We prefer to drill a hole instead of a groove. Unfortunately I have neither a caliper or drill bit index here at the moment but I think we use a 11/64" bit and touched the side on a grinding wheel slightly to get the ideal diameter. So perhaps a 5/32" groove might work?
I just cut a groove with a standard 10" table saw blade. Unless you were using hard wood you probably don't need to be more exact than that.
The stem of the cup is actually oval which allows you to tighten it in a groove by turning the cell cup.
I cut them with a "cheap" blade that leaves a groove of approximately 2.7mm or 0.106" or right between 3/32 and 1/8". The cups are a little snug the first couple of rounds but fit nicely after that. Pine works well as it has a little give to it. I use a 3/8" thick cell bar with a groove just shy of 1/4" deep. Some of the commercial bars I have purchased have a deeper grove and end up splitting the bar when the cells are pushed in tight. Probably more trouble then needed for just cell bars, but then again it is a pain when the cells drop out. Or, JZBZ makes a really nice plastic cell bar...
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