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?

On 3 Aug 2004 14:02:31 -0700, (Dominic Ostrowski) wrote:

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.

Tom Veatch
Wichita, KS USA

On 3 Aug 2004 14:02:31 -0700, (Dominic Ostrowski) wrote:

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?



over here in the united states they don't let us have riving knives
G. we have to make do with splitters.

what you are describing sounds like it would work. might not be
totally legal...

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.



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.

"Tom Veatch" wrote

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..

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.

Jeff G


--
Jeff Gorman - West Yorkshire - UK
Username for email is amgron
ISP is clara.co.uk
Website - amgron.clara.net

As far as I understand it, you don't need a splitter or riving knife
for rebating, the wood itself acts as a stiffener to prevent any blade
grab

John

On 3 Aug 2004 14:02:31 -0700, (Dominic Ostrowski) wrote:

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?

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?


John wrote in message . ..
As far as I understand it, you don't need a splitter or riving knife
for rebating, the wood itself acts as a stiffener to prevent any blade
grab

John

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.


[SNIP]
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.

Jeff G