The curved sanding block is made out of 1" thick g-10. I didn't measure or lay anything out. I just cut a chunk and ground a heavy arc on the top and a longer more subtle arc on the bottom. Each end offers a different shaped arc that can be used for a variety of inside curves. A strip of sandpaper is simply held against the block on whatever surface I want to use.
Its important, especially on thick steel and with small bits, to use some sort of cutting lubricant. A friend of mine sent me a tube of Boelube....an aerospace product made by Boeing especially for drilling and tapping difficult materials. Its a dry waxy lubricant with no smell and no oily mess. Its great and this little tube lasts a LOOOOOONG time.
I know which one he uses most. Look at the pile of debris under the Delta next to the Ryobi. Actually that is pretty ingenuous set each one up to a specific operation and no changing the settings over and over!!