Maybe there's a misunderstanding... I contend that it's best to use the normal gearing on the lathe as much as possible and vary the speed on the VFD only so much as to allow it to span the gaps from one gear speed down to the next lower one. In my case, I have the VFD programmed to allow 45 to 60 Hz. (maybe it's 50 to 60, can't remember anymore). If I can't find a good RPM in that range, I change gears and tweak speed to get what I want.
I'm not an advocate of leaving the gear in basically one fixed mid-position and using the VFDs wide range of frequency to find the speed you want. This would entail allowing frequency to range from probably 20 to 80Hz -possibly even a wider range. This I feel is bad medicine. There were a few folks here saying they pretty much did this. I think you can get away with it if you have an older leather belt drive system but, for the sake of the motor, I don't think running it below about 20% of rated speed is good for it nor do I feel running it above rated speed is good. Of course, if the motor says it's ok then, so be it. Still though, running small gears at significantly higher speeds than they were meant to go, is not a good thing.
I do know from my continuing education from Matt, that not all lathe gears are born equal. On really high end, high speed lathes, they are hardened, individually balanced and are far more sophisticated than the typical flat-root, 20[SUP]o[/SUP] involute crown gears that we are accustomed to seeing. EX: I can't remember the name of the mill but, there's a CNC mill out there that is based off the RF45 design. It has top speeds of 5000 RPM. The gears in that thing probably cost 4-5 times as much as the gears Matt has installed in his machines...
BTW, I've got VFD on my lathe for about a year (maybe less) and it's the greatest thing since sliced bread! -Would be better if I took the time to finally figure-out and program the jog function
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Ray
Ray, I know what you are saying, but we are not talking huge speed changes. Most of the changes are to slower speeds by maybe 10 or 20 rpm. Look at the new 8 and 10 inch SB lathes they don't have any back gears its all variable speed drive.
I am not trying to argue the point I just don't think the gear train on a lathe is all that weak. There are a ton of people out there with these VFD's having a lot of success and no problems. My set up is over 3 years old and I have had no problems what so ever.
One other thing to mention is with the VFD, as you know, you can set the min and max settings. This way you can get right where you want in the RPM range. I doubt that any of us want to damage our equipment.
