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+-- Thread: Concave Forstner Bits (/showthread.php?tid=7329682)
Concave Forstner Bits - gMike - 04-22-2017
A while back I bought the $399 full set of Lee Valley high speed steel forstner bits. They are great bits and hold up well to heavy use except for one thing. They leave a domed hole, i.e. the bits themselves are concave not flat. This only came to light because LOML asked for some blind holes in which to insert a tall narrow vase. The vase wobbles because the bottom of the hole is crowned. It's not that the outer spurs are longer, I expect that. The center of the bit is at least 1/16" shallower than the cutters.
I checked my other cheap set of Taiwan specials and they leave a flat bottom. For now I will finish the holes with the old bits but I guess I expected the more expensive set to be flat. In the overall scheme of things this isn't a huge deal now that I know what's going on but it kind of surprised me.
I've never seen this before and am wondering if any of you have run into this?
RE: Concave Forstner Bits - Roly - 04-22-2017
I would not thought about that issue with Lee Valley, but looking at their specs on the bits is says "Both styles have slightly sloped chipping bevels so that bits ride a shallow cone of wood to keep them boring straight. Center brads can then be shorter, increasing bit versatility in thin material. " I'll have to remember that issue and check mine. Roly
RE: Concave Forstner Bits - gMike - 04-22-2017
I guess I need to read the descriptions a little closer. As I said, it isn't a real problem but I wish I'd seen that and not found it by accident.
RE: Concave Forstner Bits - JGrout - 04-23-2017
A relatively simple fix is at hand: drill a hole in a piece of scrap then set it over the concave hole and use a top bearing flush trim bit to flatten the bottom
It solves the issue just not your reading skills
Joe
RE: Concave Forstner Bits - blackhat - 04-23-2017
So set the workpiece nice and flat and pour a little bit of poly varnish in the holes. Should level out and make a nice flat bottom.