Mortise and tenon question

[Tom Porter]

I'm planning a workbench using mortise and tenon joinery on the base. I'm pretty new to woodworking, so this is a learning process. I'm planning on starting with Douglas fir 4x4's and I'm trying to figure out the size of the mortises and tenons. A few Google searches have yielded some general rules. So, let's say I've milled the lumber to 3x3. Following a rule that tenons should be 1/3 the thickness of the mortised piece, the tenon should be 1 inch thick. A similar rule (2/3 the thickness of the mortised piece) leads to a 2 inch long tenon. But a rule from the same source indicated a width of the tenon should be 5 times the thickness, which would be 5 inches. Obviously, that's impossible with 3x3 stock. So, how wide should the tenon be? Should it just be the entire width of the stock? And as long as I'm asking, should I use blind or through tenons? If it makes a difference, I'll probably use dowels through the tenons.

As I said, I'm new to this, so feel free to correct my assumptions above if you think they aren't the way to go.

Cheers.

[Kent A Bathurst]

You are overdriving your headlights, Tom my friend. Simply put - it is not a decimal-precision issue.

The 1/3 thickness is a solid rule. Sometimes you can't - for various reasons - but generally you can. Won't go wrong.

For depth, go maybe 1/2 to 2/3 of the stile/leg. You will not have any issues. Especially if you pin them with dowels. So - 1" x 2" will be great. You will never have a problem. You don't mention the height - the 3d dimension - I will guess that the stretchers will be 3"+ in height, so make the tenon height 1/2" less than whatever that dimension is.

The wider, longer, and taller you go, the more strength and glue surface area you get. Bit - reality has to check in to the plan every so often - so, be real about it. You will be just fine.

[Bradley Gray]

+1 on all this. I usually size tenons to match one of my mortising chisels close to 1/3 to simplify the process.

[pat warner]

Indeed heavy stuff should adhere to the 1/3 thickness x 2/3 penetration mortice & tenon protocol. The booger has to stand a lot of stress.
Notwithstanding, the joinery can be compromised as long as you add some steel.
Joint connector bolts will make up for more than any liability imposed by a less than text book joint. Mending plates, screws and JCB's should not be overlooked if you'd like your offspring to use the bench. Steel will outlast glue, wedged joinery and so on.

[Lee Schierer]

I would make the tenon 1" wide, 3" tall and 2" long since it is going to be used in a work bench and will see lots of lateral forces.

[Tom Porter]

Lee, just so I'm sure I understand, following your suggestion, the height of the tenon would be the entire height of the stock? Just removing material on two sides, not all four?

Thanks for all the advice, by the way.

[Lee Schierer]

Lee, just so I'm sure I understand, following your suggestion, the height of the tenon would be the entire height of the stock? Yes

Just removing material on two sides, not all four? Yes

Thanks for all the advice, by the way.

If you don't think you can cut the top of your mortices clean and square, you could shorten the top of the tenon so there is a small shoulder to hide the top of the mortice. Of course you will need to adjust the height of the mortice to the same dimension as your modified tenon. Do your mortices first and then cut the tenon to fit.

[Kent A Bathurst]

Do your mortices first and then cut the tenon to fit.

Had Moses turned the tablets over, he would have seen this written on the back.

[william watts]

I'm planning a workbench using mortise and tenon joinery on the base. I'm pretty new to woodworking, so this is a learning process. I'm planning on starting with Douglas fir 4x4's and I'm trying to figure out the size of the mortises and tenons. A few Google searches have yielded some general rules. So, let's say I've milled the lumber to 3x3. Following a rule that tenons should be 1/3 the thickness of the mortised piece, the tenon should be 1 inch thick. A similar rule (2/3 the thickness of the mortised piece) leads to a 2 inch long tenon. But a rule from the same source indicated a width of the tenon should be 5 times the thickness, which would be 5 inches. Obviously, that's impossible with 3x3 stock. So, how wide should the tenon be? Should it just be the entire width of the stock? And as long as I'm asking, should I use blind or through tenons? If it makes a difference, I'll probably use dowels through the tenons.

As I said, I'm new to this, so feel free to correct my assumptions above if you think they aren't the way to go.

Cheers.

The general rule for tenons that I remember is 1/3 the thickness of the rail/stile not 1/3 the thickness of the mortised piece. Thats a general rule not a hard and fast law. Some times I make tenons 1/2 the thickness of the tenoned piece. If your rail is 1 1/2 in. material the mortise would be 1/2 or 3/4 in. depending on your choice of tenon size. For a work bench I would use 1/2 the rail size. As already been said cut the mortise first and the cut and fit the tenon.

[Kent A Bathurst]

The general rule for tenons that I remember is 1/3 the thickness of the rail/stile not 1/3 the thickness of the mortised piece.

If that works for you, then great.

I would not go that thin, at least not on std furniture. I make the mortises 1/3 the dimension of the mortised part, rounded up to the next size for my hollow chisel mortiser.

I make tenons that have only 3/16" - 1/4" shoulders - not 1/3 of the rail by any stretch.

May be just me - that seems thin - a std rail @ 3/4" would mean the tenon @ 1/4". I go 3/8" - 1/2" routinely. As much meat as I can get in there...............

[Tom Porter]

I appreciate all the advice and have another question. Say I'm making 2 inch deep mortises and am cutting them with a drill press or router. I know I should probably not take the depth out all at once, going partway, removing the material and then continuing until I get my depth. Is it easier to square up the corners with a chisel as I go or get the depth I want and then square them up? Bear in mind, as a novice, neither my chisels nor my skill with them is top-notch.

[Bob Snyder - Austin]

I would drill/route them to full depth, and then either square up the corners of the mortise or chamfer the corners off of the tenons. If routing them, I'm not sure you can a 2" deep mortise though. However deep you can go with a router, you want to do it a 1/4" or so at a time.

Is it possible to form/cut your mortises during assembly of the sub-components?

[Robert Engel]

For 3x3 stock, I would do a 3/4" tenon but 1" should be fine.
The forces are down in the leg. The stretchers are different.

Do not make the tenon the full width of the stock or you will see gaps unless it is absolutely perfect.
Plus a little shoulder can add alot of strength.
In your case, that would be mayb2 1/4" - 1/2" narrower, leaving a 1/8 - 1/4" overlap.

For leg-to-base MT, I would do a through tenon with wedges. That one needs to be super tight for stability.

For the stretchers I would use 1 1/2" stock (2x6 or 8) with 3/4" tenons and a 1" shoulder on top and bottom.
On my bench I used the captured bolt method but again, a through tenon with wedges or sliding wedge tenon will work just as good.

[Robert Engel]

I appreciate all the advice and have another question. Say I'm making 2 inch deep mortises and am cutting them with a drill press or router. I know I should probably not take the depth out all at once, going partway, removing the material and then continuing until I get my depth. Is it easier to square up the corners with a chisel as I go or get the depth I want and then square them up? Bear in mind, as a novice, neither my chisels nor my skill with them is top-notch.

Drill to full depth with a flat bottom bit like a Forstner.
Use a fence to keep things aligned and keep the holes at least 1/16" narrower than mortise.
Make sure you clamp the piece while you drill to keep from moving.

Clean everything up with a wide chisel.

Using a guide block clamped flush to mortise edge is a good idea.

Finally, you need a flat -- and I mean FLAT -- chisel to do a good job.

If you're uncertain about your chisels, watch a couple videos on flattening and honing chisels.
Remember, your basically chiselling crossgrain with a mortise, to it need to be razor sharp and flat or the tool will follow the grain and get messed up.

Finally (2) don't sweat the small stuff if the tenon is a little loose, glue it up and clamp it square overnight. You'll be fine.

[Phil Mueller]

Tom, I'm new to this too. Like said above, don't sweat it. Cut the mortise first. Because perfect M&T joints are still in my future, and I'm not comfortable yet with plunge routing or dropping the piece onto the spinning bit...I tend to make my mortises on a router table, cut through the top of the leg down to length. That way the tenon can "slide" in from the top of the leg, and frankly, the top is covered and never seen.

If you find the tenon is too tight...wiggle it in as best you can and pull it out. You will notice that where it is getting stuck, the wood will be shiny (grain has been compressed). Sand down the shiny areas and try again...keep going until they are snug but you don't have to use a mallet to get them in. If on the other hand, they are loose, I either shim with small slivers of wood, or use an epoxy type glue...these glues will fill the gaps versus regular yellow glue.

Just a couple of thoughts from a novice who's spent more time googling how to fix mistakes than I care to admit.