what is the difference between cnc and plc

Hello,
I am a teacher.I want to include plc programming in the course. And since I do not know much about PLC, I am trying to learn it. I want some very basic information:
What is the difference between a cnc (for controlling m/c tools) and a plc?
Is it possible to fully control a servo motor using a plc, the way it is controlled by cnc?
Thanks.
S.K.Sinha

Computer Numerical Control
Programable Logic Controller
It is possable but not practical.

What is the difference between a word processor and a CAD program?
Can you type a letter with a CAD program?
It is possable but not practical.

What is the difference between a bicycle and a freight train?
Can you haul iron ore with a bicycle?
It is possable but not practical.

Get the picture?

A (very rough) comparison between PLC and CNC is this:

• Programming PLC is as if you were writing the Excel program
• Programming CNC is as if you were using the Excel program to its full power.

I won't minimise the know-how needed for programming CNC, but this should give an idea.

Most PLCs are too slow to control a servo motor in the same way a CNC control does, although some recent ones come very close.

Kind regards,

What gets me is that someone that does not know PLC's is going to teach it.

That fall into those that can DO, those that can't teach?

CNC (Computer Numerical Control) is a particular application usually to control a multi axis machine tool such as a milling machine or a lathe but also could be a tube bending machine punch press or similar. The CNC controller is usually a purpose built controller using a micro computer but could be a general purpose computer or a PLC. The forerunner was NC (Numerical Control) before computers were readily available, it used solid state logic and relays, timers etc to provide the application.

A PLC (Programable Logic Controller) is a general purpose logic controller specifically designed to allow the user to create their own application. They select appropriate I/O (Inputs Outputs) and perhaps function modules for purposes such as communications and multi axis servo control and program the PLC to perform the required functions.

Why define it for a teacher?

rsdoran said:

Why define it for a teacher?

Click to expand...

For the same reason Ron that someone had to teach your teachers.

I tried to self teach PLC and decided my instructor was dangerous so I found me a qualified instructor.

Where did you learn PLC from?

Dan Bentler

Sorry Dan,

But I agree with Ron.

What if I decided that I wanted to teach brain surgery...
Could I learn sufficient from the web to be able to teach the subject?

Would you trust one of my pupils if you were on the operating table???

By the way, I have a sneaking suspicion that sinha_nsit is not a teacher, but actually a rather 'smart' student who has asked a homework question in such a way as to not get flamed.

Paulus said:

Sorry Dan,

But I agree with Ron.

What if I decided that I wanted to teach brain surgery...
Could I learn sufficient from the web to be able to teach the subject?

Would you trust one of my pupils if you were on the operating table???

By the way, I have a sneaking suspicion that sinha_nsit is not a teacher, but actually a rather 'smart' student who has asked a homework question in such a way as to not get flamed.

Click to expand...

I am indeed a teacher, but presently a "student" in the area of PLC. Incidently, I did not know even a "C" of CNC about 10 years back. But now I have written a book on "CNC Programming" which is so popular in India that during the past seven years, six editions have come. In the current edition I have included a chapter on "probing on CNC machines using touch probes" which is not a very common knowledge among CNC users. Now I am planning to add parametric programming using "Custom macro B" of Fanuc. I admit that there were lots of mistakes/ambiguities in the first few editions (because I was not an expert), but now the book is worth reading. I have even used a CAM s/w (Ideas) to make very complex three dimensional parts (such as a circle "swept" along a 3D spline)using a ball-end milling cutter on a 3-axis milling machine. So, you should not under-rate me or make fun of my enthusiasm about learning PLC programming. It may take one or two years but I will definitely learn. Of course, if you people can guide me a little bit, my learning process will be faster! Best regards.

sinha_nsit said:

So, you should not under-rate me or make fun of my enthusiasm about learning PLC programming. It may take one or two years but I will definitely learn. Of course, if you people can guide me a little bit, my learning process will be faster! Best regards.

Click to expand...

Hello sinha_nsit,
It was not my intention to make fun of, or under-rate you, in fact I did actually state that you may be 'rather smart'.
If you had put the above information in your original post, then you probably would have recieved much more sympathetic replies.
It is sad but true, that there are all too many posts on this forum that simply ask...
"What's the difference...blah, blah, blah?" without any sort of qualification as to what the answers will relate too.
All here on this forum are always willing to help 'genuine' cases, and even homework questions if the poster has put in some effort themselves.
If you want to learn about PLCs, then you could do a lot worse than click on the "Learn PLCs" banner at the top of this page.
Also there is...
http://claymore.engineer.gvsu.edu/~jackh/books/plcs/
...which I understand is quite highly rated (not actually read it myself - only a brief scan!!!)

Good Luck in your quest for knowledge.

sinha_nsit said:

... about learning PLC programming. It may take one or two years but I will definitely learn...

Click to expand...

You can do it in few monthes easily.. Just bring a small PLC unit to "play" with (Search the forum for learn PLC topics), And as Paulus mentioned, start with "LEARN PLCS" in the top of the page, It was my start.

Regards.

I guess theres 'PLC Snobs' just like 'coffee snobs' and 'music snobs'.

The guys is interested and wants to learn.
Whats wrong with that??

Shameful attitude from some folks on here if you ask me.
Its not like someones going to hire him to go program the safety system for a nuclear power station..

Gee give the guy a break will ya.

Funny how one minute RSDoran is a self-proclaimed 'Bubba' and the next minute hes a PLC Snob.

The number of people who want to learn plc may be much greater than the number of experts in this field. A newcomer may ask, "what exactly a plc is". For experts, this may be an irritating question, but some people may definitely want an answer, because the only thing they know is "plc is used in automation".

If somebody wants to learn a new thing, the biggest problem he faces is "how to start learning". For the benefit of such people, I think, the site should provide direct link to some free online book(s) or similar resource material, like the one suggested by Paulus (thank you Paulus for the wonderful reference you have given). Then nobody will need to ask trivial questions from experts.

If you search the forums, there are countless posts like this one.

I people would just search the forums for "new to plc" or "plc newbe" etc. before posting drama like this wouldn't happen.

Time to lighten up folks.