Usual rule of thumb is 1/4 inch a pass on soft wood with a sharp bit. I usually adjust the less for some harder hardwoods.
More important than the rule of thumb for tellling me- is what I feel, what I hear, what I see and what I smell.
If the feed rate feels like it is dragging.... If I hear it dragging... If I see smoke other than from moisture... If I smell other than the smell of cut wood, such as burning...
On large diameter bits, I test to see how much I can take off at a time. On knots and burls, I take off less at a time and crawl.
Even after all that, for some reason, after routing often for so long, it still might happen. About 4 times in 20 plus years, routing almost every day. I'll never get used to that sound. I think over half of them was my fault, thinking I could go a few more time with a dull bit, going too deep with too fast a feed rate.
Next, I use a bit insert ring somewhat close to the size of the bit. The main reason for that is to keep excess sawdust off... Secondary, (has happened with me, so) is far safety. If the bit breaks it'll slow down the pieces as it rips apart the insert ring.
I can't say that any of my router's exhaust ports have an opening large enough for a 1/4 bit to enter, like your's did.
Last edited by MAFoElffen; 07-10-2012 at 02:35 PM.