Mike et al,
There are hand grooving machines available (two on UK ebay from a dealer at the moment). They come with various width blades which can be set for any depth. I have used them and it is very satisfying seeing a new tyre appear from an old one (although if you are just going over the old groves on a worn tyre care must be taken not to go into the fabric.)
You can do one large tyre in about 15 minutes depending on the number of grooves.
Lang
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