How to Machine Plastic With Your CNC Router

Seven things you need to know about working with plastics to get the best finish from your CNC machine

Perhaps you are just thinking about CNC machining plastic, or have got a lot of experience but sometimes things don’t quite work out as well as you imagined? Here are seven tips to help you get the best results cutting and carving plastics with your CNC router.

1. Know Your Material

Plastics are very versatile, and can be used to make almost anything. Their versatility is down to their chemical composition when they are manufactured. This gives any specific plastic its physical properties, from impact resistance to conductivity, transparency and heat resistance.

First thing to remember when CNC machining plastics is that different plastic compounds will require different settings to get the best results. All plastics are definitely not the same!

  • Acrylic: Polymethyl methacrylate (PMMA)
  • ABS: Acrylonitrile butadiene styrene
  • Polypropylene (PP)
  • Polycarbonate (PC)

These are the most common plastics used in fabrication and manufacturing with their common names and acronyms, all of these can be cut and carved with a CNC router, but need very different settings to get the best results.

Machining Acrylic

Here we’re going to focus on working with Acrylic (often called by the trade names Plexiglass™ or Perspex™), which is polymethyl methacrylate (PMMA). Acrylic is  a very common material for CNC machining because it is transparent and relatively easy to carve. It’s great for sign making, fabricating PPE barriers, and even furniture and sculptures. Want to make a personalised paperweight as a seasonal gift? A block of acrylic is a good place to start.

CNC Machined 0.25” Duracast Acrylic Sneeze Guard: PPE For Business

Cast vs Extruded Acrylic

Acrylic is more shock resistant and resilient than glass, making it great for glazing, but is manufactured in two different ways that make a lot of difference to how it responds to cutting in your CNC router. It’s important to know the difference between cast acrylic and extruded acrylic.

Notice the high clarity of cast acrylic, which is more transparent than glass!

Cast Acrylic

  • Excellent Transparency
  • Uniform Structure
  • Great For CNC Machining

The PPE in our example is machined out of Duracast Acrylic. You can see it is very highly transparent, and has an excellent finish on the engraved logo and edges. Cast acrylic is made by pouring liquid acrylic into a mold, this gives it a very even and predictable structure, ideal for machining.

Extruded Acrylic

  • Good Transparency
  • Uneven Structure
  • Harder To Cut Smoothly

Extruded acrylic is pressed into sheets as it cools. This forces stresses into the plastic that make it less easy to machine cleanly and can make it less transparent.

Cast Acrylic Is Best For CNC Machining

Cast acrylic is more expensive than extruded acrylic, but is much easier to work with using your CNC router, and will give you a better finish. We recommend you use cast acrylic rather than extruded acrylic, which is more brittle, may crack when machining and has a rougher finish.

Check Thickness Before Cutting!

The cast acrylic sheet used to make the example was labeled 0.25″ thick but sheet thickness is nominal and each sheet will be a bit different. The measured thickness for this acrylic sheet was .213” – make sure to check and adjust your programming accordingly.

This CNC machined acrylic test piece used an offset of .005″ after checking the sheet thickness.

It’s essential to know the sheet depth if your design is assembled by slotting pieces of cut acrylic together. If you get it wrong you’ll end up with slots that are too loose or too tight. Adding a 0.005” offset to the measured thickness makes for a good fit in acrylic sheet. This precision is easily achievable with an I2R CNC machine.

When engraving a logo or design into the sheet with your CNC router our Z-Axis touch-off tool makes it easy to ensure you cut to the right depth, and is an excellent way of ensuring precision for surface work.

2. Choosing The Right Bit

CNC Machining Acrylic: Spiraled ‘O’ Flute

Cutting uniform extruded acrylic plastics can be done with conventional bits and standard cutting directions. A best CNC practice for cutting acrylic is to use a  spiral up cut with a single flute. We recommend an ‘O’ Flute to get the best results from CNC routing acrylic.

The Spiraled ‘O’ Flute: Ideal For Acrylic

The upward motion of the bit is ideal for chip extraction and will create an optimal clean finish. To avoid bit chatter and cutter deflection, choose a bit that has the shortest flute length for the thickness of your material and set your CNC machine speed conservatively.  For this project we used an 1/8″ spiraled ‘O’ flute.

3. Securing Your Material

It is very important to secure your material so that it’s as flush with the bed of your CNC router as possible. The aluminum T-slots that are standard on our i2R routers make hold down clamps a simple and effective way for securing your material.

When running your CNC machine, ensuring there is no flex in the acrylic will save you headaches down the road. Flex causes excess vibration which makes for jagged,rough cuts, and can crack the acrylic. Thin material may be lifted by the bit during cutting – this is a common cause of cracking and breakage. 

Using double sided tape, adhesive spray, or a vacuum hold down will reduce flexing of your material during cuts. Take extreme care when cutting very fine and thin plastics.  Double sided tape is a cost effective method which proves to be very reliable.

4. Know Your Feeds & Speeds!

When you CNC acrylic you must find the perfect balance of speed and feed rate.  Cutting too fast will result in too much friction, which will melt the plastic and quickly wear out your bit.

You can add, save, select and change your feed and speeds in the tool library on VCarve. Watch this video on Basic Guide to CNC With Vectric VCarve Pro.

For cutting cast acrylic, a spindle RPM of 17,000 is a sweet spot here for the i2R CNC router brushless spindle. A good feed rate is 59 IPM or 1500 mm/min.

5. Know Depth of Cut

The general rule of thumb when figuring out your depth of cut is to use half the diameter of your tool and adjust accordingly if you don’t quite get the finish you want. Start conservatively with plastics to control friction and avoid melting.

For this acrylic project a 0.0535″ depth of cut worked well.

6. Set Your Tabs

When using hold down clamps on your CNC router, you’ll definitely want to make use of tabs to keep your material from flying out. Don’t make your tabs too thick or you’ll have to sand them and it will leave a rough edge. The goal is tp get the tabs just right so they aren’t too thin that your material flies loose. For the thickness of this project, a length of .1″ and .03″ worked well.

7. Listen to Your Cut

Always listen to the sound the spindle makes during cutting when operating your CNC router. The noise can tell you how your spindle is performing. A loud, high pitch indicates your spindle is working aggressively. The more aggressive the cut, the rougher the finish will be.

If it sounds too loud or high pitched, slow down the cut by lowering your feed rate. In UCCNC, you can easily slow your feed rate while your CNC is running by clicking the (-) button next to the F-SET.

You can also slow down the spindle RPM in your CNC the same way by clicking the (-) button next to your S-SET. The max RPM on the i2R CNC air cooled spindle is 24,000 RPM.

Document Your Project

A general tip for all your CNC router projects. Make sure you record everything you do and learn. Keep notes on the settings that worked best for each scenario and project. Next time you do a similar project you’ll be glad you kept a good journal to reference.

Have Fun & Share Your Experience

Every project you make is a learning experience, good or bad. Get onto our online forum and share your knowledge and projects. Be patient and most of all have fun!

I2R CNC Forum

Need inspiration? Check out our social media!

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