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The motor on the Felder FD250 turns at 3350 rpm at 60 Hz. If you have a choice look into a 3 HP spindle motor to be controlled with a VFD. This would make variable speed possible.

Thanks. My normal range of mortise diameters is limited between 1/4" (for cabinet work) and 5/8" (for the occasional custom entry door). In what ways or circumstances would variable speed increase performance? Or would I have decent success with single speed basically the same as the Felder? Also, do you prefer 2-flute birdsmouth to Onsrud-type two flute spiral upcut?

Michael, I have the later model FD 250 with a 3 phase 2 speed motor and the option of a square chisel along with a chuck for birds mouth bits-1 Felder 10MM size. This older Felder is high speed as we see in this vid. I have found that 1500 or the 3K speed is more than adequate for stock removal. And, I use the 1500 speed most of the time. I get a very clean cut in hardwood with no additional chiseling required on the walls of the mortise. I think the slower speed motor is part of the Felder evolution we see and the 6500 is OMG fast. good luck check out the vid under my name with cap v and r. I do more square chisel stuff than boring with the bird mouth.

Virus 2020 time, I have been buying Onsrud (local mfg) running a 3/8" HSS end mill bit at 1500 rpm for 100MM brass stop hinges-concealed and a 1/4" carbide end mill at 30K Bosch 1.25hp router for a cavity with a waterjet cut brass hook lock on walnut gun boxes. I tested the 3/8" end mill at 3000 rpm and was unnecessary with lots more vibration in the process. Every time I setup the FD250 for hole boring or square chisel work it happens to be that one process so I am changing over a lot more. The FD250 with factory square chisel is rather pricey but for my variety of work with 3 phase 230V digital converter and 1500 vs. 3000 rpm the money on the table is worth it. No other machine on the market can match the later Felder FD250 with a set of metric and Imperial chisels in yer hip pocket. On those gun boxes I tested to skin off a thin ply of walnut for a half mortise A&E key lock-UK and had to resort to some very sharp Japan chisels to accomplish the task. Even after aligning the cast iron Felder table for better chisel throw in and out; I still had to go back in time in 100's of years and hand chisel some things. With the Felder shaper for square cuts on tenons and such and using the Felder FD250 square chisel we have an easy time with those stop pins in getting just what we need for furniture. Some orders require thinner woods milled and a rounded end slots does not leave enough meat in the mortise. One other walnut project I run requires a 6MM square chisel (smaller than a 1/4" square) in scant 3/8" thick wood so the Felder really shines. Stay safe