The weak point of these wretched controllers, FET or SCR, is this:
When you reduce the motor speed for turning large-ish diameters, you are putting a fair load on the motor, but the fan, miserable at the best of times, is now running slow. Hence reduced air-flow.
So, the poor old ( or new ) motor warms up a bit. Often these motors sit in a semi-enclosed casting. So, they cannot be said to be effectively cooled anyway, as they suck in a good percentage of 'pre-heated' air.
The whole idea of these contraptions is flawed for a machine tool like a lathe. If you reduce the average armature current through a PM motor, by whatever means you choose, you reduce the torque. Just when you need it most.
I had a Warco mini lathe, apparently you could screwcut at some 100 rpm. Well, 100 rpm is a bit fast for most screwcutting. But in any case, the chuck could be stopped quite easily by plonking my hand on it. No chance of screwcutting there .
Now, I can put my Myford with it's belts and gears, at some 100 rpm, or lower. No way would I grab that chuck, it would probably pull my arm off. And only 1/2HP ! ... but, that 1/2 HP is there all the time.
Work out the torque for yourself ... I'll even do it for you ... for nowt.
Power = torque x rpm. Motor rpm = 1425. 1 hp = 750 Watts ish. Call 1/2 HP 375W.
So, Watts = (Newton metres) x ( rads / seconds).
Motor rpm = 1425
1425 RPM = about 24 rps
24 rps = about 150 rads/ sec
So, Torque = ( 375 / 150 ) = roughly 2.5 Nm.
Multiply by 2.2 and 3.3 to get Ft lbs .... about 18 Ft Lbs.
Lowest backgear is 25 rpm ...
Still have the same 375 watts pushing it round ....
About 2.6 rads/sec. ...
Torque some 145 Nm.
908 Ft Lbs ... getting close to 1/2 foot.ton ...
More than enough to shear anyones nuts eh ???
So, assuming a 375W DC motor on a Chinky Lathe ... that's the same 1/2 HP ..
Your controller get anywhere near 1/2 Ft.Ton at 25 RPM ???
These machines with Electronic motor controllers are not like belt and gear machines.
They simply don't have enough guts.
Electronic circuits are very cheap to produce. Accurate mechanical parts are not.
Looking at the KB controller, I doubt very much if they cost much more then £5 to make.
Even at 1-off retail the parts only add up to about £9, OK, so no PCB counted in at that, but if you buy enough, they're not wildly expensive. Not small ones similar to the KB one anyway.
Not surprised Andyf's friend got that reply from Warco.
I think it is an honest answer.
Not one any supplier would boast about in the sales literature though .. wonder why ???
I hope the maths is OK ... no doubt some one will advise ..
Dave BC
