Resolving Siemens 840D Rough Block Transitions at Programmed Feedrate Changes

On SINUMERIK 840D sl controllers running CAM-generated multi-block toolpaths, a recurring symptom is a perceptible "jerk," scallop, or contour roughness at every block where the post-processor emits a new F word. The part program itself is geometrically clean, the controller is properly tuned, and the mechanical machine (in this case a Deckel Maho DMC 80U duoblock) is HSC-capable — yet the path quality is dominated by a periodic disturbance that arrives exactly when the programmed feedrate changes. This reference explains the underlying interpolator behavior, the role of CYCLE832, the custom macro CUST_832.SPF, the path-control G-codes (G60 / G64 / G642 / G645), the compressor functions (COMPCAD, COMPCURV), the feed-forward / acceleration-profile system (FFWON, SOFT), and the machine data that must be set in order to obtain a continuously smooth contour across feedrate-change blocks.

1. Problem Statement

A typical post-processed milling program on an 840D sl shows the following pattern near the rough transitions:

N11760 X119.699 Y-50.373 F2444.
N11770 X118.645 Y-49.151
N11780 X117.532 Y-47.983
N11790 X117.135 Y-47.581 F3250.
N11800 X116.331 Y-46.781 F2435.
N11810 X115.496 Y-46.014
N11820 X114.631 Y-45.279
N11830 X114.516 Y-45.182 F3250.
N11840 X114.051 Y-44.786 F2440.
N11850 X113.566 Y-44.412
...

Each line is a short linear block (1–2 mm), the geometry is continuous, but every block either initiates a new modal feedrate (F2444, F3250, F2435, F2440 …) or is the first block after a feedrate change. The measured surface bears a regular pattern aligned with the block boundaries, and the audible/visible feed change suggests the axis is decelerating, settling, and re-accelerating at every F transition.

The same symptom can be observed on a wide variety of post processors (Mastercam, NX, Fusion 360, hyperMILL, etc.) and on both 840D sl and 828D controllers.

2. Root Cause Analysis

Three independent issues typically combine to produce the symptom. Each must be addressed separately.

2.1 G60 vs G64 in the Program Header

The program header in the source contains:

N20 G40 G17 G710 G94 G90 G60 G601 FNORM

G60 activates exact-stop behavior: at the end of every block, all path axes are decelerated to zero and held within the exact-stop fine tolerance (G601 = "interpolator-end exact stop fine"). With a block length of 1 mm and a programmed feedrate of 2400 mm/min, the axis is being forced to come to rest forty times per second. This single line in the header is the most common cause of "rough at every block" complaints on HSC machines.

The correct modal setting is one of the continuous-path modes:

2.2 CYCLE832 Tolerance Selection

The header invokes:

N280 _camtolerance=0.100000 ; User Defined
N290 CYCLE832(_camtolerance,112003)
N310 TRAFOOF

The CYCLE832 call's third argument is the technology/tolerance index. With the source selection (112003), the cycle activates the Roughing (DYNROUGH) dynamic preset but, with the default CUST_832.SPF, does not activate a compressor, does not force G642, and does not switch feed-forward / soft acceleration. Tolerance is therefore not made effective. The cycle returns the dynamic settings of DYNROUGH, but the contour accuracy is still determined by the default (G60 / G601) of the calling program.

2.3 F-Word Modality in Modal Programming

In Siemens G-code, F is a modal value. Once written, it remains active until the next F word. A post processor that emits a new F on every block — or on the first block after a feed change — is harmless in continuous-path mode but actively harmful under G60. The fix is twofold: (a) get the controller out of G60, and (b) make sure the compressor reads the modal F from the previous block so the F-word is not re-interpreted as a feedrate step.

3. CYCLE832 — Parameter Reference

CYCLE832 is the SINUMERIK high-speed-setting cycle. Its syntax is:

CYCLE832 (_TOL, _MODE[, _TM])

The _MODE argument is positional: DOFTM[5].DOFTM[4].DOFTM[3].DOFTM[2].DOFTM[1], where the digits have the following meaning for current 840D sl SW (≥ V6.x):

The 112003 value in the source program decodes as 1 / 1 / 2 / 0 / 3 — that is: rough, tolerance, TRAORI + compressor, no technology preset, reserved. The cycle therefore should be requesting a compressor, but the effective behavior depends on what the loaded CUST_832.SPF actually implements on the local controller — and on the active G-code group 10 setting in the calling program.

4. CUST_832.SPF — Customizing the Cycle

CUST_832.SPF is the user-customizable cycle behind CYCLE832. It is loaded at NCK start and determines which G-codes, compressor functions, and feed-forward / acceleration profiles are activated for each combination of _TOL and _MODE. A typical HSC-oriented CUST_832.SPF excerpt for roughing looks like:

PROC CUST_832(REAL _TOL, INT _MODE, STRING _TM) SAVE DISPLOF
  ; --- decode _MODE ---
  DEF INT _MACH = _MODE MOD 10
  DEF INT _TOLB = (_MODE MOD 100)  TRUNC 10
  DEF INT _TR   = (_MODE MOD 1000) TRUNC 100
  DEF INT _TECH = (_MODE MOD 10000) TRUNC 1000
  ; --- branch on technology ---
  IF _TECH == 1 OR _TECH == 4 THEN
    ; ROUGH
    G642                          ; continuous path with tolerance
    COMPCAD                       ; CAD compressor
    FFWON                         ; feed forward on
    SOFT                          ; soft acceleration profile
    _SETTYP(1)                    ; dynamic mode = roughing
  ELSEIF _TECH == 2 OR _TECH == 3 THEN
    ; FINISH / SEMI-FINISH
    G645                          ; continuous path, geometric smoothing
    COMPCAD
    FFWON
    SOFT
    _SETTYP(2)                    ; dynamic mode = finishing
  ENDIF
  ; --- apply tolerance ---
  IF _TOLB >= 1 THEN
    $MA_SFACTOR_TOL = _TOL
  ENDIF
  ; --- transformation (5-axis) ---
  IF _TR >= 2 AND _TM <> "" THEN
    TRAORI(_TM)
  ENDIF
  RET
ENDPROC

The diagnostic rule is simple: when CYCLE832 is called, after the call the user should observe the following in the diagnostics display (or via the online form):

If any of these are absent, either the calling program has overridden them (e.g. an unconditional G60 in the header), or the local CUST_832.SPF is missing the corresponding ... statement for the requested technology.

5. Path Control Modes — G60 / G64 / G642 / G645

The path-control G-codes (modal group 10) are the primary lever for surface quality on short-block programs. Selection criteria:

5.1 G60 (Exact Stop)

Activates exact-stop at the end of every block. Two sub-modes are available via modal group 20:

Use G60 only for predictable tool-position moves (e.g., drill, tap, single-point positioning). Never in a milling contour.

5.2 G64 (Continuous Path)

Switches off exact-stop. The interpolator decelerates each axis only as required to satisfy velocity, acceleration, and jerk limits; the path is mathematically continuous across block boundaries.

5.3 G641 (Programmable Contour Deviation)

Continuous path with a maximum permissible contour deviation. The deviation is programmed with ADIS (path) and ADISPOS (positioning) — usually not required for CAM programs that already use CYCLE832.

5.4 G642 (Max Deviation with Tolerance Band)

The standard HSC mode for 840D sl SW ≤ V6.x. The tolerance band comes from the _TOL argument of CYCLE832. The interpolator distributes the path error over multiple blocks so that the contour deviation never exceeds the programmed tolerance, even on very short blocks.

5.5 G643 (Block-Internal Smoothing)

Used for low-tolerance finishing (tolerance < 0.01 mm) where the bandwidth of G642 becomes insufficient. Smoothing is internal to each block rather than distributed across block boundaries.

5.6 G644 (Maximum Dynamic Response)

The most aggressive smoothing. Uses knee-limited jerk. Best on very short blocks with high-dynamic drives. Risk: contour accuracy is reduced; not recommended for finishing.

5.7 G645 (Optimized Geometric Smoothing)

Continuous path with optimized geometric smoothing — default in CUST_832.SPF on 840D sl SW ≥ V6.x. Combines the contour-accuracy behavior of G642 with a higher level of internal smoothing, suitable for both roughing and finishing.

Selection guidance:

6. Compressor Functions — COMPCAD vs COMPCURV

The compressor replaces the original linear blocks with internal spline blocks of the same start and end point, providing a path that is smoother and shorter. Two variants are relevant:

COMPCAD is the recommended default in CUST_832.SPF for any 5-axis program and for any 3-axis program that exhibits the feedrate-change roughness described in this article.

7. Feed Forward and Acceleration Profile

Activating FFWON has two prerequisites: (1) the velocity feed-forward factor must be set in MD32610 $MA_FFW_MODE and (2) the feed-forward gain in MD32800 $MA_EEC_ENABLE / MD32810 $MA_EEC_K_FACTOR. The default installation of CUST_832.SPF assumes these have been set by the machine manufacturer (e.g., DMG MORI).

8. Machine Data Reference

The following machine data must be set consistently with the path-control G-codes and the compressor function. Modifying these without OEM consultation is not recommended; the values below are the typical HSC factory defaults on a 5-axis DMG machine.

Detailed descriptions for each data item are in the official SINUMERIK 840D sl / 828D Basic Functions manual: 840D sl Axes and Spindles (Function Manual) and 840D sl / 828D Basic Functions (Function Manual). For 5-axis / transformation-specific MDs see the 840D sl Transformations (Function Manual) and the SINUMERIK 840D sl / 828D Basic Functions article: SINUMERIK 840D sl / 828D Basic Functions – ID 64932982.

9. Step-by-Step Configuration Procedure

1. Replace G60 in the program header. Change line N20 from G60 G601 to G642 (or G645 on SW ≥ V6.x). Confirm that no later block in the program issues another G60 or G9 (single-block exact stop).
2. Verify CYCLE832 technology index. For a 3-axis roughing program with no orientation tolerance, use CYCLE832(_camtolerance, 112003) with _camtolerance = 0.03–0.10 mm. For 5-axis roughing, use 112011 (tolerance + TRAORI + compressor + mill_rough).
3. Inspect CUST_832.SPF. Open the cycle in the program directory (usually /_N_CMA_DIR/_N_CUST_832_SPF) and verify that the technology branch for "rough" includes G642, COMPCAD, FFWON, and SOFT.
4. Confirm axis-side FFW. In the diagnostics menu, check that $AA_FFW_MODE[Ax] = 1 or 2 for all path axes. If 0, the controller is not applying velocity feed-forward even if FFWON is modal — this alone can cause the symptom.
5. Check machine data values. In the SINUMERIK Operate menu Commissioning → Machine Data, verify that MD32610 and MD32810 are non-zero and consistent with the drive tuning report from the OEM.
6. Run a single-block test. Execute the program in single block with the dry-run feedrate (e.g., F500). Each block boundary should produce a smooth, almost imperceptible "soft" transition — not a hard stop.
7. Verify look-ahead. In the diagnostics menu, display $AC_LOOKAH_NO_BLOCKS_PREP and $AC_LOOKAH_NO_BLOCKS. Both should be non-zero during the contour. If both are zero, the look-ahead is not active and every F-word will be interpreted as a feedrate step.
8. Inspect the part. Run a short program at the actual feedrate and inspect the surface. The "stippled" or "scalloped" pattern that aligns with block boundaries should be gone.

10. Verification and Diagnostics

After applying the changes, use the following on-line variables in the SINUMERIK Operate "Parameters / HMI diagnostics" view to confirm the state of the controller during program execution:

For trace-based diagnosis, the controller exposes the following signals on the axis service screen (Servo Trace):

• Position command value (kv) — should be smooth across F-word boundaries when COMPCAD and G642 are active.
• Position actual value (kx) — should track the command value within the contour tolerance band (MD36400 $MA_CONTOUR_TOL, typical 0.01 mm).
• Velocity feed-forward output — should rise smoothly during the F-word transitions, not step.
• Following error — should be reduced by FFWON to a fraction of its uncompensated value (typically < 5 % of the uncompensated error).

11. Troubleshooting Matrix

Common alarm codes raised by compressor / look-ahead misconfiguration (from the SINUMERIK 840D sl Diagnostics Manual):

12. Header Templates

The following header templates produce reliable, smooth motion on 840D sl for typical CAM-generated programs. Replace the values for the actual machine, post-processor, and controller SW.

12.1 3-Axis Roughing

;SIEMENS 840D sl HEADER -- 3-axis roughing
DEF REAL _camtolerance = 0.05
N10 G40 G17 G71 G94 G90 G642 G601 FNORM
N20 $AC_MEAS_TYPE = 0
N30 TRAFOOF
N40 T122 M6
N50 S10000 M3
N60 M8
N70 G0 X97.072 Y-69.018 Z6. D1
N80 CYCLE832(_camtolerance, 112011)  ; rough + tolerance + TRAORI + compressor + mill_rough
N90 G1 Z-.7 F500 M26
N100 F1650  ; engage

12.2 3-Axis Finishing

;SIEMENS 840D sl HEADER -- 3-axis finishing
DEF REAL _camtolerance = 0.01
N10 G40 G17 G71 G94 G90 G645 G601 FNORM
N20 TRAFOOF
N30 T122 M6
N40 S12000 M3
N50 M8
N60 G0 X97.072 Y-69.018 Z6. D1
N70 CYCLE832(_camtolerance, 112022)  ; finish + tolerance + TRAORI + compressor + mill_finish
N80 G1 Z-.7 F300 M26
N90 F800  ; engage

12.3 5-Axis Simultaneous Roughing

;SIEMENS 840D sl HEADER -- 5-axis roughing with TRAORI
DEF REAL _camtolerance = 0.05
DEF STRING _TM = "ROTATION_5AX"
N10 G40 G17 G71 G94 G90 G642 G601 FNORM
N20 TRAFOOF
N30 T122 M6
N40 S10000 M3
N50 M8
N60 G0 X97.072 Y-69.018 Z6. B0 C0 D1
N70 CYCLE832(_camtolerance, 112011, _TM)
N80 G1 Z-.7 F500 M26
N90 F1650  ; engage