Machining Peek: Front & Back Turning, Cut-off, Grooving / Side Turning, and Threading.

NTK KM1 Grade Mirror Finish Insert for Front & Back Turning, Cut-off, Grooving / Side Turning, and Threading

A trend that we are seeing develop in many industries, especially in the medical implant device manufacturing sector, is a rise in the number of parts being manufactured from plastics such as Peek.  Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PAEK) family. It was originally introduced by Victrex PLC, then Imperial Chemical Industries in the early 1980s.

Its important to note that PEEK is a thermoplastic.  This polymer is capable of being repeatedly softened by an increase in temperature.  Increasing the temperature leads to a physical change.  That's why cutting tool pressure and heat will impact surface finish and tool life.
 
The medical industry has found medical-grade PEEK offers excellent strength, wear resistance and biocompatibility for components such as the dental healing caps, spiked washers and spinal implants. 

PEEK polymer is available in two basic grades: industrial and medical: 

​"Industrial-grade PEEK is a strong thermoplastic that retains its mechanical properties even at elevated temperatures. The flame-retardant material is abrasion resistant, has high impact strength and a low coefficient of friction. Industrial-grade PEEK components are used in the aerospace, automotive, chemical, electronics, petroleum, and food and beverage industries.

Medical-grade PEEK possesses those same physical properties in addition to biocompatibility, high chemical resistance and compatibility with several different sterilization methods. It is also naturally radiotranslucent when viewed using X-ray, MRI or computer tomography (CT). ​Medical-grade PEEK provides doctors with an unobstructed view of tissue and bone growth around the PEEK implant during the healing process.  Some implantable-grade PEEK polymers have a bone-like stiffness and can remain in contact with blood or tissue indefinitely."

There is also a glass and carbon-fiber-reinforced PEEK, which offer high wear resistance for components such as articulating joints and tends to wear out inserts rather quickly.

Peek parts are generally very small, ranging from 0.039" to 0.250" in diameter, which adds to the complexity of machining the non-ferrous materials.

Because of the thermoplastic properties, too much heat at the cutting edge results in Built Up Edge (BUE) and tool pressure affects part tolerance.
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That where NTK's KM1 grade insert work great. The inserts are extreｍely up-sharp edge and the mirror insert surface creates excellent surface finishes.

NTK KM1 Uncoated grade for CNC Turning Peek

• NTK KM1 Grade - uncoated
• Up-Sharp Insert Edge - high shearing / reduces heat generation
• Mirror Insert Surface - produces excellent finishes on workpiece
• Insert Radius - 0.00" up to 0.008"
• Ideal features for machining Plastics, Copper, Brass, Aluminum
• (see pages O 36-37 in 7100 Catalog)

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Things to Remember when Cutting Peek

• PEEK shapes are stronger and stiffer than most plastics but considerably softer than most metals meaning fixturing is critical. ​
• Although PEEK has a very high melting temperature sufficient coolant is required during machining to avoid overheating and localized surface melting. 

• All PEEK grades but especially those reinforced with glass and carbon fiber are abrasive on tooling. Carbide tooling can be used for short runs. Polycrystalline (PCD) tooling should always be considered long runs, tightly tolerance parts and for reinforced grades.

Keys to Succesfull Machining of Peek

1. Limiting Heat Build-up – The softening or melting temperatures of engineering plastics are roughly 1/10th those of metals.
 
2. Melting or Scorching– The thermal conductivity of plastics is low relative to metals.  Most of the heat generated by machining will stay at the surface.  Temperatures at the surface can rinse unexpectedly high. 
 
3. Loss of Tolerance – If the overall temperature of the stock changes during or after machining, expansion or contraction can cause the part to fall out of tolerance.  Softening of the stock can allow it to deflect at the surface under the pressure of the cutting tool.  When the pressure is removed, the stock will recover and fall out of tolerance. This can frequently  be managed by using lubricants and changing tooling or speed.
 
4. Controlling Deflection – Plastics inherently vary in their stiffness (modulus) and are more elastic at higher temperatures.  The entire stock can deflect under the pressure of cutting.  Proper tooling and support remains important and particular attention should be given to adequately supporting the work.

Additional Resources:

• Quoted portions of this article originally appeared in Modern Machine Shop, "A Quick Look At PEEK Machining" by Derek Korn, 6/1/2007
• Keys to Success Section from "Machining PEEK Plastics Guide" by Drake Research.