The only way to cut dovetails on a board that is wider than what fits in the
12" Leigh jig is to cut the board into 2 narrower pieces so each half will fit in the jig, then dovetail these pieces, and then edge glue them back together. It's not easy, but it can be done. You will need to make matching pieces with the same offsets to get everything to fit together and this can be quite difficult, but not impossible. It's much better to spend a few extra bucks and get a wider dovetail jig.
Nope, the board has to fit between the two clamping threaded rods. I would say go as big as you can, it makes it an easier job of doing symmetrical boxes when there will be 2 or more side my side with through DT's.
If you make lots of fine furniture, the DT jig will be an awesome acquisition. When making furniture, I do cross cutting that counts on my tablesaw.
My shop CMS can be a $39 HF unit, as all it does is shorten rough boards. My good SCMS has been banished to a shelf near the door (for site work), as I rough cut with a cordless circular or hand saw that live at the stock rack. Long parts are cut with cheap, shop made sleds that can incorporate simple hold downs and counter balancing. Sleds also allow very accurate stop blocking for identical parts.
On the other hand, if you're doing lots of trim or things like window boxes and Adirondack chairs, or site work, the DT jig will gather dust most of the time, and a good CMS or SCMS will get a workout.
A big plus for a 24" DT jig that's often missed is dual setups. When drawers have some variation of half blinds in front and through DT's in back (common), you can have the left and right setups installed at the same time. It saves time, but most importantly, dual setups can cut the error rate in a big way. This becomes very apparent when making drawers that are different sizes.
On last thought... Don't leave out factory reconditioned miter saws. Almost all of my handheld and portable power tools, including my SCMS, are recons, and I've yet to be disappointed. Maybe you can still get both...
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