Drilling Four-Inch Holes
Ways to bore large-diameter holes, and sources for oversize bits. October 26, 2005
Does anybody know where to get 4 inch and bigger Forstner drill bits?
From contributor J:
I would suggest that you use a plumbers self-feed bit. You can get them that big or bigger, however if you really need flat bottom you will have to do it in two steps. Partially drill, then remove the feedscrew and then finish to depth. Remember - you need to use a drill press and make sure your speed is slow. If you do not need flat bottom then use a hole saw.
From contributor C:
Bormax out of Germany makes 4" diameter Forstner bits in their long series. The Forstner bits have a 5/8" shank diameter and are 6-1/4" overall length.
From contributor P:
Anything larger than 4 inches for a Forstner bit sounds a little frightening. An alternative could be a circular template and top-bearing bit in a plunge router. You could always try a circle cutter if you only need a through-hole.
From contributor P:
If I only have a few holes to bore of large diameter, I use a plunge router rotated around a small center pin. I route about half way through with this method, then I use a top bearing flush trim bit and finish circle routing through. Use a smaller diameter flush trim fit than the straight cut bit in the first step. This will keep the bit from hooking up with slug as it falls out. Its a good alternative when you only have a few holes to bore.
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