I'm doing some rebating on the tablesaw (in the UK, so equipped with a riving knife rather than a splitter) 2.4m long, 75 by 100mm (3 by 4"), cutting a 12 by 40mm (0.5 by 1.5") rebate. Currently removing the riving knife (it would extend above the blade crown and usually has the crown guard attached), using a full length fence and a homemade crown guard (shaw guard like cover bolted to t slots on the fence). Safest config I could readily come up with.
My question is, would it be worthwhile/safer to buy a spare riving knife, shorten it to below the blade crown (would always be correct as it follows the blade height) ? Is this a common modification?
First, the disclaimer - I don't have the foggiest idea what I'm talking about as I have zero experience with riving knives. With that in mind, here goes:
As I understand it, a riving knife and a splitter have similar functions. They guide the work downstream of the blade and prevent the kerf from closing with the intent of preventing/reducing any tendency for the rear of the blade to catch the workpiece and throw it back in your face..
Fixed splitters are usually recommended for shop made zero clearance inserts, are effectively fixed height riving knives, and I've heard no reports that they are ineffective or dangerous. Since they are fixed height, they will sometimes be higher than the blade, sometimes lower.
Putting these observations together, I would theorize that a riving knife that was cut down to the same height or slightly shorter than the blade extension would be better than a fixed height splitter on a zero clearance insert. Especially true when the blade is extended higher than the height of the fixed splitter.
Therefore, unless there is someone who can assert otherwise from hard experience, if I were in your position, I would get out the file (or hacksaw, or whatever), shorten a spare riving knife to the point of having a small clearance between the knife and the bottom of a blind (non-through) kerf, and be damned cautious until I was reasonably certain there were no unanticipated effects.
That's what _I_ would do. I'm not recommending that you or anyone else do it. And if you do, I'm not interested in hearing from your lawyer in the event there are unanticipated effects.
a minor point here- a factory splitter or riving knife being attached to the trunnion and passing through the throat insert needs a slot in said insert. that slot does weaken the insert, making it less rigid.
I believe that the difference between a splitter and a riving knife is that the splitter can be set some distance behind the blade, yet a riving knife is curved so that its profile follows the profile of the up-running teeth.
The riving knife can also play a part in preventing the contact of the workpiece or offcut with the uprunning teeth and causing a violent ejection of same. A splitter, as above defined is less effective in this regard.
A problem with many riving knives is that they are also designed to support the crown guard, thus bringing the top of the knife above the crown of the blade and so requiring their removal for rebating and grooving.
As far as I recall, official British safety requirements say that the top of the riving knife should reach further than 25mm (1in) below the crown of the saw. Hence it seems to be regarded as satisfactory to use a slightly shorter than full-height knife.
Truly safe use, of course requires a crown guard mounted independently of the knife or a Shaw guard or tunnel guard.
Hoping this helps; there's much more about sawbench safety on my web site.
Yep, that's what I understand too. The remainig wood helps reduce any tendancy for the kerf to close up (but not eliminate it).
However when ripping a long rebate the last bit can be tricky, particularly as the cut finishes but the job has not cleared the blade to the rear.
Strikes me this would be the point a short riving knife woule make the most contribution to safety, when the job is over the rear rising teeth only and most difficult to prevent it skewing.
Surely this is such a common operation (I'm making up window profiles) that there's a widely accepted approach?
That makes good sense. I am indeed using a homemade shaw/tunnel guard. Given that rebating is such a common task I'm surprised that a shortened knife for each tablesaw isn't available from the manufacturer.
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