Thread: Felder Table Saw stopping technology

Originally Posted by BobL

I also really dislike the use of the work "accident" to describe what has a bunch of underlying causes especially where motor vehicles are concerned.
This makes it sound like it was nobodies fault.

The Italians and French have a slightly better tern which translates as "incident" or event. This removes the "it's nobodies fault" from the descriptor.

Agreed - I prefer the use of the word "crash" when it comes to vehicles - it perhaps raises more awareness of the seriousness of what has occurred.

Originally Posted by Chesand

I have just built a cross cut sled for my Sawstop. All the Youtubes,etc show a sled being built and used without the guard and riving knife as shown above.
I have modified mine to allow the guard and knife to stay on by putting a "bridge" on the back fence that allows the fence to pass over the guard The only difference is that the anti-kick back claws have to be folded up so that the sled can be withdrawn.

Love to see a picture of what you have done, I can get the knife on mine but the guard is bloody huge on my Woodman TS maybe a new post so this dosent get off topic.

I am thinking of putting a bit of Acrylic across the top of my sled/s

Originally Posted by WillyInBris

I am thinking of putting a bit of Acrylic across the top of my sled/s

Unless it is really thick acrylic can easily shatter when hit by a piece of kicked back wood.
Then the shards of acrylic become a danger.

A better choice of material would be polycabonate (PC) as even thin PC provides significant protection - think motorcycle helmet face shields.
On my TS guard the surround is 1mm, thick PC while the top is 20 mm thick acrylic.

Originally Posted by BobL

Unless it is really thick acrylic can easily shatter when hit by a piece of kicked back wood.
Then the shards of acrylic become a danger.

A better choice of material would be polycabonate (PC) as even thin PC provides significant protection - think motorcycle helmet face shields.
On my TS guard the surround is 1mm, thick PC while the top is 20 mm thick acrylic.

good point I have a fair bit of acrylic of varying thicknesses up to 25mm lying around from past laser jobs I may have some PC as well will have to have a look my housekeeping with off cuts aint the best lol.

you got any more detail links to your dust TS setup etc )

Willy
I have attached a photo of my cross cut sled. It is still a work in progress.
The "bridge " is a bit rough as I modified it as I went along. I will probably replace the top piece with aluminium angle as it would be a bit neater.
Hope that helps

cross cut sled_143954-1 (2).jpg

Originally Posted by WillyInBris

good point I have a fair bit of acrylic of varying thicknesses up to 25mm lying around from past laser jobs I may have some PC as well will have to have a look my housekeeping with off cuts aint the best lol.

you got any more detail links to your dust TS setup etc )

starts here.Ducting update.

Originally Posted by elanjacobs

Good luck trying to instantly stop a buzzer or spindle moulder without totally destroying something.

The only reason they can make it work on saws is that the rotating mass is fairly low and the diameter of the blade makes for high stopping torque. If you were to try to jam something into a 20-30kg buzzer head spinning at 5000 rpm, you could well destroy the entire machine and I'm pretty sure that's not a good business model...

Err... no. The Felder model drops THE WHOLE SAW GROUP to the bottom of the machine - probably 50-100kg of stuff gets moved in milliseconds. That is the reason why the feature is only available in the Format saws initially - even the biggest Felder saw (the 960) doesn't have enough cabinet mass to handle the drop of the group during a trigger. It also has to have electric raise and lower because as I understand it, the PCS system is an extension of the raise/lower mechanism, so that probably permanently rules it out of Hammer saws. But the technology could be easily applied to moulders, jointers, and thicknessers (but not a combo machine).

The key point is that the Felder system does not attempt to stop the rotating assembly - it just drops it out of the way (and presumably applies the usual braking system to stop the rotating stuff once it's out of harm's way). But this system works on massive 450mm saws, so the blade drops over 200mm in such a small amount of time that it beggars belief. The forces exerted trying to lift the saw off the ground during a trigger must be enormous (Newton's 2nd law, natch).

I wonder what system Altendorf are introducing. They mention it on their web site.

Originally Posted by djeddy

The key point is that the Felder system does not attempt to stop the rotating assembly - it just drops it out of the way (and presumably applies the usual braking system to stop the rotating stuff once it's out of harm's way). But this system works on massive 450mm saws, so the blade drops over 200mm in such a small amount of time that it beggars belief. The forces exerted trying to lift the saw off the ground during a trigger must be enormous (Newton's 2nd law, natch).

The following numbers were calculated here https://www.omnicalculator.com/physics/acceleration and here https://www.omnicalculator.com/physics/work-and-power

To move 200mm in 4 milliseconds (Sawstop claims under 5ms for their system to stop the blade) from rest requires acceleration of 25000 m/s^2.
To accelerate 100kg that fast needs 2.5 million Newtons of force...it'd also be travelling at 360 km/h by the end of it.
The amount of work done would be 500kJ and the power required would be...... 125 MEGAwatts

Now it's been a long time since high school physics, so I could very well be missing something big, but those numbers don't sound remotely achievable Bob, what am I doing wrong?

Originally Posted by elanjacobs

Now it's been a long time since high school physics, so I could very well be missing something big, but those numbers don't sound remotely achievable Bob, what am I doing wrong?

What I think you have done is stop the spinning blade -- which is not required once the blade has dropped below the table.
So once the blade is below the table it can spin down -- or electro brake

Elanjacobs, I admit I hadn't done the numbers. They are eye-opening. It doesn't seem feasible - 500kJ is one hell of a lot of energy, certainly beyond any feasible capacitor system I can think of (that fits in the cabinet). It must be that the motor is somehow not moved when the saw group moves, so the mass is only ~10kg or perhaps less. It's still an awful lot of mass to move that quickly though.

Originally Posted by ian

What I think you have done is stop the spinning blade -- which is not required once the blade has dropped below the table.

No, I didn't factor that in at all. The only thing calculated was moving a mass (the blade assembly) a given distance in a given time.

If I understand correctly, the Sawstop has the blade rise/fall on a pivot (as with many cabinet saws), so the force of the cartridge jamming the blade is used to drop the whole assembly on that pivot. Panel saws tend to have the blade assembly on linear rails, so they can't use that trick.

Only moving the blade sounds more realistic, if the blade shaft was offset 45 degrees from the motor shaft (as opposed to being directly above it), it could fall without the V-belt pulleys crashing. It'd still need a hell of a shove though, maybe it's held in place by electromagnets and, if tripped, the poles reverse?

Your physics looks OK EJ but it's not clear to me what specific claims relate to what about stopping a blade, dropping a blade and dropping a whole blade assembly.

The way I look at it there's no need to move a blade or blade assembly a whole 200 mm in 4ms, it just needs to be accelerated and moved away faster than a moving finger.
Rather than distance I'd start by assuming 20m/s (world record for fastest punch) as the fastest speed a finger can move and work from there.

Stopping a blade in under 5ms does not not seem that big a deal.

Originally Posted by BobL

The way I look at it there's no need to move a blade or blade assembly a whole 200 mm in 4ms, it just needs to be accelerated and moved away faster than a moving finger.
Rather than distance I'd start by assuming 20m/s (world record for fastest punch) as the fastest speed a finger can move and work from there.

That makes sense, if it's moving away from you faster than you can catch it, it doesn't even matter if the blade keeps spinning.