Hi Zuk,,, yep thats a problem,,,,, as far as the bit creaping out, You may have a worn out collet,, or maybe a dirty shaft on the bit? but seems like all the tool test the past several years in the wood working magazines,, poor ol' craftsman usually does not fair too well,,, I cannot understand why a company as old and as big as Sears cannot get better engineering for their tools,,,
I have an old craftsman,, first router I bought back around 1970,, do not use it anymore,,, hahahah,,, but I thought there would be a hex section on the shaft below the nut you could use a wrench on to hold the shaft while you turned the nut with another wrench,,, but hahah there is not,, only thing on mine is that goofy plate with a pin on it,,, break that off and ya are in trouble,,,
I am a machine repairman at work and deal with a lot of small hand tools,,, you would be amazed at how people can screw up hand tools on the assembly lines,,, Not all tools make good hammers,,, but we have peole that do not aggree wth that statement,,,
you may be able to get a small Spanner wrench on that notch in the shaft in place of the plate with the pin,,,to hold the shaft,,, then turn the nut off,,, if its on that tight,,, you could,,, BUT BE CAREFUL,,, you could use a small flame on a actylene tourch and heat the nut to expand it to get it to unscrew,, but thats an extreme fix ! and should be used as a last resort,,
I would try very hard to unscrew it with out using heat,,, for the obvious reasons,,,, and when you get it off,,, replace the chuck ,,, or better yet,, give the craftsman router its resting place on your highest shelf that is hard to get to,,, and just buy another router,,,
Good luck and wish you luck,,,
It seems like every where I go,,,,,,,,,,,,,,,,,,,,,,,,, there I am...