Thursday, January 17, 2008

Making Crown Molding

I've been working on the cove / crown molding for the past two weekends now. I've had some ups and downs with this molding. I was able to make two good pieces, out of three attempts. The third didn't turn out too good. And then I had problems when I went to miter the finished molding. But that's getting ahead of where I'm going.

I started out by mounting the crown molding bit I bought into my Bosch 1617EVS router, which was still in the router table after making the ogee molding previously. I then set the height of the bit, as you can see in the photo above. The workpiece is actually cut wider than the bit is tall. I did this so that there would still be some uncut material left that could ride on the fence & keep the board from sniping.

When I was done setting the bit's height, I then put my new fence onto the table & found the hole I'd cut in it on the band saw was too short for the bit. So I enlarged it, as you can see in the photo above. Initially, I just turned on the router & pulled the fence through the bit from back to front. At first, it seemed that the bit was binding on the fence. There was a lot of vibration & noise evident as the bit spun. So I enlarged the hole. But this didn't have any effect on the vibration. In fact, at one point, I thought that the bearings in the router were shot.

Well, it turns out that the vibration was caused by the combination of the runout in the router itself, the mass of the bit, and the speed at which it was spinning. At just the right speed, the router hit a resonant frequency and the wobble in the bit was maximized. The vibration was so intense that the clamps holding the fence in place actually loosened.

And this is what caused the first board I was running through the bit to get screwed up. The fence was moving away from the bit as I pushed the stock against it, causing the cut to get deeper & deeper. Lots of chip out occurred.

I ended up reducing the speed & the vibration was greatly reduced, though not completely eliminated. I was able to get the other 5' board I had finished without any mishaps. But the vibration did leave a rippled surface on the molding that had to be sanded smooth.

Carefully looking over the first board, I decided it wouldn't be useful. So the next weekend (this past weekend), I made a third piece. The photo above shows the third molding after the first pass. The photo below shows the same board after the first pass from the end, so you can see how much of the profile has been cut.

The photo below shows the end of the finished molding. The pencil lines show the material that has to be removed from the molding to form the final profile.

Finally, the photo below shows the surface of the molding after the final pass. You can clearly see the ripples I mentioned earlier.

That's enough for now. We'll pick this up next time.

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