Types of Chips in Metal Cutting: Continuous, Discontinuous & BUE

During any metal machining operation – turning, milling, or drilling – the cutting tool removes material and produces metal chips. The type of chip formed has a direct impact on surface finish quality, tool life, power consumption, and overall machining efficiency.

There are three main types of chips in metal cutting:

1. Continuous Chips
2. Discontinuous Chips
3. Continuous Chips with Built-Up Edge (BUE)

Understanding what causes each type – and when each appears – helps machinists optimize cutting parameters and avoid defects.

1. Continuous Chips

What Are Continuous Chips?

Continuous chips are long, unbroken ribbon-like chips formed when a ductile material is machined under favorable cutting conditions. There is no fracture or segmentation in the chip – it flows smoothly over the tool face.

Conditions That Produce Continuous Chips

• Ductile workpiece material (e.g., mild steel, aluminum)
• High cutting speed
• Large rake angle on the cutting tool
• Minimum friction at the chip-tool interface
• Small depth of cut
• Use of cutting fluid/coolant

Advantages of Continuous Chips

• Superior surface finish – smooth, uninterrupted chip flow means less surface damage
• Lower heat generation – reduced friction keeps temperatures down
• Lower power consumption – cutting forces remain stable
• Longer tool life – minimal wear due to low friction

Disadvantages

• Long continuous chips can tangle around the tool or workpiece, creating a safety hazard
• Chip disposal becomes difficult on CNC machines without chip breakers

2. Discontinuous Chips

What Are Discontinuous Chips?

Discontinuous chips (also called segmented chips) are short, broken chip segments produced during machining. Instead of a smooth flow, the chip fractures repeatedly as material is removed.

Conditions That Produce Discontinuous Chips

• Brittle workpiece materials (e.g., cast iron, brass, bronze)
• Low or high cutting speed
• Small rake angle
• High friction at the chip-tool interface
• Large depth of cut
• Low feed rate

Advantages of Discontinuous Chips

• Easy chip disposal – short segments are simple to collect and remove
• Better surface finish on brittle materials – fracture happens cleanly
• Reduced power consumption during brittle material machining
• Good tool life when machining appropriate (brittle) materials

Disadvantages

• When discontinuous chips form in ductile materials, they indicate poor cutting conditions, resulting in rough surface finish and accelerated tool wear
• Interrupted cutting can cause vibration and chatter

3. Continuous Chips with Built-Up Edge (BUE)

What Are BUE Chips?

BUE chips are a variation of continuous chips where a hardened layer of workpiece material becomes welded to the cutting tool’s nose. This hardened layer is called the Built-Up Edge. Chips then flow over and break against this edge rather than the original tool face.

How Does a Built-Up Edge Form?

When machining ductile materials at low-to-medium cutting speeds with high friction at the tool-chip interface, extreme heat and pressure build up at the tool nose. The adjacent workpiece material softens, gets compressed, and metallurgically bonds (welds) to the tool. This welded mass is the built-up edge.

As machining continues, pieces of the BUE periodically break off. Some are carried away in the chip, and some remain on the machined surface, causing roughness.

Conditions That Promote BUE Formation

• Ductile workpiece material
• Low cutting speed (insufficient to clear heat)
• Small rake angle
• High feed rate
• Lack of cutting fluid (high friction, high temperature)
• Absence of anti-friction coatings on tool

Advantages of BUE

• The hardened BUE temporarily protects the tool face from direct contact with the workpiece
• Can slightly extend tool life in the short term (before BUE breaks off)

Disadvantages of BUE

• Poor surface finish – broken BUE fragments embed in or scratch the workpiece surface
• Changes the effective rake angle of the tool, altering cutting forces unpredictably
• Dimensional inaccuracy – the effective tool geometry shifts as BUE builds and breaks
• Promotes accelerated tool wear over time when BUE repeatedly breaks off

How to Prevent BUE Formation

• Increase cutting speed (high speed clears the heat zone)
• Use a larger rake angle tool
• Apply cutting fluid generously
• Use coated tools (TiN, TiAlN coatings reduce friction)
• Reduce feed rate

Comparison Table: Continuous vs. Discontinuous vs. BUE Chips

Factor	Continuous	Discontinuous	BUE Chips
Material type	Ductile	Brittle / Hard ductile	Ductile
Rake angle	Large	Small	Small
Cutting speed	High	Medium or High	Low or Medium
Friction (chip-tool)	Minimum	Maximum	Maximum
Depth of cut	Small	High	Medium
Surface finish	Excellent	Good (brittle) / Poor (ductile)	Poor
Tool life	Long	Good (brittle materials)	Reduced (long-term)
Chip disposal	Difficult	Easy	Moderate

Factors Affecting the Type of Chip Formed

Several variables determine which chip type forms during machining:

1. Workpiece material – ductile vs. brittle is the most influential factor
2. Cutting speed – higher speeds favor continuous chips; low speeds promote BUE
3. Rake angle – larger angles produce continuous chips
4. Feed rate – excessive feed promotes BUE or discontinuous chips
5. Depth of cut – deeper cuts often produce discontinuous chips
6. Cutting fluid – proper lubrication prevents BUE and reduces heat
7. Tool material and coating – coated tools resist BUE formation

FAQs: Frequently Asked Questions

Summary

The three types of chips in metal cutting – continuous, discontinuous, and BUE – each indicate different machining conditions and produce different outcomes for surface quality, tool life, and efficiency. Selecting the right cutting speed, feed, rake angle, and coolant helps machinists control which chip type forms and optimize the machining process accordingly.