CYCLE HOLD - your thoughts and feedback

Quite a few years ago when I was working in safety I got involved between operators of machine tools (CNC) and the equipment engineers who did the design and programming. The issue was when an operator had to do something (change a cutter or whatever) he put the machine program in stop (I am going to call it CYCLE HOLD to avoid confusion with E stop). The operators wanted a switch to disconnect that one (of several) drive motor when they were near it. They feared that if the program screwed up and wanted to take off and run then they had no protection. The engineers stated that would confuse the program and the program could be relied on for operator safety. I had no knowledge of PLCs at the time and had to rely on the expertise of the engineers - I consulted several at different plants so that I was not relying on just one persons opinion and experience.

SO here I am learning PLCs and am presently designing an automated drill which has multiple steps. This is a simulated unit using small components in a school / lab setting. The goal of this is to use a PLC to control a variable speed drive and several servo valves. These will allow me to learn how to handle analog in/outs for the servo valves and control VFD speed and a stepper motor. All this will be controlled with AB 5/05 SLC.
I want to be able to interrupt the cycle at any step with this CYCLE HOLD to allow the operator to at minimum inspect clear chips and maybe change a drill. When he is done, he resets the CYCLE HOLD switch and the cycle starts up where it stopped.

IF THIS WERE real world
1. IS this a good design concept
2. WOuld I have to use a safety PLC if I were to allow the operator near the cutter or components that may move?
3. WHAT other criteria that OSHA or NFPA 79 (was it ??) that would apply to this?

IF I am going to get into this business I want to learn it right and make my mistakes in a safe school setting under controlled conditions. I have already seen my share of accidents and don't need any more.

Would like your thoughts and input.

Thanks
Dan Bentler

My $.02

#1, yes, but you may want the operator to only be able to get at the tooling when the drill is out of the part and at a home position.

#2, not in my experience, but a safety relay with a light curtain or door interlock should be used.

#3, Anymore I just go to the plant safety department, show them the application and the FMEA, and let them tell me what safety guidelines to use. It saves me time guessing because each plant is different. Plus, they feel like they are involved from the beginning (they are) and machine runoffs get a lot easier when they come in to approve the equipment.

Hope that helps.

Dan,

The degree of protection should always be proportional to the danger. If it is a life-threat such as you have in some Army nitroglycerine and TNT plants, then it is a different ball game. For those types of sfety threats, PLCs are NOT relied on.

If it is a danger to fingers and hands, I always try to put the equipment behind a fence or enclosure, and then install gates and gate switches that put the machine in a safe condition anytime the gate is opened. The gate switches should trigger the machine control circuit to stop the machine, not the PLC, but they are allowed to go to the PLC also to tell it what is going on. If you have this type of guarding, then it will be kosher to use a Machine Hold switch to stop the machine using a PLC, because once the operator enters the gate, the machine controls will always be in the safe mode regardless of what the PLC is doing or not doing, even if the operator forgets to push the Machine Hold button. This is a redundant circuit and what you want when dealing with safety issues.

If a guarded enclosure is not practical, then the next best thing is a "light curtain" that shuts off (or puts in Safe mode) the machine.

There are many things that you can do.

Some are covered and some aren't by OSHA/NFPA.

The light curtain could trip a master relay supplying some or all of the outputs, or you could have a small disconnect switch to disable some portions of the machine for suchservicing.

If you could figure out what to search, there have been quite a few threads in the past. Many have gone into extensive graet detail on chapter and verse. Some cover common sense.

You can only make the operators and repair people as safe as they want to be. And you can go crazy trying to figure out some of the regulations, or what is actually applicable.

So, figure out what you are trying to accomplish.

Determine if the program hold is a good idea. I don't like the idea of it, but don't use anything like that. So don't take my word on that one.

Make sure if you use a light curtain, that you want an automatic reset of operation or a manual reset (like a full or partial master control relay).

Provide some kind of lockout, to aid in troubleshooting the machine or be able to manually step the machine without full lockout, but have full lockout available.

Determine what safety aspects are required, what are needed, and what are desired.

Talk to the plant safety officer (if you have one), he should have all legal requirements somewhere. (Though I have yet to run across one that really did).

Ask lots of questions.

Always design to err on the safety or fail safe side. Use energize to run or operate. If power drops, so does operation.

As always, e-stop kills everything. Certain zones or functions can have "equipment" stops.

regards.....casey

Cycle Stop

What I see on MOST CNC machines is that Cycle Stop is a 'Machine Lock' IE. the program counter is stored and all axes are locked (0 Volts to motor controls, but still energized), coolant off, ETC.

The operator then has the ability to go to Manual Mode and jog any axis to clear debris, change out tooling, ETC.

When satisfied, the operator re-enters Auto Mode and hits Cycle Start. The machine assumes the co-ords when Cycle Stop was commanded and resumes programmed operation. Otherwise you issue an M00 (rewind to beginning) and drill air for awhile.
This also HELPS prevent scrapping a valuable blank part ( scrapped two 4' x 8' x 1" blanks AND an $800 tool two weeks ago!)

As an aside:
1)On your new machine are you going to use 'pecking'?
2)When you change out a drill bit, how do you determine if the new bit is the same length as the old one?
3)Will you have 'Dry Run Mode', whereby the X and Y axes move but the spindle and coolant are off and Z axis doesn't move?
These are REAL WORLD concerns and overkill for homework - but....

Incorprating all of these could give you a salable machine.

*Note* I did not address the safety issues. I've NEVER seen a light curtain surrounding a knee mill or drill press. Punches, presses and shears - yes.

Rod (The CNC dude)

leitmotif said:

The engineers stated that would confuse the program

Click to expand...

You should be able to program it so it can't be confused. That was lazy or incompenant engineering.

Sometimes I am not sure about the terms used with mills etc. Rod could you define/verify and expound on a few terms for me?
Knee
Saddle

Quill
Ram
Way
Any others you think relevant.

Somewhere along the way I learned a basic definition that the difference between a lathe and a mill is that a lathe rotates the material and a mill rotates the cutter. Sound right?

I dont see where a drill press or mill would need a light curtain.

Machine bits

Ron,
I'll try.

Think drill press
Knee mills have a vertically movable 'knee' relative to the base that has a hand crank to raise/lower the X/Y table and 'saddle' as on most drill presses. This crank is a coarse adjustment to bring the table and saddle closer to the 'quill' that holds the cutting mill tool or drill bit. The saddle (Y) sits on the knee and the table (X) sits on the saddle.
The cutting tool is usually held in a 'collet'; using a 'morse taper' 'collet' to clamp the tooling.
The rotating 'spindle' is contained in the 'quill', the Z axis. This is a 'cartridge' of various bearings allowing rotary while holding high tolerances against the side loads caused by movement of a milling cutter as it is moved in X (left/right) and Y (towrds/away from the operator). The quill on most knee mills is lowered by a rack and pinion arrangement (the drill press handle)
Z is the up/down motion of the quill. The knee/saddle/table ride on 'ways', a pair of opposing precision ground and hardened steel (slightly) parallel rails. The ways are usually angled away from each other so the saddle can not be lifted or shifted angularly. A matching set of 'guides' are machined into the machine base/saddle bottom/table bottom one of which has a very slight taper to its mate. A 'gib' is an angularly matching slightly tapered shim to fill the space between the way/guides taper. This allows the millwright to adjust the tolerance and drag of the way/guide.

That is a vertical mill - on horizontal mills the quill (Z) moves horizontally toward the operator and Y axis is up down.

Confused yet? I can draw pics if you need.

Lathes:
Where mills clamp the part on a moving table a lathe holds the part in the 'chuck' driven by the spindle motor and the tooling moves in only two dimensions. Tooling on CNC machines are held at various radial points in a 'turret'. Different tools are rotated into position to cut across the rotating part.
Imagine a drill press on it's side. Z axis is towards the rotating spindle (if the cutter were a drill to drill a hole in the center of the rotating 'stock'/part.
If the cutter is positioned outside the part then Y axis is toward the center of the part and Z axis is toward the chuck.
Again the spindle can use a collet arrangement or a chuck with a set of jaws to clamp the part. 3 jaws hold at spindle centerline, 4 jawed chucks allow for offseting the part for eccentric cutting.

Ram:
I will assume presses here.
Used to generate great forces to shear/bend/impress(form) or punch.

The ram on a punch press (makes a hole with a BLAM!) can be either a fast acting straight up/down hydraulic ram or a motor driven flywheel that drives an eccentric wheel attached to the ram (think engine connecting rod - the downward pointing piston is the ram). The ram holds the tooling 'punch' that will pierce the part into it's mating 'die.

Turret punches generally have a pair of circular steel plates. The upper plate holds the punch, the lower plate the die. The sheet metal is clamped to the X/Y table and passes petween the turret plates, when in position the ram clutch/brake is fired and one revolution of the eccentric is performed an a hole is made. Just look at the vent holes in your computer housing for an example.

Oh yeah - CNC is the HMI/PLC (electronics programs) and NC is the electrics on the machine - real world are the switches, relays, ETC

Anything else?

Respectfully,
Rod (The CNC dude)

ways - gibs

Err pic here?

I have always found that presenting the safety concerns to a safety director had me redesigning the program to the way I initially thought it should be done, meaning if that was your hand in there, what state would you want the machine in. I beleave in a light curtain or a door interlock to a safety relay and then have the operator reset when operation complete. This doesnt take much time and easier than reattaching fingers.

Rubber Ken,

Correct, to a point.

I subscribe to three levels of safety:

1) safety of the the operator and repair person
2) safety of the machine (downtime is expensive)
3) safety of the part being produced (some billets can run up in the $thousands)

But, when I pick up a hammer and a nail I do NOT want a light curtain! My hand will be inside the MACHINE. Been there.

Some, if not most machines require quick intervention by the operator to clear chips/dust/debris requiring the operator to be 'inside the machine'.
I sometimes ,not by choise, have to be inside the machine to do trouble shooting. and twice some idiot has pushed a button, without thinking, causing the machine to move (believe me, I can jump higher than Superman!). After a face-to-face DISCUSSION with said person I felt a bit safer - only a bit.

Safety works to a point --- actually happened: operator wore gloves that were physically attached to a 'safety lift device'. Operater placed part in the 200 ton stamping press and stepped on actuator. One glove hung up in the die tearing away the safety wire and the press came down.

There is no way to stop a machine (clutch/brake/E-Stop) that could have prevented this. In this case gloves that had no cuffs MIGHT have worked.

So....... Design for the worst and hope for the best and know - IT HAPPENS

How do you measure a 480/3 phase wiring inside a conduit when one wire happens to use the conduit? Left hand on one side, righthand on the other, separate at the joint and KABLAM!

Safety failure is intrinsic in every thing that moves.


(RANT OFF!)

Rod The CNC dude)

Rod said:

Some, if not most machines require quick intervention by the operator to clear chips/dust/debris requiring the operator to be 'inside the machine'.
I sometimes ,not by choise, have to be inside the machine to do trouble shooting. and twice some idiot has pushed a button, without thinking, causing the machine to move (believe me, I can jump higher than Superman!). After a face-to-face DISCUSSION with said person I felt a bit safer - only a bit.

Click to expand...

That's too close for comfort and exactly the reason LOCKOUT PROCEDURES with actual LOCKS were created. Your facility doesn't have them? Shame on management. Its an accident waiting to happen. Let them know that they might (or should) get a savings on their liability insurance if they adopt them.

lockout

I am a machine tool repair service tech - many different shops.

I have NO CHOISE as to how the machine was designed/installed/ and jerry rigged. I will tell the manager of my concern(s), but am told to "Make it run, we need production".

I don't like it one bit when I am forced to repair a machine that has flagrant safety concerns. I have walked away from several machines that were 'accidents-waiting-to-happen'. I can't in good concience get a machine running that can cause serious person injury.

All I can do is enter in the COMMENTS section of my service report that 'blah-blah could cause a danger" and HOPE I won't be sued because I was negligent by causing the machine to run.

That said; I can not say that I have EVER worked on a machine that CANNOT cause injury!!

If a machine sliding a 600 pound of 1/4" plate steel at 8 feet per/sec had a catastrophic failure of the work holders - would I be held responsible when I never touched the pneumatics?

Sorry, it bugs me too,

Rod (The CNC dude)

That's too close for comfort and exactly the reason LOCKOUT PROCEDURES with actual LOCKS were created.

Click to expand...

LOTO (Lock Out Tag Out) is not nor can be used when trouble shooting which makes it a danged if you do and danged if you dont situation because you may have to place your body in harms way. Troubleshooting usually requires the energy sources to be active and the ability to create motion where necessary.

LOTO is not used for standard operation procedures either which may also require reaching into a machine. As stated all you can do is "minimize" the possibility of harm.

LOTO is used when physical repairs are being made to release all energy from the appropriate sources and using all the necessary equipment involved to prevent energy releases or motion.

rsdoran said:

LOTO (Lock Out Tag Out) is not nor can be used when trouble shooting which makes it a danged if you do and danged if you dont situation because you may have to place your body in harms way. Troubleshooting usually requires the energy sources to be active and the ability to create motion where necessary.

LOTO is not used for standard operation procedures either which may also require reaching into a machine. As stated all you can do is "minimize" the possibility of harm.

LOTO is used when physical repairs are being made to release all energy from the appropriate sources and using all the necessary equipment involved to prevent energy releases or motion.

Click to expand...

rsdoran, I hope you don't really believe that.

Most of the facilities I call on will not allow ANYONE ANYTIME to be put in harms way. I've known of people getting fired defeating lockout procedures. Troubleshooting can be done following safety procedures; it usually takes longer but it can and is done all of time. Hell, even the automotive facilities I visit in Mexico will escort you out of the building if they catch you bypassing safety guidelines.

Why would you put yourself in a dangerous situation for something as trivial as saving a few minutes? Its not logical.

my $.02