Just completed installing an air spring on my HF mini mill. I used a method I had seen on a youtube video. Basically installing the air spring using the existing bolt used for attaching the torsion spring to the head and the left rear mounting bolt used to secure the mill to the bench. That way the height of the machine was not increased. I selected a 40 lb air spring after finding out that the kit supplied by thelittlemachineshop was approx. a 36 lb spring but I had seen comments that it did not completely support the head. I made an adjustable bracket at the top so the spring attachment point could be tweaked in order to get full height extension and be able to get down to the table on the lower end. I have done some basic testing and it appears to have solved the issue with head drop. I still need to replace the rack gear bar with the longer 14" one sold by littlemachineshop so the head travel will be able to use all of the column.
I have attached a few pictures.
I have attached a few pictures.