Mortise & Tenon - How to make it square, strong and clean

[thumbsucker]

I have been enjoying the pleasure of chopping own mortises by hand after having some practice on scraps, I have been cutting my first real project mortises into a pair of 9 cm wide legs with 3 cm wide mortises that are 6 cm high.

I however have some questions and I was wondering if people could give me some pointers. I need to point out that I do not have 3 cm wide mortise chisel and have had to make do with a smaller chisel.

The first challenge is what is the best approach to cutting a through mortise. In my first attempt I chopped from one side straight through and ended up tearing the wood out on the other side. :(( So on my second attempt I started by chopping both ends at the same time and joining them in the middle much like how tunnels are built. However I found that I had to clean up a little because their was a 2mm+/- misalignment. What is the best approach?

The next problem is a mater of how clean is clean enough, or how smooth should my faces be. Using a test tenon, I was able to tap the tenon in using a mallet. Looking into mortise I could just see tiny speaks of light shining through on the side where the tenon faces meet the mortise face.

The question is how close do I need to be across the surface face. I know theoretically I would like two glass like faces that are in contact all over. However considering that it is only my second mortise ever cut, are tiny speaks of light acceptable?

My last question is about getting the mortise square. If I had a 3-cm chisel then it would not be so much of a problem, however I can feel spots in the mortise that are raised and the spots are causing the test tenon not to be square. What is the best way to up theses high spots on the mortise face? I have been using the chisel, it is slow work, however it is coming along nicely. However, short of pushing in a test tenon every 5 minutes, how can a test if my mortise is actually square and where I need to get to squaring up the faces?

[Rocker]

Thumbsucker,

Probably the Dark Side experts can give you some more detailed advice; but I would suggest first of all that you invest in a couple of engineer's squares, one with a 100 mm arm and the other with a 50 mm arm, so that you can lay out your mortices accurately. The smaller one could be used for testing the mortice faces.

I gather that you are making a workbench, so you do not need to be unduly fussy about the appearance of the joint, I would not worry too much about slight gaps, so long as you use a gap-filling glue. Get some Techniglue epoxy from Carbatec; it will fill gaps and be very strong.

Rocker

[silentC]

First you need to make sure your layout lines are accurate. I use a marking knife and this gives you a locator for the chisel blade when you want to chop on the line.

For a through mortice you should chop about halfway through from one side and then flip it and chop from the other. I've only ever made a few by hand but what I did was to chop close to the line and then pare the walls to the line.

You can remove the bulk of the waste with a drill and then just clean up with the chisel.

Normally you would use a chisel the same width as the mortice (or in fact make the mortice the width of your chisel).

You can cut yourself a bit of scrap the same thickness as the tenon and use it as a test gauge as you cut to make sure the mortice is an even width.

I use a router :)

[s_m]

I was going to suggest a router or drill as well :B...

Steph

[SilentButDeadly]

I pre-drill all my mortices on the pedastal drill and then clean up with a chisel. I'd use the router but I haven't built the jig yet!!

[thumbsucker]

I have no router and no drill press, as i simple do not have the money nor the space. I used a marking knife and square to layout the joint.


The smaller one could be used for testing the mortice faces.

I thought this was the way to go. I have taken to sharpening my chisel really fine edge, and pairing the faces.

I make sure to keep the chisel as square to the mortise and not to twist. The legs are recycled Tallow wood. So it is not that hard, but I still need a little bit of a push.

Calling all Darksiders, and Gallot's.

[silentC]

Well, there's really nothing more to it. You have to make sure your layout lines are accurate. If you have a 2mm discrepancy, then there are only two things that could be at fault: either you started at the wrong point, or you went astray along the way (same with building a tunnel). The stock needs to be square. Your square needs to be square (many are not) - do you know how to test that?

The other thing I do is to angle the chisel away from 90 degrees so that I'm chopping a slight slope away from the finished surface in the top or bottom of the mortice, then pare it back and test, pare back and test until it is right. Until you're well practiced, I find it's better to sneak up on these things. It takes a bit longer but saves you from undercutting.

I don't know that it's crtical for the top and bottom of the mortice to be perfect, since you're really relying on the long-grain to long grain glue surface in the sides of the tenon/mortice, rather than the end grain to long grain in the top and bottom, but I like it to be a good tight fit so that it's not only relying on the glue to locate the rail.

But as we said yesterday, glue is not a great idea in a bench. You need strong, well-fitting mechanical joints.

[Kaiser Soze]

Hey SilentC

"Your square needs to be square (many are not) - do you know how to test that?"

I would like to know what you do please!

[Rookie]

You place the sqaure up against a bench edge that you know is straight. Draw a line along the square on the benchtop, then flip the square over and draw another line next to the first. The two lines should be parallel.

[AlexS]

Silent has pretty well nailed the way to set out and cut the mortices. Only things I would add are that first, you need to square a face side and face edge, and always work from these when setting out, ie, the stock of the square is always held against one of these.

Re checking your square; in a school workshop where I was teaching, more were out of square than were acceptable. Kaiser & others, the way to check a square is this.
Use a table or bench or piece of sheet material such as MDF with a straight edge. Hold the stock against the edge, and Scribe a line along the blade with a marking knife. Flip the square over, place the marking knife in the previous cut at the edge of the table, and scribe again. If the square is truly square, there should only be one line.

[thumbsucker]

you went astray along the way (same with building a tunnel)

I think that is what happened.


The other thing I do is to angle the chisel away from 90 degrees so that I'm chopping a slight slope away from the finished surface in the top or bottom of the mortice, then pare it back and test, pare back and test until it is right. Until you're well practiced, I find it's better to sneak up on these things. It takes a bit longer but saves you from undercutting.

I think that maybe the best approach, I have two more 6 more mortises to cut, I am sure I can get better with practice.

I just finished an hour in the shed and using my test tenon, a smaller square and and my pairing chisel, I have a tenon that now sits square in the mortise.


"Your square needs to be square (many are not) - do you know how to test that?"

See here for a guide on squaring a square. (http://hntgordon.com.au/checkingyoursquare.htm)