Thanks alot for that info, everyone!
My cutters and definately slotdrills, they cutting edges join up in the middle so it would appear to be centre cutting (although never tried it!)
Found a neat pic showing both styles of cutters - mine are like the ones 2nd row down and bottom row.
I've been googling around looking for machining info and found a few websites to share:
http://machine-tools.netfirms.com/index.htm http://www.frets.com/HomeShopTech/hstpages.html http://www.finelinehair.com/home/index.htm
BW - your website's one of my faves too!
http://www.cnccookbook.com/