Machining Ultem PEI — Feeds, Speeds & Tooling Guide
Ultem (polyetherimide, PEI) machines on standard CNC milling, turning, and drilling equipment—but it demands more care than acetal, nylon, or polycarbonate. As an amorphous thermoplastic, Ultem is notch-sensitive and prone to residual-stress cracking: stresses introduced during extrusion or compression molding, and additional stresses from aggressive machining, can cause delayed surface cracks or outright fracture in service. The correct protocol is: anneal stock before machining, use sharp carbide tooling, keep depths of cut and feed rates conservative, and never use chlorinated cutting fluids.
At a glance:
- Anneal all stock before roughing—critical for stress-crack prevention
- Sharp C-2 or uncoated carbide tooling; HSS acceptable for light work on unfilled 1000
- Speeds: 500–1,000 SFM for milling (unfilled); reduce 20–30% for GF/CF grades
- Feed rates: 0.002–0.005 IPT for milling; 0.003–0.006 IPR for turning
- No chlorinated solvents as coolant—ever; use compressed air or water-soluble coolant
- Post-machining stress-relief anneal recommended for precision or highly loaded parts
- Final finishing passes: ≤ 0.005" depth of cut
Why Ultem Requires Special Machining Attention
Amorphous Structure and Notch Sensitivity
Ultem is amorphous—its polymer chains are not organized in crystalline domains the way PEEK or nylon chains are. Amorphous plastics have higher transparency potential and better bond-with-solvent capability, but they are more notch-sensitive. A sharp internal corner, a tool mark, or a machining-induced surface crack acts as a stress concentrator. Under applied load or residual stress, cracks propagate from these sites.
This is not theoretical. Ultem parts that look fine after machining have cracked days or weeks later when residual stress from extrusion or machining finally finds a relief path. The mitigation is systematic annealing—before machining to relieve stock stresses, and after machining to relieve cutting-induced stresses—and design practices that eliminate sharp internal corners.
Glass-Filled Grades Add Tool Wear
Ultem 2200 and 2300 contain short glass fibers that are harder than carbide at the micro-scale. Every tool pass contacts and fractures glass fibers, and fiber ends abrade the cutting edge. For glass-filled grades, expect cutting tool life to be 40–60% shorter than for unfilled Ultem 1000. Use exclusively carbide tooling (no HSS) for GF grades, and inspect edge condition frequently. Carbon-fiber-filled Ultem CF is similarly abrasive—carbide tooling mandatory.
Pre-Machining Anneal Protocol
Annealing removes residual stresses built into the stock during extrusion or molding. This is not optional for parts with tight tolerances or applications involving sustained stress.
Use a calibrated convection oven—not a heat gun or open flame. Thermal gradients cause differential expansion and introduce new stresses if the part heats unevenly. Place stock flat on a non-reactive surface (aluminum plate works); do not stack pieces directly on each other without separation.
If you receive Ultem stock that shows visible bowing, surface crazing, or internal stress birefringence under polarized light, anneal before any machining operation regardless of part size. Stressed stock will shift dimensions unpredictably when material is removed.
Milling Ultem PEI
Tooling
Use C-2 grade uncoated carbide end mills with sharp cutting edges. TiN and TiAlN coatings are acceptable and extend tool life on glass-filled grades. Two- or three-flute end mills provide better chip clearance than four-flute tools and reduce heat buildup in the cut zone. Rake angles of 10°–15° are appropriate; more aggressive positive rake angles can cause chipping or dig-in on brittle glass-filled grades.
Avoid dull tooling at all costs. A dull edge generates heat through friction rather than cutting, and heat in Ultem—especially near the Tg—introduces surface stress and potential surface degradation. Heat also causes workpiece distortion if localized. Sharp edges cut cleanly and cool efficiently.
Speeds and Feeds — Milling
| Grade | Surface Speed (SFM) | Feed per Tooth (IPT) | Max Axial DOC |
|---|---|---|---|
| Ultem 1000 | 700–1,000 | 0.003–0.005 | 0.5× D |
| Ultem 2200 | 500–800 | 0.002–0.004 | 0.4× D |
| Ultem 2300 | 500–750 | 0.002–0.003 | 0.4× D |
| Ultem CF | 400–700 | 0.002–0.003 | 0.35× D |
These are starting parameters; adjust based on machine rigidity, fixture quality, and tool condition. Increase feed rate before increasing speed if you observe heat discoloration or workpiece temperature rise—higher feed = less heat per unit volume removed.
Roughing and Finishing
Roughing cuts can take depths of cut up to 0.050"–0.100" for material removal, but leaving 0.010"–0.020" for finish passes is essential. Finishing passes of 0.003"–0.005" depth produce the cleanest surfaces and minimize stress in the cut layer. For critical sealing surfaces or precision fit bores, use 0.002" final passes at reduced feed.
Do not use climb milling on the final pass when the part is thin or poorly fixtured—the lateral cutting forces in climb milling can flex or fracture thin-wall sections.
Turning Ultem PEI
Tooling for Turning
C-2 or C-3 carbide inserts are the standard for Ultem turning. Sharp positive-rake geometry inserts (10°–15° positive) reduce cutting force and heat generation. Avoid large nose radii on finishing inserts—they increase radial cutting force and can flex slender Ultem workpieces, producing chatter.
For glass-filled grades, use CVD or PVD coated carbide (TiAlN preferred) to extend insert life. Inspect and replace inserts before edge radius exceeds 0.002"—a rounded edge is the primary cause of heat-related surface defects in Ultem turning.
Speeds and Feeds — Turning
| Grade | Surface Speed (SFM) | Feed Rate (IPR) | Finishing DOC |
|---|---|---|---|
| Ultem 1000 | 600–1,000 | 0.004–0.007 | 0.003"–0.005" |
| Ultem 2200 | 400–700 | 0.003–0.005 | 0.003" |
| Ultem 2300 | 400–700 | 0.003–0.005 | 0.003" |
| Ultem CF | 350–600 | 0.002–0.004 | 0.003" |
Boring and Internal Turning
Internal bore work requires rigid boring bars—Ultem is stiff enough that boring bar deflection shows up as diameter error and chatter marks on the bore surface. Use boring bars with minimum overhang. For deep bores (L/D > 4), use a bar with vibration dampening. Take a final spring pass (no feed advance) to remove any remaining tool deflection artifact.
Drilling Ultem PEI
Conventional twist drills work for Ultem with these adjustments: use 10°–15° clearance angle (standard metal jobber drills create excessive heat); apply consistent feed pressure without dwelling; peck drill for holes deeper than 3× diameter to clear chips. For tight-tolerance holes (H7 or tighter), drill 0.010"–0.015" undersize and ream to final diameter—drilled holes run slightly oversize from drilling heat.
Tapping and Threading
Ultem taps well in the unfilled grade using standard ASME tap drill charts. For glass-filled grades, use carbide or HSS-Co taps and reduce cutting speed by 30%. Avoid fine threads in thin walls—thread pull-out is a failure mode in brittle GF grades. For high-cycle or high-load threaded connections, heat-set or ultrasonic metallic inserts outperform direct tapped threads in long-term fatigue.
Coolant and Cutting Fluid
Approved coolants for Ultem:
- Compressed dry air (preferred for thin parts or where moisture is a concern)
- Water-soluble coolant (4–6% concentration in deionized water)
- Deionized water mist
Prohibited:
- Chlorinated solvents (methylene chloride, TCE, TCA, chloroform): Attack Ultem aggressively, causing stress cracking and surface dissolution even in brief contact
- Petroleum-based straight oils: Can cause surface staining and are incompatible with medical and semiconductor applications
- Ketone-based fluids (acetone, MEK): Attack Ultem
If contamination with a prohibited fluid occurs, immediately flush the part with isopropyl alcohol, rinse with water, and inspect for surface crazing under good lighting before proceeding.
Post-Machining Stress-Relief Anneal
After final machining, a stress-relief anneal removes cutting-induced surface stress and reduces delayed-cracking risk in service. Protocol: 300°F (149°C), 1–2 hours depending on section thickness, followed by slow furnace cool. For tight-tolerance parts (≤ ±0.002"), anneal before final measurement—stress relief can shift dimensions slightly.
See the Ultem grades page for information on how tooling recommendations differ between 1000, 2300, and CF grades. For PEEK machining comparison—a material that machines similarly in some respects but behaves differently due to its semicrystalline structure—see the PEEK material guide.
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