I do not know if this is one of those things to which people will respond "of coursse, you idiot" or "duh" but it was new to me. I was attempting to use an Oak Park spacer fence and as far as I know I was doing everything according to Bob and Rick on the Router Workshop. However, I was using a spiral downcut bit (to avoid getting too much saw dust in my router) and wondered why it was so difficult to mak the cut. I then made the offset block according to the router workshop tip. I noticed what looked like smoke but ignored it thinking it could not be smoke. I finally picked up the block and found a lot of sawdust in it, and some of it was glowing!! I removed it and it landed in some sawdust which in a few minutes was flaming. I did have enough water with me to douse the flame safely so I do not think any long term damage was done except to the bit. I finally realized my mistake: the downcut spiral bit does not clear the sawdust so it just kept getting hotter and hotter. Downcut spiral bits should not be used with the spacer fences. I was lucky to notice the flames before they got out of control.