Making A Mortise and Tenon Joint With a Dedicated Mortiser and Tenoning Jig - by Paul Comi
If you're new to woodworking like me, you've probably read at least a couple articles on making mortise and tenon joints. The joints are a commonly used way to joint chair parts, table sides, carcase of traditional furniture and even flat panel wall treatments. Pocket hole joinery and biscuit cutting equipment has eliminated the use of this joinery to some extent, but for a very strong joint on heirloom furniture and outdoor projects, I know its going to come in handy as part of my woodworking arsenal of skills. The problem with articles I've read on this subject and even New Yankee Woodworking episodes where Norm makes the joints is that the critical aspect of how to layout the mortise and tenon parts tends to be skimmed over to focus on the actual woodworking. This will hopefully get you (and me) going making nice fitting basic mortise and tenon joints and give you confidence to start thinking about how to tackle variations on them.
When I bought my unisaw from Woodworker's Supply, I applied my 10% 1st time buyers discount to a Delta dedicated mortising machine. Frankly, I had seen Norm Abrams use one when making outdoor furniture and various traditional cabinetry projects and figured once I got more experienced at woodworking I'd appreciate having one. The truth is that until recently I didn't really know how to use it. After reading some articles and asking questions and a fair amount of trial and error, I began having a lot better luck at good fitting joints and I think the process steps I can share will get you going making good fitting joints instead of just giving up out of frustration.
A mortise is a cavity into which a tenon fits. Typically, a strong mortise and tenon joint comes from having a tenon that fits into a mortise that is one third the thickness of the material being used. For example, if I'm joining two boards that are 3/4" maple, both the mortise and the tenon will be 1/4". But, when using a dedicated mortising machine, the thickness of the joint you create is dependent on the chisels you have. You can't cut mortises smaller than the smallest chisel you have, but you can cut mortises in between two sizes of chisels by repositioning the machine's fence though you cut down on your repeatability by doing so and its less efficient that way. My mortiser is a Delta MM-300 which came with 4 chisels and since I'm using 3/4" material we're going to make a mortise and tenon joint using my smallest chisel which happens to be a 1/4" chisel.
The purpose of my article is to help make mortise and tenon joints using a dedicated mortiser and tenoning jig. If you're absolutely new to mortise and tenon joinery, read this excellent article I found online before proceeding. It covers traditional mortise and tenon joinery without the consideration of a fixed chisel size mortiser.
*Before you begin, make sure you're working with stock that is flat, square ends and has consistent thickness etc or you may find that when you put your joint together you end up with slight gaps. For this article, I am using 3/4" maple scraps and I am using my smallest chisel bit which is 1/4" and should be one third the size of my material.
PREPARATION:
Unless you set up your mortising machine to place chisel holes directly in the center of your stock, you will spend extra time later making the actual joint pieces because your tenons won't be centered either. That means you will have to set up your equipment all along the way for multiple cuts.
STEP 1. SET UP THE MORTISING MACHINE TO CHISEL HOLES DIRECTLY IN THE MIDDLE OF YOUR STOCK
If you've ever tried to find dead center of something and align your mortising machine you know that it can be a challenge. Fortunately, users at Woodnet Forums came through again with some ideas to overcome this challenge. After trying various techniques, here's the one I got from rfeeser that I found works the best.
1. Position your chisel bit as close to center as possible and
lock the fence adjustment. Chisel a hole. 
2. Rotate the stock and then chisel drill another hole in the same position. Unless you were really lucky, you'll find that the first hole was slightly different than your second one. Note that in this picture, the chisel bit is sitting directly on the material so don't be thrown off by the shadow. The offset you see is the amount off center the bit is once the stock was rotated 180 degrees.
3. With the mortising machine turned off, re-adjust the machine's fence so that the chisel bit is centered in the hole created. Look closely at the hole created by the chisel bit and you will see there is a small amount of space on both the front and back side of the bit.
4. Drill a new hole. Rotate the stock, and with the machine turned off, see if the chisel bit fits into the hole. If it doesn't run through steps 1-4 again until you're satisfied. *In this picture, the chisel slides straight into the hole without rubbing any of the sides for a correctly centered fit.
5. Now that you have found dead center, cut your piece off at the bottom of the chiseled hole you created and label this piece of stock. I cut it using my miter saw, but you can use a miter gauge and your table saw. Whatever you use, it works best to have a square cut. This set up piece will be used to set up your tenoning jig as well, so you want to protect it and make sure you don't throw it away. This picture shows the piece after being cut at the bottom of the hole:
STEP 2. SET UP THE MORTISING MACHINE TO DRILL TO THE DESIRED DEPTH
The mortising machine I have has a steel bar to control the depth of chiseling. I find that placing a piece of the stock next to the chisel blade speeds up the process. Just lower the chisel to the desired height and lock the stop. Consider using a depth that will make future layout of your actual project easier to manage. When you actually build a project using mortise and tenon joinery, you'll need to compensate for the length that the tenon will be buried into a mortise, so if you use parts that are 10" long and will have 1" tenons at either end, the actual length once assembled will end up being 8". If you use an arbitrary mortise depth, it may make planning your project parts more complicated later.
STEP 3. USE THE SET UP PIECE TO SET UP YOUR MARKING GAUGE
Now that you have both the mortising machine set up and the set up piece from that set up, you're able to efficiently and accurately layout and mark your pieces. If you're making a mortise and tenon joint that will have shoulders on all four sides of the tenon like this example, once you have your marking gauge set for the proper thickness the layout is very easy because the marking gauge setting remains the same for all marks. Here I am adjusting the marking gauge to the chiseled test piece before I trimmed the bottom of the set up piece.
If you read the article I referenced at the start, you'll no doubt know that the author extols the accuracy and precision of the Tite-Mark marking gauge. Unlike the cheap Shopfox one I'm using in this article which has a rather rough cutting/marking blade that is hard to place where you want it, the Tite-Mark has a micro adjustment feature and the marking blade is very sharp and easy to reference. I ordered mine and am waiting for it to arrive. I found it at Fine Tool Journal online where they also sell Lie-Nielsen planes and tools at 10% off with free shipping. I'm not affiliated with the site-I just think Lie-Nielsen are excellent tools and you can't beat FTJ's pricing elsewhere.
STEP 4. SETTING UP THE TENONING JIG
The last step in setting up your equipment for this joinery is to adjust the tenoning jig. Again, using the initial set up piece will produce accurate proper fitting tenons with stock that will also result in flush mortise and tenon faces. Personally, I do not like to do a lot of sanding so this preparation is is totally worthwhile and saves a lot of time later when actually making the parts.
The idea is to position the saw blade on the table saw to remove material right next to the edge of the chisel hole without actually cutting into the chisel hole area. Lock the set up piece in the tenoning jig, loosen both adjusting knob locks, gently pull the jig to the right to eliminate the play in the jig when its loose, and carefully adjust the lateral movement knob until the saw blade is nearly on the chisel hole edge like this:
I found out by trial and error that my Delta tenoning jig can come out of alignment after adjusting when I lock the controls unless I take out the play in the jig by pulling gently to the right during the adjustment phase. Here's how I hold it to take the slop out of it
Once set up properly, you will be able to run the stock through the blade, rotate it 180 degrees and run it through a second time and end up with full size grooves on both sides of the set up chisel mark like this:
When setting up the tenoning jig, intentionally keep the blade low so that you can clearly see the difference between the table saw cuts and the mortise chisel marks. Congratulations! If you've gotten this far, you're ready to start making some precise fitting mortise and tenon joints.
PRODUCING A MORTISE AND TENON JOINT WITH FOUR EQUALLY SIZED SHOULDERS
Step 1. MARK YOUR PIECES
With your marking gauge already set up, score lengthwise and crosswise lines across the end grain of the stock that will become your tenon material.
Step 2. Transfer the narrow scribed lines from your tenon piece to a piece to be mortised. I guess you could do this freehand, but I prefer to clamp the two pieces together in my woodworking vise. Then, using a try square or combination square, transfer the lines onto the piece using a marking knife or a utility knife like this:
By transferring these two lines, you are establishing the overall width of the area to be mortised.
*There is no need to layout the lengthwise lines since the mortising chisel in this case is the proper thickness for the material; however, you would need to if the chisel was narrower than the area to be mortised.
For the sake of this article, I am not marking the length of the tenons on the pieces, but depending on the joint you are making it could make things easier for you.
Step 2. Chiseling the mortise
Since you have your equipment already set up and lines to indicate where the mortise will start and end, cutting the actual mortise is easy to do. To prevent missing either of the layout lines, I chisel both starting and ending area of the mortise and then remove the material in between. It works best to chisel only half of a hole at a time because its easier on the tool it seems to produce cleaner holes for me.
Here's the finished mortise:
Step 3. Make the cheek cuts on the tenon piece
I guess you could just as easily cut the shoulders first and then make the shoulder cuts, but in my limited experience, I find that making the shoulder cuts first I end up with a finished clean line around the tenon piece. All that is left to do is remove the waste. Since the shoulders of the tenons we're doing are all the same thickness, the table saw blade will only require one height setting for all four sides. To set the blade, I use a combination square to transfer the thickness of the shoulder.
Next, I adjust the saw blade height
Again using a combination square I determine the length of the tenon by determining how deep the mortise is. Had we set the mortising machine to a depth of 1" or something simple like that, see how this step wouldn't be needed?
Using a stop block to prevent a potential kick back, I set the saw for the length of the tenon. The set up here is between the stop block and the far LEFT edge of the saw blade. If you look closely, this picture is not correct and I had to trim my tenon because it was 1/8" too thick which is exactly the thickness of my saw blade.
With the proper set up, making the cheek cuts is a breeze. Use a miter gauge and rotate to cut each of the four equal sides.
Step 4. Make the shoulder cuts
Since the tenoning jig has already been adjusted and set up, the only thing you'll need to do is raise your saw blade to cut all the way through the material. Then, just clamp your tenon piece upright in the jig and slide each lengthwise side through.
This will leave you with a nearly complete tenon piece.
The only thing left to do is remove the remaining waste. Now, where's that 17" Grizzly bandsaw I ordered in November? A bandsaw is the accurate and easy way to trim the remaining parts off your tenons. Instead I had to use the Christmas present my wife gave me-a LN dovetail saw guided by the edge of my thumbnail to start the cut.
Since I used the saw, my tenon was a bit tight so a few light strokes with a file took care of that. If you reach for a file, make sure that you take an equal amount off each side of the tenon or you will knock your joint out of alignment and the faces won't be flush when you put the pieces together.
The tenon slips into the mortise without binding. So far so good...
Ah, flush fitting parts. Now, that wasn't so bad was it!
I realize that I've only covered the most basic aspects of mortise and tenon joinery, but armed with the essentials about proper set up and techniques, you and I should be able to start making some strong, long lasting furniture in no time... Be safe