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Milling plastics: end mill speeds and feeds

Published SFM and chip load for acrylic, acetal (Delrin), HDPE/UHMW, polycarbonate and nylon with single-flute O-flute and 2-flute carbide end mills, from Harvey Tool, Onsrud, Amana, Curbell and Mitsubishi Chemical, plus chip welding, air blast and worked examples.

Bars are published SFM ranges, open dots single published values. Harvey "standard end mill" rows are for ordinary carbide end mills; the plastic-cutter rows are Harvey's 2-flute plastic end mills and Micro 100's single-flute plastic routers, which share the same SFM. Mitsubishi Chemical values converted from m/min. Sources [1]-[3], [9], [11], [12].

In plastics, the problem every source keeps coming back to is heat. Harvey Performance puts it plainly: heat has to leave the contact area between tool and work quickly to avoid melting and chip welding [13]. Mitsubishi Chemical, which makes acetal, nylon and UHMW stock, says poor thermal conductivity and low melting points make avoiding heat build-up and evacuating swarf "the main skill" [12]. Onsrud says the main difference from wood routing is that cut chips can re-weld themselves to the finished surface [7].

So the published numbers split by tool, not just by material. Harvey Tool's general table gives a standard carbide end mill only 200-600 SFM in acrylics and 200-500 in polycarbonate [1]. Its 2-flute plastic end mills, and Micro 100's single-flute plastic routers, are charted at 800-1200 SFM for acrylic, acetal (Delrin) and nylon and 1200-1600 SFM for HDPE and UHMW [2][3]. Curbell's milling table gives 920-1640 ft/min for acetal, nylon 6 and polyethylene and 980 for polycarbonate [11]. Mitsubishi Chemical gives 200-500 m/min (about 656-1,640 SFM) for nylon and UHMW and 200-400 m/min (about 656-1,312 SFM) for acetal and polycarbonate [12].

Quick reference by material

Material	Plastic-cutter SFM [2][3]	1/4" chip load, 2-fl / 1-fl profile [2][3]	Material supplier SFM	Notes from the sources
Acrylic (PMMA)	800-1200	.0083 / .0123	(not in Curbell or Mitsubishi Chemical tables)	Onsrud: extruded = soft, cast = hard plastic [4]. Chips or melts if feed is wrong [10].
Acetal / Delrin	800-1200	.0083 / .0123	920-1640 [11]; 656-1312 [12]	Onsrud: soft "mechanical" plastic, O-flute [5].
Nylon 6, 6/6	800-1200	.0083 / .0123	920-1640 [11]; 656-1640 [12]	Max feed and speed to get swarf away [12]. Onsrud lists it as both soft [5] and hard [4].
HDPE / UHMW	1200-1600	.0068 / .0101	920-1640 [11]; 656-1640 [12]	Max feed and speed to avoid wrap-around [12].
Polycarbonate	(not in [2][3]; 200-500 for a standard end mill [1])	.003 standard end mill [1]	980 [11]; 656-1312 [12]	Onsrud: soft plastic, .004-.012 for finish [8]. Avoid mineral-oil coolant [12].

Chip load for a 1/4" end mill

Solid bars are Harvey Tool and Micro 100 chart values, shown as slot / profile: the dark part is the slotting value, the light part extends to the profiling value. Hatched bars are Onsrud (series 52-600, 63-750 and 63-700, soft and hard plastic tables) and Amana catalog ranges for O-flute router-style tools. Grey is Harvey's figure for a standard metal-cutting end mill. Sources [1]-[4], [9].

Those chip loads are large for a 1/4" tool. At 1/4" Harvey's 2-flute plastic cutter runs .0072 in/tooth slotting and .0083 profiling in acrylic, acetal and nylon [2], and Micro 100's single-flute upcut router runs .0107 and .0123 [3]. Onsrud's 1/4" single-flute upcut O-flute is listed at .008-.012 in soft plastic and its 2-flute O-flute at .008-.010 [4]. Amana's 1/4" acrylic O-flutes run .008-.010 at 18,000 RPM [9]. A standard end mill in Harvey's general table gets only .003 at 1/4" (and .006 at 1/2") in acrylic and polycarbonate [1].

Onsrud's guidance on the low end is blunt. A larger chip removes more heat from the cut, so use the highest chip load that still gives an acceptable finish, and if you are making dust instead of chips, chips are being re-cut or the chip load is too low [7]. Its catalog adds that a feed rate that is too low generates heat that breaks down the cutting edge, and that after a nest of parts the stopped tool should be at or near room temperature [4]. Plaskolite says the same for acrylic from the other side: excessive feed chips the edges, insufficient feed makes heat and melts the part [10]. Onsrud's general plastic-routing starting range is .004-.012 in/tooth [6]. Light radial cuts make it worse: below a 50% stepover the chip gets thinner than the feed per tooth, and Harvey ties correcting for that to reduced rubbing [14]. See the chip thinning guide.

Single flute, O-flute or 2-flute?

Harvey Performance recommends a single-flute plastic end mill for plastics with a low melting point, because its larger flute valley makes bigger chips that carry heat away, and two or three flutes for plastics with higher heat tolerance [13]. Onsrud's catalog says single flute allows larger chip loads in softer materials, double flute gives a better finish in harder materials, and as the number of edges goes up the feed should go up too, to prevent burning [4]. Its fix for re-welding is to increase the feed or change to a single-edge tool [4].

Onsrud's O-flute tools use a high rake and low clearance to curl a chip in soft plastic; chips that don't curl properly leave visible knife marks [5]. Onsrud says the upcut spiral is used most in sheet and block work because moving chips upward avoids welding [5]. It says single-edge O-flutes shouldn't be used over 3/8" diameter because of balance, and says double-edge tools give a better bottom finish in slots and longer edges for cuts 2-4 × D deep [5]. It also explains why a metal-cutting end mill struggles: minimal rake, low clearance and little flute room aggravate melting and re-welding [5]. If parts are held with vacuum or tape, Harvey points to downcut tools to keep them from lifting [13]. See also choosing flute count.

Onsrud sorts plastics into soft, which curl a chip, and hard, which break off as a splintered wedge [6]. Its catalog puts HDPE, UHMW, polycarbonate and extruded acrylic in the soft group and cast acrylic and nylon in the hard group [4], although its mechanical-plastics paper calls acetal, nylon, UHMW and polycarbonate soft plastics for O-flute work [5]. Hard plastics get double-edge V-flutes, hard-plastic O-flutes or 2-3 edge finishers to avoid cratering [6].

Material notes

Acrylic (PMMA, Plexiglas)

Plaskolite recommends air or water mist to keep milled edges from melting, warns that improper milling can leave stresses that later craze or crack the part, and says routing its acrylic is best done dry with an air jet for cooling and chip removal [10]. Its routing range is .004-.015 in/tooth at 20,000-25,000 RPM [10]. In a Harvey case study, a 2-flute plastic cutter facing acrylic left swirl marks that a 3-flute plastic finisher avoided [13].

Acetal (Delrin) and nylon

Both sit in Harvey's 800-1200 SFM group [2]. Mitsubishi Chemical says to go for maximum feed and speed in polyamides (nylon), UHMW and softer materials to get swarf away from the tool and avoid build-up or wrap-around, and to use 2- or 3-flute cutters with a higher helix [12].

HDPE and UHMW

These are the softest group in Harvey's tables and get the highest speed, 1200-1600 SFM, but slightly lower chip loads than acetal at the same diameter [2][3]. Mitsubishi Chemical's advice for them is about the chips: maximum feed and speed so swarf doesn't build up or wrap around the tool [12].

Polycarbonate

Onsrud treats it as a soft plastic, starts with O-flutes, and gives .004-.012 in/tooth for finish [8]. Avoid plunging straight down: chips can't escape, which causes chip wrap and melting at the entry, so ramp in or enter from the side [8]. Mitsubishi Chemical warns that mineral-oil cutting fluids may stress-crack amorphous plastics such as polycarbonate and says to wash amorphous parts immediately in isopropyl alcohol and pure water if coolant was used [12].

Air, coolant and cut direction

Mitsubishi Chemical says coolant is generally not required for most machining operations (drilling and parting off excepted), that spray mists and pressurized air are very effective, and to use air blast or a vacuum venturi [12]. Onsrud adds forced air through a nozzle, or a venturi-effect nozzle that also lowers the air temperature, when a tool still runs hot or chips re-weld [7]. Both makers say to keep tools sharp: Mitsubishi Chemical wants them "always kept sharp and smooth" [12], and Harvey says a sharp edge shears the plastic cleanly and cuts down on burrs [13]. Keep the tool moving. Onsrud recommends avoiding dwell at corners and ramping into inside cuts [7].

The sources disagree on climb milling. Mitsubishi Chemical recommends climb milling over conventional: smaller break, less heat, better finish [12]. Onsrud, writing about router work, says conventional cutting generally gives the better edge in a single pass, that a separate finish pass can be a climb cut, and, for polycarbonate, that if the scrap side keeps coming out better you should reverse the toolpath [7][8]. Try both on a test piece.

Worked example: 1/4" single-flute O-flute in acrylic sheet

Amana runs its 1/4" acrylic O-flutes at 18,000 RPM, 1 × D deep: 18,000 × 1 flute × .008-.010 = 144-180 IPM (Amana prints 145-180), or 290-360 IPM with the 2-flute version [9]. That spindle speed is 0.262 × 0.25 × 18,000 = 1,179 SFM. At 2 × D depth Amana cuts the feed 25% (about 108-135 IPM for the single flute) and at 3 × D by 50% [9].

Worked example: 1/4" 2-flute plastic end mill profiling acetal

Harvey 2-flute chart, 1,000 SFM (middle of 800-1200): 1,000 × 3.82 ÷ 0.25 = 15,280 RPM, and 15,280 × 2 × .0083 = 254 IPM, at up to .35 × D (0.088") radial and 1 × D (0.25") axial [2]. On a 10,000 RPM spindle, holding the same chip load gives 10,000 × 2 × .0083 = 166 IPM at about 655 SFM, which is below Harvey's range but right at the bottom of Mitsubishi Chemical's for acetal [12]. Keep the chip load and let the feed come down with the RPM; cutting the chip load to slow the feed is what makes heat [7].

Worked example: 1/2" 2-flute slot in UHMW

Harvey 2-flute chart, 1,400 SFM: 1,400 × 3.82 ÷ 0.5 = 10,696 RPM, and 10,696 × 2 × .0101 = 216 IPM; the chart lists 1 × D axial for slotting [2]. With Micro 100's single-flute upcut router the 1/2" slot chip load is .0149, or 10,696 × 1 × .0149 = 159 IPM [3].

FAQ

What SFM should I use to mill acrylic with a carbide end mill?

It depends on the tool. Harvey Tool gives standard carbide end mills 200-600 SFM in acrylics, but its plastic-cutting end mills (and Micro 100's single-flute plastic routers) are charted at 800-1200 SFM for acrylic, acetal and nylon. Amana runs its 1/4" acrylic O-flute bits at 18,000 RPM, which works out to about 1,179 SFM.

What chip load should I use for a 1/4" end mill in plastic?

For a 1/4" tool, Harvey Tool's 2-flute plastic end mill is charted at .0072 in/tooth slotting and .0083 profiling in acrylic, acetal and nylon (.0059 and .0068 in HDPE and UHMW). Micro 100's single-flute router is higher: .0107 and .0123. Onsrud lists .008-.012 for its 1/4" single-flute upcut O-flute in soft plastics and Amana .008-.010 for its 1/4" acrylic O-flutes. A standard metal end mill in Harvey's general table gets only .003 in acrylic and polycarbonate.

Should I use coolant when milling plastic?

Mitsubishi Chemical says coolant is generally not required for most plastic machining, that spray mist and pressurized air are very effective, and that mineral-oil fluids may stress-crack amorphous plastics such as polycarbonate. Plaskolite recommends routing its acrylic sheet dry with an air jet for cooling and chip removal. Onsrud suggests forced air through a nozzle when a tool still runs hot or chips re-weld.

Single flute or two flute for plastic?

Harvey Performance recommends a single-flute plastic end mill for low-melting-point plastics, because the larger flute valley makes bigger chips that carry heat away, and two or three flutes for plastics with higher heat tolerance. Onsrud says single flute allows larger chip loads in softer materials, double flute gives a better finish in harder ones, and single-edge O-flutes should not be used over 3/8" diameter because of balance.

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