01 Steel milling
The machinability of steel is influenced by several factors such as alloying elements, heat treatment and manufacturing processes such as forging and casting.
When working with softer low carbon steels, the main challenge is the development of built-up edges and burrs on the workpiece.
When machining harder steel, it is crucial to carefully position the cutter relative to the workpiece to avoid tipping the tool.
To optimize the milling process of steel parts, it is advisable to adjust the position of the cutter to avoid coarse chips when the tool is retracted.
Furthermore, it is important to consider dry cutting as an option, especially during rough machining, and to avoid the use of cutting fluid.
02 Stainless steel milling
Stainless steel can be classified into three main types: ferritic/martensitic stainless steel, austenitic stainless steel and duplex (austenitic/ferritic) stainless steel. Each type has its unique milling recommendations.
1) Ferritic/martensitic stainless steel milling
Material classification: P5.x
The machinability of ferritic stainless steel is similar to that of low alloy steel, so steel milling recommendations can be applied.
Martensitic stainless steel, on the other hand, has higher hardening performance and requires relatively high cutting force during machining.
To obtain the best results, it is essential to use the correct tool path and arc cutting method, and a higher cutting speed Vc to overcome the work hardening effect.
Greater safety can be ensured by using higher cutting speeds, stronger materials and improved cutting edges.
2) Milling of austenitic and duplex stainless steel
Material classification: M1.x, M2.x and M3.x
The main factors contributing to wear when milling austenitic stainless steel and duplex stainless steel are cutting edge chipping caused by hot cracking, groove wear and built-up/bond edge.
In terms of parts, the main concerns are burr formation and surface quality issues.
Hot Crack
Blade tip inclination
Burr formation and poor surface quality
Rough machining recommendations:
To avoid built-up edges, use a high cutting speed (Vc = 150-250 m/min).
To minimize hot cracking problems, opt for dry cutting rather than using cutting fluid.
Finish machining recommendations:
To improve the surface quality of a material, it is often essential to use cutting fluid or oil mist/minimal lubrication. This technique results in fewer hot cracking problems during finishing, as the heat generated in the cutting area is lower.
However, when working with cermet materials, the use of cutting fluid may not be necessary, as a sufficiently good surface quality can be achieved without it.
It should be noted that if the feed fz is too low, the cutting edge may cut through the strain hardened zone, leading to more severe wear of the insert.
03 Cast iron milling
1) Gray cast iron
Material classification: K2.x
The main factors affecting wear when milling gray cast iron are abrasive/flank wear and hot cracking.
As for the components, part tipping and surface quality are the main concerns.
Typical blade wear
Tilt of the part
Rough machining recommendations:
(1) To minimize the occurrence of hot cracks, it is recommended to cut dry without using cutting fluid. Thickly coated carbide blades should be used.
(2) If the part is tilted, several things can be done: check flank wear, reduce feed fz to decrease chip thickness, use a flute with a larger positive rake angle, and consider using 65° milling /60°/45° cutter.
(3) If cutting fluid is required to avoid dust or other problems, a wet milling material should be chosen.
(4) Coated carbide is normally the first choice, but ceramic materials can also be used. The cutting speed (Vc) should be set to a relatively high speed of 800-1000 m/min, but keep in mind that burrs on the workpiece can limit the cutting speed. Cutting fluid must not be used.
Finish machining recommendations:
(1) For cutting without the use of cutting fluid, it is recommended to use carbide blades with or without coating.
(2) For high-speed finishing, CBN (cubic boron nitride) material can be used. Cutting fluid must not be used.
2) Ductile iron
Material classification: K3.x
(1) Ferritic ductile iron and ferritic/pearlitic ductile iron have similar machinability to low-alloy steel. Therefore, when selecting tools, insert geometries and materials, milling recommendations for steel materials should be used.
(2) Pearlitic ductile iron is more abrasive, so it is recommended to use cast iron materials.
(3) To ensure the best processing ability, it is recommended to use PVD coating and wet cutting materials.
3) Compacted Graphite Cast Iron (CGI)
Material classification: K4.x
The perlite content is below 90%.
This type of compacted graphite iron (CGI), commonly used for milling processing, usually has a pearlite structure of around 80%. It is used in various components, including engine cylinder blocks, cylinder heads and exhaust manifolds.
Recommended cutter guidelines for CGI are similar to those for machining gray cast iron. However, to reduce burrs formed on parts, insert geometries with sharper cutting edges and larger positive rake angles must be chosen.
Arc milling can be an excellent alternative to traditional CGI cylinder boring.
4) Austempered Ductile Iron (ADI)
Material classification: K5.x
Rough machining is typically performed on non-hardened materials and can be compared to milling high-alloy steel.
On the other hand, finish machining is carried out on hardened materials with high abrasiveness, similar to milling of ISO H hardened steel. Therefore, materials with greater resistance to abrasive wear are preferred.
When machining ADI, tool life is reduced by approximately 40% compared to NCI, and cutting force increases by approximately 40%.
04 Milling of non-ferrous metallic materials
Non-ferrous metallic materials include not only aluminum alloys, but also alloys based on magnesium, copper and zinc.
Related Reading: Ferrous vs Non-Ferrous Metals
Machinability is mainly determined by variation in silicon content.
The most prevalent type is the hypoeutectic aluminum-silicon alloy, which has a silicon content of less than 13%.
Aluminum alloy with silicon content less than 13%
Material classification: N1.1-3
The main criterion for wear is the build-up of edge/grip on the cutting edge, which causes surface quality problems and burr formation.
To avoid scratches on the surface of the part, good chip formation and removal is essential. Here are some suggestions:
- Using PCD-tipped inserts with sharp and polished cutting edges can ensure good chip breaking ability and prevent edge build-up.
- Choose an insert geometry with sharp cutting edges and a positive rake angle.
- When machining aluminum alloys, cutting fluid should always be used to prevent material from sticking to the cutting edge of the blade and to improve surface quality. The recommended cutting fluid concentration depends on the silicon content of the alloy. For silicon content <8%, use a cutting fluid with a concentration of 5%. For silicon content between 8-12%, use a cutting fluid with a concentration of 10%. For silicon content >12%, use a cutting fluid with a concentration of 15%.
- Higher cutting speeds generally improve performance without negatively affecting tool life.
- The recommended hex value is 0.10-0.20 mm (0.0039-0.0079 inches). Low values can cause burr formation.
- Due to the high feed rate of the table, a machine tool with a “pre-reading” function must be used to avoid dimensional errors.
- Tool life is always limited by burr formation or the surface quality of the parts. Blade wear is difficult to use as a criterion for tool life.
05 Superalloys and titanium alloys
Milling superalloys and titanium typically requires a machine tool with high rigidity, power and torque, which can operate at low speeds.
The two most common types of wear are notch wear and cutting edge drooping.
Excessive heat generated during the milling process can restrict cutting speed.
One possible suggestion is to maximize the use of round-blade cutters, which can increase the chip thinning effect.
Use of round blade cutters minimizes notch wear
When the cutting depth is less than 5mm, the entry angle must be less than 45°.
In practical applications, the use of a positively rounded blade is recommended.
To maintain a constant load per tooth and ensure a smooth process, as well as to avoid premature failure of individual inserts, radial and axial precision of the cutter is required.
The cutting edge must always be grooved at a positive rake angle and ideally rounded to prevent chips from sticking to it when the tool is withdrawn.
When milling, it is best to involve as many cutting teeth as possible.
Under stable conditions, this will achieve optimal productivity.
Use a superdensity toothed milling cutter.
Yellow: Tool life; Black: Tool life decreases as cutting parameters increase
Tool life is affected differently by various changes, with cutting speed (Vc) having the greatest impact, followed by ae, and so on.
Cutting fluid/coolant
When it comes to milling, unlike other materials, it is always advisable to use coolant to help evacuate the chips and regulate the heat at the cutting edge, avoiding secondary cutting of the chips.
Internally cooled high pressure coolant (70 bar) supplied through the spindle/tool is generally preferred to externally cooled low pressure coolant.
However, it is important to note that there is an exception to this rule. When milling with ceramic inserts, cutting fluid should be avoided due to the risk of thermal shock.
When using 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 edges.
To ensure a reliable machining process, the best solution is to frequently index the cutting edge. The flank wear of the cutting edge must not exceed 0.2 mm for cutters with a 90° entering angle, or the maximum must not exceed 0.3 mm for round inserts.
Typical blade wear
Be applied to ceramic blade milling cutters for roughing machining of superalloys.
Ceramic milling has a faster speed than carbide milling, typically 20-30 times faster despite a lower feed rate (about 0.1 mm/z). This results in a significant increase in productivity.
The milling process uses interrupted cutting, resulting in much lower temperatures than turning.
Therefore, a cutting speed of 700-1000 m/min is recommended for milling, compared to just 200-300 m/min for turning.
Here are some suggestions:
(1) To ensure a small positioning angle and avoid notching wear, use round blades.
(2) Avoid using cutting fluid/coolant.
(3) Do not use ceramic blades when processing titanium alloys.
(4) Ceramic may negatively affect surface integrity and other indicators. Therefore, avoid using ceramic blades when the finished part shape is ready for processing.
(5) The maximum allowable flank wear when machining high temperature alloys with ceramic inserts is 0.6 mm.
06 Hardened steel milling
This group of materials includes hardened steel with a hardness greater than 45-65HRC. Typical milling parts include stamping molds, plastic molds, forging molds, and die casting dies. Blade debris/flank wear and part tipping are the main problems.
Here are some suggestions:
(1) Use a positive rake insert geometry with sharp cutting edges. This reduces cutting force and creates a smoother cutting action.
(2) Dry cutting without cutting fluid is recommended.
(3) Cycloid milling is a suitable method that achieves high table feed and low cutting force simultaneously. This keeps the cutting edge and workpiece at a low temperature, improving productivity, tool life and part tolerances.
(4) When face milling, use a light cutting strategy with small depths of cut (ae and ap). Use an ultra-fine pitch cutter and a relatively high cutting speed.
