11mm dado over 5m. 40 times.


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Good to know, will look into collets.

Battery router - makes sense, though I confess I concluded that for any tool I'm tied to a dust extractor, I might as well also be tied to a power cable.

You finally pushed me to look up dado/groove/trench; I had no idea there were different names for with and cross grain versions. Wikipedia (so mountain of salt required) at least says that:
Cross-grain is a housing (UK), trench (EU), or dado (US)
With grain is seemingly a groove regardless of geography
Confusing it is.

Quite why our friends call it a dado beats me, as a dado is part of the plinth at the base of a column. I can understand the panelling at the bottom of a wall being a dado, but calling a slot/ groove/ trench a dado is very odd indeed.

Anyway, carry on.
I wrote a whole thing about doing it with the router table and a load of featherboards and some assistants, however I deleted it as actually I don't think that's a good solution to your situation. I have never used a 'two fence setup' although that looks like the right way to do the job here. As @paulrbarnard says it looks like something you should be able to make fairly easily.
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The up cut ones pull the waste out of the trench, but leave a slightly ragged edge. The down cut ones do the opposite and shove the chips into the trench, which I think is a drag but they leave a clean edge though.
Apart from the superior waste clearance would you say the groove’s internals are cleaner compared to a two flute cutter?
Quite why our friends call it a dado beats me, as a dado is part of the plinth at the base of a column. I can understand the panelling at the bottom of a wall being a dado, but calling a slot/ groove/ trench a dado is very odd indeed.

Anyway, carry on.
Never mind the Wikipedia definitions of trench, groove, housing, dado, etc and the differences in terminology between American and elsewhere in the world. My memories of working with American woodworkers is that they all generally describe every form of slot, groove, trench, housing, etc in a piece of wood or board a dado. I think it relates to the fact that, for the most part, they mount 'dado' blades in their table saws to cut the feature, ergo, every 'groove', or similar, must be a dado, no matter the direction of the feature on a piece of wood or a man-made board.

I also suspect the reason we nowadays read so many questions here, and elsewhere, about cutting dados posed by British participants of this forum is because their initial or primary information sources are YouTube videos the vast bulk of which are churned out seemingly en masse by our woodworking brothers, many of them amateurs, on the western side of the Atlantic.
Also, I watch many people newish to the router trying to take a single full depth cut ×hen they should be taking 3 or 5 passes. This is very relevant to slots and trenches because the chips don't clear so easily. To go single pass, the advice above about spiral bits for better clearing is valuable. :)
Any rules of thumb relating cut depth, cut width, router power, and number of passes?
Any rules of thumb relating cut depth, cut width, router power, and number of passes?
A typical recommendation is to make successive plunges (using a plunge router) at roughly half the router cutter's diameter. So, for example, plunge in ~6 mm increments for a 12 mm diameter cutter. Therefore a 25 mm deep groove would be achieved with 4 X successive ~6 mm deep plunges.

In truth, this guidance isn't particularly good because much depends upon the hardness of the wood (including things like silica), the power of the router along with the cutter's shank and cutter profile and diameter, plus its sharpness. The real guide is that whatever cutter is in use in the router, if you plunge too deeply there will be uncomfortable sounding noise emanating from the router's motor or from the cutter (chattering and smoke), or both. If that happens, you back off and take a series of lighter cuts until the full depth and profile of the cut is achieved.

With experience a router user develops a 'feel' for what sounds right and what sounds like the tools (router and/or cutter) are working too hard. Having said that I've come across plenty of numpties that think it's normal to go hell for leather at routing and think things like smoke coming from the wood being cut is quite normal... but they are idiots, or maybe not even that smart. Slainte.
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This is my £218.95 worth of twin fence, made from odds and ends lying about, mostly used for multi point lock rebates in door edges, and anything else in I need to put a "groove" in, on narrow and long stock

twin fence.jpg

Bolt the router on and line it up to a given offset and off we go, I find it far easier to take the tool to wood rather than wood to the tool!
That's apalling on every level !
Give it a week and the Chinese will make a better one, properly, in aluminium for half the price.

I'd spend the money on a used ELU and a pair of fences !

Well, not everything is about price (but I did buy mine with a 15% eBay discount, which helped me justify it to myself). And I did consider making up something myself, but this is nicely made.

As I said, I have used the two-fence method, but depending on what you are doing, the fences don’t give the side support that this jig does and they can be a bit tippy.

Anyway, having bought the jig for a particular tricky job, I am glad I now have it!

Could you laminate three pieces together to give you the profile you want? Although that is a lot of clamps.
Expensive, yes, but a delight to use.

I did raise this question earlier but the thread seems to be working on the ostrich principle.

He is wanting to form the groove in 47x75 construction timber not planed and jointed cabinet making material.

Over the 40 pieces he wants to cut, it will have some variation in width. How does a double fence cope with this?
What exactly are you trying to do with the I beams? Most building methods have been tried and the best are in regular use, you may find there's a different solution to your issue and your just reinventing the wheel, possibly as a square.
OP could be making stage props or custom interior decor for a club that needs to look like industrial beams without the strength or weight. There are plenty of non structural possibilities but it is a good question.
The OP explains what he is doing in his linked thread re external insulation (an unconventional installation).

I have just had a look at that. The routing discussion above is all very interesting, but for the problem in hand, are I-beams actually required? I cannot comment on the insulation approach, but couldn’t you make your support framework out of “C-beams” instead? You then could just glue and nail the OSB to the side faces of the two upright timbers.

Decent bit in a 1/2” router mounted in a plank/piece of ply/old worktop (attached to something heavy, probably outside) with a makeshift fence and a couple of feather boards.
Just ram it all through.
I’ve done similar on many 5m lengths of 2x1without issue but fine finish was not required.
I now have a dado blade for the table saw which is really your ideal tool for the job (or a spindle moulder).
Two fences won’t work, one fence and a feather board/s is the way to go, also I’d still prefer to ram the wood over the tool than the other way round especially if you can find an assistant (child/partner/dog) to act as outfeed support.
If you can’t source the right size bit then definitely don’t reference off different faces, use the same face and reset or use a spacer as previously suggested.
Also consider thinning the thickness of the osb edges with a quick tickle on power plane in an emergency.