Introduction
POM-H—polyoxymethylene homopolymer, commonly known as acetal homopolymer—is a high-performance engineering plastic prized for its strength, low friction, and dimensional stability. It machines cleanly, holds tight tolerances, and performs reliably in moving parts like gears, bearings, and bushings. But machining this material requires understanding its unique properties. Its high crystallinity makes it stronger than copolymer versions but more prone to chipping. Its low friction demands secure fixturing. And heat buildup can cause surface burns if parameters are not optimized. This guide covers everything you need to know about CNC machining POM-H—from material properties and tool selection to process parameters and quality control.
What Makes POM-H Different?
A High-Performance Engineering Plastic
POM-H (Polyoxymethylene Homopolymer) is a semi-crystalline thermoplastic with exceptional mechanical properties. It differs from its copolymer cousin (POM-C) in crystallinity and strength.
Key properties:
| Property | POM-H | POM-C |
|---|---|---|
| Tensile strength | 70–80 MPa | 60–70 MPa |
| Crystallinity | 85–95% | 70–80% |
| Friction coefficient | 0.12–0.25 | 0.15–0.30 |
| Moisture absorption | 0.2–0.5% | 0.3–0.7% |
Higher strength: POM-H tensile strength is 10–15% higher than POM-C, making it ideal for load-bearing components like gears and structural parts.
Lower friction: Coefficient of friction as low as 0.12 ensures smooth operation in moving assemblies without lubrication.
Dimensional stability: Moisture absorption is minimal—0.2–0.5% after 24 hours in water. Parts maintain tight tolerances even in humid environments.
Thermal stability: Continuous use up to 90°C; short-term exposure to 120°C possible.
Electrical properties: Volume resistivity of 10¹⁴–10¹⁶ Ω·cm—excellent insulator for electrical components.
The Machining Challenge
POM-H’s high crystallinity (85–95%) contributes to its strength but also makes it more brittle than POM-C. During machining, this means:
- Increased risk of chipping at edges and corners
- Requirement for sharper tools
- Need for optimized feeds to prevent cracking
What Machining Techniques Work Best?
Milling
Milling is the most common operation for POM-H, creating complex shapes, pockets, and gears.
Tool selection:
- Carbide end mills for production runs
- 2-flute for chip evacuation
- 4-flute for finer finishes
- Helix angle 30–45° improves chip flow
Parameters:
- Spindle speed: 1,500–3,000 RPM
- Feed rate: 200–400 mm/min
- Depth of cut: 1–3 mm roughing; 0.1–0.5 mm finishing
- Climb milling preferred to reduce tool wear and chipping
Toolpath considerations:
Avoid sharp corners. Use radiused transitions to distribute cutting forces evenly and prevent edge chipping.
Turning
Turning produces cylindrical parts—bushings, rollers, threaded components.
Tool selection:
- Sharp carbide inserts with 5–10° positive rake angle
- Polished surfaces to reduce friction
Parameters:
- Spindle speed: 1,000–2,000 RPM
- Feed rate: 0.1–0.2 mm/rev
- Depth of cut: 1–3 mm roughing; 0.1–0.5 mm finishing
Workholding:
POM-H’s low friction can cause parts to slip in chucks. Use:
- Soft jaws machined to part contour
- Textured surfaces to improve grip
- Light clamping pressure—enough to hold, not enough to deform
Drilling
Drilling POM-H requires chip management. Chips can clog flutes and cause overheating.
Tool selection:
- 118° point angle drills with polished flutes
- Carbide for production; HSS acceptable for low volume
Technique:
- Peck drilling—retract every 1–2 mm to clear chips
- Speed: 1,000–2,500 RPM
- Feed: 0.05–0.12 mm/rev
Grinding
Grinding is rarely needed for POM-H—its natural machined finish is often sufficient. For ultra-smooth surfaces (Ra 0.05–0.2 μm), precision grinding can be used on flat or cylindrical surfaces.
What Tools Work Best for POM-H?
Carbide Is Essential
POM-H’s high crystallinity is abrasive. Carbide tools maintain sharp edges longer than HSS.
| Tool Material | Tool Life | Best For |
|---|---|---|
| Carbide | 2–3× longer than HSS | Production runs, high volume |
| HSS | Shorter; requires frequent sharpening | Prototyping, low volume |
| Diamond/PCD | Longest; ultra-precise finishes | High-volume precision parts |
Tool Geometry
End mills:
- 30–45° helix angle for efficient chip evacuation
- Sharp cutting edge (radius < 0.02 mm )—dull edges cause chipping
- Polished flutes prevent chip adhesion
Drills:
- 118° point angle
- Polished flutes
- Split-point design reduces thrust force
Tool Coatings
TiAlN or TiN coatings reduce friction and heat, extending tool life by 20–30% . For ultra-precise finishes, diamond-coated tools minimize tool wear over long production runs.
Tool Life Management
POM-H’s abrasiveness means tools dull gradually. Replace tools when:
- Surface finish degrades
- Chipping appears at edges
- Cutting forces increase noticeably
How Do You Optimize Machining Parameters?
Cutting Speed and Feed Rate
| Operation | Cutting Speed (m/min) | Feed Rate |
|---|---|---|
| Milling | 150–250 | 200–400 mm/min (0.05–0.12 mm/tooth) |
| Turning | 100–200 | 0.1–0.2 mm/rev |
| Drilling | 50–100 | 0.05–0.12 mm/rev |
Key principle: Too slow causes rubbing and heat buildup. Too fast risks chipping on edges. Balance for clean cuts.
Depth of Cut
- Roughing: 1–3 mm—removes material efficiently
- Finishing: 0.1–0.5 mm—achieves surface finish and dimensional accuracy
Coolant and Heat Management
POM-H is sensitive to heat. Excessive temperature causes:
- Surface burns and discoloration
- Dimensional inaccuracy
- Material degradation
Recommended cooling:
- Compressed air is ideal—cools without moisture
- Mist coolant for deep cuts or high production
- Avoid flood coolant unless necessary; POM-H absorbs minimal moisture but air is preferred
Heat prevention:
- Sharp tools
- Adequate feed rates (avoid rubbing)
- Avoid prolonged cuts in one area
What Surface Finish Can You Achieve?
Standard and Precision Finishes
| Operation | Typical Ra (μm) |
|---|---|
| Standard machining | 0.8–1.6 |
| Precision finishing | 0.4–0.8 |
| Polished finish | 0.05–0.2 (grinding/polishing) |
Achieving better finish:
- Sharp 4-flute end mills for finishing
- Reduced feed rate (0.05–0.08 mm/tooth)
- Light finishing pass (0.1–0.2 mm depth)
Post-Machining Treatments
Deburring:
POM-H can form fine burrs on edges. Remove with:
- Abrasive brushes
- Hand deburring with fine-grit sandpaper
Polishing:
For cosmetic parts, buffing wheels with plastic polish achieve mirror finishes.
Annealing:
Not typically required for POM-H due to its dimensional stability. For extremely tight tolerances, stress relief at 80–100°C for 1–2 hours may help.
What Dimensional Accuracy Is Achievable?
Tolerances
| Part Size | Achievable Tolerance |
|---|---|
| Small (<50 mm) | ±0.005–0.01 mm |
| Medium (50–100 mm) | ±0.01–0.02 mm |
| Large (>100 mm) | ±0.02–0.05 mm |
Why POM-H excels: Low moisture absorption means parts hold tolerances in changing humidity. A part machined to ±0.01 mm stays within spec across seasons.
Factors Affecting Accuracy
- Tool sharpness: Dull tools cause deflection and inaccuracy
- Clamping: POM-H deforms under pressure—use light, even clamping
- Temperature: Maintain stable shop temperature (20–22°C)
- Inspection timing: Measure after parts stabilize at room temperature
Where Is POM-H Used?
Precision Parts
Measuring instruments, calibration tools, and gauges leverage POM-H’s dimensional stability. Parts hold calibration over time without moisture-induced drift.
Bearings and Bushings
Low friction (0.12–0.25) and wear resistance make POM-H ideal for moving assemblies. In high-load applications, POM-H bearings have 30% lower wear rate than POM-C.
Gears
Higher tensile strength (70–80 MPa) allows POM-H gears to handle greater loads than copolymer gears. Quiet operation, no lubrication required, and excellent wear life.
Electrical Components
Insulators, switch parts, terminal blocks. Volume resistivity of 10¹⁴–10¹⁶ Ω·cm ensures reliable insulation.
Medical Devices
Non-implantable tools—surgical clamps, instrument handles. Chemical resistance allows sterilization with common agents.
Automotive Parts
Fuel system components, door lock mechanisms, and sensor housings. Resistance to oils and fuels ensures long service life.
Industrial Machinery
Conveyor rollers, valve stems, and wear pads. Low wear rate reduces maintenance intervals.
A Real-World POM-H Machining Case
A manufacturer producing precision gears faced chipping on gear teeth edges and inconsistent surface finish. Initial parameters:
- HSS end mills
- 3,500 RPM spindle speed
- Conventional milling
- 8% scrap rate
After process changes:
- Switched to carbide end mills with polished flutes
- Reduced spindle speed to 2,200 RPM
- Implemented climb milling
- Added radiused transitions at gear tooth roots
- Reduced finishing feed rate to 0.06 mm/tooth
Results:
- Edge chipping eliminated
- Surface finish improved from Ra 1.2 μm to Ra 0.6 μm
- Scrap rate dropped to 2%
- Gear life in application increased by 25%
Conclusion
CNC machining POM-H combines the material’s inherent advantages—high strength, low friction, dimensional stability—with proper process control. Success depends on sharp carbide tools, optimized speeds and feeds, and attention to heat management. Climb milling with radiused toolpaths prevents chipping. Adequate cooling—compressed air is ideal—prevents surface burns. And because POM-H holds tolerances even in humid environments, it delivers reliable precision components across automotive, medical, industrial, and consumer applications. With the right approach, POM-H machines into parts that outperform many metals at lower cost and weight.
FAQs
How does POM-H compare to POM-C in terms of machining difficulty?
POM-H is slightly more challenging to machine due to its higher crystallinity (85–95% vs. 70–80%). It requires sharper tools and more careful parameter selection to prevent chipping. However, it offers 10–15% higher tensile strength and better wear resistance, making it worth the additional care for demanding applications.
Can POM-H be used in high-temperature applications?
POM-H has a maximum continuous use temperature of 90°C. Short-term exposure to 120°C is possible, but prolonged exposure above these temperatures causes degradation. For higher-temperature applications, consider PEEK or other high-temperature engineering plastics.
What causes surface burns in POM-H machining, and how can they be prevented?
Surface burns result from excessive heat generation, typically from dull tools, feed rates that are too slow (causing rubbing), or spindle speeds that are too high. Prevention: use sharp carbide tools, maintain adequate feed rates, use climb milling, and apply compressed air cooling to dissipate heat.
What surface finish can I expect when machining POM-H?
Standard machining achieves Ra 0.8–1.6 μm. With sharp tools, optimized finishing parameters, and light finishing passes, Ra 0.4–0.8 μm is readily achievable. For ultra-smooth surfaces (Ra 0.05–0.2 μm), precision grinding or polishing can be used, though rarely needed for most applications.
Do I need coolant when machining POM-H?
Coolant is beneficial but not always required. Compressed air is often sufficient and preferred because it cools without introducing moisture. For deep cuts or high-volume production, a fine mist coolant can be used. Avoid flood coolant unless necessary; POM-H’s low moisture absorption means air cooling is typically adequate.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology, we specialize in CNC machining POM-H for precision components across automotive, medical, and industrial applications. Our process begins with selecting the right carbide tools and optimizing parameters for your specific geometry. We use climb milling with radiused toolpaths to prevent chipping and achieve surface finishes as low as Ra 0.4 μm. Quality control includes CMM inspection and surface finish verification to ensure every part meets your specifications. Whether you need precision gears, bearings, or custom components, we deliver reliable POM-H parts that perform. Contact us to discuss your POM-H machining project.
