If you use a router bit with the guide roller on top, you can put the
template on top, cut to the exact size and set in the correct position,
and then drill a starter hole. I think this is easier than putting the
guide roller and template on the bottom, better visibility and less
chance of shavings getting into the roller.

I have no idea whether or not this would work in aluminum. If I was
going to do this in wood, I would rough cut the hole to the approximate
size, with a portable jigsaw or other saw, and then use the router to
trim the hole to the final dimension. Even in wood, a router usually is
easier to use if you only take off a little at a time. Usually, I find
some other way to guide the router, but I think the guide roller on a
template is the best solution for that size hole. If you wanted to use
a router bit without a guide roller, you could just make the hole in
your template larger than the 7" hole, increase the radius by the
distance from the edge of the router bit to the edge of the router base.

Richard



wrote:
My consternation with getting a another ~7" hole in 3/8" aluminum
plate should be satisfied after this question.

Several helpful people have suggested that I use a template and
carbide router with guide bearing to cut the hole.

Well, in my mind, I can see that the template could either be on
either side of the plate. If on top, I would need a template much
larger than 7" so as to allow the router plate's edge to be in contact
with the template. It would necessitate my drilling a hole to get
started so that the bearing would fit through. Although, after
drilling the hole, the bearing wouldn't contact anything. May in the
case of the template being on top, I don't need a guide bearing on the
router bit.

If the template is on the bottom, I would cut only a 7" hole in the
template and also cut a starting hole in the aluminum all the way
through so that the router's guide bearing will contact the template.

So which should I do?

Thanks everyone for all of the input.

Soon, I will be asking about how to cut a small taper on a 7/8" shaft
with the lathe's compound rest.