Fixing Mazak Nexus 250MSY C-Axis Sync Loss in Milling

Spindle synchronization and C-axis contouring are two different control modes on an opposed-spindle Mazak, and on a stock Nexus 250MSY they are mutually exclusive. When the Manual Milling unit starts, the control pulls head 1 into C-axis position control and releases head 2 back to spindle mode with no rotation command — that is the free-spinning sub spindle you are watching. No M code stuffed into the top of the milling unit changes that, because the mode switch is part of the unit's own start sequence. Your two real options are to drive C1 only and let head 2 hold the part cylindrically and be dragged around by it, or to have the C1-C2 synchronous software option loaded and turn each C axis on in sequence.

Read the Symptom First

Start here, before you touch the program. Watch the sub spindle position display and the sub spindle load meter at the exact block where the Manual Milling unit begins.

• Sub spindle coasts, position readout stops updating as a servo axis, load meter goes to zero — head 2 has been dropped out of position control. This is the mode-exclusivity behavior, not a fault.
• Both spindles stay in position control but buzz, chatter, or fight each other at specific rpm/diameter combinations — that is a tuning and harmonic problem, and it is a different repair.
• Sync holds through the turning unit and drops only at the milling unit — confirms the transition is the trigger, not the sync grab itself.

Symptom at the panel	Actual cause	First check
Sub spindle free-spins the instant the Manual Milling unit opens	C-axis mode change on head 1 cancels the spindle-sync pair; head 2 released with no command	Confirm whether C1-C2 synchronous option is on the machine's option list
Sync survives the manual turning unit, dies at the next unit	Turning unit runs in spindle rotation mode; milling unit requires C-axis position mode	Note the exact block number where the load meter drops
M380 or M511 at the head of the milling unit has no effect	Unit start sequence performs the mode change after the M code is output	Stop editing M codes; the fix is sequencing, not an M code
Violent oscillation between the two heads with both C axes on	Two position loops fighting through a rigid part; servo/spindle gains not matched	Turn on C1 first, then C2 — never both in one command
Chuck jaws gall or weld to the part during sync milling	Harmonic between the two drives at a specific speed/diameter	Change rpm and depth of cut; log the speeds where it appears

Why the Mode Change Breaks Sync

In spindle sync, the control runs an electronic gearbox: head 2's drive follows head 1's encoder as a velocity/phase slave. Both spindles are still spindles — speed-controlled, with a phase relationship maintained by the drive.

C-axis contouring is a completely different arrangement. The spindle motor is switched into full closed-loop position control, gains change, and the axis is fed from the interpolator alongside X and Z. The control cannot leave head 2 slaved to head 1's spindle encoder while head 1 is being commanded as an interpolated rotary axis — the reference for the electronic gearbox no longer exists in the form the gearbox needs. So the pair is broken, and head 2 drops out.

The C1-C2 synchronous option exists precisely to close that gap: it lets both C axes be commanded and fed together after each has been brought into C-axis mode. It is an obscure option and most opposed-spindle Mazaks are not ordered with it. Even with it loaded, the initial turn-on of each C axis is independent — you cannot enable both in one action.

Stop Chasing M Codes

M380 and M511 at the beginning of the Manual Milling unit will not hold the pair. That is not the fault. An M code carried by a Mazatrol unit is output as part of that unit's execution, and the C-axis mode transition is built into the unit start — the M code cannot pre-empt it. Adding more M codes only moves the symptom around and costs you setup time.

The same applies to trying to re-issue the sync command after the milling unit opens: while head 1 is in C-axis mode, the spindle-sync command has nothing valid to slave to.

Procedure: Drive C1, Drag Head 2

This is the approach that works on a machine without the C1-C2 option, and it is the one to build the part around. Head 1 owns the C axis. Head 2 grips the part cylindrically — lathe-style clamping, plain soft jaws bored to the diameter, not a milled profile jaw set that locks angular position. Head 2 is then rotated by the part itself.

1. Turn and finish the first-end features with head 1 in C-axis mode, sub spindle clear.
2. Bring the sub spindle in, sync the spindles in rotation mode, and grip. Run the manual turning unit as you already do.
3. Before the milling unit, release the spindle-sync pair deliberately rather than letting the unit break it. Take head 2 out of any commanded rotation so it is free to be turned by the part.
4. Slightly relax sub spindle chuck pressure so head 2 can be back-driven without binding, but keep enough grip to support the free end. Set clamp pressure on the low-pressure setting and verify the part cannot be pulled axially by hand.
5. Enter the Manual Milling unit. Head 1 goes to C-axis mode and indexes; the part carries head 2 with it.
6. Take milling cuts with the tangential cutting force kept modest — every N·m of milling torque is reacted through the part and through the sub spindle chuck's drag.

If the sequence cannot be expressed cleanly in Mazatrol units, drop the whole synchronized section into an EIA subprogram called from the Mazatrol program. You get explicit control over the order of clamp, sync, mode change, and index that the unit structure hides from you.

Procedure: With the C1-C2 Synchronous Option

If the option is loaded, both heads can be powered in position control and fed together — which you need for thin or slender parts, where letting one head turn free lets the milling torque wind the part up and distort it.

1. Grip both ends. Bring head 1 into C-axis mode and let it settle.
2. Bring head 2 into C-axis mode separately, as its own command. Never issue both in one block — two position loops snapping closed simultaneously on a rigid part is what produces the extreme spindle oscillation.
3. Enable the C1-C2 synchronous relationship, then command the feed.
4. Expect to ungrip and regrip one end when returning to lathe mode and back into C-axis mode — the transition out of C-axis contouring and back in is not free of angular error.

Accept that even with the option working correctly, there are speed and diameter combinations that ring. Log them and program around them.

Tune Before You Blame the Program

If both C axes are powered and the machine sets up a harmonic that tries to friction-weld the jaws to the part, that is not something you program your way out of. Servo and spindle parameters that are not matched between the two heads will make the pair fight through the workpiece, and the workpiece is a torsional spring between two stiff position loops.

• Note the exact rpm, diameter, tool, and cut depth where the harmonic appears — parameter work is guesswork without that list.
• Check for scoring or transfer on the sub spindle jaws after any event; galled jaws will keep re-seeding the problem.
• Do not compensate by cranking sub spindle clamp pressure. Higher pressure raises the coupling stiffness and can move the resonance rather than remove it.
• Servo/spindle gain and sync parameters for a machine shipped without the sync-milling application set are a factory adjustment, not a shop-floor one.

Verify the Fix

1. Single-block through the transition into the Manual Milling unit with the sub spindle chuck empty and watch the head 2 load meter. It should stay commanded through the transition if the option is active, or go to zero cleanly if you are running the drag method.
2. Put a dial indicator on a flat clamped in the sub spindle. Index C1 through 360 degrees in steps and confirm head 2 follows without lag or stick-slip. Any hesitation means the sub chuck is binding rather than back-driving.
3. Run the milling unit in the air at full programmed feed and listen. A rising whine or beat between the heads means you are near a resonance — change rpm before you cut metal.
4. Cut one part, then measure angular position of the milled feature relative to a first-end feature. Position error accumulating part-to-part points at slip in the sub spindle grip or at an ungrip/regrip step you skipped.
5. Inspect both sets of jaws and the clamped diameters after the first three parts.

Pitfalls That Repeat on Opposed-Spindle Lathes

• Profile-milled jaws in the sub spindle when you intend head 2 to be dragged — they lock angular position and turn the part into the torsional link that fails.
• Assuming the option exists because the machine has two C axes. Two independent C axes and synchronized C axes are separate line items.
• Relying on spindle sync to hold angular registration across a mode change. Re-establish reference after any transition back into C-axis mode.
• Programming the milling unit at turning-unit clamp pressure. The clamp pressure that is right for parting off is wrong for back-driving head 2.
• Editing M codes for a week. If the load meter drops at the unit boundary, the fix is sequence and option state, not the M-code field.

When to Stop and Escalate

If the sequencing works but the two heads still oscillate or ring hard enough to mark the jaws, the machine needs servo and spindle parameters retuned by the factory — that is a service call, not a programming change, and it is covered work if the machine is still under warranty. Contact Mazak service with your logged rpm, diameter, and tool data, and ask them to confirm from the machine's option list whether C1-C2 synchronous is actually installed before you build any more program around it.

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