Concave Forstner Bits
#6
  
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?
Mike


If you're the smartest person in the room, you're in the wrong room!

But not today...
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#7
  Re: Concave Forstner Bits by gMike (A while back I bough...)
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
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#8
  Re: Concave Forstner Bits by gMike (A while back I bough...)
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.
Mike


If you're the smartest person in the room, you're in the wrong room!

But not today...
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#9
  Re: Concave Forstner Bits by gMike (A while back I bough...)
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 Wink

Joe
Let us not seek the Republican Answer , or the Democratic answer. Let us not seek to fix the blame for the past. Let us accept our own responsibility for the future  John F. Kennedy 



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#10
  Re: Concave Forstner Bits by gMike (A while back I bough...)
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.
Blackhat
Common decency is as rare as common sense. I figure there was only a finite amount of both made and its getting shared out among too many folks.


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