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Machine Guarding

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JimDawson

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#1
As some of may know I'm building a small punch press for a customer. https://www.hobby-machinist.com/threads/servo-punch-press.72601/ This press is going to be out of my control and operated by unskilled or semi-skilled labor so it must be designed to be as safe as possible short of welding a large steel box around the entire unit and welding the main power switch in the Off position. :) In the die area there are no pinch points, completely finger safe. No reason to be poking around in there, but ya never know. :cautious: It will be guarded anyway with a lexan guard. And that guard will contact a limit switch that is in the E-stop circuit and must be in place to operate the machine. Designed to not be easily defeatable.

I had to add a stiffener to the press frame to keep it squared up, and in doing so created a pinch point at the wrist pin.

Without the stiffener installed
1538280483233.png


And with the stiffener in place. The bolt head moves up & down with the ram.
1538280539594.png

No problem, just build a lexan guard to cover the pocket.
1538280694051.png

But, this guard is WRONG. Mechanically it will work fine, the lexan is well suited for this application, and is completely finger safe. The guard allows easy access for removal of the wrist pin for die maintenance. Die maintenance would be done by skilled personnel, this is not an operator function. I have a fix for the problem and will implement it as the build continues, there are two solutions that I thought of.

But I wanted to post this to see if anybody else can point out the problems (there are more than one) with this design, it took me a day to realize that it wasn't right.

Time to put on your thinking caps. :eagerness: Looking forward to your comments.
 
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Norseman C.B.

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#2
If I'm seein it right it can't cycle down cuz the relief pocket is positioned wrong.......
 
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JimDawson

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#3
You are correct in that regard, it needs another 0.125 clearance, but it is nearly at the maximum down stroke in the photo.

But that's not the problem, the problem is in the human factors.
 

Martin W

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#4
I see no form of lubrication spots. Return spring?

Cheers
Martin
 
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JimDawson

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#5
I see no form of lubrication spots. Return spring?

Cheers
Martin
Lubrication will be added later. Spring return is close if I understand correctly :)

The only issue here is the guard itself. I'll post the answers in a day or so if nobody else gets it. :)
 

francist

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#6
It might be a safer guard if the mounting screws came in from the back side of the plate. Less chance for us dumb operators to take it off....

-frank
 

JimDawson

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It might be a safer guard if the mounting screws came in from the back side of the plate. Less chance for us dumb operators to take it off....

-frank
You're getting warm :)
 

derf

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#8
For those of us that have a mechanical inclination and a rational thought process, it can be quite hard at times to think like an unskilled idiot, thus the need for beta testing to see how the mechanically impaired understands a simple process.
Just spitballin'.....since the cover is transparent, you will be able to see the wrist pin moving up and down, and to an unskilled operator this situation would indicate that something is loose or broken, triggering a brainstorm about investigating the situation by removing the guard.
Curiosity killed the cat....
 

dulltool17

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#9
If the machine will still operate with the guard removed, the hazard still exists. You need an interlock, either mechanical or electrical.
 

JimDawson

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#10
If the machine will still operate with the guard removed, the hazard still exists. You need an interlock, either mechanical or electrical.
That's one of the issues. :clapping:Still at least one more that I thought of.
 

dulltool17

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#11
Well, you don't have the actual point-of-operation guarded at this point, either.
 

JimDawson

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#12
Well, you don't have the actual point-of-operation guarded at this point, either.

That is true, just because it hasn't been built yet. It will be before the machine ships.
 

francist

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#13
This may be a stretch, but is the screw pattern in the guard a square? If so, could that lead to the guard being installed incorrectly on the horizontal as opposed to the vertical orientation?

-frank
 
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Radials

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#14
This may be a stretch, but is the screw pattern in the guard a square? If so, could the guard be installed incorrectly on the horizontal as opposed to the vertical orientation?

-frank
Perhaps a small dowel pin pressed into the back of the lexan guard that has a corresponding hole on the stiffener plate so that it could only be installed one way.

Does the wrist pin rub on the guard?
 

JimDawson

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#15
Perhaps a small dowel pin pressed into the back of the lexan guard that has a corresponding hole on the stiffener plate so that it could only be installed one way.

Does the wrist pin rub on the guard?

You're close, it really needs a shoulder screw permanently installed and locktited in place in the top so it can swing closed by gravity. But there is still the problem that the machine will run with the guard removed. Overall just a bad design and not foolproof or failsafe. No, the bolt head clears the guard by about 0.025 or so.
 

JimDawson

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#16
Guys, thanks for commenting.

I have been designing machine safety systems for years, and one of my thought processes is to be the most pessimistic SOB on the planet. If anything can go wrong, it will do so worse than you ever imagined. :cautious: That little guard looked cool, but it is no good. The human factors are that someone will remove the guard while maintaining the die. Then they will lose it, lose the screws, or for some other reason the guard won't get put back on. Now you're going to say that nobody would stick their finger in there, but you would be amazed at the stupid stuff that people do.:rolleyes:

The guard over the slot will be incorporated in to the main guard over the die area, which will open the E-stop circuit when removed. All I have to do is extend the guard up a bit to cover the slot.

I added some clearance to the slot, now the die can close to the absolute minimum height.
1538454094071.png

There is quite a bit of finger clearance, the die is about 1/8'' above minimum in this pic, and the punches will never come out of the punch guide plate. But....It is still going to get a guard.
1538454546846.png
 
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