Chas
Lock miters can be tough especially in longer lengths, machining of both joints is critical, any movement of the lumber in the machining process will create a headache during assembly. I like to add about a 1/2 degree back cut on the joint for two purposes, to allow excess glue to escape during assembly and to have a closed joint on the face side.
I agree TB and good shipping tape, one that does not stretch Scotch or 3M # 355 work the best, you will need naptha to dissolve the glue on the tape so you dont raise the grain of the wood.
Lock miters are a great looking joint if exposed, but for making box newels or beam wraps I have found the haunched miter is a little easier to work with especially in long lengths. This type of joint is easier to mill on the moulder because it is milled from right and left vertical spindles, this allows for endless width sizes to be obtained within the parameters of the machinery used. With lock miters on a moulder you have to mill from left and bottom or right and top cutters the tooling can be a bit most costly.
I have attached a haunch and lock design to give you a visual of the joint. If you have any questions feel free to contact me.
Good Luck Russ @ MR
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