I have been reading many posts over the last few years about the Mini Lathe, and have come to the conclusion that many people think that because it is a 7" lathe, it will turn a 7" billet perfectly OK, and so they overload them, major problems occur, then they start to think about beefing them up even more, and even bigger problems raise their ugly heads.
I did some major work on a mini lathe, and I will tell you now, even though it had taper bearings and steel gears, for the spindle diameter (the critical part), I wouldn't use it for anything more than say 3" diameter, and even then, would be very careful on depth of cuts, purely because the lathe wasn't designed to take it.
If the gears break, then it is a sure sign that you have overloaded it to beyond the designed tolerances. People seem to think they are better than the original machine designers by fitting stronger third party parts. There is a reason for nylon gears, as a safety valve to prevent further damage.
I'm not saying you can't machine a 7" part at all, as long as the bit you are trying to machine on it isn't any larger than say 3".
When will people start to realise, if you want to start machining larger pieces, then you need to go for a larger lathe. It is just false economy to try to beef one up that isn't designed to do the job in the first place, all you will end up doing is struggling, causing yourself heartache and emptying your pockets of cash.
Small machine, small bits, large machine, large bits (and small bits), no arguments on that score, it has been that way since the beginning. I'm very sorry if you can't afford a larger lathe, but that is a fact of modern day life, if you can't afford to buy something larger, then you will have to stick to making smaller things.
DO NOT OVERLOAD SMALL MACHINES
Bogs