Wow you turn way slower than I do.
May I ask what lathe you turn on and what you "teach"? He is looking for a guideline and fairly new to turning- you should know this since your a teacher-no offense. If you have a problem with the speed guideline I have stated, go talk to Keith Rowley, he has a book called "WOODTURNING" A Foundation Course,
I am sure you are familiar with it. He stopped the speed ratings at 12" OB and 4" BC in his book. Pattern makers and Architectural turnings on a large lathe turning a large product BC or OB ,is a totally different type of turning, same principle, different tactics,tools,techniques and machinery.
EX: The OB side of my Oliver 20C pattern lathe has a 84"D capacity that is mounted on a 24"-95lb solid steel face plate which mounts to the-2-1/4" 8TPI-OBS. I could go larger if I create a pit down through my floor but it is intimidating enough to me as it is. I don't use sand bags or mounting brackets to keep my lathe from dancing all over my shop floor - it weighs 4,552 lbs , so if you
want to turn something this large that fast, that's up to you, although - I would not recommend it.The BC cap is 24" diameter (30" if I put the 3-1/2" cast iron riser blocks on the HS/TS and top tier carriage) x 106" long , again - not the same animal. The 2 tier carriage gets used "A LOT" with these type of turnings and the fine shaping , detailing and carving (if called for)is done by hand.
I always say-Safety First to anyone, then the time and experience will come after until you become comfortable- then you can increase your speed (within reason "common sense"), that was told to me by a professional pattern maker years ago after showing me on his German "Zimmerman" Lathe- absolutely beautiful massive piece of engineered equipment- all 6,248lbs of it. If you calculate the outside surface of an object with these dimensions ,at a low speed,- it is not running slow, trust me. The rpm Hz indicator is low but not the outside product.
I don't turn pens,boxes, or spindles, well- once in a while I will but I mostly do column and pediment architectural orders of the Vignola, when the opportunity arises. So before you start jumping up and down about me and my speed recommended guidelines - we don't all turn the same objects John. OK
This is my baby from 1909.