Machining acrylic is best done with a specific acrylic bit, with cutting geometry designed to minimize heat generation. Examples here, also available on Amazon for cheap (your mileage may vary): https://www.amanatool.com/products/cnc-router-bits/plastic-cutting-cnc-router-bits.html
Regarding feedrate, increasing feedrate is a common way to minimize heat generation at the cutting edge.
Increasing feedrate will increase chip load, resulting in:
a) less rubbing and more cutting if the chip load is too light
b) larger chips to carry away more heat.
The snapmaker, with its limited spindle power of a mere 50W, cannot run bits anywhere near their optimal chip load, which will result in significant rubbing as the cutting edge dulls after initial use.
In this particular instance I would think b) is unlikely to be a significant beneficial, but I can’t formulate a reason for this, just a feeling related to acrylic’s thermal characteristics.
a) is likely to be a significant benefit but only if the spindle wattage is not limiting the maximum chip load at the DoC used.
In summary:
- Use a plastic cutting bit for better performance
- Increase feedrate as close as possible to the recommended feeds and speeds for the cutter being used.
a) Because of the limited spindle power and other physical limitations the ideal speed may not be achievable in practice.
b) Limitations on the depth of cut (both axial and radial) may be necessary to prevent overloading the spindle.
For an example walkthrough of feeds and speeds from a “by the book” perspective, to illustrate how quickly you can get ridiculous numbers that are not achievable on this machine.
From here (Chipload Calculator | GDP Tooling), using the assumptions of a 12K spindle speed, 2 flute 1/8" cutter, in soft plastic (targeting the minimum acceptable 0.003"/tooth)
That is achievable with a feedrate of 2000mm/min, approximately as fast as the machine is physically capable of moving. Due to the vibrations induced from moving so quickly while machining this would likely result in failure.
At a more modest 600mm/min that is only a chip load of 0.001", which is likely to generate significant rubbing, resulting in melting through the workpiece instead of cutting as the cutter is trying to remove only 1/1000 of an inch (0.025mm) per tooth.
That company has a tooling guide here: https://cdn.dynamixse.com/blog/wp-content/uploads/sites/3/2018/06/CNC-Tooling-Guide-by-GDP-Tools.pdf
Page 47 describes a method to find the optimal chip load:
FINDING YOUR OPTIMUM CHIP LOAD
When the chip is too small, the cutting action will generate heat in the cut
and will cause the cutting edges of the tool to deteriorate prematurely. A
larger generated chip within the reference range for a particular material
will achieve the longest tool life.
You can find your optimum chip load, which will allow you to maximize
productivity and get the best tool life and lowest cost per panel, if you
follow the steps below:
Start using the recommended chip load and slowly increase your feed rate
until the finish quality becomes unacceptable. Then slowly decrease feed
rate again until desired finish is restored. Make note of your feed rate.
Next, decrease the machine RPM’s until the finish deteriorates. Once that
occurs, increase RPM’s until finish is once again restored. At this point,
you have found the “sweet spot”.
While this is good practical machining advice, it unfortunately does not translate well for new users on a snapmaker. This is standard practice, however, for new materials and new tools.
