Special Scraping Drill Bits
Using a regular twist drill in plastic is a problem because the cutting edge takes too big a bite on each rotation of the drill and pulls itself into the work. The best way to drill plastic is with special drill bits designed for plastic. These bits use a scraping action, shaving off thin layers instead of slicing into the material, and they have a much more acute angle on the tip so that the hole is more gradually enlarged than with a metal cutting 118 or 135 degree bit. These bits do not pull into the work, but rely on you to press them through. One brand is "Plexi Point," shown below.
Regular high speed drill bits can be ground to cut plastic. See this site for details.
You can cut plastic with a hole saw, but the balance between enough force to cut and enough to melt the plastic is tough to judge, and it is very easy for things to get out of control. As discussed in the sheet metal section, it is best to clamp the work between pieces of scrap material to ensure that it is supported completely. Also, see below on peck feeding to keep things cool.
Step drills are good both for sheet metals and plastics. A step drill starts with a small hole and sequentially enlarges it in small step with a single cutting edge until the desired size is reached. Different drills have different step sizes to handle different thicknesses of material. Some drills have a spiral flute, which improves the removal of waste.
Zero Flute Countersinks
We have had some success with enlarging holes using a zero flute countersink and cutting all the way through. The single cutting edge on these countersinks is not really scraping, but takes a controlled fine cut on each rotation.
Cooling and Lubrication
The most common "gotcha" when cutting plastic is most plastics melt when they get only moderately hot. And it is hard to avoid its getting hot with all of the friction involved with drilling and cutting. Though it can get messy, water is a good lubricant and coolant for cutting plastic. Just make sure you dry everything off afterward to avoid rusting the steel surfaces and cutting tools.
Time vs Pressure
With practice, you can learn to feed a drill or saw at the right compromise speed to avoid melting from friction. Feeding too fast generates more friction, and can jam melted plastic into saw teeth or drill spiral, causing even more friction. It is important for chips to fly clear so they don't contribute to this friction. On the other hand, a lot of heat is removed during the cutting process when hot chips leave the cutting area, so hesitating to the point of not cutting fast enough will also build up heat.
One way to reduce the problem of melting is to make the cut with a series of short feeds instead of one long feed, giving the tool and work a chance to cool with the tool out of contact with the work between "pecks." This works well for drilling or sawing, especially if you are cooling with water between cuts.
Backing up the Work
When a cutter, whether a drill or a saw, finishes its cut and leaves the plastic (break through,) there are a lot of forces pushing out the back side, which can cause chipping and cracking. It is very helpful to have a scrap of wood or plastic in contact with (clamped to) the back side of the work so that the break through happens on a material you don't care about. Previous Tutorial: Drilling Sheet Metal