I have an 18" tall ribbed cactus which was being eaten away about 4" from
the top and I was afraid that the top part would simply die. So I cut the
top off above the bad bit and cut the bad bit off the main stem and tried to
graft the top onto the bottom, if you see what I mean.

I held the top bit hard to the bottom bit with a weight, hoping it would
graft itself and grow together. Unfortunately, someone hit the thing and
knocked off the top bit and I now see that the cut ends, top and bottom,
have simply calloused over without trying to grow together.

What should I do to make this work?

Thanks

Rob Graham