Machining PEEK: Properties, Challenges, and CNC Tips

Machining PEEK: What Every Engineer Should Know Before Cutting This Thermoplastic

PEEK is one of the most difficult plastics to machine—however the experience of machining it successfully and efficiently is immensely satisfying. With heat resistance, chemical inertness and tensile strength similar to metals, PEEK is the engineering plastic of choice for replacing metal components for use in everything from spinal implants to jet engine seals. Take a quick tour through the tooling, feeds and coolant choices and the annealing techniques that make or break a successful PEEK CNC machining operation.

What Is PEEK and Why Does It Matter in CNC Machining?

Polyether ether ketone (also written polyetheretherketone) or PEEK is a semi-crystalline polyaryletherketone (PAEK) thermoplastic polymer. It was first synthesized by Victrex in the 1980s and has established itself as one of the most high-performance engineering plastics for CNC machining processes. It is easy to understand why the machinist treat it differently to other thermoplastics such as nylon or acetal with these properties.

The PEEK material is unique in several quantifiable ways. Its continuous operating temperature is 250 °C—around 5x POM. Tensile strength hovers around 90-100 MPa, similar in strength to a high-alloy aluminum. Density only reaches 1.32 g/cm. Combined, these properties make it ideal for light weight load-bearing structural components, such as those in the aerospace industry.

Its chemical inertness extends across hydrocarbons, strong acids and many organic solutions. When used in medical applications, the biocompatible Nature of PEEK（as docμmented in ASTM F2026-17) means it is suitable for direct seating in spinal fusion cages, dental abutments and fixtures, etc. In electronics, its very low outgassing characteristics, stable electrical dielectric thermally in cycling.

The key quality that makes this high performing polymer both desirable and impossible to machine is this same thermal stability. PEEK does not exhibit the typical thermoplastic. Instead, it maintains its strength right up to its transition temperature of 143 °C, then transforms abruptly. It is this narrow temperature window that keeps the heat generated during machining constrained at the tool-workpiece cutting zone, as described in the following.

Key takeaway: With its ability to tolerate high heat, chemicals and moisture easily, in addition to its biocompatibility and mechanical properties, PEEK has few applications to replace—yet those same properties pose some distinct machining challenges.

Why PEEK Is Difficult to Machine — 3 Core Challenges

Machining PEEK is entirely feasible but punishing to any shortcuts taken. Three material properties account for most of the issues machinists face; each one must be countered with these machining techniques to prevent scrap parts or deformation after CMM inspection.

Main point: The three issues thermal isolation, internal stress, and price (cost of material) are additive in nature. Solving only one and neglecting the others will still result in rejects. A successful process will tackle all three issues at once.

Cutting Tools for PEEK: Carbide, Diamond, and PCD Options

Tool choice for PEEK is really practically limited to what grade you are cutting. Unfilled glass filled, and carbon fiber filled PEEK all cut with very different processes and not optimizing that tool type will result in factor 10 differential wear. Below is a simple decision guide used by most production shops.

Carbide inserts C-2 for unfilled PEEK will work well, but must be finely ground and sharp, – ie not sintered edges. Asharp carbide rake will cause too much friction and overheat the PEEK, softening above its melting point (> 138 °C). It is best to replace dull tips as they begin to round off, rather than wait until they are visibly worn.

Glass filled PEEK (roughly 30% glass fiber in a PEEK matrix) is abrasive enough to degrade carbide edges in a few dozen parts. While the cost of PCD tooling is higher, longer tool life (5-10x) makes it cheaper per part for production runs above 50. Silicon carbide cutting tools can sometimes be suggested for ceramic composites however these are not compatible with glass filled PEEK.

Requires diamond-tipped tools for milling. Carbon fibers are harder than glass ones in CF-PEEK, and they are oriented in directions that significantly accelerate localized wear of non-diamond edges and tips. CVD diamond coated end mills are now all common as standard milling tools for CF-PEEK parts.

Pre-embargo key lesson: Choose tool based on what PEEK you’r machining: carbide is fine for unfilled, PCD for glass-filled. Diamond for carbon-fiber variants. It doesn’t matter as much how much the actual tool costs–what will matter more are the economics of the part that run out on a production batch.

PEEK Machining Parameters — Speeds, Feeds, and Coolant Strategy

The correct machining parameters for PEEK exist at a narrow range. Too slow means too much heat enters the workpiece as the chip dwells. Too fast, and the workpiece gets too hot, and a lack of chip flow packs heat back into the cut. Drake Plastics—a rare hat trick as far as PEEK companies go, as they extrude and machine stock shapes—sets out reliable “ramp-up” data production shops can tweak:

The recommended speeds and feeds for milling PEEK at 500-700 SFM on the surface at 0.010-0.040 in/tooth give nice chip flow that helps carry heat away. MDFN is also a nice cut depth for tending force levels and providing reasonable throughput. Going deeper by too much—more than 0.100 in/ pass—tends to cause deflections on thin sections, while blowing out the thermal limits of heat transfer up those same walls.

Turning speeds (400-600 SFM) and feeds (0.005-0.015 in/rev) reflect material removal with minimal force settling. Wraps are best to clean sections, but perforations or other interrupted cuts lead to more attention of sharpness on inserts. Drilling requires peck cycles to clear chips from the hole before pack heat together. Chunks too big over 3 X-dia will bake the edges and lend to additional tear.

Surface finish on machined PEEK is typically Ra 1.6–3.2 μm with standard parameters and knife-sharp tooling. That level meets most industrial sealing and bearing needs. Since tighter surface finish standards exist—Ra 0.2–0.4 μm—secondary polishing with abrasives of fine grains is needed. Pre-finishing on machine may be performed on the order of 2500-25000 mm/min after care of work table, pathway, and fixturing, whereas dedicated hand finishing with abrasives yields a more consistent part.

Lecreator’s precision PEEK parts manufacturing center includes a dedicated PEEK production cell with a controlled environment and dedicated tooling to maintain the precision this material requires:

In the fundamental lessons, start from published data from Drake Plastics, choose air as a default coolant and specific no flood delay different tool profiles accordingly and expect you’ll need some post-machine polishing in finer surface qualities from a secondary abrasive set.

1.2.3 Processing Characteristics

PEEK Grades and Their Machinability: Unfilled, Glass Filled, and Carbon Fiber

Pursuing that thought further, not all PEEK is created equal. Grade selection determines tooling, parameters, and cost- and the wrong grade for an application can be downright expensive. Four general groups are used to classify most CNC-machined PEEK parts- and each presents different machinability factors to the shop:

Unfilled (natural) PEEK machines the least amount of dirt. The Victrex PEEK 450G data sheet specifies a tensile strength of 100 MPa with a 30-40% elongation at break. Such ductility results in the formation of smooth chips which tend to evacuate without fracturing.

Natural PEEK is the standard by which seals and bearings are made along with electrical insulators where dirt resistance (abrasion resistance) is of little importance. This is the starting point for the vast majority of PEEK plastic components unless a derivate is specifically needed.

Glass filled PEEK (usually 30% by weight) increases the rigidity modulus form 3.6 GPa to about 11 GPa and enhances creep resistance at elevated stress level for long durations. Disadvantage of the glass fiber reinforced stock is abrasive effect. Carbide tools which last over 500 parts for unfilled PEEK may last only 50-80 parts on the glass filled stocks before the edge dullness compromise surface finish quality.

PCD tooling will be the cost effective decision for batch sizes above 30-50 pieces.

PEEK CG50 CF has tensile modulus over 200 MPa and density below 1.45 g/cc. It is often used in aerospace structural applications and high performance racing products where weight saving outweighs other considerations. Carbon fibers are much harder than glass fibers and tend to produce fine abrasive dust during machining.

If diamond tooling is not used then it had better be optional! Tool wear data collection while machining CF-PEEK should be more controls than for other PEEK grades.

Medical grade PEEKperASTM F2026-17 is chemically identical to Natural PEEK, but manufactured with certified feedstock traceability and biocompatibility testing. Machine parameters and tooling is the same used for Natural PEEK. Where it differs is in the processing environment dry machining only, no coolant contamination, with fixturing dedicated to the Medical grade material and not cross-contamination with Natural PEEK.

The potential Medical grade PEEKapplications are: spinal fusion cages, cranial implant plates and dental abutments. All of these fall under the cytotoxicity and implant-site testing.

The industrial grade PEEK astm.org/d8033-22.html”>ASTM D8033-22 meets standard mechanical and electrical specifications and does not include certification for biocompatibility. This industrial grade PEEK has found common use in ASTM injection molded PEEK pieces for high-volume manufacturing but CNC has remained the standard for prototype and low-volume manufacturing.

What to remember: Choosing grades is an engineering decision, not a purchasing decision. First choose the correct filler grade for your load case. Then determine the tooling and control parameters to suit that grade.

PEEK Machining Tolerances and Post-Machining Annealing

Tolerancing ability of machined PEEK components falls into three categories-the part feature geometry, the part’s heat treatment state-annealed or unannealed, and the degree of stock removal with respect to the equipment. Whereas for metal components machinability closely translates to tolerances, PEEK does not work the same.

Reducing from 0.05 mm to 0.025 mm triples or doubles part price for similar reasons as above; more annealing between rough and final; tighter process controls; inspection by CMM post-machining on every part. At 0.012 mm it jumps even further: 4-5X due to controlled-environment machining, CMM verification, multi-annealing cycles.

This correlation between tight tolerances and annealing is straightforward: no stress relief the PEEK part machined to 0.025 mm could relax by 0.02-0.05 mm in days or weeks after the part leaves the machine, as the stresses build back in. This could result in the part falling out of tolerance even after it leaves the machine.

Two-Stage Annealing Protocol for PEEK

The annealing process as suggested by Boedeker Plastics involves a two step process which aims at relieving the machining stress as well as improving the crystallinity of the part for dimensional stability:

For PEEK parts and PEEK features with tight tolerances, many shops use a rough-anneal-finish cycles: rough machine to 0.5mm of final dimension, anneal, and machine to final tolerance. This keeps the finishing cut from being relaxed away by stress relaxation after machining. Extra cycle time is costly, but provides significantly higher yields for tight tolerance features.

Surface cracks can form if the heating is omitted for heavily machined specimens. These micro-cracks, while unnoticeable without dye penetrant inspection. Micro-cracks of this type can also be discovered through pressure testing.

For fluid seal and vacuum conditions, annealing is not an option but a specification.

The important point is that, for PEEK, annealing is not optional. For any dimension that needs to be held in 0.025mm or less, or for any part with walls that are thin in comparison with stock size, the rough-anneal-finish cycle is the sure way to achieve dimensional stability and no surface cracks.

Where Machined PEEK Parts Are Used — Industry Applications

Applications span multiple industries where no other polymer offers the combination of temperature, chemical and load resistance. Different application areas take advantage of different subsets of PEEK properties and consequently have different machining requirements.

However small or large the PEEK service component production run, skill and experience are required of the CNC machine shop that machines the parts. At Lecreator, 98% of our plastic machining jobs pass the first time and our customers enjoy total traceability of the imported raw material.

Summary: No other polymer is seeing such an increasing use across various industries as PEEK because each application demands a unique set of performance attributes which no other machinable polymer currently offers. From aerospace to medical to semiconductor applications, each niche has its own machining idiosyncrasies but the end result is consistently a high performance assembly.

Frequently Asked Questions About Machining PEEK