Machining Delrin — Speeds, Feeds, Tooling & Tips (POM-H)

Machining Delrin is the most straightforward CNC plastic operation most shops encounter. It cuts cleanly without smearing, produces short chips that evacuate easily, holds ±0.001–0.002" dimensional tolerances on a well-maintained CNC lathe or mill, and requires no special tooling for standard grades. The challenges are grade-specific: Delrin 570 (glass-filled) accelerates tool wear dramatically, and Delrin 100AF generates a dry lubricant dust that requires chip collection. This guide covers the full workflow for all Delrin grades.

At a glance:

• Delrin machines dry for most operations; coolant is optional but helpful for deep cuts and thin walls
• Turning SFM (Delrin 150/500): 400–800 SFM; feed 0.004–0.012 IPR
• Milling SFM: 300–600 SFM; chip load 0.003–0.006 IPT
• Tooling: sharp uncoated carbide for standard grades; PCD for Delrin 570 production runs
• No pre-annealing required in most cases; optional for very tight tolerances in large cross-sections
• Avoid laser cutting — formaldehyde gas is generated; use waterjet, bandsaw, or router instead
• Target keyword: "machining delrin" — US volume 90/month, CPC $2.50

Why Delrin Machines Differently from Other Plastics

Delrin is semicrystalline (75–85% crystallinity), which produces a fundamentally different chip character than amorphous plastics like polycarbonate or ABS. Amorphous plastics tend to smear under tool pressure when heat builds up; Delrin's crystalline structure fractures cleanly, producing short, curled chips rather than long strings or gummy smears.

The comparison to aluminum is apt: Delrin machines at similar surface speeds (slightly slower), requires sharp tools, and holds tolerances comparable to 6061-Al for moderate cross-section parts. The key differences from aluminum are lower thermal conductivity (heat stays in the cut zone longer), significantly higher CTE (more thermal expansion per degree), and much lower stiffness (workpieces spring away from the tool under load in thin sections).

Delrin vs. Acetal Copolymer Machinability

Standard (unfilled) Delrin 150 and acetal copolymer (Celcon, Hostaform) machine nearly identically. The differences are minor:

• Delrin 150 tends to produce slightly shorter, more controlled chips
• Acetal copolymer has a slightly lower melting point (165°C vs. 175°C) — in thin-wall turning, this means the surface can glaze slightly sooner if SFM is too high
• Otherwise: same speeds, same feeds, same tooling for both materials

For machining acetal copolymer specifically, see the acetal copolymer machining guide.

Pre-Machining Preparation

Do You Need to Anneal Delrin?

Delrin stock shapes carry less residual stress than PEEK or polycarbonate, so pre-annealing is not routinely required. Exceptions:

• Very tight tolerances (±0.0005" or tighter) on large cross-sections (>3" diameter rod or >1" thick sheet): consider a light stress-relief anneal at 200°F (93°C) for 2–4 hours
• Parts with thin walls after machining that were cut from thick rod: residual stress relief can cause post-machining distortion if the stock was stressed; pre-anneal if distortion history with similar geometry

Annealing protocol for Delrin:

• Temperature: 200–220°F (93–104°C) — do not exceed 250°F (121°C)
• Time: 1 hour per inch of cross-section, minimum 2 hours
• Cooling: slow cool in oven to room temperature; do not quench

Stock Inspection

For rod diameters above 3.0 inches, inspect the cross-section for centerline porosity before setting up a part that uses the core material. Delrin (POM-H) homopolymer has a known tendency for porous cores in large cross-sections. If centerline porosity is unacceptable for the application, substitute acetal copolymer rod.

Turning Parameters

Recommended Speeds and Feeds (CNC Lathe)

Turning Tooling

Standard grades (Delrin 150/500/100AF): Sharp, positive-rake uncoated carbide (C-2 grade). High-speed steel (HSS) can be used for short runs or prototyping. Keep tool edges sharp — a worn insert dramatically increases cutting force and surface temperature, leading to glazed surfaces.

Delrin 570 (glass-filled): TiAlN-coated carbide or PCD inserts. Glass fiber is abrasive; carbide tool life in production turning of Delrin 570 is 30–50% of that for unfilled grades. Monitor insert condition closely.

Key turning tips:

• Maintain continuous chip flow; interrupted cuts spike temperatures
• Support parts longer than 6× diameter with live center or steady rest
• Flood coolant is not required but is beneficial for deep cuts and thin-wall finishing
• Clear chips frequently; re-cut chips produce surface marks and add dimensional error

Milling Parameters

Recommended Speeds and Feeds (CNC Mill)

End Mill Selection

Use 2- or 3-flute end mills with sharp edges and helix angle 30–40° for good chip evacuation. Uncoated carbide for standard Delrin; TiAlN or PCD for Delrin 570. Avoid 4+ flute mills in roughing passes — chip evacuation is critical in plastics.

Milling tips:

• Climb milling gives slightly better finish on Delrin; conventional milling is also acceptable
• For slots and pockets deeper than 3× tool diameter, use flood coolant or compressed air blast for chip clearing
• Thin walls (<0.060"): reduce chip load by 50%; use multiple light passes rather than one deep pass
• Delrin 570 (glass-filled): use dust collection; glass fiber particles are respiratory irritants

Drilling

Delrin drills cleanly with standard practices, but chip evacuation is critical in deep holes.

Drill Parameters

Drilling tips:

• Use a spot drill or center drill to locate holes — Delrin offers low tactile resistance on entry
• Peck drill (0.5–1× diameter peck increments) for holes deeper than 5× diameter
• Drill 0.005–0.010" undersize and ream to final diameter for precision bores
• Coolant or compressed air improves chip evacuation in deep holes; dry drilling is acceptable for shallow holes

Hole Tolerances After Drilling and Reaming

Tapping and Threading

Delrin accepts taps well. Thread quality is good, and thread strength is adequate for moderate fastener loads.

Tapping guidelines:

• Use spiral-flute (gun) taps for blind holes; spiral-point (gun tap) for through holes
• Drill to 70–75% thread engagement — full 100% engagement increases tap breakage risk without significantly increasing pull-out strength in Delrin
• Tapping fluid or light cutting oil recommended; dry tapping is workable for coarse threads in unfilled Delrin but increases tap wear
• For threads in Delrin 570 (GF): use carbide taps or premium coated HSS; glass fiber accelerates tap wear significantly
• For repetitive fastener removal/installation: consider thread inserts (Heli-Coil, E-Z Lok) in coarse threads

External Threading

Single-point threading on a CNC lathe produces excellent thread quality in Delrin. Alternatively, thread milling provides good results for both internal and external threads in larger-diameter Delrin parts.

Sheet Fabrication: Sawing, Routing, Waterjet

Bandsaw

Standard bi-metal bandsaws cut Delrin sheet and rod efficiently. Blade pitch: 6–10 TPI for sheet up to 0.5"; 4–6 TPI for thicker sections. Feed slowly to avoid melting at the cut face.

Router / CNC Router

Routing Delrin sheet is the standard shop method for flat-blanked parts. O-flute (single-flute) or 2-flute upcut carbide bits work well. SFM: 400–700; chip load 0.003–0.006 IPT. Vacuum table or double-sided tape for workholding on thin sheet.

Waterjet Cutting

Waterjet is the preferred method for cutting large sheet blanks and complex 2D profiles from Delrin. It produces clean edges without heat-affected zones and no chemical vapors. Tolerances achievable: ±0.010–0.015" for standard parts; ±0.005" for precision waterjet.

Laser Cutting — AVOID

Do not laser-cut Delrin or any POM. Formaldehyde gas is generated when polyoxymethylene is exposed to laser energy. This is a serious occupational health hazard. CO₂ laser systems with activated-carbon filtration may reduce — but not eliminate — the exposure risk. Waterjet is the safe alternative for Delrin sheet cutting.

Surface Finishing

As-machined Delrin has a surface finish of Ra 32–63 µin with standard turning parameters; finishing passes can reach Ra 16–32 µin. For functional surface requirements:

• Sealing surfaces (o-ring grooves, face seals): Ra ≤ 32 µin standard for compressed o-ring seals; Ra ≤ 16 µin for dynamic seals
• Bearing bores: Ra 16–32 µin in finished bores for optimal bushing run-in
• Cosmetic/aesthetic surfaces: Ra 63 µin as-machined is typically acceptable; bead blasting produces a uniform matte at Ra 100–200 µin

Delrin cannot be solvent-polished (it resists virtually all common solvents). Mechanical polishing with progressively finer abrasives is the only path to mirror-like surfaces.

More related guides

Compare to other materials

• Delrin vs. Acetal Copolymer
• Machining Acetal Copolymer
• Machining PEEK

Applications

• Delrin Gears and Precision Parts
• Bushings and Bearings in Delrin

Frequently asked questions — Delrin FAQ