The Haas ST-10 and ST-20 are the same lathe family at two different swing capacities, and most of their documented wear patterns are identical. One failure mode is not shared, and it happens to be the more expensive one.
What both share
- Backing plates: both machines show documented cases of broken/trashed chuck backing plates on both chucks by the time of teardown.
- Rails and rail covers: both show rail covers and rails needing replacement, alongside general chip, cable, motor, and linear-guide wear.
- Bed twist: both have documented bed-twist correction requiring a laser-level tool and machinist jacks.
- Family-level spindle bearing risk: grinding or whining at high RPM is the most common Haas spindle failure across the CNC lathe line generally, not specific to either size.
None of this is a reason to prefer one over the other. It is a reason to inspect both the same way: check the backing plates, listen at speed, and ask when the rails were last serviced.
What is different: the ST-20's turret
The ST-10 has its own documented spindle issue: a torn-down unit found catastrophic front spindle bearing failure with a destroyed cage and damaged journal, plus a separate hydraulic chuck-clamp fault traced to foot-pedal proximity switches reassembled into the wrong connector plugs after a repair.
The ST-20 has a distinct, size-specific failure: a documented turret crash that slipped the servo coupling, producing a 0.012-inch turret offset and off-center drilling and tapping. This is not a wear-out failure, it is a crash-damage failure mode, meaning it correlates with how hard a previous owner ran the machine and whether tool offsets were managed correctly, not simply with age or hours. It is also a family-level risk: a sticking clamp/unclamp solenoid valve on a Haas lathe turret has separately caused index stalls with an alarm.
Price context
Observed listings are thin for both (2 real listings each), so treat these as directional, not a full market picture: ST-10 $39,000-$50,000, ST-20 $59,000-$70,000. The roughly $20,000 gap tracks the larger swing capacity, not a documented reliability difference.
How to actually inspect one
- On either: check backing plates, rail condition, and listen for spindle noise at speed. Ask for bed-level/twist history if the machine has had a hard life.
- On an ST-20 specifically: run a test part and check that drilled/tapped features land on center. A 0.012-inch turret offset from a prior crash will show up exactly there, and it is a real, documented failure on this size.
- On an ST-10 specifically: if the chuck will not clamp, do not assume the worst before checking the foot-pedal proximity switch wiring, a documented cause turned out to be a simple reassembly error, not a part failure.