Yes, you need a 2 speed motor and appropriate switch-gear. Probably difficult to source and quite pricey. A alternative is a 3 phase motor with a variable frequency drive ( VFD ) if you want to keep the full speed range.

High speed isn’t essential for carbide tooling but a cheaper option would be a bigger drive pulley on your present motor but the lower speed range would also rise. Bear in mind that the ML7 spindle isn’t rated for very high speed, say 1200rpm max.

A VFD is a very good option. On my S7, I very rarely go over 700 rpm. It is really handy to be able to just go a little faster or slower. With my setup, I mainly use the fastest pulley on the headstock and the lowest from motor to clutch.  I got a 1hp (750watt) motor and matching VFD. The slowest I run it is at 7 hz for power tapping, about 90rpm, and the Max I run it is 60hz or about 780rpm.  Mostly I use it at 30 to 50 hz range. Can’t remember when I last ran it in high gear for turning a part. I did do a test in high gear for the VFD starting the lathe with the clutch engaged when a member was having issues with their VFD quitting and not able to start with that load. Mine worked fine. But I do have mine on a delay or ramping start, and have it on a ramping slowing (breaking) to slow down. Just can’t be too aggressive on the slow down, as it may make a heavy setup want to unwind.

I use a lot of carbide inserts. You don’t actually need to go very fast with them if you have the correct geometry for the material being cut.

The ones I mainly use are the Ground ones, both coated and uncoated.  I do not use the pressed inserts that are not sharp on the edges. These require very high rpm to work correctly and I am not running my lathe that way. I have TNGG16 inserts, triangle 16mm format, DCGT11 inserts, 11mm format in a 55deg angle, CCGT09 inserts, 9mm Rhomboid 80 deg angle inserts. Mostly in 0.2 and 0.4 mm radius and a few in the 0.8mm radius. For steel, Aluminium, Titanium, and they cut Hard bearing shells too.

For Brass I use a ceramic insert in the 16mm TN holder. They last a very long time, or I use the flat Cast iron carbide inserts, again mostly in the 16mm TN holder.

I try and use the industry standard inserts, and have cut down the holders to fit the Myford Dickson holders. Using the industry standard sizes, give you the greatest choice of insert geometry and the best prices. Also, as technology moves ahead, these are the types that see the newer coatings or carbide matrix advances. The newest inserts from Seco, like  the TH1000 grade will cut almost anything, similar with the new Ground Mitsubishi inserts in the 9005 grade will cut almost anything, including hard steels at 60Rc with interrupted cutting. And they don’t have to run high speed to get a really good finish. Both the Th1000 grade and the 9005 grade are very similar characteristics. Really good for cutting Hard bar, that is hard on the outside and a tough centre.

Hope this helps you in some way.

Neil