How using the right cutter ‘pitch design’ can Make or Break your face milling operation

All workpieces can have challenging features to machine, such as interrupted and rough surfaces, excessively large blank stock, thin walls and faces, high surface finish requirements, and many more. Using the correct ‘pitch’ on your milling tool greatly impacts the machining process and dramatically affects vibrations, chatter, surface finishes, and productivity levels.

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How can you choose the correct ‘cutter pitch’ design for your next Face Milling Operation?

To answer this question, we first need to discuss the term ‘Pitch” and the terms Close, Course, and Normal Pitched cutters.

‘Pitch’ refers to the number of inserts in an Indexable Milling cutter and can be classified as having coarse, close, or normal pitch. These insert pitches can also be placed equally around the cutter or at an unequal irregular distance, known as the differential pitch.

Now, let’s talk about what Close, Course, and Normal Pitched cutters are, starting with “What exactly is a Normal Pitched Cutter?”. Well, when tool designers develop a milling cutter, two central elements define the number of cutting inserts used. (amongst others).

• The first element is the amount of steel behind the insert mounting area, ensuring it has enough strength to handle the cutting forces and temperatures generated throughout the cutting process.
• The second element is the amount of space given in front of the cutting inserts, ensuring there is sufficient chip evacuation space to handle the volume of removed material.

These two elements eventually govern the number of cutting inserts used on a milling cutter design, which we name “Normal Pitched Cutters.”

Using the “Normal Pitched Cutter” design as the reference, if the designer purposely adds more cutting inserts, we name them “Close Pitched Cutters.” And, if the designer purposely reduces the number of cutting inserts, we name them “Course Pitched Cutters.”

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So, what is the purpose of Differential pitch?

Milling cutters with differential-pitch configurations (unequally spaced inserts) break up the harmonics caused during the cutting process. This design reduces vibrations in the machining process and increases tool life and surface finish possibilities. The differential-pitch configuration performs well when face milling large widths of cuts and long overhangs.

The benefits of Differential Pitch are:

• It helps to reduce vibrations in the machining process
• It increases tool life and surface finish possibilities.

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Most standard indexable milling cutters incorporate the differential pitch design. However, “Close Pitched Cutters” typically do not have a differential pitch.

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When should I use each of these designs?

“Normal Pitched Cutters” are used for everyday use. Most of today’s indexable milling operations use the Normal Pitched Cutter design as they provide reliable performance and versatility when used within their recommended cutting parameters. These cutter types are ideally suited for:? ?

• Regular facing operations with typical feed rates and depth-of-cuts.
• Milling machines with average torque and power.
• Workshops wanting to minimize their overall tool inventory, as this design is ideally suited for all workpiece materials and any facing application.

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“Close Pitched Cutters” have more inserts than ‘normal pitched cutters’ offering better tool life and higher productivity due to the spread of the total tool wear amongst more inserts. They are mainly used in high-production environments on high-powered machines, taking small depth of cuts and high feed rates.

These cutter types are ideally suited for:?

• High-production environments.
• Finishing operations that have smaller depths of cut.
• Machines with high torque and power.
• Stable operations where vibration isn’t a concern. ?

?Precautions:

• They don’t typically have differential pitching to reduce chatter.
• Close-pitched cutters offer us higher productivity, but we must limit the cutting conditions to smaller cut depths and ‘feeds per tooth’ than ‘normal pitched cutters.’

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“Course Pitch Cutters” have a reduced number of inserts, resulting in fewer teeth in cut during the machining process. This reduction reduces cutting forces, deflection, and deformation and lowers the risk of vibrations, which leads to better overall surface finishes. Also, you can increase your DOC to improve productivity due to the lower cutting forces.

These cutter types are ideally suited for:?

• Facing operations that have poor workpiece stability.
• Heavy roughing operations that require several large ‘depth-of-cuts’.
• Machines with low torque and power.
• Reducing the risk of vibrations in unstable workpiece setups.
• Finish passes in unstable operations –

Precautions:

• Don’t underestimate their finishing capabilities in unstable operations.

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Let’s sum it up.

Selecting which pitch design suits you and your needs comes down to your application and desired Production Economy goals. Determining the potential manufacturing issues, such as interrupted and rough surfaces, thin walls and faces, or high surface finish requirements, will help you select the right tool for your machining needs.