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I have been getting vibration trying to cut a box joint using a 1/2" trim bit. The cut is big since the wood thickness is 1", the cut is 1" high by 1/2" diameter. Material is ash.
How can I get a smooth cut when I need to hog out such a large amount of material?
The usual advice for hogging out this kind of joint is to make a jig and use a table saw with a dado set or successive overlapping cuts. The jig would have a movable stop so you can cut the mating sides. This is also the best way to make the throat of the slot relatively square, which you will not get with any ordinary router tool.
Using a router would be a little more tricky. Again, I'd build a jig with shims (and a stop), and take successive cuts, removing shims until you are all the way through. I mean, you could hog all of it in one pass, but you won't get very nice edges.
Care will have to be taken to get the cutting tool RPM right (the bit manufacturer will tell you their recommendations) and maybe make sure any bearings riding on the jig are in good shape. Good bits often have sealed bearings, and consequently high prices. Otherwise you will have to carefully apply light oil to the bearings every so often.
There are commercial jigs that make cutting the slots/fingers really easy, but there are also DIY plans out there that are probably fine (though I've never used such a jig).
Another trick is to use two routers: one hogs out the middle in one pass, making a terrible looking cut but removing most of the waste. then follow that up with your sharpest tool that cuts to your lines, either in one go, or successive cuts. You may be able to get away with a single cleanup "arc" pass, where you have a smaller, faster tool that follows the material edge in a loop so there isn't any climb-cutting. Or you deliberately climb-cut so you get a nicer edge.
Key here will be testing your setup on some scrap to tune for best results.
I think vibration you're getting is a combination of factors. Most trim bits are straight bits and straight bits tend to vibrate more, specially when engaged for their full length. You're also trying to move a lot of relatively hard wood, so your feed rate is important. Using a spiral bit and feeding slow and easy should help. Also, hogging the bulk of waste with something else will help tremendously, either cutting waste on a bandsaw\jigsaw or even roughly kerfing it on a table saw.
