Ron, many of the dovetail bit sets designed for use with the Leigh jig are 8mm shank tooling because of the extra strength it provides over what is a longer cutter due to the nature of the jig without clearance issues that can come with the larger .5" shank bits. I'm talking about "designed for Leigh" tooling. There are certainly other options, but the 8mm collet is necessary if one has the tooling I mention, including the bits that come with the jig from the manufacturer.
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The most expensive tool is the one you buy "cheaply" and often...