Clear acrylic is one of the most widely used plastics in the world—and for some good reasons. It's five times stronger than glass, which means it's shock-resistant and shatterproof. And it is slightly more transparent than glass.
While other plastics can become yellow or brittle with age, clear acrylic does not deteriorate after being under UV light for long periods. It is also water-resistant and not subject to corrosion.
Because of their durability, resistance, and transparency, acrylic parts are being used in many industries today. So, it would make sense that any manufacturer with CNC routers in its shop should know more about the machining processes involved with acrylic.
Here a few things to keep in mind before you start machining transparent acrylic:
Which type of acrylic is better for machining?
Acrylic blanks are manufactured using two methods: extrusion and casting. The extrusion process is used to make acrylic sheets, and both sheets or blocks can be cast. They come in thicknesses up to one inch (25mm), which is ideal for producing signs or flat emblems.
The extrusion process consists of melting acrylic pellets in a special tank and extruding the resulting liquid through an extrusion nozzle. Casting acrylic is like casting any other material in that the melted pellets are poured into a mold and extracted after the mold cools.
Extruded and cast blanks do not behave the same during machining. Extruded acrylic has more leftover tensions, making it prone to cracks and brittleness. Those tensions are the primary reason that machining extrusions is so challenging.
Another issue: extruding makes acrylic anisotropic, which means, in essence, that machining along the "X" axis will be different than machining along the "Y" axis.
So, as a general rule of thumb, cast acrylic blanks are the wiser choice for CNC router machining.
Holding down acrylic parts on the router table
Make sure the clear acrylic is held down firmly. If it's not, there is a higher chance that it will vibrate or flex, ruining your cut or spoiling the finish.
There are several methods for fastening acrylic, but you need to consider the size of the piece. Even though acrylic is relatively rigid, you must still be careful that larger sheets are held down along the entire area.
While smaller parts can be clamped to the table (below) or in jigs and vises, longer pieces are held most securely on a machine with a vacuum table.
Otherwise, the material will have to be glued or fastened with strips of double-sided tape. There are spray adhesives available, but some may have chemical properties that are not suitable for acrylic.
Photo credit: Halo Signs
The essentials of machining acrylic
Once you've fastened the acrylic properly, you can begin machining. Machining acrylic on a CNC router is similar to machining polycarbonate.
Here are a few suggestions to help you along:
Use the right cutting tools
You won't get the best results with the same cutting bits you use for wood. It would be best if you bought bits that are designed for acrylic since they will have flutes that ensure optimal chip removal.
Router bits for machining acrylic will typically have one to three flutes, with single and double flutes being the most common.
The length of the cutting edge should not be more than three times the diameter of the tool, and the shank diameter should always be the same as or larger than the tool diameter. Keep your bit sharp since a dull one will not give you the best edges.
Photo Credit: Amana Tool
The picture above shows a ½ inch diameter solid carbide slow spiral 0-flute router bit. It has 1¼" of flute length and a ½" shank diameter. It meets all of the criteria for an ideal cutting bit for clear acrylic.
Also, standard drill bits with the 120-degree (included angle) tips must be ground to 60 degrees to allow the bit to enter and exit the acrylic without chipping. (See the image below).
Image Credit: Acrylite
Spindle speeds are important
The recommended spindle speed for cutting acrylic is in the range of 15,000 to 20,000 RPMs. If you choose the fastest speed, you might need to adjust your feed rate upward to prevent the material from melting. Find the best combination of speeds and feeds by experimenting.
The cutter diameter should determine feed rates
The ideal feed rate range for machining acrylic is usually 75 to 300 inches-per-minute and will depend on the size of your cutting bit. For example, an 1/4" diameter bit works better on the low end (75-125 IPM) while a 1/2" bit would work well at 200-300 IPM. Maintaining the proper combination will save both the bit and the material.
Another rule of thumb: Use the largest bit possible and stay aggressive with the feed rate. By doing this, you will prevent the tool from staying in the same place too long, generating heat from friction, and ruining your surface finish.
Depth of cuts also require some experimentation
You will hear recommendations that the depth of roughing cuts should be anywhere from half the cutter's diameter to twice the diameter.
If your cutters are breaking, you'll have to make multiple cuts, but you won't know that without experimenting somewhat.
Ramping is safer than plunging
Entry points can be a problem. Even though acrylics do not have the "chip wrap" problem as softer plastics, they tend to "walk" during a plunge since they don't have a centering point like a drill.
A ramped entry can work well and doesn't show the entry melt that is associated with direct plunging by router bits. Use the ramp feature in your program to make sure to come to the starting point at an angle, rather than plunging straight down.
This method is smoother and prevents distortion when the cutter breaks through the material surface.
Coolants help keep the acrylic cool
With any CNC acrylic cutting process, it's essential to keep the material's temperature to a minimum. This is the reason acrylic sheets are often cooled before machining since it allows for better chip removal and a lower starting temperature.
You can use standard coolants, but make sure that if they contain oils, those oils don't react negatively with acrylics.