Adding a Hold to Run/ Enable/ Safety Circuit to Standard Power Tools

Folks.. we have a little station here that uses a basic circular saw (one you can buy at Home Depot, Canadian Tire etc).

The station is currently setup so that the saw is fastened on a movable axis. To run, the operator must place one hand on a palm button to enable the 120vac ciruit, the other hand on the saw handle trigger button, and then he lowers the saw on the axis to cut the material. So basically the saw won't run unless both buttons are pressed, kind of like 2 hand control, but in this case only one safety/ enable signal is actually being monitored.

At this point (for 10 years or so), the enable signal was just powering a basic ABB 120VAC contactor (not saftey rated) and then 120vac to the saw through the contacts.

What we've been asked to do is upgrade the circuit using safety rated components to Cat 3.

Has anyone worked on a circuit as such? We are thinking simply adding a dual channel palm button, safety relay to a saftey rated contactor, and then if the conditions have been met, 120vac to the saw.

Will this be enough? The main goal is to make sure that the operators hands are away from the saw cutting area as we chop, and if either button is released the saw stops.

Thanks,
4

You need to consider the possibility of the button and or switch being made by a weight or maybe taped down. Has the safety relay/logic got "Anti tie down" capabilities/Redundancy checks. Is it also third party safe. Regards Badger.

Fourpaw said:

We are thinking simply adding a dual channel palm button, safety relay to a saftey rated contactor, and then if the conditions have been met, 120vac to the saw.

Click to expand...

That sounds good but make sure the safety relay is an anti-tie down safety relay. This way it cannot be defeated with tape, weights, etc.

Also keep in mind that in Ontario if you make changes to a safety circuit on a machine you have to get a PSR done on it. So the person to ask "will this be enough" is the Professional Engineer that will be performing the PSR.

I would have to do more digging than I really want but on dual chanel Saftey relays there are anti tie down wiring configurations. This means the switch has to change states each time the other changes states. The problem I see here is that the switch on the saw is actually part of the power circuit. As far as I can see that switch would have to be removed or bypassed and your safety switches would have to power the saw directly.

As Clay points out, the one nagging issue is the saw's trigger button. If this ever fails in the ON position, then the saw will start when just the palm button is actuated. NOT GOOD!

The 'correct' solution is to replace the saw's trigger with a dual-channel safety switch, which of course probably isn't feasible. You might want to take the saw apart and see what kind of switch is in there. Probably just a SPST, but you might get lucky and find a DPST. If so, you could remove it from the 120V circuit, and instead, wire it into the dual-channel safety circuit. Even though it's not a 'safety' switch, the safety relay should do a sufficient job of checking its proper operation.

Even if its SPST, you can still make it safer by wiring it to a separate single-channel safety relay. I would then wire the 2 safety contacts of this relay into the dual-channel's safety circuit. This should prevent the saw from restarting if the trigger switch ever failed closed.

I'm wondering if a standard two-hand control is appropriate for this application? Normally, both switches must be activated within 0.5 seconds of each other. In this application, I'm picturing the operator pressing the saw's trigger switch a considerable time (i.e. longer than 1/2 second) after actuating the palm button.



-Eric

IF the objective is to keep operator hands out of saw THEN

I think a good way to do it is "standard" two hand control where there is anti tie down and two buttons must press within say one second of each other. The output of this circuit would go to a relay coil that has a foot switch in series with the coil.
OPERATION SEQUENCE
Position lumber and set clamp (if a feature)
Operator presses two hand controls, then presses foot switch (suitably guarded of course) to start saw.
Release of foot switch lets saw coast
Release of either or both of two hand controls brings saw to immediate stop. Immediate stop may be difficult - say one or one half second?

Dan Bentler

Or talk the boss into getting one of these:
http://www.sawstop.com

I know it's a table saw, but maybe they can customize one for your application.

Don't circular saws have two handles to help prevent it from kicking back. I take it the axis this is mounted to prevents this. Just thinking if the saw is running the operator releases one of the two switches to remove a object that fell in the path of the saw,does the saw blade come to a complete stop in the time the operator has to reach their hand in front of the blade?

Many saws are required to have anti kick back dogs. These will stop the wood from kicking back. Kick back is commonly encountered in ripping and caused by the outfeed side closing in on the blade. Some saws have a guide to prevent these.

I have never seen the unit Okie referred to but based on their advertizing it looks good to me. From little I have read once the safety stop is activated the unit has to go back to factory for rebuild.

Dan Bentler

The makita shown here is pretty much what we have... just that we want to make sure the free hand is on a separate enable button thats tied into the circuit.

http://www.istorya.net/forums/other-electronic-devices/344368-chop-saw.html

Lots of great points above... in the end we want the enable button to act as it normally would in a dual palm circuit. Resetting the circuit after each saw chop might need an extra feedback, and we don't really want to defeat the saw trigger as is as that would mean voiding the warranty. So in this case, the operator must use one hand to enable, and the other hand to trigger and lower on the part.

leitmotif said:

...From little I have read once the safety stop is activated the unit has to go back to factory for rebuild.

Dan Bentler

Click to expand...

Nah, they'll sell you a new brake mechanism (pretty reasonable too I think), but you may need a new blade too.

Fourpaw said:

The makita shown here is pretty much what we have... just that we want to make sure the free hand is on a separate enable button thats tied into the circuit.

http://www.istorya.net/forums/other-electronic-devices/344368-chop-saw.html

Click to expand...

So, it's not a circular saw (to me that means hand held "Skil" saw), but it's a miter saw. Hence the confusion about the two hand handle that a circular saw has.

Does your particular model have a trigger lock on it? If so, I would say to heck with the waranty, and use the trigger as part of the two hand safety, running power to the motor separately from your safety contactor.

One lost finger will buy a boat load of warranties.