SINUMERIK 808D Retrofit for Fanuc 6T Lathes: Drive & I/O Guide

1. Overview of the Fanuc 6T to SINUMERIK 808D Retrofit Question

A common retrofit scenario involves 1978-1983 vintage CNC lathes originally fitted with Fanuc 6T or 6TB controls. These machines were typically delivered with brushed DC servo motors (Fanuc Models 5, 10, and 20) controlled by Fanuc DC servo amplifiers (A20B-1000 series or equivalent). Auxiliary functions (M-codes, tool changer, chuck, tailstock, coolant, lubrication) were handled by a third-party PLC - in many cases an OMRON SYSMAC C-series, a GE Series One, or a manufacturer-specific controller (for example, Okuma Howa's SCY-M1, Hitachi Seiki's proprietary ladder, or Shizouka's discrete relay logic).

When Fanuc 6T PCB-level maintenance becomes uneconomical, three retrofit paths are available:

1. Repair the Fanuc 6T in place (often unobtainable DRAM, bubble memory, or PAL replacement)
2. Replace with a Fanuc-compatible modern control (e.g., 0i-TF or 30i-TB) with full drive changeover
3. Replace with a third-party control package (SINUMERIK 808D, Centroid, LinuxCNC, Mach3/4, MASSO)

The SINUMERIK 808D is the lowest-cost production CNC in the Siemens portfolio and the only one designed for sub-$2000 retrofit budgets. It is sold as a pre-engineered bundle (PPU + MCP + cables + drive bus) targeting the small-lathe and small-mill segment. This article examines whether the 808D bundle is suitable for replacing a Fanuc 6T-era lathe, with detailed drive, motor, encoder, and I/O compatibility analysis.

2. SINUMERIK 808D System Architecture

The 808D system consists of the following components, connected as shown in the topology diagram below.

Key hardware components:

PPU (Panel Processing Unit): The 808D PPU integrates the CNC kernel, PLC runtime, operator panel, and drive interface. Two display variants are available: PPU141 (8.4-inch) and PPU161 (10.4-inch). The PPU runs the SINUMERIK Operate Basic HMI and contains the PROFINET IRT interface for the V70 drive bus, plus a separate pulse interface for V60 drives.

MCP (Machine Control Panel): A pre-wired panel with axis-selection switches, feed/spindle/rapid override rotary selectors, mode keys (JOG, MDA, AUTO, EDIT, REF), NC start/stop, and emergency stop. The MCP is connected to the PPU via a dedicated DB9 cable. The MCP is recognized in the 808D PLC as inputs at fixed addresses (see Service Guide chapter 4).

HHU (Handheld Unit): The MPG handwheel is an optional peripheral. It is recommended for setup mode and is connected via a dedicated cable. For a lathe retrofit, a 100-detent handwheel is typically selected.

Drive bus: The PPU connects to SINAMICS V60 or V70 drives via a pre-fabricated 808D drive bus cable. The bus topology is line (not ring) with a maximum length determined by the PPU specification, typically 15 m for V60 and 50 m for V70 (PROFINET copper).

S7-200 PLC: The 808D does not include integrated discrete I/O. All physical machine I/O is wired through an external S7-200 CPU (CPU 224, 224XP, or 226) and EM-series I/O modules. The PLC communicates with the PPU over PPI/MPI. Refer to the SINUMERIK 808D Service Guide for the I/O address assignments.

The 808D PPU carries a finite number of onboard I/O points (typically 24 DI / 16 DO on the PPU itself) plus optional PP 72/48 I/O modules for additional I/O density. In a typical 2-axis lathe, total I/O requirements are 64 DI / 32 DO, requiring at least one PP 72/48 expansion module and an external S7-200 for low-voltage signaling.

3. SINAMICS Drive Compatibility: V60 vs V70

The 808D supports two drive platforms. Selection determines the entire downstream drive/motor chain.

For a Fanuc 6T lathe retrofit, the V60 platform is the most cost-effective but the V70 platform provides better absolute-encoder support, more rigid position loops, and integrated spindle control. The recommendation depends on axis count, axis performance requirements, and the desired encoder technology.

For a 2-axis lathe (X/Z) with separate VFD spindle:

• V60 + 1FL3: $2,000-3,000 total for both axes
• V70 + 1FK7: $4,000-6,000 total for both axes

For a 3-axis lathe with C-axis and servo spindle:

• V70 + 1FK7 + 1PH8: $7,000-12,000 total

4. Motor and Encoder Selection

The 808D does not support arbitrary third-party servo motors. The motor must be a SIMOTICS series that has been pre-engineered into the SINUMERIK 808D drive database. The 808D service tool (Create MyConfig) maintains the list of valid motor-drive pairings. Cross-reference any selected motor against the current Service Guide motor database before procurement.

For a Fanuc 6T-era Okuma Howa NC4PS retrofit, the original Fanuc DC servo motors (Model 5, Model 10) must be removed. They cannot be re-driven by the 808D/Sinamics V60/V70 platform because:

• The 808D does not have analog ±10V output for DC servo amplifiers
• The 808D does not have resolver excitation outputs
• The 808D does not have ±15V or ±24V tacho interfaces
• The Fanuc DC servos use a proprietary brush-and-tape winding that is not compatible with the SIMOTICS stator design

The replacement motors must be sized to match or exceed the torque envelope of the original Fanuc motors. The NC4PS typically used Fanuc Model 10 (10A continuous, ~10 Nm) on Z and Model 5 (5A continuous, ~5 Nm) on X. Replacement choices:

• X-axis: 1FL3042 (V60) or 1FK7022 (V70)
• Z-axis: 1FL3044 (V60) or 1FK7032 (V70)

Mounting adapters are required because the SIMOTICS frame sizes and shaft diameters differ from the Fanuc DC servos. A custom adapter plate (steel or aluminum, 10-15mm thick) must be machined to bridge the original Fanuc mounting pattern (typically a 145mm or 175mm square flange) to the new motor bolt circle. The pinion gear or pulley must be re-bored or replaced to match the new motor shaft (typically 14mm, 19mm, or 24mm keyed, depending on frame size).

Encoder feedback from the SIMOTICS motors is integrated; the absolute position is detected at power-on with V70/EnDat. There is no need for the external encoders that the original Fanuc system required. For V60 (incremental TTL), a homing sequence is required at every power-up.

5. Spindle Drive and Encoder Strategy

The original Fanuc 6T spindles were typically Fanuc spindle motors (Model 12 was a 12 kW DC spindle with separate spindle amplifier). On a retrofit, three approaches are possible:

Approach 1: VFD + Asynchronous AC Motor (lowest cost, recommended for hobby/small shop):

• Sinamics V20 (1.5-30 kW) or V90 (higher-end)
• Standard 4-pole AC induction motor (e.g., Siemens 1LE, Marathon, or surplus industrial)
• Spindle speed command from 808D: ±10V analog output on the PPU (channel 1)
• Spindle encoder feedback: incremental TTL, wired to the PPU X412 connector
• Encoder mounted on spindle (Belt drive or direct-mount)
• Cost: $800-2,500

Approach 2: Sinamics V70 + 1PH8 PM Servo Spindle (high performance, C-axis capable):

• 1PH8083 or 1PH8107 PM spindle motor
• Sinamics V70 on the same PROFINET bus as the feed axes
• Spindle encoder: integrated EnDat 22-bit
• Cost: $5,000-9,000 (single-axis V70 spindle drive)

Approach 3: Sinamics G120 + 1LE AC Motor (mid-range, vector control):

• Sinamics G120C or G120P
• 1LE induction motor (4 or 6 pole)
• Spindle encoder optional
• Cost: $1,500-4,000

For a Fanuc 6T-era retrofit, Approach 1 is the most cost-effective and is the recommended baseline. The 808D's spindle analog output is documented in the Service Guide (chapter "Spindle setup") and uses machine data:

• MD 30130: CTRLOUT_TYPE (0 = analog, 1 = digital)
• MD 30200: NUM_ENCS (number of spindle encoders)
• MD 30350: SIMU_AX_VDI_OUTPUT (simulated axis for V/Hz mode)
• MD 32000: MAX_AX_VELO (max spindle speed in rpm)
• MD 32010: JOG_VELO_RAPID (rapid jog speed)
• MD 32250: RATED_OUTVAL (rated analog output in %, typically 100% = 10V)
• MD 32260: RATED_VELO (motor rated speed at 10V output)

For a 10/15HP (7.5/11 kW) spindle motor, the V20 11 kW variant is the recommended pairing. Configure the V20 as follows:

• P0300 = 1 (motor type: asynchronous induction)
• P0304 = 380 (motor rated voltage, 3-phase)
• P0305 = 21.0 (motor rated current, A)
• P0307 = 11 (motor rated power, kW)
• P0308 = 0.84 (motor power factor)
• P0310 = 50 (motor rated frequency, Hz)
• P0311 = 1460 (motor rated speed, rpm)
• P1900 = 2 (motor identification, full ID at next run)
• P0756 = 0 (analog input 1 type: 0-10V)
• P1000 = 2 (setpoint source: analog input 1)

6. PLC Integration via S7-200

The 808D does not provide integrated discrete I/O sufficient to wire a complete lathe. The I/O subsystem consists of:

1. PPU onboard I/O: 24 DI / 16 DO on the X100-X103 connectors
2. PP 72/48 expansion module: 72 DI / 48 DO per module (up to 2 modules = 144 DI / 96 DO)
3. S7-200 external PLC: I/O points connected via PPI to the PPU

The 808D PLC is a Siemens 808D-PLC programming environment. It uses ladder logic (LAD) or function block diagram (FBD). The PLC program structure for a 2-axis lathe is typically organized as follows:

OB 1 (cyclic main):
  CALL FC 100 (MCP read)
  CALL FC 101 (HHU read)
  CALL FC 102 (turret indexing)
  CALL FC 103 (spindle control)
  CALL FC 104 (coolant/lube)
  CALL FC 105 (chuck/tailstock)
  CALL FC 106 (alarm handling)
  CALL FC 107 (S7-200 I/O scan)

DB 14 (axis data):
  DBX0.0 to DBX0.7: axis enable bits
  DBX1.0 to DBX1.7: axis ready bits
  DBW2: actual position feedback (X)
  DBW4: actual position feedback (Z)
  DBW6: actual velocity (X)
  DBW8: actual velocity (Z)

For the S7-200, the recommended CPU is the CPU 224XP (14DI/10DO onboard) with the following I/O:

• EM221: 16 DI expansion
• EM222: 16 DO expansion
• EM223: 16 DI / 16 DO expansion

The S7-200 communicates with the 808D PPU via PPI (point-to-point interface) or MPI at 187.5 kbaud or 1.5 Mbaud. The 808D treats the S7-200 as a slave I/O station. The default PPI slave address is 2.

The 808D PLC can also interface to the S7-200 as a master, requesting data blocks and writing outputs. This is the typical configuration for handling the 808D's auxiliary functions while the S7-200 manages the lower-level I/O scan.

For tool turret integration, the 808D includes a standard PLC example program (in the 808D Toolbox) that handles 4-station, 6-station, 8-station, and 12-station turrets. The turret indexing sequence is:

1. M6 (tool change) command
2. PLC reads current tool from DB 200.DBB 0
3. PLC reads requested tool from DB 200.DBB 1
4. PLC calculates shortest path (or fixed-direction)
5. PLC energizes turret motor contactor
6. PLC monitors turret position feedback (proximity switches or absolute encoder)
7. PLC clamps the turret with a drawbar/clamp signal
8. PLC signals "tool change complete" to the 808D via M30 or M299 (operator-defined)

7. Tool Turret and Auxiliary Functions

The original Okuma Howa NC4PS used a mechanical 8-station tool turret (Curvic-style coupling) with a hydraulic clamp. The turret was driven by a hydraulic motor and indexed by a position-sensing cam. The 808D can drive this turret directly via the S7-200 PLC outputs to the hydraulic solenoid valves.

For the original Fanuc 6T-era machine, the OMRON SYSMAC SCY-M1 PLC handled the MST codes:

• M03/M04/M05 (spindle CW/CCW/stop)
• M06 (tool change)
• M08/M09 (coolant on/off)
• M10/M11 (chuck open/close)
• M12/M13 (tailstock advance/retract)
• M30 (program end)
• M98/M99 (subprogram call/return)

In a retrofit, the same M-codes are exposed through the 808D PLC. The 808D CNC interprets the M-code from the part program and passes it to the PLC via fixed-address signals. The 808D PLC then energizes the appropriate output.

The 808D PLC addresses for MST signals are documented in the Service Guide (see chapter "PLC interface signals"):

The 808D ladder logic example in the Service Guide shows the canonical M-code processing function (FC 36 "M_DECODE" or similar). The decoded M-code is compared against a pre-defined list; if matched, the corresponding output is energized with appropriate on/off delay.

For the Okuma Howa chuck open/close issue, the troubleshooting path on a 808D retrofit is:

1. Check M10 output signal (DB21.DBX32.4 = M10 = chuck open, DBX32.5 = M11 = chuck close)
2. Verify the S7-200 input from the foot pedal (M10 button contact)
3. Verify the chuck solenoid driver (24V DC, 2.5A typical)
4. Verify the chuck pressure switch (input to the S7-200)
5. Verify the chuck position feedback (open limit and close limit prox switches)

8. Service Mode and Commissioning Procedures

The SINUMERIK 808D Service Guide documents a structured commissioning procedure that the retrofit installer must follow to bring the system online. The key steps are:

Step 1: Hardware installation

• Mount PPU, MCP, HHU in the operator panel
• Mount V60/V70 drives in the electrical cabinet (observe 100mm clearance above and below)
• Wire incoming power (3-phase 400V AC for drives, 24V DC for PPU)
• Connect PROFINET or pulse cables from PPU to drives
• Wire encoder feedback from motors to drives
• Wire I/O from the machine to the S7-200 and PP 72/48

Step 2: Power-on checks (per Service Guide chapter 4)

• PPU powers up, displays SINUMERIK Operate Basic splash
• 7-segment display on PPU shows "6" (ready state) or "0" (initial)
• All drives show green RDY (ready) LED
• No red SF (system fault) LEDs illuminated
• MCP LEDs illuminate in self-test pattern
• Check whether the PPU is in the program test state - verify by inspecting the indicator light of the "Program test" key on the MCP; if illuminated, exit the mode before continuing

Step 3: PLC initialization

• Create the 808D PLC project in the Service Tool
• Set the S7-200 PPI address (default 2, baud 187.5 kbaud)
• Load the standard PLC example program (lathe with turret)
• Configure MCP key mapping (default is standard 808D mapping)
• Configure HHU handwheel resolution (default 100 IPR)

Step 4: Drive commissioning

• For each V60/V70 drive, run the auto-configuration wizard
• Set motor type from the database (1FL3 or 1FK7)
• Set encoder type (TTL or EnDat)
• Run the encoder calibration
• Set the position loop gain (Kv factor, default 1.0)
• Set the velocity loop gain (Kp, default 0.5)
• Set the current loop gain (default from motor database)
• Set the maximum velocity (MD 32000 in NC data, or drive-side p1082)
• Set the maximum current (drive-side p0640)

Step 5: Axis referencing

• For incremental encoder axes, the reference point approach must be performed
• The 808D supports the standard reference cam + reference mark method
• MD 34010: REFP_CAM_NC (NC-side reference cam)
• MD 34310: ENC_REFP_MARKER_DIST (distance from reference cam to reference mark)
• MD 34100: REFP_SET_POS (absolute position at reference point)
• The reference point is approached in JOG mode with rapid override

Step 6: Spindle commissioning

• Configure the spindle motor in the VFD (V20 or V70)
• Set motor rated power, voltage, current, frequency, RPM
• Run the motor identification routine (V20 p1900 = 2, full identification)
• Set the speed scaling (MD 32250 = 100%, MD 32260 = motor rated speed)
• Configure the spindle encoder (if used) for rigid tapping or thread cutting
• For rigid tapping: MD 30450 = 1, MD 30460 = 1
• For thread cutting: MD 30300 = 1 (is_modulo), MD 30310 = 360 (modulo range)

Step 7: Software limit switches

• Set positive and negative software limit switches per axis
• MD 36100: POS_LIMIT_MINUS (-)
• MD 36110: POS_LIMIT_PLUS (+)
• Set the software limit switch activation (default enabled)

Step 8: Tool table and zero offsets

• Configure the tool table (G43 zero offsets)
• For each tool, set the geometry offsets (length, radius)
• For each tool, set the wear offsets
• Save the tool table to the PPU non-volatile memory

Step 9: Test program

• Run a simple G-code program in JOG or AUTO mode
• Verify linear interpolation (G1)
• Verify circular interpolation (G2/G3)
• Verify tool change (M6)
• Verify spindle start/stop (M3/M5)
• Verify coolant (M8/M9)
• Verify program end (M30)
• Verify alarm and reset behavior

The Service Guide includes a comprehensive checklist for each of the above steps. The checklist must be completed and signed off before the machine is released to production.

9. Bill of Materials and Cost Analysis

For a Fanuc 6T-era 2-axis lathe retrofit with SINUMERIK 808D + V60 + 1FL3 + V20 VFD spindle, the recommended bill of materials is shown below. Catalog numbers vary by region and are subject to change; confirm against the current Siemens product configurator before procurement.

Adding 40-80 hours of installation labor at $75-150/hour, the all-in retrofit cost is $11,000-21,000. This is approximately the same cost as a used Fanuc 0i-TF retrofit but with a 5-year newer control platform.

For comparison, the original question referenced a $1,000-1,500 used 808D on the secondary market. At that price, the PPU alone is a small fraction of the total cost. The 808D drive/motor chain is 5-7x the cost of the PPU itself. A "control-only" retrofit on a used 808D is not economically viable; the savings on the PPU are dwarfed by the mandatory new drive/motor chain.

10. Step-by-Step Retrofit Workflow

1. Survey and document the existing machine. Photograph every connector, every wire, every terminal block. Document all motors with nameplate data, encoder type, and connector pinout. Save the ladder logic from the existing OMRON SCY-M1 PLC (or equivalent) for reference.
2. Remove the Fanuc 6T control. Disconnect the back panel, the operator panel, the drive cabinet, and the spindle amplifier. Remove all wiring. Tag and bag every connector for potential reuse as machine-side connectors.
3. Remove the old DC servo motors and spindle motor. Disconnect each motor, remove the mounting bolts, and document the shaft coupling and pinion/belt drive configuration. The motors will not be reused.
4. Inspect and refurbish the machine mechanicals. Inspect ballscrews, bearings, lubrication system, hydraulic system, way surfaces, spindle taper, and chuck. Any mechanical issue will be much harder to fix after the new control is installed.
5. Install the new SIMOTICS servo motors. Mount the new 1FL3 or 1FK7 motors with the custom adapter plates. Torque the mounting bolts per the motor data sheet (typically 25-50 Nm). Connect the drive power cables (X1 on the V60/V70) and the encoder feedback cables (X2).
6. Install the new spindle motor and VFD. Mount the new 1LE or 1PH8 spindle motor. Connect to the V20 or V70 VFD. Mount the spindle encoder to the spindle housing and connect to the PPU X412.
7. Install the SINAMICS V60/V70 drives. Mount the drives in the electrical cabinet. Wire incoming 3-phase power. Wire the drive bus from the PPU to the drives. Wire the motor power cables. Wire the 24V control power.
8. Install the SINUMERIK 808D PPU and MCP. Mount the PPU in the operator panel. Mount the MCP adjacent to the PPU. Connect the PPU to the drives via the drive bus. Connect the MCP via the dedicated cable. Connect the HHU (handwheel) via the dedicated cable.
9. Install the S7-200 PLC and I/O modules. Mount the S7-200 in the electrical cabinet. Wire the S7-200 to the PPU via PPI cable. Wire the EM expansion modules. Wire all discrete I/O from the machine to the S7-200 inputs and from the S7-200 outputs to the machine.
10. Wire the control cabinet. Wire all 24V DC power supplies, contactors, overload relays, fuses, and circuit breakers. Wire the E-stop circuit (PILZ or equivalent safety relay). Wire the cabinet lighting and cooling fan. Wire the three-phase power distribution.
11. Initial power-on. Apply 3-phase power to the cabinet. Verify all drives power up. Verify the PPU powers up. Check for any alarm or fault condition. Clear the alarm history.
12. Commission the drives. Run the auto-configuration for each V60/V70. Verify the motor rotates in the correct direction. Set the position loop gain and run a square-wave test to verify following error is within tolerance (typically 0.01-0.05 mm for a lathe).
13. Commission the PLC. Load the standard PLC example program. Customize the MCP key mapping, the HHU handwheel resolution, the turret indexing sequence, the chuck/tailstock logic, and the coolant/lube logic. Compile and download the PLC program.
14. Verify the MST signals. Run a test program with M03, M04, M05, M06, M08, M09, M10, M11, M30. Verify each M-code triggers the correct machine function. Verify the M-code addresses in the PLC are wired correctly.
15. Verify the safety chain. Test the E-stop button. Verify the drive enable signal drops when E-stop is pressed. Verify the spindle stops within the safety time. Verify the safety doors are interlocked with the spindle rotation.
16. Test program execution. Load a simple G-code program. Run the program in AUTO mode. Verify the cuts are within tolerance. Run the program at 50%, 75%, 100%, and 110% rapid override to verify feed-forward and following error.
17. Final acceptance. Run a series of acceptance tests per the machine builder's specification. Document all test results. Sign off the acceptance certificate. Release the machine to production.

11. Verification and Acceptance Testing

Verification of a SINUMERIK 808D retrofit follows a structured test plan. The test plan should be documented in advance and signed off at completion.

Mechanical verification:

• Axis travel: Verify X and Z axis full travel matches the machine's mechanical envelope (±0.5 mm)
• Backlash: Measure X and Z backlash with a dial indicator; should be <0.01 mm for a ballscrew-driven axis
• Repeatability: Run a 10-cycle position test; should be <0.005 mm bidirectional per ISO 230-2
• Spindle TIR: Measure spindle taper runout; should be <0.005 mm at 50 mm from spindle face
• Tool turret indexing: Cycle 100 tool changes; verify all positions index within ±0.005 mm
• Chuck clamping: Verify chuck clamping force is within the original spec (typically 15-25 kN for an 8-inch chuck)

Electrical verification:

• Insulation resistance: Megger test each motor and drive at 500V DC; should be >10 MΩ
• Ground continuity: Verify cabinet ground bond is <0.1 Ω to the main ground bus
• Drive bus integrity: Verify PROFINET or pulse signal integrity with an oscilloscope
• E-stop response: Verify E-stop trip time is <50 ms (per ISO 13849-1 PL d)
• Safety relay test: Verify the safety relay contacts open within the specified time
• I/O wiring verification: Continuity check of every I/O point

Software verification:

• Position loop bandwidth: Run a frequency response test; should be >30 Hz for a 1kg axis
• Following error: Run a circularity test (G2/G3 with G62/G64 smoothing); following error should be <0.02 mm
• Spindle speed accuracy: Set 1000 rpm and 5000 rpm; actual should be within ±0.5%
• Thread accuracy: Cut a test thread; pitch should be within ±0.01 mm
• Tool change time: Measure M06 cycle time; should be <3 seconds for a Curvic-style turret
• Program execution: Load a 1000-line test program; verify execution time matches the predicted time within ±2%

Documentation:

• Save the as-built wiring diagram (CAD)
• Save the as-built PLC program (LAD/FBD source)
• Save the as-built machine data (NC data, drive parameters, PLC DBs)
• Save the parts list with serial numbers
• Save the test results
• Save the operator manual and maintenance manual

12. Frequently Asked Questions