Homing, limit switches and soft limits
Short answer
Homing drives the machine to a fixed physical reference so it knows where it is in its own space. That gives you soft limits, which stop a job before it crashes, and it means a lost position can be recovered rather than starting over.
What homing actually accomplishes
Homing moves each axis slowly toward a switch at a known end of travel, backs off, and touches it again more slowly. When it finishes, the controller knows the machine's real position in machine coordinates. Everything useful follows from that.
- Soft limits become possible. The controller can refuse a move that would run past the end of travel, because it knows where the end is. This turns a crash into an error message.
- Work zero survives a power cycle. Because work zero is stored as an offset from machine home, re-homing restores your relationship to the workpiece.
- A lost position is recoverable. If a job fails part way, you can re-home, restore the work offset, and restart from a sensible point instead of throwing the workpiece away.
- Tool changes get easier. Many tool change routines rely on going to a known machine position.
Soft limits and hard limits
| Soft limit | Hard limit | |
|---|---|---|
| What it is | A number in the controller: "do not go past here" | A physical switch that triggers when hit |
| When it acts | Before the move starts, or as it approaches | At the moment of contact |
| Result | The job refuses to run, or stops cleanly | Emergency stop, position lost |
| Needs homing? | Yes. Without homing the numbers mean nothing | No |
| Good outcome? | Yes. It saved you | Barely. It prevented damage, but the job is gone |
Many machines use the same physical switches for both jobs. Homing uses them to find the reference, and during a job they act as hard limits.
Common homing problems
- Homes in the wrong direction. The homing direction is configured per axis. If it drives away from the switch it will run to the end of travel and stall. Stop it and fix the direction setting.
- Triggers early or randomly. Usually electrical noise, especially with a VFD spindle and unshielded switch wiring routed alongside motor cable. Separate the wiring, use shielded cable, and ground the shield at one end.
- Not repeatable. Mechanical switches have a small amount of variation. A machine that homes to a slightly different place each time will show it as parts that are slightly off between sessions. Inductive or optical sensors repeat better than cheap microswitches.
- Homes fine, then soft limits fire during a normal job. The machine's configured travel dimensions do not match reality, or your work is positioned near the edge of the bed and the toolpath needs to run the tool past the workpiece edge.
If your machine has no homing switches
Adding them is usually cheap and it is the upgrade that most changes how it feels to use the machine. Until then, make a habit of parking the machine at the same physical spot when you finish and treating every startup as a fresh setup.