How to Machine Plastics: CNC Guide for Acrylic, Delrin, and Polycarbonate

Machining plastics is different from machining metals. The cutting behavior, tool requirements, and feeds and speeds are all different. Plastics melt instead of chip under the wrong conditions. They expand more with heat and they require sharp tools to cut cleanly. I have machined thousands of plastic parts and the most common mistake beginners make is treating plastic like soft metal as covered in our [Feeds and Speeds Guide]( G90 G54 G00 X0 Y0 G01 Z-0.1 F10 G01 X1.0 F20 M30 /posts/feeds-and-speeds-beginners/).

This guide covers the four most common engineering plastics acrylic, Delrin, polycarbonate, and nylon with specific feeds and speeds, tool recommendations, and troubleshooting for each material.

General Principles for Plastic Machining

Sharp tools are essential for plastic machining. A dull tool generates friction and heat instead of cutting cleanly. The heat from a dull tool can raise the temperature of the plastic above its melting point in milliseconds. Once the plastic melts it flows into the flutes of the tool and solidifies. This creates a clogged tool that cannot cut at all. The tool must be removed and cleaned or replaced. Carbide tools are recommended for plastic machining because they hold a sharper edge much longer than HSS tools. A carbide end mill can machine hundreds of plastic parts before needing replacement while an HSS tool may dull after only a few parts.

The geometry of the cutting edge matters for plastics. A sharp cutting edge with a high positive rake angle shears the material cleanly. A negative rake angle pushes the material ahead of the tool which generates heat and causes melting. Most standard end mills have geometries optimized for metal cutting. For plastic machining consider using tools specifically designed for plastics with high positive rake angles and polished flute surfaces. The heat melts the plastic which welds to the tool and ruins the surface finish. Carbide tools are recommended because they hold a sharper edge longer than HSS. Use tools with polished flutes to reduce friction and prevent material from sticking to the cutting edge.

Single-flute end mills are preferred for most plastic machining. The single large flute provides maximum chip clearance. Plastic chips are long and stringy and they fill the flutes of multi-flute tools quickly. When the flutes clog the tool stops cutting and starts melting the plastic. A single-flute tool evacuates chips efficiently and prevents clogging.

Coolant is not always needed for plastic machining but compressed air blast is essential. The compressed air clears chips from the cutting zone and prevents them from melting onto the tool. If coolant is used it must be water-based and compatible with the plastic. Some plastics absorb coolant and swell. Test coolant on a scrap piece before using it on production parts.

Climb milling produces significantly better results than conventional milling for plastic materials. In climb milling the cutting edge engages the material at the maximum chip thickness and shears it cleanly. In conventional milling the cutting edge enters the material at zero chip thickness and rubs across the surface before cutting. This rubbing generates heat and causes melting. For plastic finishing passes always use climb milling. For roughing passes climb milling is also preferred but conventional milling can be used for the first pass to avoid work hardening on some materials.

The workholding approach for plastics is different from metals. Mechanical clamping can deform soft plastics. Use vacuum workholding or double-sided tape for thin plastic sheets. For thicker blocks use soft jaws in a vise with light clamping pressure. Support thin walls and unsupported sections to prevent vibration and chatter. Climb milling shears the material cleanly while conventional milling pushes the material ahead of the tool which causes melting and poor surface finish. Use climb milling for all finishing passes on plastic parts for best results.

Acrylic

Acrylic also known as PMMA or plexiglass is the most common plastic for CNC machining. It is rigid, transparent, and machines well with the right techniques. Acrylic is brittle and can crack if the cutting forces are too high or if the tool is dull.

The single most important rule for acrylic is to use a single-flute end mill at all times. Multi-flute tools clog immediately and the heat buildup causes the acrylic to melt and weld to the tool. A single-flute upcut spiral bit evacuates chips before they can melt. The feed rate should be high enough to produce a clean chip but not so high that the tool deflects.

Recommended feeds and speeds for acrylic with a quarter-inch single-flute carbide tool are 12,000 to 18,000 RPM and 50 to 100 inches per minute feed rate. The chipload should be 0.003 to 0.008 inches per tooth. Use a radial depth of cut of 30 to 50 percent of the tool diameter and an axial depth of cut of 0.5 to 1 times the tool diameter.

Air blast is essential for acrylic machining. The compressed air clears chips and keeps the cutting zone cool. Without air blast the chips melt and weld to the tool within seconds. The tool becomes clogged and the surface finish degrades rapidly. Stop cutting immediately if you see melted acrylic on the tool.

Acrylic produces a flame-polished transparent edge when machined with sharp tools and proper feeds and speeds. This is one of the unique properties of acrylic machining. The cutting action generates enough heat to locally polish the cut edge making it transparent without any secondary finishing. The edge appears as clear as the original material surface. If the edge appears cloudy, frosted, or white the feed rate is too low causing the tool to rub rather than cut. Increase the feed rate until the edge becomes clear again. This visual feedback makes acrylic one of the most satisfying materials to machine because you can see immediately whether your parameters are correct.

Finishing passes on acrylic should use a light radial engagement of 3 to 8 percent of the tool diameter. This produces a clean polished edge. Roughing passes can use 30 to 50 percent radial engagement but the edge will be frosted. Plan your toolpaths to leave at least 0.01 to 0.02 inches of material for the finishing pass that produces the polished edge. The edge is transparent and does not need secondary finishing. If the edge appears cloudy or frosted the feed rate is too low or the tool is dull.

Delrin

Delrin also known as acetal or POM is one of the easiest plastics to machine. It is rigid, stable, and produces clean chips without melting. Delrin has natural lubricity which reduces friction at the cutting edge.

Delrin can be machined with standard end mills. A 2-flute carbide end mill works well for most operations. The material does not melt as easily as acrylic so multi-flute tools can be used. Recommended feeds and speeds with a quarter-inch 2-flute carbide tool are 10,000 to 16,000 RPM and 60 to 120 inches per minute feed rate. Chipload should be 0.004 to 0.010 inches per tooth.

Delrin holds tight tolerances very well and does not expand or contract significantly with temperature changes or moisture absorption. This makes Delrin the preferred plastic for precision machined components like gears, bushings, and bearing retainers. A Delrin part machined to plus or minus 0.002 inches will maintain that tolerance over time and across normal temperature ranges. The same part in nylon would change size as it absorbs moisture from the air.

Delrin machines to a smooth surface finish without secondary operations. The surface finish after machining is typically 16 to 32 microinches Ra which is suitable for most applications without sanding or polishing. The natural lubricity of Delrin also means that machined surfaces have low friction and good wear characteristics. This is why Delrin is commonly used for moving parts like gears and slides. A part machined to plus or minus 0.002 inches is achievable with reasonable care. Delrin also machines to a smooth surface finish without secondary operations.

Delrin is available in different grades. General-purpose Delrin 150 is suitable for most machined parts. Delrin 570 is glass-filled for higher stiffness and better dimensional stability at elevated temperatures. Glass-filled Delrin is more abrasive and requires carbide tooling.

Polycarbonate

Polycarbonate also known as Lexan is a tough impact-resistant plastic. It is more difficult to machine than acrylic or Delrin because it is gummy and tends to melt and stick to the tool. Polycarbonate also stress cracks if machined too aggressively.

Sharp tools are critical for polycarbonate. A dull tool generates heat that melts the material and causes it to gum up on the cutting edge. Use single-flute or 2-flute carbide end mills with polished flutes. The polished surface reduces friction and prevents the polycarbonate from sticking.

Recommended feeds and speeds for polycarbonate with a quarter-inch single-flute carbide tool are 10,000 to 14,000 RPM and 40 to 80 inches per minute feed rate. Chipload should be 0.003 to 0.006 inches per tooth. Use lower feed rates than acrylic to prevent heat buildup.

Air blast is essential for polycarbonate. The material generates significant heat during cutting and the chips must be cleared immediately. Without air blast the chips melt and weld to the tool within seconds. The tool becomes clogged and the surface finish degrades.

Polycarbonate is prone to stress cracking along machined edges if the tool is not sharp enough or if the feed rate is too aggressive. The stress cracks appear as small white lines or crazing along the cut edge. These cracks can propagate over time and eventually cause the part to fail. If you see stress cracking on a polycarbonate part the first thing to check is tool sharpness. A sharp carbide tool with polished flutes should produce crack-free edges. If the tool is sharp and cracking still occurs reduce the feed rate by 20 percent and try again.

Polycarbonate is also sensitive to coolant and chemicals. Some coolants cause polycarbonate to craze or crack on contact. If you must use coolant for polycarbonate machining test it on a scrap piece first and leave it for 24 hours to check for chemical reaction. In most cases air blast is sufficient cooling for polycarbonate and avoids any risk of chemical damage. The cracks appear as small white lines along the cut edge. If you see stress cracking reduce the feed rate and check the tool sharpness. Annealing the polycarbonate before machining can reduce stress cracking.

Nylon

Nylon also known as polyamide or PA is a tough material that absorbs moisture from the air. Nylon is more flexible than acrylic or Delrin and requires different machining techniques.

Nylon absorbs moisture which changes its dimensions. A nylon part machined to size in a dry environment may swell and become oversize in a humid environment. For precision parts machine nylon in the same environment where the part will be used. Alternatively dry the nylon before machining and seal it afterward to prevent moisture absorption.

Recommended feeds and speeds for nylon with a quarter-inch 2-flute carbide tool are 8,000 to 12,000 RPM and 40 to 80 inches per minute feed rate. Chipload should be 0.004 to 0.008 inches per tooth. Nylon cuts cleanly with sharp tools and produces a good surface finish.

Nylon produces long stringy chips that can wrap around the tool and workpiece during machining. These chip strings can entangle the tool and cause breakage if not managed properly. Use air blast directed at the cutting zone to blow chips away before they wrap. A chip breaker or pecking strategy that interrupts chip formation can also help reduce stringy chip problems. Use air blast to clear chips and prevent wrapping. A chip breaker toolpath that interrupts the chip formation can also help.

Comparison Table

Material	RPM	Feed IPM	Chipload	Flutes	Tool Coolant
Acrylic	12-18K	50-100	0.003-0.008	1	Air blast
Delrin	10-16K	60-120	0.004-0.010	2	Optional
Polycarbonate	10-14K	40-80	0.003-0.006	1	Air blast
Nylon	8-12K	40-80	0.004-0.008	2	Air blast

Common Problems and Fixes

Melting is the most common problem in plastic machining. If the plastic melts around the cutting edge the feed rate is too low, the tool is dull, or the wrong tool type is being used. Increase the feed rate to produce a thicker chip that carries heat away. Switch to a single-flute tool with polished flutes. Apply air blast to keep the cutting zone cool.

Poor surface finish is usually caused by a dull tool or incorrect feeds and speeds. Check the tool for wear and replace if needed. Increase the spindle speed and decrease the feed rate for a lighter chipload on the finishing pass. Use climb milling instead of conventional milling.

Chatter in plastic machining is caused by tool deflection or insufficient workholding. Reduce the tool stickout to increase rigidity. Use a larger tool diameter if possible. Ensure the workpiece is securely clamped with no vibration.

Cracking in acrylic and polycarbonate is caused by excessive cutting forces or thermal stress during machining. The material expands when heated by the cutting action and contracts when it cools. If the thermal stress exceeds the material strength cracks form along the machined edge. Reduce the feed rate and depth of cut to minimize heat generation. Use sharp tools with polished cutting edges that shear rather than rub. Ensure the material is at room temperature before machining. If cracking persists consider annealing the material before machining by heating it slowly to 180 degrees Fahrenheit and cooling it back to room temperature over several hours.

Hole drilling in plastics requires different techniques than metal drilling. Standard twist drills designed for metal tend to grab and pull themselves into plastics which causes cracking around the hole exit. Use a drill bit specifically designed for plastics with a sharper point angle typically 60 to 90 degrees instead of the standard 118 degrees. Peck drilling with frequent retracts clears chips and prevents heat buildup. Support the back side of the hole with a backing plate to prevent breakout cracks.

Threading in plastics is different from metals because plastic threads strip more easily. For tapped holes use a larger tap drill size to produce deeper threads. Typically 75 percent thread engagement is used for metal but 60 percent is sufficient for plastics. For high-stress applications use threaded inserts instead of tapping directly into the plastic.

Plastic machining is a skill that improves with practice. Start with Delrin which is the most forgiving plastic and work up to acrylic and polycarbonate as your techniques improve. Each material has its own personality but the principles of sharp tools, proper feeds, and chip evacuation apply to all plastics. Once you master these principles you can machine any plastic successfully.

For more feeds and speeds information see our Feeds and Speeds Guide and our CNC Router Bits Guide. For tool selection see our CNC End Mill Selection Guide.