Any woodworkers (or others) how to cut this groove?

Use a drill to cut a hole at each end of the slot, then chisel it out.

That’s going to be tough. A router would be your best option, but obviously you don’t have one. I think you could use an exacto knife, along with a metal ruler to score the outline of the groove you need. Go over each line several times, cutting deeper with the knife on each pass. Then use your chisels, carefully. Good luck!

@augustlan That sounds like the way to go—I may be able to get all the way through it with that—I have a couple other tricky square shapes, but not so small, and this may be the way to go.

I forget that with wood this thin, you can just cut it, carefully, with a knife.

I would cut a “v” cut with a saw or knife to the depth you need. Once the v portion of the wood is cut out, take your knife or saw and cut vertical cuts along each v edge to the depth you need. At this point use your chisel to cut the other 2 v portions out. This will help reduce any variation in depth when you use your chisels. Sharp chisels are very important. Good luck.

The classic way would be to drill the ends of the slot, as @steve6 said, then use a straightedge and sharp knife to score a line along the two edges of the slot. Do the scoring on both sides of the wood if the back side needs to look clean too.

The scored line will serve as a precise guide for precisely aligning the edge of the chisel. The idea is to use one of your wider chisels, say the 1”, to cut down partially along the scored lines. Hold the chisel with the beveled side of the edge toward the interior of the slot and the blade held perfectly vertical. Make the edge drop into the scored line. If the chisel is sharp, you can just use firm pressure to cut down into the wood. You don’t want to go all the way through, as that would likely split the piece. Cut down along all the scored lines, then use your 1/8” chisel, held with the bevel down, to cut out the top layer of wood that the larger chisel has freed.

Eventually, you’ll reach the depth of the side cuts and will have to use the 1” chisel again to deepen the side cuts. Keep repeating the cycle til you’re through the piece.

Use the 1/8” chisel to square off the round ends of the slot (you may need to tap it with a hammer to cut cross-grain like this.

Oh, one other technique that should work well here:

It’s called “chain drilling” because it involves drilling a series of closely spaced holes along the length of the slot (the holes should run together). Then you use the large chisel to cut straight down along the edges of the slot. With this method, you can cut all the way through the thickness of the wood at one go without risking splitting the piece.

@Harp—excellent solutions!

Chain drilling sounds like it would likely require a drill press to do properly (drilling overlapping holes tend to walk into one anytime I haven’t had the piece clamped onto a drill press), but it’s a good solution to think about.

I have some larger square holes (I’m building a control panel for a steampunk flying machine, ergo the oak), so I think I’ll start there trying them with @augustlan ‘s method and go from there if it doesn’t seem like it’s working.

@bpeoples “drilling overlapping holes tend to walk…”

You can get around this problem by using a brad point bit

Good luck!

@bpeoples: Do you have a power drill??

@critter1982 Here’s the inventory:
18v Dewalt drill
5” RO sander

1/8” through 1–1/2” japanese chisels
Various files from a chain-saw round up through what I believe is known as “Large Bastard” =)
(2) pull saws, both pretty large, one with very fine teeth
(1) yankee screwdriver, only with flat screwdriver bit
(1) #5 1/4 Jack plane with very nice blade.

And some other random assorted things you’d expect to find. (Like a straight edge and a sharp knife)