I want to mount a Qi phone charger flush into my desk. So I need to put a circular hole into the desk that's about four inches across and a quarter-inch deep. What is the appropriate tool for this job?
A 4" forstner bit would require a drill press, so you would need to put the desktop on a drill press table to drill out the hole. Depending on where the hole is located (distance from edge of desktop), this could be very difficult with most drill presses (a radial drill press would be great for this, I love mine). If you try to use a 4" forstner bit in a hand held drill, you are likely to break your wrist - that is a LOT of torque (I did try this once with a smaller 1 1/4" forstner bit - it was a bad idea)
Moving on - you can use a 4" hole saw if you are ok with having a hole left in the center of your 1/4" deep depression. To make sure you don't drill too deep, place a piece of painters tape around the hole saw, 1/4" above the top of the teeth. Stop the cut when the bottom of the tape meets the top of the desktop.
You can use a router, but you will need to follow a template. Don't try to freehand "color inside the lines" with a router, you will probably ruin your desktop - definitely use a template. With regard to the type of router, I have a few routers - a Tritan plunge router, a cheap Ryobi 2hp plunge router and a Dewalt trim router. I dig the Dewalt for small jobs like yours. You are only going 1/4" deep, so buying a plunge router seems excessive, and the less expensive ones are really more of a pain to use, they plunge poorly. Get a good quality trim router like the Dewalt. You will need to do your first cut at 1/8" and your second cut can remove the rest. With regard to bits, get a good straight plunge bit with a top bearing (to follow the template).
The router is the best power tool for this, but making an appropriate template will be a challenge. Finding a base for the router that has a pivot point so that you can make a circle is a better way (easier) to go. However...
I'm not surprised that no one has suggested a bit and brace. This is "old" technology, but in this case I think it's the best solution. It won't require a drill press, a template or even an extension cord! It's cheaper than buying a new router and bits and such, and it will do the job.
OK, it's not a lot cheaper. In fact, it may be more expensive. But it will do the job. Just google "bit and brace adjustable" because you'll need to buy an adjustable bit to make a hole the size that you want.
You are really asking two questions here:
- how do I make a large circular cut that needs to look good
- how do I hollow out a large area to a pre-set depth
Forstner bits try to do both at once, but it has been well-discussed that a 4 inch model requires a lot more force than your wrist can provide. They're not cheap ($400 !!) and you will likely never use it again.
A 4-inch hole saw comes in cheap-and-nasty or really expensive, and need a 1/2 inch drill. Depth control is difficult. But a fly cutter will do exactly what you want: engrave a slot in the desktop X mm from the center.
Once you have the outer slot cut you carve out the center with a router and a square bit, any convenient size. Set your depth (1/4 inch can be done in one pass, run it a tiny bit deeper to allow for glue) and just work from one side to the other.
While the router and template solution is a good one (particularly if you need to make a bunch of these holes) it does have the small complication of requiring you to first make a template.
For a single hole, a simpler solution would be to route out progressively larger holes using a combination of a rabetting and pattern bits.
The process starts with a more reasonably sized forstner bit (one that you can handle freehand and that may actually be useful in the future). This hole is then enlarged using a rabetting router bit, running the bearing around the existing hole. Next the pattern bit is used, with its bearing running along the larger hole made by the rabetting bit. This can be repeated as many times as necessary.
As the brief article linked above mentions, you have to calculate out the the final size using the bearing offsets and the starting hole diameter. They also mention building up a surface for shallow non-through holes of the type you describe, although I would suggest it a better idea to use this technique on some 3/4 MDF to perfect a 4" hole, and then use that as the template for routing the actual desk. The template can be attached to the desk using double stick carpet tape.
Finally, if this is a desk you intend to keep longer than the charger, it would be convenient to make up some filler blanks to pop into the hole before you loose track of the template.