Two days later I removed the clamps and the bowing condition was a little better. However, I still don't know 'why' this happened.

Now it's time to machine the ends to length. What I'm shooting for is 17 1/2" and as you can see I'm right at that dimension according to my scale. And yes it seems weird to use something other then some kind of 'precision measuring equipment' to hit a dimension. But I don't have 18" calipers so this is the next best thing. Side Note: When I do use a scale like this, I also use magnification to help me see the lines better.
 

As you can see I've stopped short of the end because I didn't want to split the wood. At this point I used a left-hand spiral end mill to machine off the remainder. And yes, this left-hand cutter needs to be run in the opposite rotational direction when you cut. This process works great for a large piece like this.
 

Every time I buy wooden dowel material (four feet long pieces), they seem to be different sizes. Plus they are out-of-round as well. When they're oversize, I've found using my drill motor and sandpaper works pretty good for reducing the outside diameter.
 

The distance between the arrows is about 1/16". This was planned while putting in the dowel holes to make sure the side pieces overhang the bottom piece. The reason for this is again, I don't own large calipers. However, I can measure this small distance with my smaller six inch calipers.

What I do is make a clean-up cut on my side piece, put the dowels in the holes and then place the side piece on the bottom. Then I can measure the amount that sticks out (overhang), place it back in my milling machine and remove that exact amount.