holdfast diameter?

Just wondering, what would be the diameter range of a holdfast shaft to work well on 3/4" hole? My bench in progress is about 3.75" thick out of soft maple. I have a 5/8" holdfast that doesn't seem to be able to hold (even after some sand-paper roughing). I hit it with a mallet, and it just bounces up and down and doesn't catch. I am wondering that perhaps 5/8 is too thin, but before I go about buying new ones from Gramercy of LN, I want to make sure I am not doing something wrong.



Thanks!

(73d)

holdfast diameter?