Table saws: is kickback feasible with a riving knife?

[nanscombe]

...

Do try and allow the teeth gollet to "clear" the stock being sawn when at all possible as this can cause the blade to heat up and as the sawdust cant get out quick enough, and, in reality a lot more dust is produced!

Understand gullet, but trying to visualise what you mean by "clear" the stock? Do you mean that the teeth should come out of the top of the stock sufficiently that you'd just see the gullet of those teeth?
...

I would imagine that it means allowing the gullet between the teeth to clear the sawdust out of the way rather than allowing it to collect between them.

 

[sploo]

Sploo, the finer teeth saw kerf, mainly for man made materials will probably be less than a rip saw, though, much depends on the maker and price.

As I'm just starting out with a table saw, I'd bought blades from Axminster. All the 205mm blades have a 2.2mm kerf (http://www.axminster.co.uk/axcaliber-co ... saw-blades). I've got a 32T "general purpose" blade with the saw, and added a 24T rip and 48T cross cut (though haven't used either yet).

One of the reasons I've got the TS is for cleaner cuts (vs the bandsaw), so getting the "right" blade for the job does matter to me. I'm assuming it's pretty much always a bad idea to use a blade with a high number of teeth for ripping, even though the finish is claimed to be better? I've not got the 64T "extra fine" blade, but I assume that would be for cross cutting like the 48T?

I get the impression that this size of blade is mostly used in budget machines; hence most of the available blades are more toward the budget cost/spec end of the range - though there are a few Freud Pro blades that might fit (between 180mm and 216mm diameter), but I don't know if they'd be significantly better than the Axy contract blades.

Out of interest; what blade should I use for ripping MDF? Or ply? High tooth count for a clean cut, or low tooth count to clear saw dust? Or is the answer "either; it depends on whether you want fast and messy or slow and clean"?

Can't you use one of the riving knives you already have and just bolt the guard to the top of that and adjust the saw clearance?
Yes, the saw will perform best when the saw blade gullet just clears the stock, and the most teeth are cutting, many saws instructions will advise this. but if you're guard is in place, should be difficult to see.

The knife has a long slot in the bottom, to slide over two bolts under the table surface, so you can adjust the height. Obviously however, there's a limited range of heights you can use, as you want the curve on the knife to match the curve on the blade.

There's a hole drilled into the top/rear area of the knife, through which the blade guard is bolted. The issue is that the height of the guard prevents a full depth cut. You'd either need to fashion new knife that had more material at the top (to get the guard bolt hole higher), or lift the standard knife up (but then the knife curve wouldn't match the blade and you'd have a big gap).

The fence should should lead, as in, head in the direction of the saw blade, just enough to keep the saw effectively cutting a straight line, possibly in the same token as the to-in of a car's steering.
when you can cut a piece of timber,parallel, without any back sawing at the riving knife, and no gap at the fence, That's it! Regards Rodders

I'm struggling with that idea. I understand that if the fence stops at the blade's leading edge then there's no way material could be pinched between the blade and fence, but everything I've ever read about table saws has indicated that getting the fence perfectly parallel to the saw blade is an absolutely critical part of the safety setup of a saw.

Maybe I'm worrying too much about the short fence. I've got it in my head that sheet material may twist towards the fence (specifically towards the non-existent fence at the blade, because it's not being supported). I guess that once some material has passed through the blade, then even the trailing edge shouldn't start to twist, at the rest of the material will be on either side of the blade. Guess I just need to try a few more ripping cuts of sheets.

 

[sploo]

Yes. It's happened to me. Kickback happens very fast and with a lot of force. Assume it WILL happen EVERY time you use the saw riving knife or not.

Could you give details of the setup? In almost all anecdotal cases of kickback I've heard, it usually starts with something on the lines of "well I was doing something dumb and..." - i.e. was there something you'd identify that you shouldn't have done, that could have avoided the kick? (I'm asking in order to learn, not criticise BTW)

I would imagine that it means allowing the gullet between the teeth to clear the sawdust out of the way rather than allowing it to collect between them.

Yep. I think I've got that now. Makes a lot of sense, and I know bandsaw blades are prone to wander if you don't get sufficient dust clearance.

 

[andersonec]

If I could add my penny'worth.

Firstly I would like to say that if the material comes straight back at you then the blade is not high enough and there is not enough downward pressure on the material and because your blade is not sharp enough, the teeth are pushing the material straight back before it enters the material and pushing it back before the downward cutting effect can happen.

Secondly, the advice of standing to the side is wrong,,,if the riving knife is not fitted and the material is tilted into the rising rear teeth of the blade it will lift and come at you to from the side of the blade at a tremendous rate of knots.

Thirdly,, a short fence which just passes the gullet of the teeth is a must, the full length of your fence must not be used, this is to allow sideways flexing of the material so that it does not pinch onto your riving knife (or saw teeth if the knife is not fitted) The front of the fence is to guide the material straight and the important distance is between the fence and the cutting edge of the saw blade, once the material has been cut the rest of the fence is not required.

If the material does pinch onto your riving knife (as in a previous post who said they then needed a hammer) the thing to do is to pull the material back a few centimeters, switch off the saw then place s small wedge in the previously cut kerf to open the cut.

Here is a video (by an American of all people) showing what happens during a so called 'kickback' One of the best video's out there.

http://www.dailymotion.com/video/x7d2vt ... _lifestyle

Andy

 

[sploo]

Firstly I would like to say that if the material comes straight back at you then the blade is not high enough and there is not enough downward pressure on the material and because your blade is not sharp enough, the teeth are pushing the material straight back before it enters the material and pushing it back before the downward cutting effect can happen.Andy

Thanks for the thoughts (I'll watch the video when I get a break).

Playing Devil's Advocate on the above though; if the blade is higher then there's more chance of the rear edge lifting material up onto the top of the blade and throwing it at you. There's also more chance of material pinching the blade body and getting thrown. I suspect it'd be very difficult to get a serious kickback if only a small amount of blade were protruding, because it'd be mostly the teeth, which I'd hope would cut into material pinching it - rather than being grabbed and thrown.

Agreed completely on dull blades though; just bad in any scenario.

 

[blackrodd]

Firstly I would like to say that if the material comes straight back at you then the blade is not high enough and there is not enough downward pressure on the material and because your blade is not sharp enough, the teeth are pushing the material straight back before it enters the material and pushing it back before the downward cutting effect can happen.Andy

Thanks for the thoughts (I'll watch the video when I get a break).

Playing Devil's Advocate on the above though; if the blade is higher then there's more chance of the rear edge lifting material up onto the top of the blade and throwing it at you. There's also more chance of material pinching the blade body and getting thrown. I suspect it'd be very difficult to get a serious kickback if only a small amount of blade were protruding, because it'd be mostly the teeth, which I'd hope would cut into material pinching it - rather than being grabbed and thrown.

Agreed completely on dull blades though; just bad in any scenario.

andersonec, well found, at least it looks as if one american is concerned with safety.

Mr advocate, that's exactly right! And when you're 2.2mm blade is going at 120mph, as the blade slightly oscillates, due, in part to the speed of revolutions you will find the saw kerf closer to 3mm.
And as the blade is (hopefully) lower there is a smaller area to "pick up" the saw blade.

Regards Rodders

 

[PAC1]

36 years ago I was taught to set the blade no higher than with the gullet just clearing the material being cut. I understand this is still HSE advice.
Standing in a direct line of fire is for Darwins. I was lucky as an apprentice when the push stick hit the blade. It sent me flying but I received only bruising. I now stand to the left (which is difficult as I am left handed).
It is interesting that some manufacturers still supply long fences as standard rather than short fences. I made a plywood short fence to sit over the supplied fence.
As with Jacob two push sticks are better than one.

 

[sploo]

andersonec, well found, at least it looks as if one american is concerned with safety.

Mr advocate, that's exactly right! And when you're 2.2mm blade is going at 120mph, as the blade slightly oscillates, due, in part to the speed of revolutions you will find the saw kerf closer to 3mm.
And as the blade is (hopefully) lower there is a smaller area to "pick up" the saw blade.

3mm strikes me as quite a lot of wobble I must say. Certainly worth checking on a cut to see what I actually get at some point.

36 years ago I was taught to set the blade no higher than with the gullet just clearing the material being cut. I understand this is still HSE advice.
Standing in a direct line of fire is for Darwins. I was lucky as an apprentice when the push stick hit the blade. It sent me flying but I received only bruising. I now stand to the left (which is difficult as I am left handed).
It is interesting that some manufacturers still supply long fences as standard rather than short fences. I made a plywood short fence to sit over the supplied fence.
As with Jacob two push sticks are better than one.

I'd always thought of the ideal height as having just the tips of the teeth protruding, but the gullet argument makes a lot of sense. Interestingly, having just checked the saw, I reckon the guard basically covers the teeth - i.e. you can't lower the blade such that the material would cover the teeth; thus it ensures the gullet will just protrude. I do wonder if that was possibly a concious design decision - especially if it's HSE advice.

I've got three push sticks so far (a shorter one that came with the saw, and a couple of longer ones from other machines). The only thing they're not great for is applying pushing pressure on the top of a sheet, so I might make a hybrid push stick/push block (i.e. a block on a stick with a rubber foot) - that way I can move material from the top, but still keep my hands well away from the blade.

Long fences; I'm sure I've watched Norm Abram rip a length of pine on a saw with no knife or guard on a full length fence. So it must be safe then :wink:

 

[Graham Orm]

Yes. It's happened to me. Kickback happens very fast and with a lot of force. Assume it WILL happen EVERY time you use the saw riving knife or not.

Could you give details of the setup? In almost all anecdotal cases of kickback I've heard, it usually starts with something on the lines of "well I was doing something dumb and..." - i.e. was there something you'd identify that you shouldn't have done, that could have avoided the kick? (I'm asking in order to learn, not criticise BTW)

I would imagine that it means allowing the gullet between the teeth to clear the sawdust out of the way rather than allowing it to collect between them.

Yep. I think I've got that now. Makes a lot of sense, and I know bandsaw blades are prone to wander if you don't get sufficient dust clearance.

It was a few years ago now, I wasn't doing something dumb, and that's the reason to always expect it. I think I was pushing too hard on the side of the blade away from the fence. I now only push the wood on the side of the fence to avoid trapping the blade. It was over before I had chance to even jump. The wood passed me by thankfully and penetrated an MDF 9mm board 4 feet behind me.
Always stand to one side of the blade. I was already doing this which is why I wasn't injured. Left me shaking though.

 

[nanscombe]

...
andersonec, well found, at least it looks as if one american is concerned with safety.
...

To give them their due both Marc Spagnuolo (The WoodWhisperer) and the sometimes maligned Matt VanderList (Matt's Basement Workshop) do seem to advocate the use of riving knives on table saws, so that's another two. :lol:

 

[andersonec]

It was a few years ago now, I wasn't doing something dumb, and that's the reason to always expect it. I think I was pushing too hard on the side of the blade away from the fence. I now only push the wood on the side of the fence to avoid trapping the blade. It was over before I had chance to even jump. The wood passed me by thankfully and penetrated an MDF 9mm board 4 feet behind me.
Always stand to one side of the blade. I was already doing this which is why I wasn't injured. Left me shaking though.

Right,,,the way to push your work across a table saw is to push it against the fence and forward. try not to put any undue pressure onto the blade, have your push-sticks to hand and use them once the edge of the work has landed onto the table,

Andy

 

[Graham Orm]

It was a few years ago now, I wasn't doing something dumb, and that's the reason to always expect it. I think I was pushing too hard on the side of the blade away from the fence. I now only push the wood on the side of the fence to avoid trapping the blade. It was over before I had chance to even jump. The wood passed me by thankfully and penetrated an MDF 9mm board 4 feet behind me.
Always stand to one side of the blade. I was already doing this which is why I wasn't injured. Left me shaking though.

Right,,,the way to push your work across a table saw is to push it against the fence and forward. try not to put any undue pressure onto the blade, have your push-sticks to hand and use them once the edge of the work has landed onto the table,

Andy

Andy thanks for the input, but if you try to push towards the fence with a push stick from the opposite side of the blade to the fence you'll trap the blade and end up like I did.

 

[Jacob]

Two push sticks;
Right hand one close to the line of the saw blade (but not on it you don't want to cut it in half). Basically behind the workpiece. Could be to the right of the blade if there is enough room for it to pass between it and the fence - which could be handy as you can flip the off-cut forwards away from the blade
Left hand one wherever convenient to steady the workpiece but always to the left of the blade. Could be on the back edge close to the right hand one, or on the left hand edge etc etc - according to the shape of the workpiece.
2 sticks and the fence triangulates it and should be stable.
Sticks should be ply or mdf (?) so they don't splinter or shatter - they just get trimmed if things go wrong. And it's good to have a few of them hanging about so you can replace them easily and always find one.

 

[heimlaga]

I agree with Jacob. That is the two push stick method I use though only one of them was visible on my picture. Sorry for missing it.

If I could add my penny'worth.

Firstly I would like to say that if the material comes straight back at you then the blade is not high enough and there is not enough downward pressure on the material and because your blade is not sharp enough, the teeth are pushing the material straight back before it enters the material and pushing it back before the downward cutting effect can happen.

Secondly, the advice of standing to the side is wrong,,,if the riving knife is not fitted and the material is tilted into the rising rear teeth of the blade it will lift and come at you to from the side of the blade at a tremendous rate of knots.

Thirdly,, a short fence which just passes the gullet of the teeth is a must, the full length of your fence must not be used, this is to allow sideways flexing of the material so that it does not pinch onto your riving knife (or saw teeth if the knife is not fitted) The front of the fence is to guide the material straight and the important distance is between the fence and the cutting edge of the saw blade, once the material has been cut the rest of the fence is not required.

Andy

I don't agree with Andy on the standing position:
I have had a metre long piece of two by five inch pine coming straight back and flying out the door and landing out in the yard 10 metres away. The blade was set as high as possible which is 100 mm above the table so 50 mm of it was inside the overarm guard above the workpiece. The kickback barely lifted the overarm guard. There were some internal stresses in the wood so it pinched the blade before reaching the riving knife and kicked straight back. If I hadn't stood at the left corner of the saw out of the immidiate firing range I would have ended up either badly hurt or dead.
I have also hade 4 or 5 similar but much less violent kickbacks going straight back in 17 years of woodworking as far as I can remember. Always when ripping thick solid wood with sharp blades in high position and the riving knife in place.
I have never had a kickback going to the side. Maybe because I never remove the riving knife and have a fairly solid overarm guard both on the saw in my workshop and on the site saw.


I totally agree with Andy on the use of the short fence but I want to add more details:
The short fence is not for sawing sheet materials. Only for ripping thick solid wood and for crosscutting tennons and such together with a mitre gauge or a sliding table to avoid small offcuts being pinced. Sheet material is sawn with a long fence.
The reson for the short fence for heavy ripping is as Andy says that solid wood migh have internal stresses which may sometimes make it bend sideways when ripped. Then it may pinch between a long fence and the blade. It doesn't matter if a thin stip of wood pinches that way because there is very little force in it but when resawing slabs and a solid 2x3 or 2x4 bends and pinches between blade and fence something may go very wrong.

All dedicated rip saws for industrial use have the blade fixed in it's highest position and a short fence. Like this one:
https://www.youtube.com/watch?v=luS0q1GmjxM
Mine is not yet ready for use.

 

[andersonec]

I don't agree with Andy on the standing position:
I have had a metre long piece of two by five inch pine coming straight back and flying out the door and landing out in the yard 10 metres away. The blade was set as high as possible which is 100 mm above the table so 50 mm of it was inside the overarm guard above the workpiece. The kickback barely lifted the overarm guard. There were some internal stresses in the wood so it pinched the blade before reaching the riving knife and kicked straight back. If I hadn't stood at the left corner of the saw out of the immidiate firing range I would have ended up either badly hurt or dead.
I have also hade 4 or 5 similar but much less violent kickbacks going straight back in 17 years of woodworking as far as I can remember. Always when ripping thick solid wood with sharp blades in high position and the riving knife in place.
I have never had a kickback going to the side. Maybe because I never remove the riving knife and have a fairly solid overarm guard both on the saw in my workshop and on the site saw.

What you experienced there was the timber being 'ejected' it has rubbed on the front side of the blade which is making a downward and forward motion, maybe there was not enough 'set' on your teeth, the kerf was not wide enough to allow the bade free passage through the timber, or the timber has pinched before it has got fully across the blade, the timber has been pushed straight back towards you on the table.

What is commonly known as 'kick-back' is when the timber contacts the rear rising teeth, the timber is then lifted which brings it into contact the topmost teeth which are spinning towards you this then causes it to be very violently thrown forward and to the left, your timber was just pushed straight forward, you will know the difference when and if it happens, this one can do some really serious damage.

Glad to hear you always use the riving knife and a sturdy guard above which can also help prevent the timber from coming into contact with the uppermost teeth as long as it's base is well below the top of those teeth which are the ones which do the hurling of the timber.

Andy

 

[Owl]

I certainly wouldn't like to be around if this joker had a kick back. The blades measure 9ft and reached a speed of 130mph.

 

[sploo]

Here is a video (by an American of all people) showing what happens during a so called 'kickback' One of the best video's out there.

http://www.dailymotion.com/video/x7d2vt ... _lifestyle

Just watched. Yea, very good indeed. What I almost find weird is how a "European riving knife" seems to be held as this slightly mythical thing that guys in the US may have heard about, but really aren't quite sure it's for them. Granted the knife on the TS200 causes some problems as it's above the height of the blade, and grinding one down would mean the standard guard couldn't be used, but essentially, they're not obtrusive for most cuts, and reduce the chance of nasty kickbacks significantly.

Right,,,the way to push your work across a table saw is to push it against the fence and forward. try not to put any undue pressure onto the blade, have your push-sticks to hand and use them once the edge of the work has landed onto the table

I've just come across this: http://www.tablesawpushstick.com/index.htm

It's pretty close to what I was thinking of; a hybrid push stick/push block - with some of the advantages of both. I'd probably design it with your hand even further away, but essentially it looks good (and the rubber foot would allow good control). Although you'd miss some of the benefits, it wouldn't be hard to make one from 3/4" ply, and fit some sort of rubber foot.

Andy thanks for the input, but if you try to push towards the fence with a push stick from the opposite side of the blade to the fence you'll trap the blade and end up like I did.

Yea, that's where I think a push block excels (being able to apply sideways pressure to keep something on the fence, from the side closest to the fence). But of course for a thin rip, you hand is too close to the blade - unless you use something like the tool I've linked above.

I totally agree with Andy on the use of the short fence but I want to add more details:
The short fence is not for sawing sheet materials. Only for ripping thick solid wood and for crosscutting tennons and such together with a mitre gauge or a sliding table to avoid small offcuts being pinced. Sheet material is sawn with a long fence.
The reson for the short fence for heavy ripping is as Andy says that solid wood migh have internal stresses which may sometimes make it bend sideways when ripped. Then it may pinch between a long fence and the blade. It doesn't matter if a thin stip of wood pinches that way because there is very little force in it but when resawing slabs and a solid 2x3 or 2x4 bends and pinches between blade and fence something may go very wrong.

Thanks for the info.

I've certainly felt much safer doing cross cuts using the mitre gauge and a stop block (so the material to the right of the blade is never near the fence). I'll take heed of using the short fence for ripping thick stock.

I do have a plan to try to build a sliding table as part of the table surface the saw will go into. That would mean cross cuts and medium sized rips (I think I might be able to get 60-70cm of cutting "length" with my design) would not be using a fence at all.

What you experienced there was the timber being 'ejected' it has rubbed on the front side of the blade which is making a downward and forward motion...
What is commonly known as 'kick-back' is when the timber contacts the rear rising teeth, the timber is then lifted which brings it into contact the topmost teeth which are spinning towards you this then causes it to be very violently thrown forward and to the left...

I like this distinction. The Kelly Mehler video linked on page 2 of this thread was the first time I've come across it (the difference between "ejection" and "kickback"). Ironically, that kinda answers the original question of my thread - which is that (that definition of) kickback should really never happen with a riving knife, but ejection could still occur. I'm struggling to think of how you could get kickback with the knife in place (and especially with a sturdy guard).

 

[sploo]

I certainly wouldn't like to be around if this joker had a kick back. The blades measure 9ft and reached a speed of 130mph.

More to the point, I want to see the push stick designed for the tree you'd be heaving through it :mrgreen:

 

[Jacob]

...
I've just come across this: http://www.tablesawpushstick.com/index.htm

It's pretty close to what I was thinking of; a hybrid push stick/push block - with some of the advantages of both.....

No it looks like a really cra p idea. The disadvantages of both.
Just stick to simple push sticks like heimlaga's above.

PS the worst aspect of that utterly stupid american push-stick is that you hand and forearm are well over or past the blade before the end of the workpiece! Ferkin dangerous!
With sensible push sticks you are always well away.

 

[Owl]

Sploo, I glued fine sandpaper onto my push sticks, they grip the wood but don't mark it.