safety issues on design

[rookie77]

When designing a program with clamps holding down the material in a saw, would you have the e-stop

a)release the clamps and stop and raise saw

b)stop and raise the saw but keep the clamps on

c)stop everything where it was

d)other..........

There has been a lot of discussion here about it and there are several pros and cons to all of them. Looking at other saws we have here, the e-stop typically just stops the process.

But just because its the way its always been done doesn't make it right, .....right?

 

[realolman]

I have no credentials in this subject, but I think just stop the saw, and release the clamps
I don't know if there are standards for an "Emergency Stop" or not... like maybe it has to kill all the power

I would say whatever seemed to you , being familiar with the situation, to be the safest

 

[James Mcquade]

lets look at your options.

you are near the saw and the estop is pressed.
the clamps open, the blade is still turning, the material may or may not sit there. the material in all likelyhood will move and go everywhere, would you like to get hit by flying objects? of course not.

in regards to the other options, i am unable to answer since i do not know the details of your system.

you must do a risk assessment for all possible cases.
if you can ask what if and do not have an answer, plan for it.
i stress this because we had an incident with a rather large saw and we lost power for only a second.

remember safety first.

regards,
james

 

[Geospark]

Quite right James...

"would you have the e-stop..."

I don't like guessing in these matters. I would not know what I would want it to do until I know the known hazards and potential risks involved.

Forget pros/cons....think Risk Assessment.

What are the risks involved in using each of the suggested emergency stopping methods?

Go back a step...

What are the risks involved while it's moving, requiring an emergency stop?

Which stopping method is safest? i.e. creates the least dangerous hazard?

Forget saving the material here, we are thinking of the operator.

Regards,
George

 

[Caveman]

I'm curious about the saw lift. Is it pneumatic? If yes what type of valve? I would think it should be a two position, single soliniod 4 way.
As far as the clamps. Valve should be 3 position 2 soliniod 4 way, all ports blocked in center position. But that creates an issue if an E-stop to unclamp is needed.

Again and Again, Risk Assesment

 

[rookie77]

Clamps are 24v DC
Saving material is not the issue, safety is. Thats why I was looking for input.

I'm looking at having the saw stop, and then the clamps release

Potential hazards, from most likely to least

Blade is dull, blade breaks, material not sitting square in clamps, shirt or hand stuck under clamp (two handed start, this not likely) hand or shirt stuck in saw

 

[dcooper33]

Clamps are 24v DC
Saving material is not the issue, safety is. Thats why I was looking for input.

I'm looking at having the saw stop, and then the clamps release

Potential hazards, from most likely to least

Blade is dull, blade breaks, material not sitting square in clamps, shirt or hand stuck under clamp (two handed start, this not likely) hand or shirt stuck in saw

How do you plan to stop the saw? Is there a motor brake, and if so, what type, or do you plan to just let the saw coast to a stop? (I wouldn't recommend the latter. Normally saws have good bearings, sharp teeth, and lots of momentum. This = lots of cutting potential whether powered or not)
What kind of safety devices do you plan to use? (i.e. safety relays, isolating contractors, estop buttons, light curtains, etc)
There are so many things to consider when designing a safety system. Without knowing exactly what hardware you are using, and without seeing the physical layout, no one worth his salt is going to feel comfortable being very specific with design advice.
One thing I would say is use controlled stop (not coast!) and a fail-safe (energize release, spring-applied) electromechanical brake.

[SOAPBOX]
As has been said, assess, assess, assess and test, test, test! Never forget the potential consequences of cutting corners on a job like this.

I also have to mention, since you said something in the OP about designing a "program to estop" your machine:
NEVER TRUST A PLC TO SAVE A HUMAN'S LIFE!!
Your safety system must be hard-wired, safety-rated, ideally redundant, and hopefully fail-safe (in the event of a broken wire or power loss, the machine reverts to a safe, de-energized condition)
[/SOAPBOX]

 

[ASF]

I also have to mention, since you said something in the OP about designing a "program to estop" your machine:
NEVER TRUST A PLC TO SAVE A HUMAN'S LIFE!!
[/SOAPBOX]

Unless it's a GuardLogix PLC

Aside from that little nitpick, fully agree with the post above

(what do you mean, we can't afford a whole guardlogix PLC just to control a dropsaw?)

 

[rookie77]

Lol, OK, so I'll try to post something by Monday, basically the saw will function very similarly to how it was designed.

It does have a brake, it will have hard wired e-stop and will function in every way like the manufacturer intended except that there will be clamps added to hold the material down. and assisted lift. (Right now, manual lift, vice clamp)

I'll do my best to post a rough draft of how I expect it to work. I look forward to the input

I guess what I was looking for now is a little discussion on scenarios that could happen and how I would like the program to respond, I am very much concerned about safety and hence the starting off this thread, more people involved, more sides to the story



I am very new to PlC programming, but quite experienced in control wiring and industrial machinery.

 

[dcooper33]

Unless it's a GuardLogix PLC

I knew that was coming. Thought about throwing a caveat in there, but didnt want to muddy the water.

@rookie77,

Looking forward to seeing your draft. It sounds like your saw is already controlled without a PLC, so I am curious to see what all the PLC will control, other than the hold-downs. Be sure to list inputs and outputs.

Blade is dull, blade breaks, material not sitting square in clamps, shirt or hand stuck under clamp (two handed start, this not likely) hand or shirt stuck in saw

So as far as scenarios,

1. Blade Dull- this may be the trickiest to sense. Possibly could be determined if you had a CT on the motor power (higher amps = worn blade working harder?) Lot of potential pitfalls here, especially if you cut a lot of different materials.
2. Blade breaks (or stalls) - If you have a very high-speed prox and a high-speed counter card then you may be able to detect the transitions between blade teeth, and if it stops spinning or breaks then the prox will stop changing states, and therefore stop the motor. There are probably lots of other methods for this too.
3. Material not square in clamps - You could use prox switches here too (inductive if metal, capacitive if wood or plastic) to determine if the material is fully seated and square. You could also use a prox of some kind to verify that the clamps are up or down
4. Personnel caught in moving equipment - This is the biggie. This is obviously the worst case scenario. This is the part where you can use the PLC in a supplementary monitoring fashion if you wish, but you must have these conditions accounted for in your safety circuit. I would look at having the saw and clamps all be behind a guard, with a safety switch that verifies the guard is closed. If the guard isn't closed, the clamps can't shut, and the saw can't run. A light curtain may be a good option here too. It's hard to say at this point.