Dado cutters - Please, what is so dangerous about them

[SketchUp Guru]

The issue of the riving knife being ineffective in conjunction with a dado set continues to come up. I can't see how a riving knife would make any difference when using a dado set. As I understand it, a riving knife is supposed to keep the kerf from closing up on the rising teeth at the back of the blade during a rip cut. A dado cutter isn't used for through cuts so there's no kerf to close up.

My experience has always been with a full length fence against which the stock bears until beyond the blade. Maybe the riving knife is used to keep the stock way from the blade with a short fence? I don't know.

From the Wikipedia: A dado is a slot or trench cut into the surface of a piece of machineable material, usually wood. When viewed in cross-section, a dado has three sides. A dado is cut across the grain and is thus differentiated from a groove which is cut with the grain.

From that definition, which is the one I learned years ago, The rip fence doesn't come into play anyway. Just nitpicking. :lol:

 

[Scrit]

I was thinking more of individuals rather than businesses....

Actually the comments I recall were from discussions amongst hobbyists rather than professionals.

Scrit

 

[Scrit]

It seems to me that the only one which cannot be overcome is the riving knife issue which as I understand it, leads to potential kickback dangers which I completely agree should not be taken lightly.

I may have got the wrong end of the stick here but isn't the point of the riving knife to keep the two pieces of wood on either side of the cut apart when you are cutting completely through the stock? So that the rising blade doesn't kick back the wood.

In the case of a saw with an overhead guard, such as an Altendorf or Felder panel saw or a large rip saw, for example, it also serves the purpose of safeguarding the operator from kickbacks caused when work is accidentally lifted whilst pushing through - and believe me that does happen. You can get exactly the same on a dado head

Scrit

 

[RogerS]

In the case of a saw with an overhead guard, such as an Altendorf or Felder panel saw or a large rip saw, for example, it also serves the purpose of safeguarding the operator from kickbacks caused when work is accidentally lifted whilst pushing through - and believe me that does happen. You can get exactly the same on a dado head

Scrit

Scrit - not doubting you for one moment but can you explain a bit further as I don't see how the riving knife will safeguard the operator in this case.

Thanks
Roger

 

[Scrit]

Actually those nuts hold the wheel onto the hub, the hub is held in place with a single (castellated I believe it's called) nut. Nit picking I know but from an engineering perspective, it could be achieved on a table saw arbour if the manufacturers chose to do so.

And a castellated nut doesn't go on and come off as often as a nut on a saw and would have a clevis pin through it, wouldn't it? For our machines this would prove impractical. In any case the industry standardised on an approach many years ago for conventional sawing - two pins either side of the arbor OR a lock nut OR a pinned cutter set OR a keyed arbor. They're out there and in use on European kit (since the early 1990s). Dado heads are increasingly an anachronism as there are other faster, safer, more appropriate approaches available to us.

What you have completely overlooked is the tooling design. A "safety" head has the advantage over a stacked saw set that it will tend to throw a finger, hand, etc. out from the cutter in the event of a contact - a stacked set will do the opposite and will pull the finger, hand, etc. aggressively inwards. Hence the need for effective guarding. This is why the few European manufacturers selling compliant tooling - Kity, Scheppach and Felder, etc (and formerly deWalt, too) - have adopted a non-stacked saw / interlocking cutterblock approach rather like the tooling used on spindle moulders, and which incidentally on a RAS is much less prone to climbing the work (from experience of both).

Scrit

 

[Sawdust]

Actually those nuts hold the wheel onto the hub, the hub is held in place with a single (castellated I believe it's called) nut. Nit picking I know but from an engineering perspective, it could be achieved on a table saw arbour if the manufacturers chose to do so.

And a castellated nut doesn't go on and come off as often as a nut on a saw and would have a clevis pin through it, wouldn't it? For our machines this would prove impractical. In any case the industry standardised on an approach many years ago for conventional sawing - two pins either side of the arbor OR a lock nut OR a pinned cutter set OR a keyed arbor. They're out there and in use on European kit (since the early 1990s). Dado heads are increasingly an anachronism as there are other faster, safer, more appropriate approaches available to us.

What you have completely overlooked is the tooling design. A "safety" head has the advantage over a stacked saw set that it will tend to throw a finger, hand, etc. out from the cutter in the event of a contact - a stacked set will do the opposite and will pull the finger, hand, etc. aggressively inwards. Hence the need for effective guarding. This is why the few European manufacturers selling compliant tooling - Kity, Scheppach and Felder, etc (and formerly deWalt, too) - have adopted a non-stacked saw / interlocking cutterblock approach rather like the tooling used on spindle moulders, and which incidentally on a RAS is much less prone to climbing the work (from experience of both).

Scrit

Scrit,

I don't understand this, why would one push the fingers away and one pull them in, is it something to do with the angle of cut?

Thanks
Mike

 

[Kane]

I was thinking more of individuals rather than businesses....

Actually the comments I recall were from discussions amongst hobbyists rather than professionals.

Scrit

Interesting - maybe US woodworkers aren't the suing type

 

[SketchUp Guru]

Interesting - maybe US woodworkers aren't the suing type

Nah, that's not true. I don't think woodworkers in the U.S. are the sorts to sue unless maybe they are working in a commercial or school situation.

It never occurred to me to sue anyone when I slit myself with the chisel.

Of course one could sue and win if they spilled hot coffee in their lap while driving their car away from the drive through window at McDonalds. :roll:

 

[Kane]

Interesting - maybe US woodworkers aren't the suing type

Nah, that's not true. I don't think woodworkers in the U.S. are the sorts to sue unless maybe they are working in a commercial or school situation.

It never occurred to me to sue anyone when I slit myself with the chisel.

Of course one could sue and win if they spilled hot coffee in their lap while driving their car away from the drive through window at McDonalds. :roll:

Exactly my point lol

 

[Scrit]

I don't understand this, why would one push the fingers away and one pull them in, is it something to do with the angle of cut?

In limiter tooling (as in a "safety head" on a spindle) the "gullet" in front of the cutter face is fairly small, the body of the cutter is circular and in addition you have only two or four cutters. If you manage to jamb a digit into such a cutterblock the distance you can push into the gullet (and by default the amount of "bite" a cutter can take out of timber when machining) is limited. The body of the cutter behind the cutter being round and the direction of the block rotation will tend to throw the digit back out towards the infeed side of the saw - a combination of friction contact between the hand/digit and centrifigal force. A stacked dado set on the other hand has no body behind the individual rakers to do this, does not have a limited "gullet" and that combined with the attack angle of the raker means the tendency id to pull inwards/down wards.

I've experienced this first hand with a router cutter on an overhead router - 1/30 second contact approximately with a 2-flute cutter going at 20,000 rpm = 22 lacerations and 13 stitches. I think that a non-limiter cutter would potentially have amputated my left thumb. I also know two guys who've lost digits in circular saw accidents because saw blades seem to have a tendency to pull-in.

RASs have a different set of problems. If you trap a finger between the workpiece and the dado set, even if that is a safety head, the saw will tend to "climb" the work - and your finger - and you will be lucky not to get away without an amputation. This is why the HSE advice is that dado heads on RASs must be close guarded to within a few millimetres of the surface of the work piece.

What you don't experience on a RAS is kickback - what you do get instead is climbing. The old way around this used to be to fit a air/hydraulic piston onto the arm to "smooth" out the cut - for us trades types these are banned now, I believe. I've noticed from spindle moulder work that it is almost impossible to "overfeed" a limiter cutter block as the body prevents you doping so and I've no doubt that the same is true of a safety grooving head on the RAS. Incidentally for safety's sake I'd recommend fitting a spring return mechanism onto your RAS - that will return the saw head to the home or parked position at the rear of the saw in the event of you letting go of the handle for any reason.

Scrit

 

[Scrit]

...not doubting you for one moment but can you explain a bit further as I don't see how the riving knife will safeguard the operator in this case.

On a panel saw with an overhead guard (e.g. Felder/Altendorf) or a large rip saw similarly equipped it is possible to "deep", e.g. saw a groove without breaking through and with the riving knife left in position during the cut. This is because the riving knife is wider than the body of the saw and nattopwer than the kerf. On the machines I've mentioned it is also a few millimetres "shy" of the top centre of the blade in height. I sometimes use the panel saw to saw 3.2mm wide slots for thin back material this way and you need to take care that you don't accidentally lift the workpiece as you push it across the saw - doing so can tip the work down onto the rising teeth, a kickback situation if the riving knife were not present. If we accept that it is probably impracticable to install and adjust a riving knife with a dado set on a saw, then the way to avoid this form of kickback has to be to use a hold down (Shaw guard) or a power feeder to ensure that the workpiece cannot be accidentally lifted/tilted

Scrit

 

[Sawdust]

I don't understand this, why would one push the fingers away and one pull them in, is it something to do with the angle of cut?

In limiter tooling (as in a "safety head" on a spindle) the "gullet" in front of the cutter face is fairly small, the body of the cutter is circular and in addition you have only two or four cutters. If you manage to jamb a digit into such a cutterblock the distance you can push into the gullet (and by default the amount of "bite" a cutter can take out of timber when machining) is limited. The body of the cutter behind the cutter being round and the direction of the block rotation will tend to throw the digit back out towards the infeed side of the saw - a combination of friction contact between the hand/digit and centrifigal force. A stacked dado set on the other hand has no body behind the individual rakers to do this, does not have a limited "gullet" and that combined with the attack angle of the raker means the tendency id to pull inwards/down wards.

Thanks for that explanation

I've experienced this first hand with a router cutter on an overhead router - 1/30 second contact approximately with a 2-flute cutter going at 20,000 rpm = 22 lacerations and 13 stitches. I think that a non-limiter cutter would potentially have amputated my left thumb. I also know two guys who've lost digits in circular saw accidents because saw blades seem to have a tendency to pull-in.

Ouch!

RASs have a different set of problems. If you trap a finger between the workpiece and the dado set, even if that is a safety head, the saw will tend to "climb" the work - and your finger - and you will be lucky not to get away without an amputation. This is why the HSE advice is that dado heads on RASs must be close guarded to within a few millimetres of the surface of the work piece.

Whenever I use my RAS, I never allow my left hand to get in the line of the blade, if I'm machining a small piece, I hold it in place with push stick. With a dado in place I would adopt the same practice. I allow quite a safety margin when doing this as if there is a nasty dig-in it can jump forwards. This can be pretty unnerving but at least it doesn't launch the wood at the operator.

Mike

 

[wrightclan]

Interesting - maybe US woodworkers aren't the suing type

Nah, that's not true. I don't think woodworkers in the U.S. are the sorts to sue unless maybe they are working in a commercial or school situation.

It never occurred to me to sue anyone when I slit myself with the chisel.

Of course one could sue and win if they spilled hot coffee in their lap while driving their car away from the drive through window at McDonalds. :roll:

I'll second that. I was raised the non-suing type. About 16 years ago (doesn't seem that long :shock: ), I was in a cycle race, when I was forced off the road about 10 yards from the finish, by another cyclist. There were a couple of things that made the crash far worse than it otherwise would have been. First, the road had just been resurfaced, leaving about a six inch drop from the tarmac onto the gravel shoulder. More importantly, there was a car parked at the side near the finish. My front wheel went into the wheelwell of the car, throwing me back onto the road for about five other cyclists to come tumbling over me. I required medical attention, and my expensive bike was ruined.

Now it is against USCF (United States Cycling Federation) regulations to have a car parked that close to a finish. Several of my teammates kept reminding me of that rule. It never occurred to me, until years later, that they may have been hinting that I could have pretty good grounds for a lawsuit. To me, the crash was just part of doing the sport I loved. Two weeks later, I was racing again.
Brad

By the way, Dave, the race was in Cottage Grove

 

[johnelliott]

Of course one could sue and win if they spilled hot coffee in their lap while driving their car away from the drive through window at McDonalds. :roll:

Anyone who wants to know more about the most famous case in American legal history could have a look here
http://freerepublic.com/focus/f-chat/1287038/posts
It's quite short but very interesting.

John

 

[SketchUp Guru]

By the way, Dave, the race was in Cottage Grove

And you didn't invite me to watch? :roll: :lol: I was living in Woodbury then.

 

[wrightclan]

By the way, Dave, the race was in Cottage Grove

And you didn't invite me to watch? :roll: :lol: I was living in Woodbury then.

What, to watch me crash and burn? [-X

I've heard about you types. The ones who watch the Tour de France just for the sprint crashes--my brother is one of those :roll: :lol:

I was living in Burnsville at the time. I may have cycled by your house a couple of times, while out training.

Brad

 

[Scrit]

What, to watch me crash and burn?.....

.....I was living in Burnsville at the time.

Might I say, "how appropriate"? :lol: :lol:

Sorry, Mike, OT. I'll get me coat

Scrit

 

[wrightclan]

What, to watch me crash and burn?.....

.....I was living in Burnsville at the time.

Might I say, "how appropriate"? :lol: :lol:

Sorry, Mike, OT. I'll get me coat

Scrit

Stepped right into that one :lol:

Brad

 

[Anonymous]

The issue of the riving knife being ineffective in conjunction with a dado set continues to come up. I can't see how a riving knife would make any difference when using a dado set. As I understand it, a riving knife is supposed to keep the kerf from closing up on the rising teeth at the back of the blade during a rip cut. A dado cutter isn't used for through cuts so there's no kerf to close up.

Yep. that's true.

I still say that my dado took as long to mount and set up in the saw as a router takes to change the bit, clamp the guide, cut the dado and glue the wood.

I see no reason for using a dado head cutter set and got rid of mine.

 

[Inspector]

I suppose it's a good thing that these forums were not in existence when I started woodworking 30+ years ago. I probably wouldn't have started. That said, being on this side of the pond, I have 2 dado cutter sets for my saw. They are a lot quieter than a router and even compared to a normal blade.

The first is a cheap 6" wobble type that I never really liked. Not because of safety reasons, but because it doesn't leave a nice edge when crosscutting (splinters). It is also, because its was cheap, a pain to set the width. Maybe I might feel different with some of the newer high end blades that are now sold. I use it more for cutting coves than for dados.

My second set is a 10" (yes they are available in that size here) stacking type that I bought a few years after the wobbler. It leaves a clean cut and is simple to set the cut width. I like it and I'm not going to let anyone pry it out of my fingers as long as I still have them.

I don't use sheet goods very often, so my use of the blades is on hardwoods when plowing grooves. And while I understand why most use routers for that kind of cut, when you want to plow a 2" or deeper groove a router takes too long. I do understand the risks that can be encountered with this kind of cutting (especially the wood closing around the blade) so I stay mindful of the hazards and work accordingly. My choices are not for others and If you don't have a lot of time on saws then don't get your experience with a dado.

Since I have never used splitters (they were not with or were so badly designed and weren't used) or riving knives I have learned to work without them. If decent ones were on I would use them. North American makers started out making saws without them in the dark ages, and in spite of Americas claims to be innovators and on the leading edge they are extremely resistant to change. They didn't want to put on riving knives because it would mean making new castings etc. And that costs money and if you don't have to, then why bother. They like the expression "If it ain't broke, don't fix it".

A riving knife could probably be made for a stacking dado set if it already has a good one with the saw. With a few holes through the main knife, and knife profiled leaves matching the thickness of the blades. Then what ever stack and shims are selected for the blade, the corresponding leaves are added to the riving knife to proved the same width. If the knives are not strong enough to add to.......then....... never mind.

I have used dados on the RAS, but prefer the table saw because it is more accurate. The RAS types that I've had access to are of the hobby/contractor type and not all that rigid.

There are as many sides to a debate as there are participants, and I'll let you be the judge of your own choices and actions. I may however make a couple signs to hang at the entrances to my shop when dadoing stating. "controversial WOODWORKING IN PROGRESS. NO BRITS ALLOWED".

:lol: :lol:

The one thing that nobody has thus far mentioned is that a dado set move a lot more air and is cooler to work around in the summer.