Most routers these days that can handle 1/2 bits also come with a collet for both 1/2 & 1/4" shanks so hopefully, you'll be able to find one for your router.

EDIT: Try Searspartsdirect.com to find the part number for your router. That might not be the cheapest price but might help you find the right one.

 

Get a quarter inch collet for your router.

 

I've tried a 1/4" adapter that goes in the 1/2" collett and it worked pretty well but on heavy blind cuts, like dadoes, it seemed to let the bits climb a little bit. I now have 1/2" and 1/4" colletts for both of my routers. They were inexpensive and well worth the cost to eliminate the extra piece in the setup.

I now only buy 1/2" shank bits though. The extra "beef" makes for a better cut. So my advice is to get a 1/4" collett (not adapter but a seperate collett) and use the 1/4" bits you already have but buy 1/2" ones going forward.

If you can't find a reasonably priced collet for your particular router, get a good quality adapter like this one.

 

I have had to use a 1/2" to 8mm adapter for use with my dovetail jig. The only problem I noticed was that I had to tighten the collet a lot more. The first time I used it, I was routing a 6" wide board for testing and then I noticed that things didn't sound right. The router sounded like it was running without a bit. Guess what? I t was running without a bit.

Wayne