Lello,
How could I
not have noticed your D300.
Your problem was probably due to not enough focus stages.
As the workshop doubles as a photostudio but is in a bit of a mess at the moment I will try a series tomorrow of my lathe. That is 44 inches long, so a series shot on the diagonal should be an interesting challenge for a non macro stack. A bit like your pencil suggestion but on a larger scale.
Don