SINUMERIK 840D SL Spindle Displayed as Rotary Axis: MD Fix

Problem Description

On a SINUMERIK 840D sl control, an operator may observe two distinct but related symptoms on a configured spindle channel:

1. The spindle actual value and setpoint are displayed in degrees (°) on the HMI basic display, axis monitor, and service screens, instead of the expected revolutions per minute (rev/min or rpm).
2. The spindle feedrate override only switches between two states: 50% and 100%, regardless of the rotary switch position on the HMI or the override from the PLC.

In a correctly configured spindle these values are shown as rotary speed (rpm) and the override follows the gray-coded 0%–200% range with discrete steps of 1% (or 0.1% in fine resolution). Both symptoms are caused by the same root cause: the machine axis is recognized as a plain rotary axis and the spindle PLC interface is not properly activated.

Root Cause Analysis

The SINUMERIK 840D sl differentiates between three fundamental axis types that share the same axis container and the same NCK machine data namespace:

• Linear axis – default; no special axis type flag set.
• Rotary axis – geometric values are programmed in degrees (°).
• Spindle – a rotary axis with an additional speed-programming interface (S, M3/M4/M5, SPOS, M19, M70).

When a spindle is set up only as a rotary axis, the NCK displays the current speed as an angle (°/min or °/s, depending on MD 30300 IS_ROT) rather than in revolutions per minute. Additionally, the feedrate/rapid traverse override is processed as a rotary-axis override, which is binary by default (50% / 100% / 150%) unless the dedicated spindle override interface is enabled by the PLC.

The two symptoms therefore point to two missing links:

1. Axis → Spindle assignment is missing. Machine data MD 35000 $MA_SPIND_ASSIGN_TO_MACHAX must be set to the spindle index (1…n) for the axis that shall act as spindle 1.
2. Spindle override activation is missing. The PLC must set interface signal DB36.DBX1.7 (Override active) for the spindle, otherwise the NCK clamps the override to the minimum value of the OVR rotary switch position (default 50%) and 100%.

Required Machine Data Overview

The following NCK machine data must be consistent for the spindle channel axis. The machine axis index (AX1…AXn) used here is the 6th machine axis. Spindle 1 is being defined on the 6th machine axis.

Sources: SINUMERIK 840D sl Axes and Spindles Function Manual (06/2019) and the SINUMERIK 840D sl List Manual (parameter list of machine data).

Channel Axis Assignment (CHANAX vs MACHAX)

SINUMERIK 840D sl maintains two tables:

• Channel axis table (MD 20070 $MC_AXCONF_MACHAX_USED and MD 20080 $MC_AXCONF_CHANAX_NAME_TAB) — the logical order of axes inside a channel (channel axes 1…6, sometimes called AX1…AX6 inside the channel).
• Machine axis table — the actual hardware/NCK axes (NC axes 1…n) defined by MD 30150 $MA_AXCONF_INPUT_AX or default order in the drive assignment.

If the spindle is supposed to be the 6th channel axis (CHANAX index 5, zero-based) but the channel axes are not defined continuously from index 0, a gap is created in the channel axis list. In that case the control ignores axes defined after the gap unless:

MD 11640 $MN_ENABLE_CHAN_AX_GAP = 1

With this MD, the NCK allows the channel axis list to contain undefined entries, so the spindle at channel axis 6 (CHANAX 5) is still recognized. Without it, the control does not bind the spindle to any channel axis, the PLC interface does not fire, and the override defaults to 50% / 100%.

PLC Interface for the Spindle Override

For spindle 1 on the 6th machine axis, the relevant PLC interface is in data block DB36 of the basic PLC program (blocks are typically generated by the SINUMERIK Operate / 840D sl toolbox). The signals are:

Step-by-Step Configuration Procedure

1. Verify the drive assignment. Confirm the spindle drive is configured and the motor is properly assigned via drive parameters (drive parameters are accessible in Start-up → Drive configuration). The drive must be present and commissioned before the NCK accepts a spindle definition.
2. Set the rotary axis MDs on the machine axis used as spindle.
   Open Start-up → Machine data → Axis MD and edit:
   MD 30100 IS_ROT_AX = 1
MD 30110 ROT_IS_MODULO = 1
MD 30120 DISPLAY_IS_MODULO = 1
MD 30300 IS_ROT = 0
3. Assign the axis to spindle 1.
   MD 35000 SPIND_ASSIGN_TO_MACHAX = 1
   This is the central link that turns the rotary axis into a spindle. Each channel may have multiple spindles; the value 1 means "this axis is spindle 1". To turn off the spindle assignment, set the value to 0.
4. Allow gaps in the channel axis list if needed.
   If the 6th channel axis (the spindle) is the last logical axis and indices 0…4 are not defined continuously, set:
   MD 11640 ENABLE_CHAN_AX_GAP = 1
5. Save and reset (NCK reset).
   The MD changes require a warm restart of the NCK. After reset, the spindle will appear in the HMI's axis/spindle selection list and the actual value will be displayed in rev/min.
6. Activate the spindle override from the PLC.
   In the basic PLC program (S7-300/400 / S7-1500 soft PLC), write the following signals continuously:
   DB36.DBX1.7 := TRUE;    // Override active
   DB36.DBB18 := GrayCode(OverrideValue);   // 0…200%
   The override is a gray-coded byte, so the byte 0x00…0xFF is interpreted as 0…200% in 256 gray-coded steps, but only the discrete positions of the rotary switch (typically 32 positions in 5% steps) are valid.
7. Compile and download the PLC project.
   Use SIMATIC Manager / TIA Portal to build the project. The S7-300 (or S7-1500) must be reset to load the new blocks.

Verification Procedure

1. Open the Spindle Service screen from the HMI: Diagnosis → Service displays → Axis/spindle service. The actual speed should now read in rev/min, not degrees.
2. Operate the spindle from MDI with S1500 M3. The actual speed should rise to ~1500 rev/min and the display should read "1500 rev/min" (or simply 1500 with the rpm unit), not a degree-based value.
3. Turn the spindle feedrate override rotary switch on the HMI to 50%, 75%, 100%, 120%. The actual active override should follow the switch position. If it snaps to 50% or 100% only, the PLC interface signal DB36.DBX1.7 is not being held high continuously — the PLC cycle must not pulse it.
4. Use the HMI Diagnosis → Trace (or BTrace) to record the Override field and verify the gray-coded value at DB36.DBB18.

How the Override Is Interpreted

The gray-coded byte at DB36.DBB18 is the position of the rotary switch or the override value from a handwheel. It is NOT a linear 0…255 byte. The 840D sl basic PLC decodes the gray code, then the NCK clamps the resulting value:

• If DB36.DBX1.7 = 0 (Override inactive), the NCK internally forces the override to 0% (spindle stop) on the high byte and 50% (50% of programmed speed) on the low active switch position. Hence the binary behavior you see on the HMI: 50% / 100%.
• If DB36.DBX1.7 = 1, the gray-coded byte is decoded and applied; full 0…200% range becomes available.

The behavior is identical for the axis feed override at DB36.DBB0, but the signal that activates the axis override is DBX0.6 on the axis DB (DB31…DB35) — this is a common source of confusion.

Troubleshooting Matrix

Related Spindle Function Codes

Once the spindle is correctly recognized, additional NC commands are available. Reference: SINUMERIK 840D sl / 828D Fundamentals Programming Manual – Positioning Spindles (SPOS, SPOSA, M19, M70, WAITS).

• SPOS[1] = 90 – positions spindle 1 to 90° (requires MD 35000 link).
• SPOSA – asynchronous spindle positioning, block-change is not delayed.
• M19 – orientation stop (legacy, equivalent to SPOS with position from MD 35300).
• M70 – switch to axis mode (positioning axis), then back to spindle via reset.
• WAITS(1) – wait for spindle 1 to reach its commanded speed before continuing.

Safety and Commissioning Notes

FAQ