My house has central forced air heating, with the usual cheap furnace registers in the floor. I thought I'd experiment with making a wooden one.
I decided on four slots 9/16" (14mm) wide. That way I still have enough material in between to make the grill sturdy.
But the straight pieces of wood between the slots would still be relatively vulnerable to breaking, for example, if a chair leg was on just one of them.
So I figured I'd have two 3/8" (10 mm) dowels going across them to form a sort of grid. It's best to drill these holes before cutting the slots, so I wouldn't run into any tearout from the drill.
I didn't have a long enough drill of that size. So after I drilled as far as it would reach, I used a thin long drill to drill a pilot hole all the way through, then drilled in from the other side to join up the holes. Thanks to the pilot holes, the larger holes lined up perfectly.
Most people would cut the slots with a router, but cutting those slots in thick hardwood is slow, noisy and dusty, with many shallow passes.
I figured I'd make the slots by cutting either side of each slot with a table saw, then drill a hole at either end to free up the waste piece.
My homemade table saw happens to be very convenient for making plunge cuts...
... because it doesn't have a depth adjustment wheel. It only has a lever, so I can just grab that lever and raise it while the saw is running. I set up my cuts so that I had to raise the blade to full depth. I just raised the blade all the way in each position.
The wood is placed against the rip fence for each cut, with a stop on the front and the back to control the length of the cut. After each cut, I insert a spacer between the workpiece and the fence to move the workpiece to the left to cut the other side of the slot. After that, I move the fence for the next slot.
Unfortunately, I set the fence in slightly the wrong place for one of my slots. I repositioned it correctly and cut the slots again, but I'll have to fix the extra slots.
Drilling a 9/16" (14 mm) hole on either end of each slot to free the waste piece.
Now fixing my mistake. I cut a strip of wood to the same thickness as the blade's kerf and glued it in the extra saw slot that I didn't need. The other erroneous cut was in the middle of the waste piece I removed.
If the infill piece matched the colour more closely it would be very hard to tell there was a mistake at all!
Smoothing the transition from the saw slot to the hole a bit. I wanted to avoid over-cutting the slots on the bottom side, so the saw slots don't quite come to the middle of the hole on the top (the bottom side of the cut is always a bit longer because of the round shape of the saw blade)
Now gluing a dowel into the holes I drilled earlier. I put glue in every hole, then tapped in a dowel. To get more glue into the joints, I applied more glue to the exposed parts of the dowel when it was most of the way in, then tapped it all the way.
I cleaned up the excess glue by rubbing sawdust against it, then scraping that off with a chisel.
Cutting the register to its final size.
I need a rabbet all the way around the grill so it will drop into the opening in the floor.
I made these rabbets with two cuts on the table saw.
Test fitting it in the floor.
The grill still has some sharp corners sticking up. I didn't want to make the part that sticks above the floor any thinner, fearing that it might make the grill too weak.
My solution was to taper the edges. But my homemade table saw doesn't have a tilt function, so I improvised by placing a scrap of wood against the fence, on the bottom, so that my workpiece leaned against the fence. This worked quite well.
Finally, sanding all the edges on the top, so it doesn't feel uncomfortable to step on without shoes, and no rough edges to snag socks.
I finished it with three coats of varnish. Now in the floor. Looks much improved over the old one.