For what it is worth, cutting holes is best done with hole-saw bits. But, for those that have a rotary tool a clean opening can also be achieved.
This requires a sideways (spiral) bit, and a circle-cutting attachment:
1) Measure the radius. Ensure that the measurement bisects the center-point and includes the full width of the cutting blade. Additionally, with acrylic you may want to shorten the radius by 1/32” to account for slight chipping that will be sanded away at the time of clean-up.
Using the rotary tool and the attachment as the measuring device for the pilot holes is a sure-fire way to get it right the first time. Yes, one can make all measurements with a ruler/tape measure and drill the holes without using the rotary tool as a guide, but I for one tend to bork it up… this technique saves me a lot of
’ing… it hurts after awhile.
Drill the center-point hole, then use the pre-measured rotary tool to gauge the distance for the
It is no real issue to start and stop a cut due to obstructions, but when at all possible I set up the work so that the entire cut can be done in one swoop.
When it comes to smaller panels, it is the C-clamps that get in the way. This image shows the approximate location of the C-clamp footprint and therefore allows me to set the cut up so that the rotary tool can spin full 360-degrees without being hindered.
Set up and ready to rock-n-roll!
When cutting holes with the rotary tool, it is preferable to have the pilot holes be the exact same size as the spiral bit and the center-point guide. This alleviates the “varying radius sloppiness” (potential). If there is play in the rotary tool and attachment when it is sitting ready to start the cut, then when the cutting commences always have a slight amount of outward pressure. Doing so will ensure a clean and consistent arc.
One pitfall with cutting with a center-point guide is that at the end of the cut the rigidity is lost. To ensure that the bit does not jump and gouge into the piece for the project, change the intention from being outward, to that of a decreasing radius.
This is the point at which I change the cutting intention.
This will leave a small barb in the opening:
That is removed using this:
The rest of the cut will look similar to this:
To remove the barb, tap the area lightly with the sanding drum. If your rotary tool has no speed-control, take care as it is easier to remove more material than it is to put it back. If indeed the tool is on/off only, then switching to hand-sanding may be preferable.
Barb removed:
Once the barb has been leveled, use the rotary tool and sand the entire inner edge to remove any chipping on the edges. Uniform removal is recommended, so it one side needs a 1/32” removed, then remove that from the entire opening. Once cleaned up the edge will look similar to this:
The next step is profiling (if applicable), sanding and polishing.