how to cut acrylic
using a cnc router
by Susan Mattine

here’s more than meets the eye when it comes to cutting acrylic. If you are not satisfied with the quality of the cut you are getting, read on for suggestions for how to improve it.

There are some nuances to consider. For example, did you know that cast and extruded acrylic cut quite differently? Or that tool selection and machine acceleration play a fundamental role in the quality of your finished product?

The keys to your success will be to set up your machine properly, choose the right material, select the correct tool and use the correct cutting strategy. This article will show you how.

Machine configuration and setup
A strong machine is a critical starting point to a successful project. When searching for a high-quality finish, there are four components to consider during the buying process:

  1. A strong, rigid base is essential. Purchasing a heavy-duty router allows you to couple a strong base with a stiff, solid gantry. This structure supports demanding processes, offering a robust and reliable unit while helping to minimize vibration and movement.
  2. A smooth control and drive system have arguably the most important role in producing a quality outcome.
  3. Helical rack systems and servo drive motors help to deliver smooth movement, keeping vibration to a minimum. The machine can cut at a higher speed, producing higher volumes without losing quality.
  4. Finally, a competent vacuum hold-down system is highly recommended. The vacuum hold down ensures material stays in the correct place throughout the cutting process. Even the slightest material movement can put the software program, spindle and material out of sync, invariably ruining the piece completely.
the AXYZ Trident, a versatile router-knife hybrid
Choose a strong, versatile machine when cutting acrylic. Shown here is the AXYZ Trident, a versatile router-knife hybrid.
two collets
Collets come in all shapes and sizes.
Material selection
Material selection is a key component when working with acrylic. When selecting between cast and extruded acrylic, cast is recommended. Cast acrylic generally has good optical clarity, greater surface hardness and machines more cleanly than extruded.
Collet selection
Collets are often overlooked as a possible reason for poor quality cutting. It’s not uncommon to find machine operators using collets that are many years too old. These may appear to be fine, but they wear out. After three to six months of use, collets won’t hold a router bit as straight and true as when they were new. Therefore, collets should be changed regularly. Replacing collets like this will also help to prolong tool life.

Make sure you select the correct type of collet to fit your spindle, as different spindles take different collets.

The most important factor is to ensure you match the collet shank with the tool shank diameter. Be aware that manufacturers often quote sizes in metric and imperial. Make sure you read the data correctly to avoid mistakes.

Tool selection
There is an enormous and sometimes bewildering choice of CNC router tooling available. There are single, twin and triple flutes, straight, compression and spiral flutes. There are even special coatings that can be applied. This makes the task extremely difficult. Selecting the right tool for the job can make the difference between a perfectly smooth and clean cut, or something that looks like it has been hacked out with a saw.

When operating with performance plastics, a high-quality router bit is essential. An upward spiral, single flute geometry usually gives best results on acrylic. The upward geometry provides a smooth and consistent finish and ensures optimal chip extraction. For best cut quality, always choose a tool with the shortest flute length that will cut through your material. This helps to reduce cutter deflection and chatter.

Tool positioning
When fitting the bit into the collet, make sure the cutting flutes are not inside the collet. Leave a short section of the shank exposed. Equally important is to ensure the tool is not protruding too far out of the collet. If the tool is protruding too far then the material will show vibration marks and produce a poor cut quality. This results in reduced tool life.
Cutting process
For best results, the recommended strategy is to split the cutting process into two parts — a roughing cut and a finishing cut, each done with a different tool. This allows you to preserve your finishing tool for a longer period, which in turn enables you to achieve that higher quality finish.

The dynamics of this process are simple enough: the roughing cut does most of the work, reducing the load on the finishing cut. The roughing pass churns out most of the material, whereas the newer finishing cutter removes the remaining material and leaves a smooth, polished edge. Many application engineers adopt this strategy when cut finish holds the highest priority.

When creating the program for the rough cut, leave about 0.02″ (0.5 mm) on the Z axis so that you don’t cut fully through the material and the part is held in place. Also, offset the tool with an additional allowance of approximately 0.008″ (0.2 mm) around the profile. By doing this you leave only a small amount of material to remove for the finishing tool. This reduces cutter loading and chip volume and minimizes the chances of vibration spoiling the edge finish. Set your cut speed to approximately 60 ipm (1.5 m/min) and cut in a “climb” or clockwise direction.

Conclude the process by running the finishing pass, this time cutting all the way through. Remove the profile allowance and reduce the cut speed to about 40 ipm (1 m/min) in the same direction. The finishing pass should produce little if any noise, with higher noise resulting in a rougher cut. If the noise increases at any point of the finishing pass, stop the machine, check your work and reassess the situation. You may have to reduce the feed speed or acceleration rate or fit a new tool.

If your quality starts to diminish you should consider replacing your finishing tool. This will need to be changed on a regular basis to maintain highest quality.

Getting an edge on quality
A smoothly cut edge on acrylic could make the difference between customer satisfaction and disaster, especially with the highly visible barrier applications so prevalent today. Use the tips in this article to set your company up for success, maintain your tools and machinery properly and delight your customers with their cut acrylics.
Suzanne Fenton is the communications and public relations associate at the AAG head office. For more information about routers, contact AXYZ Tailored Router Solutions at 5330 South Service Road, Burlington, ON L7L 5L1 Canada; phone (905) 634-4940 or (800) 361-3408, enquiries@axyz.com or www.axyz.com.