That's a half inch shank for CNC routers hence the spoilboard part of what it is. This is a good option too: http://www.amazon.com/gp/product/B0035GWKCW/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B0035GWKCW&linkCode=as2&tag=woodworkweb-20