PLC vs CNC ??

Hi all,

can anyone tell me more about a CNC machine? i knew it means computer numerical control machine and it usually consist of Servo motor and servo drive. We do have many CNC machines but i have yet to learn them since im currently doing PLC machines.

I understood CNC machine apply high accuracy works. but i wonder if a PLC is able to work as well as CNC? let's say for simple work.

please advice. thanks!

For moving an axis from point A to point B, a motion control module in a PLC can quite often do an adequate job for considerably less money than a CNC axis controller. For a simple application like that, a CNC controller is more than you need. You'd be paying for a lot of features that you won't use.

A CNC controller is a servo controller optimized for metal cutting applications. It employs a set of standard language elements known as 'G' codes. There are software packages available that can translate drawing files into G code programs. Many PLC servo modules can't be programmed with G codes.

Sutton,

consider this: a lot of CNC machines (and robots as well) are controlled by a PLC. The user enters the programming of the CNC machine on some kind of HMI or operator panel and the PLC translates this to control signals for the motors on the machine.

Of course there also CNC machines and robots controlled by PC's or by dedicated circuitry, but in general you can't tell from the outside which system controls the CNC machine.

Bottom line to this story: yes, a PLC is able to work as well as CNC.

Kind regards

CNC's are motion centered with plc functions. PLC's are controllers with motion functions.

With G code, CNC's are much easier to program for motion than PLC's. They also have the capablility storing anywhere from tens to thousands of individual programs that can be entered from the HMI or other source. Behind the CNC is usually a plc that controls stuff like clamps, hydraulics, tool changers, etc.

The CNC is no more accurate than a PLC with a good servo amp attached.

Thanks guys for your reply. So jvdcande you mean a CNC machine has a programmable controller which is similiar to a PLC we usually use? I understood there is a controller behind the machine. But i wonder if it is a more advance controller? With higher speed processor?

So how can i get information of a G code? would you please introduce me some website to go through? I do have the softwares of the machines but i have yet to go through them since i dont have basic knowledge of a CNC machine.

Steve, >>There are software packages available that can translate drawing files into G code programs<< well, how to do the drawing? is it by using the software given by the manufacturer? and how amazing can the G code works! I look inside the machine and well, it works like a robot!!

Rick >>The CNC is no more accurate than a PLC with a good servo amp attached.<< Do i need a higher speed pulse PLC to do this? I consult the sales engineer last week, and he said i need to buy PLC with high speed pulse to get the accuracy of the machine motion. In this case, I think i need a PLC that consists many timers and relays? Well, I had never do a rack PLC. the most i do is PLC with 100 I/O, so forgive me i need to look into this.

Thank you!

So how can i get information of a G code? would you please introduce me some website to go through? I do have the softwares of the machines but i have yet to go through them since i dont have basic knowledge of a CNC machine.

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http://www.linuxcnc.org/handbook/gcode/g-code.html

I think the link Steve posted makes it pretty clear

PLC motion controllers and CNC machine have completely different instruction sets. CNC machines are optimized for moving a tool using multiple axes in a coordinated way. CNC have vector like commands whereas PLC motion controllers don't. PLC motion controllers have a more generic command set allow changing the position, velocity, acceleration, and deceleration for a single axis on the fly. In general a PLC would have to activate two motion blocks at once to make a coordinated move whereas the CNC can move multiple axis with just one line of code.

jvdcande said:

consider this: a lot of CNC machines (and robots as well) are controlled by a PLC.

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Name one PLC that can do the traditional CNC type job using G code. I sure there maybe one or two, but I have not heard of any and I try to keep up on motion controllers in the PLC world.

A PLC can be made into a CNC machine, but it would never match a CNC machine at what it does because the axes really need to be coordinated at a very low level and not in ladder logic. Most here are trying to figure out how a timer and counter work. I can't imagine anyone trying to do vector motion in two or three dimensions in ladder.

On the other hand I could not inmage a CNC machine moving lumber in saws in a sawmill either.

Peter,

I know for sure one of the welding robots in our training center at Vilvoorde (near Brussels) is controlled by a Siemens S5 PLC equiped with motion modules from Siemens. It was because the technician installing it showed my colleague the programm, and the programming tool. This was our first meeting with S5 for Windows (IBH-Softec) as an alternative for STEP5.

And in my training center at Bruges I have programmed a little manipulator to perform a NC like program. Here too, the controller is a Siemens S5. This contollers is however completely steered by pneumatic cilinders and therefore doesn't need motion cards.

Kind regards,

The usual way for a plc and servo to interact is having the servo amp or a specialized card in the plc close the control loops. The PLC just supervises.

Most PLC's are unsuitable for motion control because of slow scan times and non-deterministic scans.

Most CNC controllers have a main motion control engine that runs the programs like Peter said. They then have a "back end" plc to do the more plc like functions. This is for supporting functions of the machine.


There are lots of programs that translate drawings into G code. The source drawings can be in lots of different packages, Autocad, Solidworks, Pro-engineer, Inventor, etc.

CNC's and PLC's are two different tools meant for two different jobs.

Show me a PLC setup than can mill a 3 dimensional curved surface and I'll show you an engineer that wasted a lot of money.

Show me a PLC setup than can mill a 3 dimensional curved surface

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I think I know at least one that comes pretty darned close: Yaskawa MP920. No G-codes though. Programming ain't going to be easy. But if you compare hardware costs - it might be a wash.

Hi there,

I always say, use a PLC were it's build for and the same goes for CNC.
There are always some people with examples where a PLC is used for a
not plc job. I also used ones a plc were is was better to use a robot, it was a 3 dimensional welding(robot?). We used 3 fast countercards togheter with the encoders and at the end of the day it worked, but it had been better if we used a small robot.

best regards

LadderLogic said:

Programming ain't going to be easy. But if you compare hardware costs - it might be a wash.

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The software cost will be much higher. Time is money, you know.

Fritz_frederix said:

I always say, use a PLC were it's build for and the same goes for CNC

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...and the same goes for a PC!...

beerchug

-Eric

Rick Densing:

That's exactly what I meant. CNC hardware is generally way more expensive than PLC. Considering development and debug time - could be a wash...

If you don't buy Fanuc, you will save money with a CNC.