Usage experience shows that the effective service life of a CNC system in a CNC machine tool is 6 to 8 years. Driven by the rapid development of electronic technology, major global manufacturers of renowned CNC systems basically launch a new generation of products every 3 to 5 years. Consequently, after a CNC system has been in use for 6 to 7 years, it becomes extremely difficult to source spare parts even if repairs are needed.

   With the advancement of electronic manufacturing technology, the proportion of CNC systems and supporting accessories (such as servo units and motors) in the total cost of an entire machine tool has been decreasing. For instance, in the 1970s, this proportion reached 50%; after the 1980s, it gradually dropped to below 1/3 to 1/4, and for some large-scale machine tools, it accounts for only around 1/10. Therefore, with a relatively small investment, many key pieces of equipment can be restored to their optimal performance.

  In recent years, China needs to carry out numerical control transformation on more than 100,000 general-purpose equipment, and there are more than 4,000 old CNC machine tools. If the transformation funds can be secured, this will be a vast market worth tens to hundreds of billions of yuan. Therefore, in the machinery manufacturing industry, especially in the machine tool manufacturing industry, it is necessary to make full use of its own technical strengths to quickly capture the market and make its own industrial contributions to the rapid development of the national economy.

1 Feasibility Assessment of CNC Retrofit Projects

Key considerations in feasibility assessment include: whether a specific piece of equipment requires CNC retrofitting, the purpose of retrofitting, the desired application indicators, the required financial and human resources, the risk factors involved in the retrofitting project, and the final cost-performance ratio.

After an old device has been in use for a certain number of years, it will inevitably experience frequent failures, difficult maintenance, reduced production efficiency, unstable machining accuracy, and even complete shutdown. When facing such equipment, a decision must be made: should it be completely scrapped or undergo CNC retrofitting through medium or major overhauls? Before making this decision, it is first necessary to assess the residual service value of the existing equipment. For example, the main electromechanical components of a machining center include: ① CNC system and operating subsystem; ② servo system (including motors); ③ machine tool electrical system; ④ mechanical structure (such as bed, column, guideways, and ball screws); ⑤ tool magazine and robot system; ⑥ automatic pallet changer (APC), etc. Each subsystem can be evaluated for its residual service value based on actual conditions.

For instance, for a CNC system installed more than 20 years ago, it is impossible to renovate it given the unavailability of spare parts, so its residual value is zero. Regarding the servo system, based on its wear level, if it is to be retained, it should be considered that it retains over 50% of its residual value after renovation. However, to ensure compatibility with the newly replaced CNC system and achieve optimal servo performance, complete replacement may be considered even if the old servo system is not fully damaged or its expected remaining service life is not long—thus determining the residual value of the servo system.

  Mechanical components generally account for a relatively high proportion of residual value. Multiple engineering practices have shown that the wear of major mechanical parts of high-quality machine tools is limited. We have retrofitted two American horizontal machining centers, both manufactured in the late 1970s. One has an X-axis travel of 1.5m, and the other 2.5m; the straightness and distortion of the guideways were adjusted to 0.008mm/1008mm/1000mm without the need for further grinding of the guideways themselves. Two vertical machining centers produced by our institute 8 years ago were used continuously for over 6 years in a user's factory, operating two to three shifts (24 hours a day). Recent measurements during renovation at our institute showed that the linear wear of the one-meter guideways was only 0.003mm. The wearing parts of the robot tool magazine system are concentrated on a few components (such as grippers and bolts), and even replacement costs are relatively low. Therefore, mechanical components always retain high residual value.

For a typical device, the mechanical components have over 60% residual value, and the electrical components have 20% to over 50% residual value. These are all factors worth considering for renovation.

The second step is to set the target expectations for the upgraded equipment, i.e., what level of machine tool accuracy and performance should be achieved after the renovation. Some people believe that the renovation indicators should be linked to the qualified inspection standards of similar new machine tools, even including requirements for appearance quality, which has a reasonable aspect. However, it must be considered that the basis is an old piece of equipment with defects; if the requirements are set too high, the investment costs will inevitably increase significantly. Combining numerical control renovation with medium and major repair plans for the equipment is more reasonable, as it can achieve a relatively ideal usage effect. Therefore, if the target value after renovation is not considered from the overall perspective of machinery, electricity, hydraulics, and even appearance quality, but is instead viewed as simply replacing an electrical system, problems will inevitably remain after the renovation.

After determining the renovation target value, it is necessary to estimate the investment of financial resources and human resources, i.e., the evaluation of investment funds. The amount of investment funds is closely related to the level of the formulated renovation target. The main renovation costs invested are composed of the following items: ① CNC system and related servo systems; ② Replacement of machine tool electrical components and accessories; ③ Precision repair and maintenance of machine tool mechanical components; ④ Maintenance of machine tool auxiliary systems (such as hydraulic systems, cooling systems, etc.); ⑤ Repair of machine tool appearance quality; ⑥ Debugging and testing after machine tool renovation; ⑦ Technical labor costs required for machine tool renovation. When restricted by renovation costs, it is common to modify the renovation target value, reduce some secondary requirements to reduce investment costs.

2、Basic Steps for CNC Retrofitting

1. Undertaken by the equipment owner itself, which requires the owner to have strong technical renovation capabilities.
2. Collaborative implementation by the equipment owner and trusted sister units invited to complement the owner’s technical weaknesses.
3. Full entrustment to a single contractor for the entire project.