PLC control vs CNC control - difference?

leitmotif

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Join Date
Nov 2004
Location
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Can someone with basic knowledge take on "basic" CNC and learn as he goes?

I finished 6 months at Seattle Community COllege studying PLC - I do not pretend to know it all and have some learning curves ahead of me -- real education and learning starts in the field I know.

Starting to talk with outfit that is a fabrication shop generally plate steel. Have presses and laser cutter, and of course lots of welders (MIG). They say they want CNC guy. I need to know if I can realistically do the job for them.

Let me as the question this way
Is CNC control basically PLC control?

Granted there is a lot of position sensing with CNC (ie X,Y,Z, etc axis)
AND the program changes with the part to be make design / configuration which may require changing the tool list.


Thanks

Dan Bentler
 
Dan,

Are they looking for someone to program their CNC equipment with G-Code to run the machine? or are you talking about programming the motion control system for an OEM machine builder.

G-Code programming would be nothing like PLC programming. It is all about part geometry, feed speeds, tool compensation, etc. There are a lot of CAD/CAM packages to aid in creating the CNC codes to run the machinery.

Most CNC control systems have proprietary motion control algorithms in them that define the speed, acceleration,etc. to move to the desired target that must be tuned to the piece of equipment they are applied to. The tuning takes into account the mass, inertia, hysterisis, tolerance, etc. of the system. There will also be a bunch of special M-Codes that will be machine specific for cutting on motors, valves, etc.

Hope this helps, I am not an expert on any of this, just have some expering in writing G-Code and PLC programming.

Glenn
 
glenncovington said:
Dan,

Are they looking for someone to program their CNC equipment with G-Code to run the machine? or are you talking about programming the motion control system for an OEM machine builder.

G-Code programming would be nothing like PLC programming. It is all about part geometry, feed speeds, tool compensation, etc. There are a lot of CAD/CAM packages to aid in creating the CNC codes to run the machinery.

Most CNC control systems have proprietary motion control algorithms in them that define the speed, acceleration,etc. to move to the desired target that must be tuned to the piece of equipment they are applied to. The tuning takes into account the mass, inertia, hysterisis, tolerance, etc. of the system. There will also be a bunch of special M-Codes that will be machine specific for cutting on motors, valves, etc.

Hope this helps, I am not an expert on any of this, just have some expering in writing G-Code and PLC programming.

Glenn

REPLY I would be in a "plant maintenance" position in a production shop they do not build machinery -- part of my job would be taking care of the CNC machines. I do not think I would be doing the G programming and would have to be trained on this if that is what they want me to do.

It helped - thanks

Dan
 
Most CNCs are canned systems. The core is usually unaccessible, and the manufacturers won't give you much in the way of manuals and drawings either. The CNC often has a space for a small ladder, usually of limited instruction set, for running things like auxilary coolant pumps, collets, etc on the machine, but by and large the CNC will be "black box."

From the kind of shop it is and the kinds of equipment listed, they are looking for a G-code writer - someone to describe the path the machine travels, not really program how the machine actually makes the described motion.

However, anyone who can think logically and pick up the manual can quickly pick up onConnection: close the G-code. An understanding of machine shop techniques can helpful.
 
I would not classify them as a machine shop. Basically they take sheet steel cut to shape and brake it to get 90 corners and weld - primary product seems to be garbage dumpsters. Maybe not glamorous but like
OBs (kids gotta get born)
morticians (gotta die too)
sewage plant maintenance (no more comment needed otherwise I'll get
censored)
it HAS job security also

Dan Bentler

Thanks for help guys.

Dan Bentler
 
And they're right across the street from what used to be the best greasy spoon diner in the industrial south of Seattle. Do they still serve the milkshakes in the big steel cups ?

I'd say that you have the right learning background and basic skills to take on CNC maintenance for a plate shop. Start by rounding up all the user manuals you can find, and settle in to do some reading.
 
Ken Roach said:
And they're right across the street from what used to be the best greasy spoon diner in the industrial south of Seattle. Do they still serve the milkshakes in the big steel cups ?
REPLY You sure know local geography. Yep milkshakes are done in the steel cup and served in milkshake glass. If you get the WSBBB (whiner special blueberry + blueberry + blueberry - real berries I might add) it fills the glass nearly twice. Let me know if you are in the area and I'll buy you one.

I'd say that you have the right learning background and basic skills to take on CNC maintenance for a plate shop. Start by rounding up all the user manuals you can find, and settle in to do some reading.

REPLY I think as with any job in maintenance there will be lots of reading and midnite oil.

Thanks Ken

Dan
 
Being a "Plant Maintenance" guy myself I can tell you more than likely when they refer to CNC 'plant maintenance' they're not after G coders. They want someone versed in troubleshooting the machine to get it back into production, quickly! It's not a laptop job, be prepared to get really dirty.........
 
quaizywabbit said:
Being a "Plant Maintenance" guy myself I can tell you more than likely when they refer to CNC 'plant maintenance' they're not after G coders. They want someone versed in troubleshooting the machine to get it back into production, quickly! It's not a laptop job, be prepared to get really dirty.........

REPLY
I got my coveralls and boots.

Thanks

Dan
 
Be prepared to spend a lot of time reading the manuals. My personal experience with Fanuc manuals is that they are among the worst at translating from Japanese to anything even remotely resembling English. I've read and reread the same paragraph a dozen times and, while I can understand every individual word, I have no clue what its trying to tell me.

Another annoying detail is that technical support from CNC controller manufacturers is geared toward the OEMs and not toward the end users. If you try to call them, you're apt to be told "take that up with the machine builder".

Despite all that, a background in machinery controls and industrial strength electricity is as good a preparation as any for dealing with CNCs. They're nothing more than sophisticated motion controllers. They also tend to have pretty decent dignostics built in, so once you learn the terminology the patient does a good job of telling the doctor where it hurts.
 
Dan,

I doubt you will ever see a ladder in the CNC world unless you are doing retros or are an OEM.

You will get an error message that will be almost totally useless, example: "Error 420 X-axis loss of feedback" - translation = the servo drive tripped out, reset thermal overload, check for binding, dirt, electrical sag or spike, ETC. (Fanuc)
Or "Error 212" - translation = slug door open, relatch and hit cycle start.
On rare occasion you won't GET an error message - This is where you earn the big bucks.

The laser will be your main problem. They are probably burning 1/4 to 3/8 plate. So about a 1500 watt laser - LOT'S of dross to clog up every ball screw around. And a BIG table to move the sheets around.
The resonator, it's power supply and the optics will require a lot of your attention. That would be good knowledge for you to have.
****Warning****
Production lasers emit infrared. Carry a sheet of acrylic as a shield and as a warning, the plexi will melt or flame but not pass IR.
Production laser resonators generate high DC at lots of Amps - 4 foot clearance when at power (those doors do have safety interlocks for a reason)

The press brakes will have a back gage:
A CNC control for position - usual brands are Hurco, AutoBend, CNC99
Usually a DC motor drives a ball screw with tach/encoder feedback
A simple DC variable drive
The press proper may be hydraulic and can have a linear encoder for position height per ram tied back to the control.
The press control cabinet is usually just a couple of contactors and some ice cubes - simple relay logic for you.

No 2 CNC controls are equal, all are quirky in some way, but you will quickly learn them.

Summary:
1) A CLEAN machine runs
2) Setup a PM schedule for #1. The laser will require frequent water flushing, it's chiller will get filthy, the resonator will suck up dross that will do weird things to the plasma.
3) Throw out all cans of WD40 - it attracts dust and welding dross. I use a non-flammable brake cleaner and follow up with a moly based chain lube. Keep the ballscrews clean and LIGHTLY oiled.
4) LOT'S of disposable rags
5) A box of blue nytril gloves - you have clean hands when jumping back to the electronics.
6) Measure power supplies at the caps (DC and AC ripple - over .02VAC is a bad cap - measure power supply on a random IC in the control as above.
7) clean/dry air supply to air valves
8) connectors age - unplug and replug several times
9) servo drives often have fault indicating LEDs - OT=over temp, OV=over voltage, ETC
10) clean filters on the controls OFTEN - those puppies can get hot!
11) check hydraulic fluid and filters often

I'm sure I left a lot out - PM me if you wish
I'm curious what laser system they are using

Rod (The CNC Dude)
 
Dan,
Some other thoughts:
You won't be writing G-codes per se, but you will have to learn how to program the back gages for troubleshooting. Pretty simple
And you'll have to know how to drive the laser table for t-shooting, the operator should drive the laser proper. That is a two man job.

Most production lasers I've worked on had a punch to create a starting hole, else the laser has to perforate the sheet and that can cause an unacceptable surface crater. So you may have a turret punch with a laser - good fun

Mitsubishi RF resonators are really cool in pulse mode

BTW,
Laser service calls are $100/hr. and you earn EVERY cent!

Rod
 
Steve Bailey said:
My personal experience with Fanuc manuals is that they are among the worst at translating from Japanese to anything even remotely resembling English. I've read and reread the same paragraph a dozen times and, while I can understand every individual word, I have no clue what its trying to tell me.

One place where I worked had a Japaneese lathe. The first line of the manual said:

"We hope this sincerely machine reaches up your needed."

:eek:
 
I got a job running CNC EDM machines back when I was in college. They had no one doing maintenance, and one day when one broke down I got the books out and ran through the diagnostics until I found the problem. Suddenly I was the designated fix it. I was fortunate enough to have several identical machines, so when one broke I could swap boards around until I moved the problem. After a while I learned the little quirks and certain problems I knew right away what they were, for example, the encoders on these machines would go out frequently. I’ll give a big second what Rod said about it all. It was that experience with CNCs that steered me into automation.
 

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