Troubleshooting 904-1 Cold Start Y-Axis Limit Stops

The Y axis shows about 10.09 inches of negative travel, then stops as though it reached a software limit. You can jog the axes to their markers and enter CS, but the Y marker sits at the operator end of travel instead of near the center. Start with that marker position. Replacing or tuning the new servo drives will not correct a cold-start reference established from the wrong physical location.

Identify the stop before changing anything

Jog at low speed while watching the displayed Y position and the remaining mechanical travel. Stop immediately if the table or saddle approaches a physical obstruction.

A repeatable stop at a displayed coordinate points first to position-limit logic. A hard mechanical stop feels or sounds different and must never be used to prove the software limit. That is not the fault described here.

Check the Y marker location first

Move the Y axis only far enough to inspect the marker relative to the full stroke. The expected correction in this case is to place the Y marker at the center of travel, not at the operator-side end.

1. Determine the usable mechanical stroke without contacting either end.
2. Locate the physical midpoint of that stroke.
3. Compare the midpoint with the current Y marker position.
4. If the marker is already centered, leave it alone and inspect the cold-start reference relationship next.
5. If the marker is at the end nearest the operator, correct the marker/reference relationship before investigating the replacement drives.

The marker tells the control where a repeatable physical reference occurs. Cold start uses that event to establish the axis coordinate system. If the event occurs near one end while the control expects it near the center, one direction receives too little usable coordinate range. The control can then stop the axis even though visible mechanical travel remains.

Separate a moved marker from a changed cold-start position

Two conditions can produce the same symptom: the physical marker moved, or the stored cold-start position was established from the wrong location. Measure both sides of the marker before deciding which one to change.

1. With the machine in a safe state, place the Y axis at the current marker.
2. Compare the available physical travel toward the operator with the available travel away from the operator.
3. If the marker is near one end, repositioning it toward the center is the resolving branch.
4. If the marker is physically centered but the displayed coordinate range is strongly offset, re-establish the cold-start reference from the centered position.
5. If neither condition is present, inspect the configured travel limits and feedback direction through the machine's documented service procedure.

Do not silently reverse feedback, swap motor connections, or alter limit values to make the axis travel farther. Those changes can hide the reference error and remove the protection that stopped the axis at approximately 10.09 inches.

Re-establish the centered cold-start reference

Use the machine's documented service method for changing the marker or stored cold-start position. The evidence identifies CS as the cold-start entry, but it does not provide the service keystrokes or parameter-writing sequence. Read those from the machine documentation or obtain them from official support rather than guessing.

1. Place the Y axis at the physical center of its usable stroke.
2. Re-establish the cold-start position using the documented service procedure.
3. Move the Y marker to the centered location required by that procedure.
4. Jog away from the area, then approach the marker in the normal cold-start direction.
5. Jog every axis to its marker and enter CS.
6. Confirm that cold start completes without forcing the Y axis toward a mechanical end.

Keep the axis clear of clamps, fixtures, and tooling while changing the reference. Use reduced jog speed near either end. The control's displayed position is not trustworthy as a measure of remaining physical clearance until the new reference has been verified.

Test both directions after cold start

Verification must prove that the centered reference gives usable travel on both sides. A successful CS alone is not enough.

1. Record the displayed Y position immediately after cold start.
2. Jog slowly in the negative direction and confirm that the previous stop near 10.09 inches no longer occurs with substantial mechanical travel remaining.
3. Return through the center and jog slowly in the positive direction.
4. Confirm that each direction stops by normal control logic before physical contact.
5. Repeat the cold-start cycle and verify that the same marker produces the same displayed reference.
6. Run a low-speed positioning check at several points across the stroke before returning the machine to production.

If the stop moves or the reference changes between cycles, inspect the marker mounting, sensing repeatability, feedback coupling, and mechanical looseness. If the axis is repeatable but the usable travel remains unequal, compare the stored reference and travel-limit settings with the machine documentation.

Avoid fixes that mask the reference error

Do not start by retuning the two replacement servo drives. Drive tuning affects response and stability; it does not relocate a physical reference marker or correct an offset cold-start coordinate.

• Do not increase a software limit merely to pass the 10.09-inch stop.
• Do not use a mechanical end stop as a reference.
• Do not move the marker without first identifying the usable stroke center.
• Do not test at rapid speed until both directions have been checked at low speed.
• Do not change feedback polarity to correct an apparent positive/negative mismatch without the documented commissioning procedure.

FAQ

Stop if the axis approaches a physical end, motion becomes unstable, the documented reference procedure is unavailable, or the new reference is not repeatable. Escalate to the machine manufacturer's official support channel with the displayed Y position, marker location, travel available in both directions, and the results of repeated CS cycles.