CNC machining plexiglass (also known as acrylic) can produce crystal-clear, professional-grade parts—but only if you use the correct tools, feeds, speeds, and cooling methods. The single most important fact to remember is: plexiglass is not forgiving. Too much heat or aggressive cutting will cause melting, chipping, or cracking. This guide gives you the exact, proven parameters and step-by-step workflow to get flawless results on any standard CNC router or mill.
01Choose the Right Tool for Plexiglass
Using a standard wood or metal end mill will almost certainly ruin your workpiece. Instead, use tools specifically designed for plastics:
Solid carbide single-flute (O-flute) router bits – Best for most plexiglass jobs. The large flute space clears chips efficiently and reduces heat.
Two-flute “up-cut” plastic cutting bits – Good for thicker sheets (≥10 mm) but requires careful chip evacuation.
Compression spiral bits – Ideal for double-sided finishing (no burrs on top or bottom edge).
Tool material: Always use solid carbide. High-speed steel (HSS) dulls quickly on plexiglass, leading to friction and melting.
Common mistake example: A workshop tried to use a standard 2-flute wood router bit at 18,000 RPM. Within 10 seconds, the chips welded themselves into a molten lump around the bit. After switching to a single‑flute O‑flute bit at 16,000 RPM with the feed rates below, they achieved clean, transparent edges.
02Critical Feeds and Speeds (With Verified Numbers)
The following data is derived from recommended parameters by plastic material suppliers and CNC tool manufacturers. Start at the lower end of the feed range and adjust upward only when you see clean, powder‑like chips (not stringy or melted).
Rule of thumb for chip load: For single‑flute bits on plexiglass, maintain 0.1–0.2 mm per tooth (0.004–0.008″).
Example: 16,000 RPM, single flute, feed 1,600 mm/min → chip load = 1600 / (16000 × 1) = 0.1 mm – perfect.
03Cooling and Chip Evacuation – Non‑Negotiable
Heat is the #1 enemy of plexiglass machining. You must actively cool the cutting zone:
Use compressed air jet – Direct a steady stream (≥2 bar) at the cutting tip. This removes chips and cools the tool. For most hobby and pro shops, air is sufficient and leaves no residue.
Mist coolant (water‑soluble, 5‑10% concentration) – Recommended for deep cuts (over 6 mm depth) or high‑volume production. Never use straight oil – it causes crazing (micro‑cracks) on plexiglass.
Flood coolant – Acceptable only with non‑oil based synthetic coolants. Ensure complete drying after machining.
What never to do: A common operator once used no cooling and a dull bit on 8 mm plexiglass. The material melted, fused to the bit, and the spindle overloaded – destroying both the workpiece and the collet.
04Workholding and Tabbing – Prevent Cracking
Plexiglass is brittle. Clamping too tight will induce stress cracks. Follow these practices:
Vacuum table – Ideal for thin sheets (≤6 mm). Ensure the entire sheet is covered – any uncovered area may lift and shatter.
Double‑sided tape (acrylic‑compatible, e.g., 3M 468MP) – Excellent for small parts. Apply tape over the entire back surface, not just edges.
Clamps with soft jaws – Use rubber‑coated or wooden jaws. Tighten just enough to hold the sheet – no more.
Leave tabs (bridges) – For parts that cut out completely, add 2‑4 small tabs (1 mm thick, 3 mm wide) to prevent the finished part from moving. Remove tabs manually with a fine saw or sandpaper.
Real‑world example: A sign shop cut 5 mm plexiglass letters using only perimeter clamps. Halfway through the job, the sheet vibrated loose, and the bit grabbed it – cracking three letters. After switching to a vacuum table and tabs, zero waste occurred.
05Step‑by‑Step Machining Workflow
Follow this exact sequence to eliminate guesswork:
Step 1 – Prepare the material
Remove the protective paper layer only from the top surface (leave bottom paper to prevent scratching on the table).
Clean the surface with a soft brush – no dust or grit.
Step 2 – Secure the sheet
Use vacuum or double‑sided tape for thin sheets (<6 mm).
Use soft‑jaw clamps for thick sheets – place clamps no more than 100 mm apart.
Step 3 – Set toolpath strategy
Climb milling (conventional direction) – Always use climb cutting on plexiglass. Conventional milling lifts the chip and increases heat.
Ramp entry – Never plunge directly into the material. Use a 2‑3 degree ramp (e.g., 2 mm horizontal movement per 0.1 mm depth) for internal cutouts.
Lead‑in/out arcs – Add 2‑3 mm radius arcs to avoid tool marks at entry points.
Step 4 – Run a test pass
Cut a small square (20×20 mm) at your chosen feeds/speeds.
Inspect the edge:
Matte white edge → feed too slow (increase by 20%).
Melting or burrs → speed too high or feed too low.
Crystal clear edge → perfect.
Step 5 – Machine the final part
Apply compressed air continuously.
For depths over 3 mm, use multiple passes (see table above).
After cutting, clean edges with fine sandpaper (400‑600 grit) wet‑sanding, then flame polish (quick pass with a propane torch) for optical clarity.
06Avoiding the Three Most Common Failures
07Post‑Processing for Professional Clarity
Raw CNC‑cut plexiglass edges are slightly frosted. To restore full transparency:
1. Wet sand with 400 → 600 → 800 grit sandpaper, always under running water.
2. Flame polish – Pass a propane torch (blue flame, not yellow) quickly along the edge at 50‑100 mm per second. Do not stop – heat builds up and causes bubbles.
3. Alternative for small parts: Use a cloth wheel with fine plastic polishing compound (e.g., Menzerna).
Important safety note: Flame polish only after all machining is complete. Never flame near uncut sheet – it will warp.
08Final Actionable Recommendations
To consistently achieve clear, burr‑free plexiglass parts on your CNC machine:
Always use a single‑flute O‑flute carbide bit. This single change eliminates 80% of melting problems.
Start with these proven parameters: 16,000 RPM,1,600 mm/min feed, 2 mm depth per pass, with compressed air. Adjust from there based on your chip color (white powder = good; stringy or melted = bad).
Never skip cooling. No air? No machining.
Run a test cut on a scrap piece every time you change material thickness or tool diameter.
Replace your bit after every 100 linear meters of cutting in plexiglass – dull bits cause chipping.
Core takeaway: Plexiglass CNC machining is 100% predictable and repeatable when you respect heat management and chip load. Follow the exact numbers and methods above, and you will produce flame‑polish‑ready edges straight off the machine.
