Yep, I'd ask the same question as Glenn -"How many are you going to make?" If it's a small number, just drill them out and clean up with a chisel.
For a large number, the absolute best is a horizontal slot mortiser - but they tend to be somewhat expensive. Laguna is suppose to have a good one at a reasonable price (I think it was about $1,000).
[Just a comment about those attachments for the drill press. My experience has not been good. First, there's the time to put it on and take it off. Second, the arm of the DP is not long enough so it's tough to make the mortises. Also, many of those attachments are not that good, although with some work (sharpening) you can make them work decently. But I'd recommend a dedicated mortiser rather than a DP attachment.]
Last edited by Mike Henderson; 06-18-2009 at 12:49 AM.
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