Cleaning up a dado wall

[yoboseyo]

I am making a through mortise and tenon, but the mortise piece is in 2 thin pieces that I'll laminate together, so I'm cutting a dado through one piece, fit the tenon, make sure it's flush, then glue the other piece on. Seems like a foolproof way to do a mortise and tenon.

Anyway, the dado is too tight. I need to make it wider using fine adjustments to sneak up to fitting the tenon. I know it's easier to clean up the tenon, but the tenon is even thickness and the dado isn't, so I'd rather do the dado for exactness.

I've tried clamping a piece to use as a chisel guide to chisel straight down, but that didn't work so well because either the timber is too hard or I can't get my chisel sharp enough.

I can try to cut the dado on the tablesaw again, but the whole reason it's not fitting right is because I have a really crappy mitre fence, so I'm not confident it can cut a square dado.

Any ideas? I have the option of using a router with a flush trim bit, but the router is my least favorite tool to use and it's a huge chore to break it out for simple tasks.

[Mountain Ash]

This is the sort of job that the Stanley #79 and #98/99 are designed for. There are also wooden bodied versions, of which I have both a LH and a RH but both of mine need a bit of fettling.

[jpdv]

Or the newer Veritas version: Veritas Side Rabbet Plane - Lee Valley Tools (https://www.leevalley.com/en-us/shop/tools/hand-tools/planes/joinery/60012-veritas-side-rabbet-plane?item=05P4401) left and right hand in one...

[Chief Tiff]

A couple of questions; what’s the timber and what chisel are you using? You may need to increase your bevel angle if the wood is too hard or lower it if the wood is soft pine. I would persevere with the chisel and guide block.

Side rebate planes excel at widening grooves rather then dados, the very short nose makes it awkward to start the cut square and you risk tearout as the blade exits the other end. If you do have access to one then I would fit guide blocks on either side of the dado to straighten the start and support the exit of the cut.

[yoboseyo]

A couple of questions; what’s the timber and what chisel are you using?

at a guess, grey ironbark. It's very hard and siliceous - it leaves a shiny surface when planed. Can't really feel the grain when working it. I've used a range of chisels of different widths. They are Berg and Titans. Bevel edged

I'll take your tip of trying a higher angle. I should be able to put a secondary bevel on the chisel pretty easily

[yoboseyo]

Is it true that oil makes it easier to take end grain shavings?

[Mountain Ash]

I've seen it done on YouTube so it "must" be true.:D

[bruceward51]

I find it hard to understand why a sharp chisel will not do the job. A sharp tool will cut a whole mortise in almost any timber that I have worked with. You need to use a suitable persuader (mallet). I assume your Titans have rings at the top of the handle to prevent mallet damage?
If I was doing this, I would first mark the line with a sharp marking knife, then chisel back to that. I regularly cut dados with just a knife and chisel, before refining the bottom with a router plane. Hardwoods don't seem to be any harder to cut than pine with these methods - in fact I prefer working in hardwood. You do need a solid work surface. A bench with any bounce is counter-productive. Better to work on a concrete floor than a lightweight bench.
I can see what you are doing with the laminated construction, but most old-style joiners would just cut the mortise from both sides of the timber, meeting in the middle. They would clean up the mortise to make sure the walls were parallel, but most would do any fitting on the tenon, because it is far easier to refine the tenon than the mortise and you can get a really good friction fit. Your nice clean cuts on the tenon will not be seen once the joint is assembled!

[AnthonySeiver]

Others have already said these things but worth consolidating. Start with a marking knife to precisely measure and mark where the dado shoulder should go, it's too easy to blow past without a definite line and then your joint is sloppy (unless you want it to be tusk or pegged mortise and tenon joint). Also your freshly sharpened chisel can go into the knife line for super-precise and square shoulders. On chisels steeper secondary bevels are your friend in Ironbark (one of the hardest timbers). Clean up the dado bottom with a router plane (not the electric one) or chisel as a flat bottom and clean square corners will make fitting a charm.