for milling processing, usually has a pearlite structure of about 80%.

Typical parts are engine cylinder block, cylinder head and exhaust manifold.

The recommendations for milling cutters are the same as when machining gray cast iron; however, insert geometries with sharper cutting edges and larger positive rake angles should be selected to minimize the burrs formed on the parts.

Arc milling may be a very good alternative to traditional CGI cylinder boring.

4) Austempered ductile iron (ADI)

Material classification: K5.x

Rough machining is usually performed in an unhardened state and is comparable to high-alloy steel milling.

However,  the processing object of the finish machining is a hardened material with very high abrasiveness, which is comparable to ISO H  hardened steel milling.

Materials with higher resistance to abrasive wear are preferred.

Compared with NCI, the tool life when machining ADI is shortened to about 40%, and the cutting force is increased by about 40%.

04 Milling of non-ferrous metal materials

Non-ferrous metal materials include not only aluminum alloys, but also magnesium, copper and zinc-based alloys.

Machinability is mainly due to the difference in silicon content.

Hypoeutectic aluminum-silicon alloy is the most common type, with a silicon content of less than 13%.

Aluminum alloy with silicon content less than 13%

Material classification: N1.1-3

The main wear criterion is built-up edge/bonding on the cutting edge, which leads to burr formation and surface quality problems.

In order to avoid leaving scratches on the surface of the part, good chip formation and chip removal are essential.

Suggestions:

1) The use of PCD tipped inserts with sharp polished cutting edges can ensure good chip-breaking ability and prevent build-up edge.

2) Choose a positive rake insert geometry with sharp cutting edges.

3) Unlike most other milling applications, cutting fluid should always be used when machining aluminum alloys to avoid material sticking to the blade cutting edge and improve surface quality.

  • Silicon content <8%: use cutting fluid with a concentration of 5%
  • Silicon content <8-12%: use cutting fluid with a concentration of 10%
  • Silicon content> 12%: use cutting fluid with a concentration of 15%

4) Higher cutting speeds generally improve performance without negatively affecting tool lifespan.

5) The recommended value of hex is 0.10-0.20 mm (0.0039-0.0079 inch).

Low values will cause burr formation.

6) Due to the high table feed, a machine tool with “pre-reading” function should be used to avoid dimensional errors.

7) Tool lifespan is always limited by burr formation or surface quality of parts.

Blade wearing is difficult to use as a tool life criterion.

05 Superalloy and titanium alloys

Milling of superalloys and titanium usually requires a machine tool with high rigidity, high power and high torque to run at low speeds.

Notch wear and cutting edge tipping are the most common types of wear.

The generated high heat will limit the cutting speed.

Suggestions:

To use round blade milling cutters as much as possible to increase the chip thinning effect.

The use of round blade milling cutters minimizes notch wear

The use of round blade milling cutters minimizes notch wear

When the cutting depth is less than 5mm, the entering angle should be less than 45°.

In practice, it is recommended to use a positive round blade.

Both radial and axial accuracy of the cutter is necessary to maintain a constant load per tooth and a smooth process as well as to prevent premature failure of individual inserts.

The cutting edge should always be grooved at a positive rake angle and optimally rounded to prevent chips from adhering to the cutting edge when retiring the tool.

During milling, the number of cutting teeth actually involved in cutting should be as many as possible.

Under stable conditions, this will achieve ideal productivity.

To use superdensity tooth milling cutter.

superdensity tooth milling cutter

Yellow: Tool lifespan; Black: Tool lifespan decreases as cutting parameters increase

Changes will have different effects on tool life; cutting speed Vc has the greatest impact, followed by ae and so on.

Cutting fluid/coolant

Unlike most other materials when milling, it is always recommended to use coolant to help chip evacuation to control the heat at the cutting edge and prevent secondary chip cutting.

Preference is always given to the internally cooled high-pressure coolant (70 bar) applied through the spindle/tool rather than the externally cooled and low-pressure coolant.

Exception: When milling with ceramic inserts, cutting fluid should not be used due to thermal shock.

When using cemented carbide blades, internal cooling will bring benefits

When using cemented carbide blades, internal cooling will bring benefits

Blade/tool wear

The most common causes of tool breakage and poor surface quality are groove wear, excessive flank wear, and chipped edge lines.

The best solution is to frequently index the cutting edge to ensure a reliable machining process. The flank wear of the cutting edge should not exceed 0.2mm (for milling cutters with an entering angle of 90°), or the maximum should not exceed 0.3mm (for round inserts).

Typical blade wear

Typical blade wear

Be applied in ceramic blade milling cutter for rough machining of superalloy.

 

The speed of ceramic milling is usually 20-30 times more than that of cemented carbide milling, although the feed rate is lower (about 0.1mm/z), which will result in a substantial increase in productivity.

Due to the use of interrupted cutting, the temperature of the milling process is much lower than that of turning.

Therefore, the cutting speed of 700-1000m/min is used for milling, which is compared to only 200-300m/min for turning.

Suggestions:

1) Round blades are mainly used to ensure a small entering angle and prevent notch wear.

2) Do not use cutting fluid/coolant.

3) Do not use ceramic blades when processing titanium alloys.

4 Ceramics will have a negative impact on surface integrity and other indicators.

Therefore, do not use ceramic blades when the shape of the finished part is ready to be processed.

5) The maximum flank wear when machining high-temperature alloys with ceramic inserts is 0.6mm.

06 Hardened steel milling

This group contains tempered steel with hardness greater than 45-65HRC.

Typical parts for milling include stamping molds, plastic molds, forging molds, and die-casting dies.

Blade debris/flank wear and workpiece tipping are the main problems.

Suggestions:

1) Use a positive rake insert geometry with sharp cutting edges.

This will reduce the cutting force and produce a smoother cutting action.

2) Dry cutting without using cutting fluid.

3) Cycloid milling is an appropriate method, which can achieve high table feed and low cutting force at the same time.

So that the cutting edge and workpiece can be kept low temperature, which is beneficial to productivity, tool lifespan and part tolerances.

4) In face milling, a light cutting strategy should also be adopted, which is to keep small depths of cut: ae and ap.

Using ultra-close pitch milling cutter and relatively high cutting speed is also needed.

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