Concave Forstner Bits

[gMike]

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?

[Roly]

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

[gMike]

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.

[JGrout]

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

[blackhat]

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.