SIEMENS 840D CNC System Application in NAXOS Reducer Grinder Retrofit

Literature Overview

This paper by Liu Changjie and Wang Yang of Tianrun Crankshaft Co., Ltd., published in Manufacturing Technology and Machine Tools (2006, No. 1), describes the retrofit of a NAXOS reducer grinder with a SIEMENS 840D CNC system. The retrofit involved replacing the original control system with a modern CNC platform, incorporating step drive technology, form grinding wheel systems, and PLC-based process control. The project aimed to improve product quality, machining accuracy, and production flexibility. While focused on crankshaft manufacturing, the retrofit methodology and CNC system integration principles are relevant to pipe fitting grinding operations, particularly for precision machining of reducer bores and sealing surfaces.

Retrofit Scope and System Architecture

Original Equipment Condition

The original NAXOS reducer grinder was a specialized machine designed for grinding the big-end and small-end bores of connecting rods. The machine featured a form grinding wheel system capable of grinding the complete bore profile in a single pass, but was limited by an aging control system that could not support modern quality requirements or process flexibility.

SIEMENS 840D System Integration

The SIEMENS 840D CNC system was selected for the retrofit based on its advanced interpolation capabilities, high-speed axis control, and extensive PLC functionality. The system architecture includes:

System Component	Function	Specification
CNC controller	Motion control, interpolation, cycle management	SIEMENS 840D sl
PLC	Process sequencing, auxiliary control	Integrated SIEMENS PLC
Drive system	Axis positioning, spindle speed control	Step drive with encoder feedback
HMI	Program editing, monitoring, diagnostics	SIEMENS SIMATIC HMI
Tool management	Form grinding wheel indexing, dressing	Automated tool changer interface

Form Grinding Wheel System

The form grinding wheel system is a critical component of the reducer grinder, as it determines the final bore geometry and surface finish. The form grinding wheel is shaped to match the complete bore profile, allowing the entire bore to be ground in a single pass. This is advantageous for reducing cycle time and ensuring bore alignment, as the grinding wheel simultaneously contacts both the big-end and small-end sections of the bore.

The grinding wheel dressing system is automated, with the dresser tool positioned by the CNC system to maintain the wheel form accuracy throughout production. Dressing intervals are programmed based on production count, ensuring consistent wheel form without manual intervention.

Technical Implementation Details

Motion Control and Axis Configuration

The retrofit involved configuring the SIEMENS 840D system for the specific axis requirements of the reducer grinder. The machine typically requires the following axes:

X-axis: Grinding wheel infeed (radial direction)
Y-axis: Grinding wheel traverse (axial direction)
Z-axis: Workpiece indexing or positioning
A-axis: Workpiece rotation (for form grinding)

Each axis is equipped with a step drive motor and encoder feedback, providing closed-loop positioning accuracy. The SIEMENS 840D system supports high-resolution encoder feedback (up to 250,000 pulses per revolution), enabling positioning accuracy of less than one micron.

PLC Process Control

The PLC component of the SIEMENS 840D system manages the process sequencing, including:

Workpiece loading and clamping
Grinding wheel selection and indexing
Dressing cycle execution
Grinding cycle execution with programmed parameters
In-process measurement and quality verification
Workpiece unloading and completion signaling

The PLC program is written in ladder logic or function block diagram, with separate routines for each process step. Interlocks and safety functions are implemented to prevent machine damage or operator injury during automated operation.

Quality Control Integration

The retrofit includes integration of in-process measurement systems, such as bore diameter gauges or laser displacement sensors, with the SIEMENS 840D system. Measurement data is acquired during production and compared to programmed specification limits. If measurements exceed tolerance, the system automatically adjusts grinding parameters or signals for operator intervention.

Statistical process control data is collected and displayed on the HMI, enabling real-time monitoring of process capability. Trend analysis of measurement data allows predictive maintenance of grinding wheels and dresser tools, preventing quality degradation before it occurs.

Relevance to Pipe Fitting Grinding Operations

The retrofit methodology and CNC system integration principles described in this paper are directly applicable to precision grinding operations in pipe fitting manufacturing:

Reducer Bore Grinding

Precision reducers, particularly those used in instrumentation, hydraulic systems, and aerospace applications, require bore diameters held to tight tolerances (typically IT6 or better) with excellent surface finish (Ra 0.4 micrometers or better). A CNC-controlled grinder with SIEMENS 840D or equivalent system can achieve these requirements with high repeatability and minimal operator intervention.

Sealing Surface Grinding

Flanges, socket-weld fittings, and butt-weld fittings with sealing faces require precise surface finish and flatness control. CNC-controlled grinding with in-process measurement can ensure that sealing surfaces meet the required surface quality for reliable gasket sealing.

Form Grinding of Complex Profiles

Some pipe fittings, such as those with internal threads, groove faces, or complex contour profiles, benefit from form grinding with shaped wheels. The CNC system controls the grinding wheel form and the workpiece positioning to produce complex internal geometries that would be difficult or impossible to machine with conventional turning or boring operations.

Engineering Practice Applications

Machine Tool Retrofit Strategy

For pipe fitting manufacturers with aging grinding equipment, the retrofit approach described in this paper offers a cost-effective alternative to purchasing new machines. The key steps in a successful retrofit include:

Assessment of existing machine condition: Evaluate the mechanical condition of the machine bed, guideways, spindles, and workholding to determine if retrofit is feasible.
CNC system selection: Choose a CNC system with capabilities matching the machine's mechanical precision and the production requirements.
Drive system upgrade: Replace aging drive motors with modern servo or step drives with encoder feedback for precise positioning.
PLC program development: Develop PLC programs for process sequencing, safety interlocks, and quality control integration.
In-process measurement integration: Install and integrate measurement systems for real-time quality verification and process adjustment.
Operator training: Train operators on the new CNC system, including program editing, monitoring, and basic diagnostics.

Quality Improvement Through CNC Control

The transition from manual or semi-automatic grinding to fully CNC-controlled grinding offers significant quality improvements:

Consistent grinding parameters: CNC control ensures that grinding speed, feed rate, and infeed depth are precisely controlled for every workpiece, eliminating operator variability.
Automated wheel dressing: Programmed dressing intervals maintain wheel form accuracy throughout production, preventing quality drift.
In-process measurement and compensation: Real-time measurement and automatic tool offset compensation maintain bore diameter within tolerance without manual adjustment.
Data collection and analysis: Production data collected by the CNC system enables statistical process control, trend analysis, and continuous improvement.

Study Insights and Reflections

This paper demonstrates the value of CNC system retrofit in extending the useful life of specialized machine tools while significantly improving their capabilities. The SIEMENS 840D system, with its advanced interpolation, high-speed axis control, and integrated PLC functionality, represents a robust platform for retrofit applications. The step drive technology provides sufficient positioning accuracy for most grinding applications while offering cost advantages over servo drive systems.

For pipe fitting manufacturers, the retrofit approach offers a practical path to modernizing grinding operations without the capital investment required for new equipment. The key is to carefully evaluate the mechanical condition of existing machines and select appropriate CNC systems that match both the machine capabilities and production requirements. In-process measurement integration is particularly important for achieving the tight tolerances and surface finish requirements of precision pipe fittings.

The paper also highlights the importance of operator training and process documentation in ensuring successful CNC implementation. Even the most sophisticated CNC system will underperform if operators are not adequately trained in program editing, monitoring, and diagnostics. Similarly, thorough process documentation ensures that grinding parameters are consistently applied and that quality issues can be systematically investigated and resolved.

Summary

The SIEMENS 840D CNC system retrofit of a NAXOS reducer grinder demonstrates a successful approach to modernizing specialized machine tools through advanced control system integration.