Need to make a big hole in Aluminum plate

I used a hardware store saw and arbor that wasn't very round or concentric, but the main issue was side wall friction and chip clearance. It took over an hour. Simply the wrong tool for the job. I might have a photo in the archive somewhere. It was a pretty ugly result. That was ten years worth of learning ago, and I haven't dared consider it since.

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Edit:. I found a pic of the plate, but I must have skipped the pic after the hole saw debacle.
 
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A hole saw will work fine if, as mentioned above, peck and use WD-40 or Kerosene. Also for that thickness I would cut it from both sides.
Edit: A quality hole saw and arbor.
 
After cutting with a hole saw, if you are not happy with the surface finish, you could then use your Rotary table idea to take a finish cut to clean it up.
 
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Personally I would enter the piece on the rotary table with a couple of 1" parallels under it. then center where the hole is to be bored under the quill . Next drill a center hole through the plate with the same size bit as the hole saw pilot. Be sure to note the coordinates if you have a DRO, or zero the dials if you don't have a DRO.

Using a hole saw slightly smaller than the finished hole make a slight to serve as an outline. Move the mill table to align the caw cut with the quill. Rotate the table drilling several 3/16"holes around the perimeter using the saw cut as a centering point. Reinstall the hole saw, move the mill table back to the zero point and cut through the plate using the pecking method and plenty of WD 40 as a lubricant and flush. Once the core is removed insert a mill in the quill and move the mill table to the proper position to cut the desired diameter. Rotate the table to finish the cut. Depending on the size of the original hole and the size of the end mill used for finishing you might want to make more than a single finish cut.
 
The steady I have for my 12" Logan can handle up to nearly 4" diameter. I would not want to limit myself to much less than that.

Also I would use the rotary table for producing the hole. The attached photo shows a part I made on my rotary table. The rounded portion fits inside a 4" diameter sailboat boom (so its would be ~3-5/8") and was hollowed out using a 5/8" carbide end mill.

 
The steady rest I made could handle 5" pipe. I made it that big so I could face the ends of pipe if I needed to.
It all depends on usage.
 
Here's one I made for my 10" SB, and then an adapter for my 13". It's been a long time ago, but I think I just drilled some 3/4" holes and freehanded the rest on the mill.
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