Are any of the small lath mill combos decent? what are their limitations.
I have one and wouldn't go quite as far as Richard in saying they don't do anything
well. However, I don't think they are suitable for a production environment, particularly the milling functions, where one will be constantly switching back and forth between milling and turning. And my experience suggests functionality is certainly a function of how well they are tuned.
Mine is a larger one, with 1.5hp variable speed motors. Low speed torque was an issue until I replaced the cheap SCR controller with PWM ones. (I took the opportunity to re-wire things so both motors could be run at once, providing a powerfeed for the mill). Lathe spindle speed is now from 1 to about 3,000 rpm over two ranges with usable power at all speeds. I had to replace the nylon headstock gears with metal ones to handle the torque. I also modified the change gears a bit for faster switching of ratios. and re-ground the jaws of the cheap Chinese chucks for better run-out. Finally, I replaced the supplied tool post with a QCTP and mounted the compound to a plate so it could be centred on the table. Other than that, the lathe is pretty decent. Last night I was turning 1" SS rebar (416 or monel, I think) down to 3/4" with 0.0003" tolerance over 6" in 25 thou passes. The steel smoked like the devil, but the machine didn't falter.
The mill is definitely the weak half of the equation, and is as much a compromise or afterthought as any milling attachment for any lathe. It suffers from all the faults of any round-column table-top milling machine (some 3-in-1 designs vary in this regard, having different mounting schemes like bridges or dovetails. I can't attest to the efficacy of those designs). Look at it this way: if you are used to a Bridgeport, you will be disappointed. If you are used to a drill press, you will be pleased. My biggest complaint is lack of rigidity, The 4" or so round column is inadequate to the task, and the quill doesn't have enough overlap or a good way to make fine adjustments to the play -- it's OK when locked near the top of it's travel but pretty iffy in the lower half. Making fine depth adjustments when plunging is a bit of a trick -- the "fine" adjuster is too coarse and the lock must be judiciously loosened to keep things aligned while allowing movement.
The motor will handle course heavy cuts, but the round column won't -- too aggressive and the tool will rotate the column causing all kinds of horrible results. Finally, there is no adjustment of table height. The column height can be adjusted to a certain degree, but things must be re-indexed each time as there is no way to keep it from rotating when changing the height. The range of adjustment is limited to 4" so short workpieces need to be raised up off the table with a riser block, accessory tilt table, or similar trick to keep from having to drop the quill very far. It would also be nice if the table had a third T-slot, running down the centre. On the plus side, the mill spindle speed is up to 5,000 rpm over two ranges, overall costs are lower since it shares the table, DRO, etc. with the lathe and the overall footprint is smaller than two separate machines of equal capacities.
With time and care and judicious cuts, it is possible to work within 1 thou tolerances even in tough materials, but the mill in particular is definitely more suitable for a home hobbyist with limited space and cash but lots of time than for production.
(Mind you, an Afghani gunsmith fixing AK-47s with a file in a cave would increase his production a thousandfold, so I could be wrong in that assumption