Replace cnc with plc


Thread Starter


I need some advise. I have worked with many types of plc's. Mostly I have programmed A-B PLC-5. Also I have worked with motor drives. I considered myself to be a bit of an automation specialist. Until I started a new job working with old cnc machine tools. Is it practical to yank all of the printed circuits and replace with a plc and variable frequecy drives for the spindle
motors. If so what HMI would you recommend. Any help will be greatly appreciated.

david mertens

I would recommend the replacement of the cnc machines control system by a system that is meant to do just that. All mayor brands offer cnc
control systems (e.g. GE-FANUC, siemens, etc). The advantage of those systems are not only in standardisation, flexibility and operator
training, but also in the posibility to export data from CAD systems directly into the control system. Even if this posibilty is currently
not used, it may offer enormous advantages in the future.
It depends on the application. If the machine has a fixed cycle and only needs parameters to be changed for different parts, then a PLC with decent motion control capabilities could replace a CNC. This is typical of volume production machines.

The advantage that a CNC has is that an operator can write an arbitrary program with any sequence desired by typing in a list of codes. This is typical in a job shop or tool room. This would be difficult to duplicate with a PLC. The PLC would need many subroutines and it would branch to them based on data in a data table. Unless you are doing many systems like this, you are probably better off upgrading to a modern CNC control.

If it were me, I would use a PC with Visual Basic or C for my HMI. The reason is that you can write a lot of the specialized setup and data handling routines in a language that is well suited for that purpose. The typical PLC programmer seems to prefer writing all of their logic in the PLC and using the MMI as a dumb graphics terminal. Your pick, I suppose.

Let us know what you decide,

Bill Sturm

If the application is somewhat routine, than a PLC replacement may be in order. Our customer had a circa 1990 CNC controlled 13 axis
machining center. Cust requirement was to install an AB SLC based modular system. We used 13 HSRV modules across two local racks. (Note:
you can't place an HSRV in slot 0 of an expansion rack.) We used a custom VB front end to handle all the database management, recipe control, and part programming. The system basically machined the same parts with only slight profile changes (i.e. one to three step drill advancing) This was all done through downloaded blended move profiles. The main impetus was cost. Cust was dealing with a CNC system with $3000 4 chan analog cards. SLC cards are 8 channel for about $900. Most CNC components were expensive $$$ and had long lead times. PLC based system parts were generally in stock at distributor. Going from CNC to PLC has its caveats, but if it meets the application, the cost savings can ge great.
Sounds like a nice system for a reasonable cost. I assume that you did not need any multi-axis coordinated moves.

Bill Sturm