painterbug-
I am assuming that the hollow metal pole is the central vertical pole that goes from the floor to the top of the spiral stair. I'm also assuming that it is a fairly decent gauge (thickness). I'm also assuming you're talking about a relatively short length (maybe 3' or so?) of vertical 2x8 that you want to attach. The easy part is getting it attached; the more difficult part is having it stay that way.

As for attaching, I would drill the 2x8 in 3 places (top, middle, bottom), then drill matching holes in the pole and tap threads into the holes; then bolt the 2x8 on. If you want to be fancy, you can countersink the bolt holes in the 2x8 and then cover the bolt heads with wood plugs to hide them. If the pole is not heavy enough gauge, then you might have to drill holes on both sides of the pole and put carriage bolts of the right length through the pole and use nuts to join things together rather than tapping threads into the pole.

The harder part is that you've attached a flat surface to a curved surface, and, if there is any chance of flexing, this connection could loosen over time. You could consider cutting a cove up the whole backside of the 2x8 so that it matched the curve of the pole. Then you would have much more bearing surface and could even add some epoxy where the two surfaces meet. Since you're competent enough to build a custom railing, I assume you have a table saw and could do the coping on that. If the coving operation would take too much wood out of the backside, you could just do a smaller radius cove than the pole and at least you would have a pair of vertical bearing points on either side of the bolts and this would also be very sturdy.

At any rate, please re-post and say whether this is what you had in mind...or provide a clearer description so I can take another go at it.

Here is a link that might be useful: How to Cut a Cove

Gosh thanks kudzu,

Unfortunately, while I am good, I am not that good. Just a girl with a jig saw, a drill and a dream.

Actually the 2x8 is the top rail and bottom rail with the ornate found iron pieces (very old spindles) in between. Like an iron sandwich if you will. Those spindles had about an inch or so of tapered ends to fit into holes I drilled into the 2x8's. So really I was speaking of attaching 2 butt ends (the 8" side vertical) of the 2x8 into the pole at the top and bottom.

I found some self drilling hex screws with rubber washers for metal and used those with some hefty brackets. 4 brackets on the pole side, with 8 of these screws into the pole alone. I then used wood screws for the wood side of the brackets. I hope it holds and seems to be tight. Your way sounds much prettier and probably more secure. Yes, it bugs me that a flat surface is against a curved one, but I did the best I could.

At any rate it is there ...for now...and the good thing is the knowledge is there in case I need to redo or repair. If that time should come, I might try my hand at coping.

Thanks for responding.

painterbug-
I'm impressed. Sounds like you did pretty well for someone with limited tools. You fooled me into thinking you were some
crusty old woodworker with lots of tools! That should hold fine unless you've got kids running up and down and torquing the railing...