Replies

1. 
   forestgirl | Sep 28, 2006 08:24am | #1

   So, in the kick-back thread, all the talk of a riving knife saving the day when cutting a rabbet (rebate) should be taken cautiously?  Boy, I swallowed that one, didn't I? ;-)

   forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 
   

1. 
   SgianDubh | Sep 28, 2006 10:36am | #2

   fg, you only have to look at the two pictures I attached in my first post to see that the riving knives on those machines go over the top of the blade.

   To cut rebates, or other non through cuts on those machines you'd need to remove the riving knife. Slainte.Richard Jones Furniture

2. 
   Mumda | Sep 28, 2006 02:37pm | #3

   My description of the riving knife had this disclaimer: "...and this answer is speculation since I have a US saw without a riving knife..."So I too was mis-informed and I dragged you down with me. If other people have read my post, I may have created a whole society of mis-informed riving knife myth believers. Now, Richard is trying to destroy my new society...how should I feel about this?

1. 
   forestgirl | Sep 28, 2006 07:40pm | #7

   "Now, Richard is trying to destroy my new society...how should I feel about this?"  I'm not sure, but one thing is certain -- Richard doesn't care!  ROFL!!!forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

2. 
   BobMc | Sep 28, 2006 06:34pm | #4

   Not sure how he got sucked into the brawl, Riving Knife, considered a good guy by most, and a hero by many, staggers out of the Old Knots Tavern.  Black and blue from the trouncing, Riving stops for a breath of dust-free air when BAM! he's hit over the head.  Seems some one started a rumor he'd been hovering over Terrie D'Chick, and those who knew her well didn't like it.  "Next time I stop in at Old Knots, I'm coming armed with the facts, and a pen."  Thinking how the pen is mightier than the sword, and certainly a knife, Riving figured that next time he could kickback at the table and enjoy it.  Then he started thinking of TDC, and, caught off-guard, his mind spun off on a tangent.

   <I'd better go get my first cup of coffee before I spin further out of control emoticon>

   :)

1. 
   BG | Sep 28, 2006 06:47pm | #5

   BobMc,
   Why do I get the sense that this Terrie D'Chick has long 'come hither' eye lashes....

1. 
   Rennie | Sep 28, 2006 07:02pm | #6

   Why do I get the sense that this Terrie D'Chick has long 'come hither' eye lashes....

    

   Careful now....be on your guard!Rennie

   A man is a fool if he drinks before he reaches the age of 50, and a fool if he doesn't afterward. Frank Lloyd Wright

2. 
   BobMc | Sep 28, 2006 08:20pm | #9

   Could be; I do know that among lisping woodworkers that she's a "popular myth."  As for me, I always thought that TDC was a well-centered, sensible woman.  She apparently has a good sense of timing as I have read how finding TDC is the first step to time both the cam and the ignition after an engine rebuild.  When I bought my jointer, the salesman told me to find TDC to set the blades evenly.  One guy even told me that you know that you've found TDC by the fact that any movement away will be your downfall. So yeah, she's probably got "come hither" eye lashes that make you want to get as close as possible.

   What?  Both Terrie and Riving just said "Hi Jack," even though I'm Bob.  Apologies to all.  <Gonna post it regardless emoticon>

    

    

    

2. 
   forestgirl | Sep 28, 2006 07:42pm | #8

   Ohmigod, that's funny!  You could write "Short Stories for Woodworkers" in Workbench or somethin'!  LOL!forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-) 

3. 
   SgianDubh | Sep 29, 2006 01:41am | #10

   I like that Bob. How much better would it have been after that coffee you mentioned?

   Mythbusters plc. Slainte.Richard Jones Furniture

1. 
   eddiefromAustralia | Oct 02, 2006 03:11am | #11

   Hi RJ,Perhaps worth mentioning another myth that's gaining currency in that riving knives prevent kickback.***********
   They don't.
   ***********(but they do make the saw far less likely than one without a riving knife to produce a kickback.)Cheers,eddie(Who still has a saw kick back about once per year, complete with riving knife and guard)

   Edited 10/1/2006 8:13 pm by eddiefromAustralia

1. 
   DonaldCBrown | Oct 02, 2006 04:08am | #12

   "eddie(Who still has a saw kick back about once per year, complete with riving knife and guard)"How do you manage that? I would think the guard, mounted atop the knife, would prevent the stock from lifting up and then being thrown back. If I'm wrong about that, then how does it manage to be thrown back without being lifted?

1. 
   eddiefromAustralia | Oct 02, 2006 04:22am | #13

   Hi Donald,Good to see you still hanging around.As you allude to, kickback happens when the stock contacts the rising rear teeth of the blade and is thrown back or upwards at the peripheral speed of the blade. The riving knife helps prevent these teeth from getting too much bite.However, they still can nibble a bit. When they do, the potential is there for kickback. It throws straight back out (along the table) the gap between the saw and the fence (approx 80% of my personal experience), or can bounce off the table and get airborne slightly - can also climb up and over the blade at times, especially with an overhead guard (that isn't attached to the blade). First thing I was taught when I was apprenticed was where to stand. I'd say that all kickbacks I've seen have travelled waist height or lower - never up in the air to any great degree. I'm guessing that it went up but then was deflected down by the guard - been a while since I've had a kickback like this.I don't know the exact mechanism as it happens too quickly. Usually if someone's been tinkering with the saw and/or fence, so the timber jams. Usually, but not always. It can happen for a number of reasons.Edit II: Thinking about it, I've gotten more kickback when the blade's set correctly (so the gullet just clears the surface of the timber.) Gives a better cut, but the peripheral force is directed more horizontally than vertically.Edit III: Just to explain to those others that aren't used to the crown guard, the guard is set to be approx 3/8" above the blade at the leading edge. On a crown guard (mounted on the riving knife), this puts the gap between the guard and the timber at the rear teeth of the blade approximately 1-1.5" high. Enough to allow the timber to rise and kick back. The guard doesn't have enough force to hold the timber down, so it will kick back.Anyway, back to work - no more edits - will check in tomorrow.Cheers,eddie

   Edited 10/1/2006 9:45 pm by eddiefromAustralia

2. 
   bakesale1 | Oct 02, 2006 05:33am | #14

   Eddie do you use a short fence for ripping?

3. 
   eddiefromAustralia | Oct 02, 2006 06:51am | #15

   Hi Bakesale,Yes, I do.We would have been literally kicked from here to kingdom come if I didn't - and thinking back, the kickback was due on some to the saw that we had to use. The 'short' fence extended past the rear teeth.Cheers,eddie

4. 
   Lataxe | Oct 02, 2006 04:41pm | #16

   Eddie,

   My saw (a Scheppach - German) has a riving knife that is 3mm thick - 0.2mm less than the blade thickness.  It also follows the curve of the blade very closely (2mm gap or thereabouts).  This seems to prevent true kickback, although it is possible to get the workpiece to bounce a little if it is fed very badly, with lots of sideways pressure .

   The fence is set so there is a tiny taper away from the riving knife at the back.  This gives the workpiece 0.1mm "room" at the back of the blade, which also helps.

   The riving knife can also be tilted side-to-side as well as front-to-back, via its mounting bolts.  This allows you to set it very accurately with reference to the blade.

   Finally, just to dispel a little Internet myth that may be growing here: many European saws come with a riving knife that does allow cuts over the blade even when there is also a mounting point on that knife for a blade guard.  My Scheppach is like this.  Take off the guard via its mounting slot (5 seconds via a Bristol lever) and the work can be part-cut over the top of the blade with riving knife still in place.  The knife top is about 1mm lower than the blade top.  (The knife moves with the blade of course).

   The previous saw I owned (an Emco) came with two riving knives: one was large with a guard mount, which was higher than the blade top.  The other was a sickle-shaped knife that was just lower than the blade top and was meant for overcuts.  It took about 2 minutes to swap these riving knives.

   Perhaps there are some European machines with less versatile riving knives. However, as such knives are just a bit of shaped, flat ,  tempered steel that have mounting points on the saw arbor, it would be very easy to make one suitable for overcutting, should such an accessory not be offered by the manufacturer.

   Lataxe

5. 
   mufti | Oct 03, 2006 07:43pm | #20

             Just a small point but thin kerf blades need a thinner riving knife and I make them as needed. However it then has to be shimmed on the mount or there will be tears before bedtime.

6. 
   eddiefromAustralia | Oct 03, 2006 10:34pm | #21

   Hi Mufti,Riving knife thicknesses are:Thicker than the saw plate (so the wood won't close up on the plate under internal tension unless it's really bad and closes on the plate before the leading edge touches the riving knife) and;thinner than the kerf (for the obvious reason.)Cheers,eddie

   Edited 10/3/2006 4:08 pm by eddiefromAustralia

7. 
   mufti | Oct 04, 2006 09:38am | #22

                 Thanks Eddie, as usual I did not say what I intended. My point was that when switching blades it is prudent to switch knife, and then align it central to the blade. Knives are easy to make and as you say a standard knife will not fit a thin kerf blade. 

8. 
   DonaldCBrown | Oct 03, 2006 03:20am | #18

   Eddie, thanks for your informative response.I guess I don't know how to resolve all the vectors involved in kickback when both a riving knife and a crown guard (rigidly attached to the knife) are used.But you have the experience, so you're obviously correct. I'm glad you weren't standing directly behind the work piece.Donald

3. 
   citrouille | Oct 03, 2006 02:42am | #17

   Are we confusing a riving knife with a splitter here ?
   A splitter (commonly used to hold the guard) tilts but doesn't go up and down with the blade and it has to be removed when making blind cuts.
   A riving knife is usually dagger shaped and does go up and down with the blade as well as tilting.

   C.

1. 
   SgianDubh | Oct 03, 2006 03:30am | #19

   "Are we confusing a riving knife with a splitter here?"

   I'm definitely not, but you might very well be confusing the two citrouille. Please go back to my opening post, the one you responded to, read it again, and have a look at the picture attachments of riving knives. They are not dagger like. They go over the top of the saw blade, rise up and down with it, and the guard is attached directly to the knife.

   Yes, certainly, some riving knives are dagger like. Many aren't. Slainte. 

   Richard Jones Furniture

   Edited 10/2/2006 8:41 pm by SgianDubh