Pekka,
My need for deep holes was several. A #46 hole one and one half inch deep and a .2180 hole 13 inches deep. I had bought a metal bench that had an old Atlas lathe attached to it. Missing parts, needed a lot of work. I was going to use the bed for something else until I took a close look at it. It was in good shape so I took it apart, replaced the missing parts, cleaned, repaired and painted it.
I added a variable speed DC motor and changed the pulleys to achieve 4000 rpm. I checked with Timken, the maximum rpm for the bearings with oil is 7200 rpm. I added a variable DC motor to the lead screw so I could slow down the feed rate. I made a collet system to replace the chuck. (This was also deep hole drilling. Step drilled from both ends, then reamed).
I made a pressure vessel to supply air and coolant like the Sterling unit. I then added a high pressure pump from a small pressure washer to increase the coolant pressure.
I purchased some carbide 'v' with a through coolant hole from EBay. I bought some 'v' tubing from Drill Masters.
I hard soldered a gun drill the length I needed, 18 inches. Ground it to size on the lathe and the T&C G.
Hooked everything up and that is where I stopped. I have a few details to finish. I have not made a test hole as of yet, other things got in the way. One thing I am having trouble with is to keep the solder out of the coolant hole. I purchased some anti-flux and have been doing some reading.
The material I am drilling is 4140. I am going to purchase some stress proof before I continue so I can see which is better. I believe the stress proof will be.
The long, curly swarf is to be avoided. It will jamb and destroy the gun drill. The higher the rpm, the slower the feed rate, the smaller the chip, the more success.
Long term project.