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Dare I mention Dados?

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Chris72

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Hi, As I'm to understand it,the main danger to using dado cutters is that there often used without a riving knife.Obviously if you allow your piece being machined to come away from the fence, and contact upcoming blades, you are going to get kick back.For some time now I have been using a variable width knife which is adjusted on two set screw bolts.It only takes a couple of minutes to adjust the knife to fall in line with the right hand blade.I should point out that on my saw the riving knife rises and falls with the blade. Im also aware there is still a risk using dado blades because no blade guard can be used.I get around this by using a macrelon guard which also bolts conveniently to my rip fence.
Chris.
 

Garrett in Victoria BC CA

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I was thinking of raising it myself, although with a different slant.

Tablesaws in North America do not have riving knives, a subject of increasing debate. However, they do have - I believe - longer arbors than European saws to accommodate dado sets up to 13/16" wide.

Dado-ing is, in fact, one of the safer cutting actions because none of the blade is exposed. Every woodworker makes them by running the work against the fence, and I've never heard of an accident.

I grew up working part-time and later full-time in my father's architectual woodworking and store fixture factory where three full-time trim sawyers were employed. Quite literally thousands of metres of dados were cut in the above fashion without incident.

Cheers, Garrett
 

Scrit

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Chris72":8699zc6n said:
...as I'm to understand it, the main danger to using dado cutters is that there often used without a riving knife.
Actually, Chris, riving knives are mainly there to stop work binding at the rear of the blade around the rising teeth.

Chris72":8699zc6n said:
...Obviously if you allow your piece being machined to come away from the fence, and contact upcoming blades, you are going to get kick back.
Don't forget that you also run the risk of kickback if you hit a knot or crack in the timber, too. Another cause of kickback is if you inadvertently let the work rise up off the cutter then attempt to press it down at the rear of the piece, thus running the risk of contacting the rising teeth (operator error)...

Chris72":8699zc6n said:
I'm also aware there is still a risk using dado blades because no blade guard can be used. I get around this by using a macrelon guard which also bolts conveniently to my rip fence.
Agreed. From the statistical evidence published the danger of overbalancing after an event such as a kickback and then setting ones hand down on top of the spinning blade set is probably the greatest serious injury risk of all. The difficulty is in making a guard which is substantial enough to withstand your weight pressing down onto it above the dado set. If you've ever used a properly set-up spindle moulder you'll be aware that it is necessary (in trade shops mandatory requirement) to use Shaw or SUVA guards to press the work both down onto the table and into the fence [Edit: although the requirement for side pressure only really applies if machining narrower stock]. Router tables really need the same sort of thing which can generally be accomplished by the use of featherboards due to the smaller sections usually being worked. If you use a spindle without such guards there is a risk of the work being kicked back at you. Such a kickback can be dangerous, but often isn't because the work tends to be thrown outwards, away from the cutter and not to be able to make any further contact with it. In the case of a saw table with a dado set you find most users have neither side nor top Shaw guards to force the work down onto the table and into the fence. This means that in the event that the work does get kicked back they are running the risk that gravity combined with your physical pushing may combine to bring the workpiece back down onto the rising teeth which may then result in a further serious kickback - after which they potentially lose their balance in the act of pushing forward, and it's, "Well hello, surgeon" - and maybe a three-fingered salute.....

Trade shops have for many years used power feeders in place of Shaw guards on spindle moulders, and we are now also required to use anti-kickback (limiter) design cutterblocks on spindle moulders. These two combined deliver a much safer way to machine timber, but as yet there doesn't appear to be anyone making anti-kickback dado sets (assuming that is even possible). By guarding the top of the cutter you are protecting yourself from part of the problem, but is your guard substantial enough to withstand you pressing down on it bearing in mind the fact that a dado blade will very quickly grind its way through 6 or even 8mm of polycarbonate or acrylic (or for that matter 1mm of steel)? Don't underestimate how aggressive dado blades are.

I've done a risk assessment on this and come to the conclusion that the only half-safe way to use a dado blade on a table saw is to use it in conjunction with an industrial power feeder and that even then there are questions about effective guarding of the cutter before and after the workpiece is being machined (which probably answers Garretts comments about the blade being exposed - it is exposed at both the beginning and end of the cut). I know that Felder have a couple of "dado" blades available for their saws (Luna used to do these as well), but these are specialised chip-limiter grooving cutter designs much more like spindle moulder tooling (in fact they can be used on spindles) and are limited in width. In any case I believe that they are designed for use with a boom-type crown guard which is mounted independently of the riving knife, something very few hobbyist saws have available for them.

Probably the only safe way to use a dado head is to use it on a radial arm saw. In that situation the likelihood of kickback is minimised as the work is constantly being pressed against the fence by the cutting action of the blade and down against the table by the same motion meaning that it has nowhere to go (i.e. nowhere to kick back to). In that case the main danger is of climbing onto the work (where the cutter starts to flex the arm of the saw and literally climb up onto the surface of the workpiece). This can be countered by installing a damper and deWalt have these available for most of their saws, new and not so new.

The last and possibly unseen danger of dado heads is under braking. Modern saws should have a power off brake fitted which brings a spinning blade to a halt in 10 seconds or less (again the trade is required by law to retro fit brakes to ALL saw benches). This is a CE regulation, so any new saw, DIY or pro, will have this feature. Braking a large dado head means that the head needs to be secured using two nuts or a pinning mechanism to ensure that the head doesn't come adrift in an emergency stop situation.

Sorry for the turgid response, but this is quite a complex issue.

Scrit
 

tombo

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I am going nuts waiting for my new saw and dado blade. Cough... If i am honest i will use it unguarded :whistle: . Will it be more dangerous than a router... probably, but its a risk i am willing to manage.

tom
 

luthier49

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I consider a dado as more safer than a sawblade. The reason being that dados are used mostly to cut housings, grooves and rebates. The blade is totally covered when actually cutting. It`s when your ripping wood that part of the blade is exposed and a lapse of concentration could cause an accident. When ripping wood, inherant stresses in the wood can cause the wood on the out feed side to bend inwards therefore nipping the saw blade at the rear which causes the blade to lift the wood and throw it back at the operator. The riving knife plays a part in counteracting this affect. The riving knife should be sharp at the front and slightly thicker than the kerf of the sawblade. I`m still stuck with my Coronet Consort for dado and moulding block work because there`s not a reasonably priced cast iron saw available in this country that allows me to do these operations. To be able to use a moulding block is extremely important to me as the photos show. I build mandolins and guitars, and the machining operation below is cutting a dovetail on a softwood test mandolin neck. As you can see by the finished dovetail, this would take a considarable time to make by hand, machining takes but a few minutes and is extremely accurate.

 

Chris72

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once again scrit is the god of machining! Top marks chap, spot on!
 

Chris72

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But nobody speaks of the riving knife!, is this the dark side of the force?!.
Sorry working nights in Scotland!
 

Scrit

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Robert

luthier49":1p94qcaa said:
The blade is totally covered when actually cutting.....It`s when your ripping wood that part of the blade is exposed and a lapse of concentration could cause an accident.
But it's exposed at the beginning and end of the cut and you don't seem to consider the possibility of kickback :? When ripping the blade should be protected by an effective crown guard which is much more difficult to achieve with a dado set.

luthier49":1p94qcaa said:
The riving knife should be sharp at the front and slightly thicker than the kerf of the sawblade.
The riving knife should be THICKER than the body of the saw blade and THINNER than the kerf to prevent it from binding in the cut. For example a typical 300mm TCT blade typically has a kerf of 3.2mm and a body of 2.7 or 2.8mm - my panel saw has a standard riving kinife of 3.0mm supplied by the manufacturer.

luthier49":1p94qcaa said:
I`m still stuck with my Coronet Consort for dado and moulding block work because there`s not a reasonably priced cast iron saw available in this country that allows me to do these operations.
Your cutter block became illegal 2 years ago (see PUWER 98 : Selection of tooling for use with hand-fed woodworking machines on the HSE site) as it's a non-limiter design (i.e. prone to kickback) and is just downright dangerous. That means that if you ever get a finger in it you will lose it, unlike a kickback design which will injure you but probably won't amputate. As to there not being a suitable saw, surely a more suitable machine to perform this operation on would be a spindle moulder? Or even an overhead router or router table? If you want to put yourself at risk, that's fine by me. But I don't think it's good practice to encourage others to follow your lead and if you are a professional instrument maker you really do need to do a risk assessment to find out where you are going wrong.

In wood machining we probably run a greater risk than any other occupation/hobby of injury through contact with sharp spinning blades - one of the reasons that employers insurance is so expensive. Insurance companies being the way they are they might well refuse to pay out in the event of an accident if they perceive that you have put yourself at undue risk. My signature at the bottom is partly the result of personal experience and I can assure everyone here from personal experience that reconstructive surgery is not a pleasant way to spend one's afternoon after having had the accident!

Scrit
 
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