I use a core box router bit. I have them in 1/8″, 1/4″, 3/8″, 1/2″, 3/4″ ans 7/8″. They are carbide tipped and cut like hell. They are super dangerous because they will grab a hold of the wood sometimes and you can take off a whole branch or finger and not even know it.
You can cut branches off like a pruner and cut a three inch tree in half in about 30 seconds. The bits can be found in any hardware store like Home Depot or Lowes and sell for about $13.00 to $25.00.
The grinder is just a die grinder from Harbor Freight. One can spend 200.00 on a Makita if you want but I have used mine for over eleven years with no problems what so ever. It comes with a spare pair of brushes for the moter which I have not had to change. The Harbor Freight model….$35.00 on sale. 28,000 RPM’s and 1/2 horse at 10 amps. I have used the Makita and this is as good or better. I prefer it more than the Makita because the makita does not have a good handle at the nose like this has. This unit is also aluminum on the business end and has two ball bearings. I like it, good luck.
PS…Two hands on the die grinder at all times. This is not a dremel that you can hold a branch while you do a little carving. One of these bits would sever your hand right off!