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TilingLogistics
My builder mate has also bought a mortar rake, but I don't think that you can get a bit less than 8mm. To give you an idea check out this link. http://search.ebay.co.uk/search/search.dll?from=R40&_trksid=m37&satitle=mortar+rake
Being adventurous and practical I would experiment using a standard router and making up a guide for it. I found this out once when I wanted to bullnose some travertine....so tried a router and it worked a treat.
Hope this gives you an idea.
Thanks for the info
Kev