Let's Talk Chips

Well ... it's the name we have chosen for the blog ... so let's start the talk with that subject. I would like to devote this first article to the highlighting of some of the key points about chips ... what they mean ... how they are made ... because a deep understanding of these points is essential to good chip making.

Lets start with the Types of Chips :

• Discontinuous Chips : Discontinuous chips are small broken chips and may form when cutting at low speeds or when cutting a material containing points of stress concentration, such as cast iron that contains graphite flakes or free-machining steel. These chips are a series of chip segments that have broken at areas of stress concentrations in the workpiece material.
• Continuous Chips : At speeds at which the temperature at the chip / tool interface is relatively low, the chip being created can split and a portion of the chip is left behind. This portion of the chip attaches itself to the to the face of the cutting tool and is commonly referred to as the "built up edge" or BUE. This left-over material then acts as a cutting edge and will usually continue to grow until it reaches a size where it then passes off with the chip. This process repeats itself many times during the cutting process producing a variety of continuous chips. This process is a major factor in surface finish since the BUE varies in size and depth and effects the depth of cut at the cutting tool edge and thus the surface finish produced during removal.
• Seconday Shear in Chipmaking : As the metal is being removed and the chip created, the chip rides up the face of the cutting tool. When the sticking stress of the chip on the cutting tool face equals the stress of the chip removal at the material, additional stress is transformed internally into the chip being created. This is commonly referred to as secondary shear. Since secondary shear is due to high stress on the tool face, it is always accompanied by an increase in cutting forces. The remedy for secondary shear is normally an increase in the shear angle of the cutting tool.

General Considerations in Chipmaking

• Chipbreakers : Chipbreakers are often added to the face of a cutting tool to insure that the chip is broken periodically. The distance the chipbreaker is away from the cutting edge and the style of chipbreaker determine the length of the chip produced and the frequency of those breaks. Chipbreakers can be seperate from the cutting tool or insert or built directly into the tool or insert.
• Chip Color : Contrary to popular belief, the color of the chip is not a direct reference to the heat of the chip or the heat present at the time the chip was produced. Rather, the color of the chip is a reflection of the oxide coating of the chip and only a minor reflection of the cutting temperature involved in the creation of the chip. The amount of oxide coating can be effected by the use or non-use of coolant ( more oxygen is present at the cut ), feedrate ( chips produced under a high feedrate remain hot longer ), as well as the materials ability to oxidize. 
• Cutting Forces : 
• Cutting forces are less with discontinuous chips than with continuous chips.
• A built-up edge may decrease or increase cutting forces
• The greater the secondary shear zone on the face of the tool the greater the cutting forces.
• Cutting forces are decreased by reducing the chip thickness.
• Surface Finish : 
• The main components that effect surface finish are built-up edge, feed marks caused by a secondary cutting edge (BUE) and tool instability or chatter. 
• Feed marks are often eliminated by a reduction in feedrate and an increase in the tool nose radius of the tool.
• BUE can often be reduced by increasing the cutting speed, reducing the tool face friction or through the use of coolant containing free machining additives. 
• Chip Control : 
• Long, unbroken chip are a danger both to the operator and to the workpiece.
• Broken chips allow for easier removal from the machine both in terms of maintenance and in cutting.
• Chipbreakers should be employed where necessary to force the breaking of chips.

So there you have it ... what I consider some of the essentials of Making Chips ... I hope you will find these ideas helpful and informative.

Information in this article was taken from Kentech's KipwareEDU® - Mechanical Engineering Version. If you found this information helpful ... we invite you to explore all our versions of KipwareEDU® in more detail ... just click here.