The haunched tenon that Larry mentions is a good workaround for the 3/8" deep groove, even if it's a "loose" tenon for practicality. It's about glue surface for the joints.
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The haunched tenon that Larry mentions is a good workaround for the 3/8" deep groove, even if it's a "loose" tenon for practicality. It's about glue surface for the joints.
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The most expensive tool is the one you buy "cheaply" and often...
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Depending on exactly how thick your plywood is, the 6x10 mm tongue and groove set for the Euroblock cutters (nearly always called #16 and #17) might just work, and you can buy both the tongue and the groove knives for about $40 in total. A 3/4" bore aluminum head to take the knives is about a hundred bucks but there are about 170 different knife profiles you can run in them. 6 mm is just a fuzz under 1/4" (6.35 mm) but most plywood is a touch thin today anyways. 10 mm isn't quite 1/2" but it's close and might be close enough for you.
If you really need 1/4" tongues and grooves 1/2" deep, get a 1/4" groover to cut the grooves (~$50 for a decent one) and a 1/4" spacer and a pair of 3/8" or taller groovers or rabbeting cutters to cut the tongues. That will probably cost you about $200 in all. The one thing you will have to do with that setup vs. the stacked cutters is you will have to set your tongue depth yourself rather than simply coping past stacked cutters which will cut the piece to length if you have the piece set a little "long" with your coping sled or sliding table.
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