I was wondering what would be the best steel to use for my thumb. I use my backhoe for everything you can imagine demolition. Clearing small lots etc... i was told.i would be good with 1/2" thick mild steel but to me that seems light for what i need it for, but at the same time i dont want something so heavy that install and removal would be damn near impossible what is every ones opinion on this. Also how thick should i make the pins
Go look at the Amulet Clamp hoe thumb. No hydraulics, it's all mechanical. I made a copy, used 1" thick T-100 or HY-100, which is a high strength steel with about every element known to make a better product in it. Then it's heat treated and stress relieved. This stuff will bend if you go crazy with the hydraulics, but will spring back to the original shape. Depending on how powerful your machine is, you might get away with .750" thick. This uses the same size pins as the OEM bucket. My Dig-more on the boom prevented the installation of a hydraulic thumb; it has the bucket attached to a male slider into the boom, so no way to have the Dig-more and a hydraulic thumb at the same time.
I am planning on building one as well. The non hydrolic types have several drawback the main one being that you put stress on the boom that may cause issues. You also can only clamp mount if your machine is made of cast ductile iron like case. I like designs that have their own hydraulic cycling. You could go 1/2 if you known how to harden and temper a huge chunk of metal. I myself would go with 3/4 or larger. I will probably use 1 inch for the jaws. You said you use it for demo so you might even think about having some blades that bolt to the jaws that would allow you to cut as well as grab .