Milling is a form of machining using rotary cutters to remove material from the workpiece. Milling allows one to manufacture high-precision parts in different sizes and forms.
We are distinguishing 2 types of Milling machines: Vertical and Horizontal Milling Machines. The Vertical Milling Machine has a vertical spindle and rotates by staying on the same axis. In this case, the spindle is expendable for drilling or cutting operations.
The Vertical Machine has also 2 more types: Turret Mill and Bed Mill.
The Turret Mill has a table that moves perpendicularly and parallels to the spindle axis while the spindle remains fixed. Usually is suitable for small workpieces.
The Bed Mill’s table moves in the horizontal direction and the spindle moves up and down. This type of machine is used for big workpieces.
The Horizontal Milling Machine has also a working table just like the Vertical Machine, but in this case, the tool is operating horizontally or at a specific angle if the machine permits.
Anatomy of Milling Tool
Milling Tool Holder
Tool Holders are used to adapt different types of tools and also secure firm the tool to the milling machine spindle.
- End Mills
End Mills are the most frequently used milling cutters that have teeth at the end or on the face of the circular disc. They are available in a wide range of lengths, diameters, and types. This type is mostly used in vertical milling. They are made from High-Speed Steel (HS, HSS) or Cemented Carbide
End Mills can be Centered Cutting or Non-Center Cutting.
Center Cutting End Mills have cutting edges on the end face and also on the sides while the Non-Center Cutting Mills have cutting edges just on the sides. Center Cutting Mill can be used in various applications like drilling, ramp, and plunge milling. The Non-Center Mill is used for side cutting and contouring.
End Mills types and characteristics
Flutes are spiral-shaped cutting edges allowing a defined path for the chip to escape when the end mill is down in a pocket. More flutes mean more cutting edges while the channel for chip evacuation becomes narrower. The most frequently used mills have 2, 3, or 4 flutes.
The chosen number of flutes is related to the chip load. What does it mean chip load?
Chip load is defined by the measurement of the removed material’s thickness by each cutting edge during machining. The material type and not at least the milling machine capability influence the chosen flutes number. For example, aluminum is a softer material that produces large chips, which can be jammed and break the cutter so for this reason a 4 flutes mill is very rarely used in this application. In the case of hard material with short chips are recommended to use multiple flutes which improves even the finished surface.
- 2 flutes: the most commonly used one. It has the greatest amount of space for chip evacuation, so is perfect for soft materials. Used in slotting and pocketing in non-ferrous materials like aluminum.
- 3 flutes: can be achieved a better finish in hard materials. It’s used in slotting and pocketing in both soft and hard materials providing great strength during cutting.
- 4 or more flutes: recommended for finish milling. The higher number of flutes allows faster feed and smoother surfaces. The chip removal could cause difficulties.
End Mill Materials
The primary materials used for manufacturing end mills are High-Speed Steel HSS and tungsten carbide.
End Mill Coatings
Coatings increase the surface hardness of the tool expanding the tool life and permit faster-cutting speed.
- Titanium Nitride (TiN): this coating is used on alloy steel, aluminum, plastic. It has a gold color.
- Titanium Carbonitride (TiCN): better wear resistance than TiN coating, so is a good choice for materials like ductile cast iron, stainless steel, aluminum, and plastic. The coating color is blue-gray.
- Aluminum Titanium Nitride (AITiN): best for very high feeds and temperature applications. It can be used for cast iron, stainless steel, nickel-based alloys, titanium. Not for Aluminum. It has purple-gray color.
2. Face Mills
Face Mills have a large diameter with a solid body, that holds carbide inserts that can be replaced when they wear out or a brake. A higher number of inserts on a tool means more efficient material removal. They are used in horizontal machining, facing a large surface. Face Mills prepare in general the top of the part for further milling operations.
Cutter with 45° angle:
- most frequent used
- reduce vibrations
- allows increased productivity
Cutter with 90° angle:
- thin-walled components
- weak-fixtured components
Round inserts and large radius cutters:
- are versatile used in both face milling and profiling operations
- strongest cutting edge
- smooth cutting
3. Drills, Taps, Reamers
Twist Drills are used to create holes in different sizes. In comparison to end mills with the same sizes, these are more efficient in material removal and come in many more sizes. In some cases, is better to drill first the slot than machine the remaining material.
Twist Drills have a conical cutting point at the tip of a cylindrical shaft with one or more helical flutes. Flutes create a path for chip evacuation. The point angle is also important it can be smaller or larger.
A smaller point angle refers to an easier centering in the material. A larger point angle means shorter tapping time but higher contact pressure is required and the centering is more difficult.
The most frequently used material for Twist Drills is High-Speed Steel (HSS) and Solid Carbide. Drills have also coating for extended tool life and increased cutting speed.
The well-known coating used for drills is Titanium Nitride (TiN), which came for a higher cost but increase the tool life. TiN is used to harden the cutting surfaces and has a gold color.
Indexable Drills are used mostly for large holes, having a solid body with replaceable inserts. Ordinary drills drill holes with a diameter less than 20 mm. Indexable Drills drill accurate holes more than 13 mm.
Spot Drills and Chamfer Mill Tools
These drills were designed for special applications, therefore, have very short flutes.
Spot Drills are very rigid because their purpose is to spot a hole for further drilling. It makes a little dimple in the workpiece to be easier for the drill to center the hole and remain on the right path (straight hole).
The Chamfer Mill tool as its name said is designed to create chamfers, V-cuts, undercuts, preparation for welding, and deburring operations along the workpiece edges.
Reamers are used to enlarge an existing hole precisely adding a high-quality surface finish. This operation requires a hole with a close diameter to the final size because the reamer can remove just a bit of material.
Cut TAP (left image) Roll Form TAP (right image)
Taps are designed to create internal threads in a pre-existing hole. There are 2 types of taps: one requires material cutting Cut Tap and the other one deforms the material Roll Form Tap. The Roll Form Tap instead of material cutting is forced into the hole creating a stronger tap. Roll Form technology can be applied in metals that cold form well such as stainless steel, light metals, light metal alloys.