Buy the heavy duty blades for whatever groover you use, you will not regret it. The blades that came with the Ideal Knife I have were pretty weak so I upgraded to the heavy duty blades and it cut my Boggers like butter. Here is a link to an article...
http://www.izook.com/reviews/heatknife/heatknife.htm
Also, let the tires sit in the sun before grooving them, it will help.
Hope this helps...
Jeff