Tom.
For general turning, drilling, boring work, you will never need such a low speed as 70 rpm. Even with big parts – the larger the work, the slower the rpm, to keep your surface speed at the tool correct. But on a 13” lathe, even if you were turning a 13” diameter that’s only 238 feet/min. Roughly speaking that’s a third of what HSS will put up with in steel.
The slow speeds really come in when you are thread cutting. The slower the spindle speed, the slower the carriage travels. That would mostly be a problem with flying – course leads. You have to stop, or retract the tool, particularly cutting up to a shoulder. The faster the spindle is going, the quicker you have to react.
All in all thou, 70 rpm’s is not that fast, and would be pretty normal on that size lathe. It’s still only just less than 1 turn per second. You would be hard pressed to find any thing big enough, that could be turned in such a machine that would require any thing slower.
Vic, doesn’t cut it as a location. If you are any where near the big town. Your welcome to come and have a play with a Nardini, I have down the back. It has 25 – 40 & 63r.p.m in the bottom end. I’m in Thomastown, and in all day Saturday. Stuffed if I’ve ever used the bottom, end, except for maybe shaving the last little bit out of a acme thread, with a polished tool, and dark oil, just to bring the shine out on the flanks. My email is in the profile.
I’m guessing with those brands, the vendors don’t have them under power. You’re welcome to come and have a look and play with 25 rpm. I doubt you will feel the need for it, against 70rpm.
Regards Phil.