Hiya all,
I've been noodling on a piece of bronze I've had laying around. It's going to be a 3/4" bore mortar. (posted a while back about it) I've had an undersized hole drilled 90% of the final depth for a few months now. I've been trying to think of a good way to make a spherical radius at the bottom (2-3/8" deep) without spending a ton of cash.
I thought about a long ball nose end mill, but was afraid it might leave bad cutter marks in the bore and it wouldn't be easy to get into the tail stock of my bench top lathe. I thought about somehow making a custom boring bar to hold a 3/8" radius carbide insert. I also considered custom grinding a radius on the end of a 3/4" drill bit. I've given it a few months to think about it. Finally, I came up with the answer the other day.
First, I bored the hole straight in the mortar and left it just undersized. Then I ran a 3/4" reamer down it for size and finish. (note: at this point the bore still has a drill point at the bottom). I bought a 3/8" radius carbide router bit from my local home improvement store. Then I took a piece of 5/8" round stock and drilled 1/4" hole in one end for the shank of the router bit. I turned some of the other end down to 1/2" to fit the Jacobs chuck in the tail stock. I also drilled and tapped for two 10-32 set screws to hold the router bit in. So, I ended up with a fancy drill bit, of sorts, with a 3/8" spherical radius at the end. Then I took a piece of brass/bronze round stock that I had laying around and made a 5/8" ID, 3/4" OD, w/ 7/8" shoulder bushing. (You could probably buy a sintered bronze bushing that would do the same thing) The bushing fit snug over the 5/8" shank of the drill I made and also fit snug in the 3/4" reamed bore. I wanted the drill guided to prevent chatter and keep it centered, but I didn't want steel touching the bore. Thus I made the brass/bronze sleeve.
So, I used the drill with the bushing to guide it and slowly drilled down (periodically checking depth and cleaning out chips) until the bore-radius transition felt smooth with a dental pick. A final polish with some Scotch-Brite wrapped around a piece of round stock.
According to my digital calipers the bore ended up at .754" at the breach. I didn't bother getting out the I.D. micrometer to check deeper. There are a couple tooling marks that were deeper that you can still see, but it's good enough for who it's for. I'll try to take a digital picture of the tool tomorrow and post it. However, for those of you playing along a home I am sure you can picture it from my description. I am not sure where you are going to get large router bits for big bores, but maybe this idea will spur on a idea in your head for a project you are working/planning on.
Aaron
