EE was attempting to install an oxygen tank guard tube measuring 11"" o d pipe w/1/2"" thick walls x 34"" long on the #5 mainten. truck. EE had it supported w/a 10"" long piece of 6"" channel, channel iron slipped out from under the pipe before the ee could tack weld it in place causing the channel iron to strike him on the rgt thumb & index finger and hyper extended his lft thumb.