Making Tongues Fit Dadoes

[Steve H Graham]

I'm trying to make a simple shelf to hold a heavy lathe chuck, with a tray on top for other tools. Some of the boards will fit into dadoes. I am putting little shoulders on the boards so they will cover the edges of the dadoes.

I have found that it's a real pain to get the boards to seat in the dadoes. I'm a little annoyed about this, because the books and videos I've seen make it look simple. I thought I should ask a few things.

1. Is it a good idea to make the dado slightly deeper than the height of the tongue that sits in it? I used the same router bit, set at the same height, to cut tongues and dadoes, and danged if the tongues didn't refuse to go in all the way. On Youtube, everything just slides into place in ten seconds.

2. Is it usually necessary to relieve the top edges of the dado? It seems like the bit gives you a sharp 90-degree corner there, but the inner corner of the tongue will have a radius on it, so you're trying to push a sharp edge into a rounded corner.

3. Is it usually necessary to relieve the edges of the tongues, for the same reason mentioned above?

4. Is there any hope of getting good glue contact at the bottom of the dado, or is that something I should forget about? Seems like there is so much fiddling to be done, the edge of the tongue and the bottom of the dado are pretty unlikely to be flat and in good contact. Quite honestly, I feel like I should shoot a small amount of something like 5200 in there, since it will fill a gap and allow some movement.

I get the impression that people who make videos hide their mistakes and omit the fine details that make woodworking work. Their parts go together like greased Legos.

I am suddenly very interested in getting a real router plane. I learned a few things about them today.

I have a makeshift plane I made with a 1/2" chisel. When I tried to use it, I learned that a 1/2" chisel will not go in a dado cut by a 1/2" router bit. It's too big. That was certainly interesting. I also learned that unless the board with the dado in it is incredibly flat, you are going to need a router plane, because it will not be easy to get a dado of uniform depth.

I've seen people use saws to round the ends of tongues and tenons, but I roughed with a saw and then used a file. If this is a stupid idea, please let me know. It seemed less aggravating, although it created a danger that the file would cut into the shoulder. I feel like I should use a Fein Multitool next time, since it will give me more control than a saw, with no danger of false starts.

[Kent A Bathurst]

I'm trying to make a simple shelf to hold a heavy lathe chuck, with a tray on top for other tools. Some of the boards will fit into dadoes. I am putting little shoulders on the boards so they will cover the edges of the dadoes.

I have found that it's a real pain to get the boards to seat in the dadoes. I'm a little annoyed about this, because the books and videos I've seen make it look simple. I thought I should ask a few things.

1. Is it a good idea to make the dado slightly deeper than the height of the tongue that sits in it?
Absolutely. A "must do". One reason - to be sure you have enough clearance. Second - as a reservoir for excess glue. Which answers your question #4 below: Nope. No hope. don't even worry about it.

2. Is it usually necessary to relieve the top edges of the dado? It seems like the bit gives you a sharp 90-degree corner there, but the inner corner of the tongue will have a radius on it, so you're trying to push a sharp edge into a rounded cornerfers with a
Necessary? I dunno. A smart thing to do? Absolutely - slight chamfers with a block plane takes care of it - a "lead it" for the fit, if you will. I always do it.

3. Is it usually necessary to relieve the edges of the tongues, for the same reason mentioned above?
Same exact answer as above in #2.

4. Is there any hope of getting good glue contact at the bottom of the dado, or is that something I should forget about? Seems like there is so much fiddling to be done, the edge of the tongue and the bottom of the dado are pretty unlikely to be flat and in good contact. Quite honestly, I feel like I should shoot a small amount of something like 5200 in there, since it will fill a gap and allow some movement.
Asked and answered: fugeddboutit.

I get the impression that people who make videos hide their mistakes and omit the fine details that make woodworking work. Their parts go together like greased Legos.
Really? Ya think they game the system to make themselves look like Gandalf? Shame on them.

I am suddenly very interested in getting a real router plane. I learned a few things about them today.
Nothing wrong with that.

I have a makeshift plane I made with a 1/2" chisel. When I tried to use it, I learned that a 1/2" chisel will not go in a dado cut by a 1/2" router bit. It's too big. That was certainly interesting. I also learned that unless the board with the dado in it is incredibly flat, you are going to need a router plane, because it will not be easy to get a dado of uniform depth.

I've seen people use saws to round the ends of tongues and tenons, but I roughed with a saw and then used a file. If this is a stupid idea, please let me know. It seemed less aggravating, although it created a danger that the file would cut into the shoulder. I feel like I should use a Fein Multitool next time, since it will give me more control than a saw, with no danger of false starts.
A good block plane will be a better choice, and can be used on many other tasks.

See embedded notes, Steve...........Full Disclosure - I rarely do this task with routers, but that matters not - the issues are the same.

You figgerd it all out. You're good.

[Greg Sznajdruk]

Steve: When I first read your post it appeared that youwere asking about dados. But as I read further began to wonder if you were asking about mortice and tenon.

Cutting dados in making a shelving unit for example, the shelves fit to the bottom of the dado without relief. The dado should be cut so that it accommodates the shelve.

This video maybe of interest.

https://www.youtube.com/watch?v=7j4vGS4HY-8

Greg

[Steve H Graham]

Thanks for taking the time to write that, Kent. It's like a textbook.

Greg, I am always a little unsure of the nomenclature. The slots I made do not go all the way to the edges of the board; they terminate 1/4" from the edges so the joint and the end grain won't be exposed. I debated putting square ends on the slots vs. rounding the ends of the tongues, and I decided rounding would be faster.

It's amazing how much you can learn from making one slot and one tongue. I learned that the depth of cuts made with handheld routers is more uniform than the depth you get with tables, because a table router won't ride on the surface of the board and go up and down with it. If the board is bowed up in the middle, the cut will be shallower there. I guess I should have applied some pressure to it to flatten it out as it passed over the cutter.

On the up side, I learned that I can use my table saw fence with the router, which is in the extension. I just have to put a sacrificial MDF fence on the router side so the bit never comes near the aluminum fence.

[Kent A Bathurst]

Steve - those are known as blind dadoes, or stopped dadoes - interchangeable terms. Blind or stopped in that they do not run clear to the end.

On the round v square........ Don't overdrive your headlights, brudda...........

Run the router bit for the dado - you get round ends.

Cut the tenons - you have square ends. You have to cut some off at each end, since you have blind dadoes.

You can cut them close to dead-nuts length fit, and then use a chisel to chamfer the corners - they ain't round, but who will know? They will hold fine in glueup. Very fast - just a quick whack down followed by one cleanup paring cut per corner. Faster to do than to type.

Or - just cut the off square end, short enough so they don't interfere with the rounded end of the blind dado. It will work fine in glue up. Who will know?

I personally never get carried away with squaring the rounded corners so I could fill that last little bit of hole with wood from the tenon, nor creating perfectly rounded ends on the tenons. It will not matter in terms of holding performance, and life is too short for that noise. IMHO.

BTW - I wholeheartedly endorse stopped dadoes and your method of slight shoulders on the tenons to cover the joint line. I do that on every dado/mortise/tenon I do. Much cleaner look, from my perspective.

[Steve H Graham]

Thanks again. I'll beat on it again tomorrow.

[Shawn Pixley]

2. Is it usually necessary to relieve the top edges of the dado? It seems like the bit gives you a sharp 90-degree corner there, but the inner corner of the tongue will have a radius on it, so you're trying to push a sharp edge into a rounded corner.

3. Is it usually necessary to relieve the edges of the tongues, for the same reason mentioned above?

I've seen people use saws to round the ends of tongues and tenons, but I roughed with a saw and then used a file. If this is a stupid idea, please let me know. It seemed less aggravating, although it created a danger that the file would cut into the shoulder. I feel like I should use a Fein Multitool next time, since it will give me more control than a saw, with no danger of false starts.

I've done a lot of stopped and shouldered dados / "tongues" but never with a router bit. I didn't realize they round the corners. I would cut mine on the tablesaw or use a handsaw and shoulder plane or router plane for the tongues. The dado is done with saw and router plane.

Kent gave very solid advice. If you have a shoulder plane, you could clean up the tongue for a better fit.

[Kent A Bathurst]

I've done a lot of stopped and shouldered dados / "tongues" but never with a router bit. I didn't realize they round the corners. I would cut mine on the tablesaw or use a handsaw and shoulder plane or router plane for the tongues. The dado is done with saw and router plane.

Kent gave very solid advice. If you have a shoulder plane, you could clean up the tongue for a better fit.

The rounding is where you come to a stop with the router bit while cutting the dado. Of course you knew that the router bits would leave a rounded end to the slot - you likely misunderstood the issue..

On the shoulder plane - sure - great tool. However - if the OP does not have a block plane, that is higher on the versatility scale, IMO, and will put paid to this task very easily. I'd suggest a LA rabbet block, but that's just me.......... a chisel is needed for the last few inches of the dado's edges............

I'se gots me a very nice shoulder plane. Plus 4 block planes, which cannot be explained rationally. Harold is expecting me to send one to him [skew block; using his cosmic tool string theory mind meld], but I'm not yet entirely down with that.....

[Shawn Pixley]

I'm not a router guy. The more I hear of these rounding issues, the less likely I am to join the router club.

[Steve H Graham]

My shoulder plane is way too long to use on the inside edges of dadoes. The blade's edge is maybe 5" from the back of the plane and 2" from the front. I used sandpaper.

[Pat Barry]

I'm confused about where the rounding is occurring. With a router bit I would expect nice sharp corners at the lead in to the dado / mortise or on the mating part of the tongue / tenon. What Kent described is totally different (and unavoidable with a router). For what Kent described you either clean up / square up the ends of the dado with a chisel, or you make your tenon rounded over to match the dado / mortise created by the router bit, or maybe just shorten up the tenon a bit I suppose.

[Steve H Graham]

Here it is.

I am really bad at routing. Somehow I pulled away from the fence and got a booboo. I guess I need to make things more slippery.

08 25 15 lathe shelf base dado.jpg

[Kent A Bathurst]

Here it is.

I am really bad at routing. Somehow I pulled away from the fence and got a booboo. I guess I need to make things more slippery.

08 25 15 lathe shelf base dado.jpg

Yer fine. In this type of operation, the work piece is lifted only when the bit ain't freakin' moving.

Your sandpaper deal is fine also - I use the block plane on the tenon, and most of the groove/dado, and then go in with a sharp chisel to slice the rest in one quick motion.

Whatever.

[Kent A Bathurst]

I'm confused about where the rounding is occurring. With a router bit I would expect nice sharp corners at the lead in to the dado / mortise or on the mating part of the tongue / tenon. What Kent described is totally different (and unavoidable with a router). For what Kent described you either clean up / square up the ends of the dado with a chisel, or you make your tenon rounded over to match the dado / mortise created by the router bit, or maybe just shorten up the tenon a bit I suppose.

Sorry to correct you. my friend.............

What I said is that I NEVER do either of those, because it ain't worth the effort.....you are adding nothing to the integrity of the joint.

I leave the dado alone. Period.

Then I do one of two things:
1. Use a chisel to whack the corners of the tenon - make it fit inside the routed half-circle. OR - even better:
2. Cut back the tenon an extra 1/2" - whatever it takes so that it does not interfere with the rounded end.

By definition - you have to grab your saw to trim the tenon for a stopped/blind dado. So - just cut it short by a little bit more.

Either 1 or 2 means you have a gap between the tenon and the top/end dead center of the rounded end of the dado.

So. Freakin'. What. ?.

That will not cause joint failure. Making a perfect fit for that last tiny bit will not be the deciding factor in joint success. Ergo - FIDO: [Forget] It. Drive On.

[John Donofrio]

Here it is.

I am really bad at routing. Somehow I pulled away from the fence and got a booboo. I guess I need to make things more slippery.

08 25 15 lathe shelf base dado.jpg

Which direction were you feeding the stock? It matters because the leading edge of the bit tries to either pull the work into the fence or push it away depending on which side of the bit you are feeding from. Even knowing this I found out the hard way how easy it is for the work to wonder a bit from the fence when feeding the wrong way.