Tuning mortise and tenon joint to be flush

[Dave Rosner]

Hi all - I am building a table saw extension table and using it as an excuse to hone my M&T joint making skills. I did pretty well with the tenon widths and shoulders. All the joints fit nice and snug but easily installed with some hand pressure. My tops of my rails and posts are all aligned as well.

The problem is that on some of my joints one side of the joint is slightly proud of the other. In other words where I have a rail with a tenon connected to a table leg with a mortise there is a 1/16th inch offset between the two pieces on their plane - one piece is proud of the other. The gap is NOT between the joint - that is tight and I used a shoulder plane to adjust that.

So I guess I didn't have my jig set properly for edge location when I did the mortises.

Maybe im searching using the wrongs terms but I can't find anything that describes how to fix this problem. Lots of advice on adjusting shoulders and tenon width, etc. But nothing on how to fix when the faces of the pieces aren't flush.

Any advice?

[John Gulick]

Is your stock the same thickness? Are the mortises in the center of the workpiece? Is the tenon in the center of the workpiece?

As far as I can tell if the answers are yes,yes and yes the faces will flush out

[Andy Giddings]

Can't you just plane the protruding face(s) flush with the other parts of the frame? I wouldn't adjust your tenons or mortises as they are already a good fit and you'll end up chasing your tail

[John Gulick]

I like chasing my tail to get it correct

[John TenEyck]

Cut the offending tenon face until the stretcher and leg faces are flush. Then glue another piece of stock on the other side of the tenon and recut that face so it fits the mortise.

John

[Peter Aeschliman]

Two things:

1) Since you're doing this for practice, it's important to do a root cause analysis in order to learn. John's questions will send you down that path.
2) To fix what you have now, I would just hand plane a taper into the face of the piece sitting proud of the other. 1/16" shouldn't take that much work, and it's easy to sneak up on it. If you don't tend to use hand planes, then the upside is that you get to learn another new skill!

Have fun.

Peter

[Dave Rosner]

Thanks for the replies!

I used a router with an edge guide to create the mortises. I did my best but it was tricky work to get it perfectly set so I must have been slightly off. I also tried a method where I created a template out of mdf that was the right size and offset to use a bushing. Problem with that method is you have to glue on a guide and I must have been off on that one too.

A slight taper won't matter and I'm always looking for an excuse to use a hand plane. Plus if I trim the tennon then I need to mill a perfect piece for the other side and that feels tricky to get right without a lot of work.

[John TenEyck]

Thanks for the replies!

I used a router with an edge guide to create the mortises. I did my best but it was tricky work to get it perfectly set so I must have been slightly off. I also tried a method where I created a template out of mdf that was the right size and offset to use a bushing. Problem with that method is you have to glue on a guide and I must have been off on that one too.

A slight taper won't matter and I'm always looking for an excuse to use a hand plane. Plus if I trim the tennon then I need to mill a perfect piece for the other side and that feels tricky to get right without a lot of work.



No, glue on an piece thicker than you need and then recut and tune the tenon to fit your mortise.

John

[Doug Hepler]

Dave,

It sounds like you have a solution. I'll offer a comment regarding root causes and preventing this in the future. I have learned that I can't use an "industrial" approach when cutting M&T joints. The mortises often get cut slightly off one way or another, despite being cut with a jig as accurately as I can. I cut the mortises and then I mark out each tenon to fit, almost as if I were going to cut them by hand. So, I don't necessarily center the tenon on the work piece. I position the tenon to accommodate any variation in the actual position of the mortises. This will also take care of any variation in the thickness of the tenon stock.

Doug

[Bill Dufour]

You are looking at it wrong. Change the design. On much of what I have made I purposely make it so the tenon piece is below flush by 1/8". That way if it is off by 1/32" you can not see it.
I use this idea in table aprons, chair bases etc.
I actually use biscuits not tenons but the design is the same..
Bill D.

[Derek Cohen]

Scribe the shoulder line around the rail - it must meet exactly, which means that all is square.

Now pare away the waste with a chisel, working to to line, and finally in the line. This should result in a perfect tenon.

Regards from Perth

Derek

[John Gulick]

In our sash/door shop we set up our mortiser (either chain or chisel) first, making sure the mortise is "exactly" in the center of the work piece. Next we do the same at one of our PM tenoners. We always keep a short sample (6") or so for checking as we run additional stiles or rails. The mortiser will cut an accurate dimension and the tenon can be fit within a few thou to get the desired fit.

Every ten pieces we check with the go/no go gauge

[Charles Lent]

It's very important to mark the face side of each piece before you start, and then do all of your setups and measurements from that face side. It's also very easy to make tenons slightly off center and then get the piece installed with the back facing the front or inverted end for end.. Do all of your measurements and setups, even glue ups with the pieces facing the same way, also label the tenon ends to match the mortises, mortise A to match tenon A, mortise B to match tenon B, etc.

Charley

[John Gulick]

It's very important to mark the face side of each piece before you start, and then do all of your setups and measurements from that face side. It's also very easy to make tenons slightly off center and then get the piece installed with the back facing the front or inverted end for end.. Do all of your measurements and setups, even glue ups with the pieces facing the same way, also label the tenon ends to match the mortises, mortise A to match tenon A, mortise B to match tenon B, etc.

Charley

+1 Charley, the faces must be kept with the faces and each joint matched to it's mate and glued. No rocket science, just a little time to get things right

[Derek Cohen]

I see that my earlier reply mis-understood the question and replied about tenon shoulders being misaligned when it is the tenon cheeks which are the problem.

While a rule for all woodworking, one technique that is emphasised when using hand tools is that you always work from a reference side. It is unnecessary to centre mortices or tenon; all that is important is that all marking out (such as scribing with a gauge) is done from the reference side. All joints should go together if cut accurately.

To repair this off-set mortice and tenon, determine which is out-of-line with the referenced side. It could be either. If it is the mortice, then one could fill the mortice with a loose tenon, and re-make it. Or one could plane the one side of the tenon cheek to the correct position and add a shim to the other side. The former is a stronger joint than the latter, but (depending on how you make your joints) the latter could be easier to do.

Regards from Perth

Derek

[Pat Barry]

I don't think half the people responding are clear on your problem. Seems to me that you need to either cut the mortise a tad bit deeper or just cut off the end of the tenon enough for it to seat properly. Note: I'm not sure which half of the group I'm in.

[Mike Cutler]

Dave

Go back to your original jig to cut the mortises. If I were betting, I'd bet that it is 1/32nd off from centering the mortise. This could be caused by the jig being off, or the bit not centered in the bushing, or both. If you now cut the tenon, and are not paying attention to the orientation of that material, and you are using the mortise as a reference, the joint will be 1/16th off if you reverse the tenon material, and if not, you will find that you might be 1/32nd off. Measure the distance form the material face on both sides to the mortise wall, using a dial caliper, and make sure that they're exactly the same. Any deviation could be doubled when the tenon is cut.

To fix what you have now, I would make the tenon thinner so that both reference faces are in the same plane, and use a gap filling structural epoxy, like T-88, to assemble the joint(s).

MDF is a horrible material for router jigs. I know everyone uses it, and maybe they have access to a higher density MDF than I do, which is why it works for them, but for me, I gave up on MDF years ago. I use either Baltic Birch ply, or Lexan. MDF has a "ratty" edge which compresses under load, and after a while will "wear" along the reference edge. A guide bearing will make it happen even faster.

The M&T Joint is an easy joint in concept, but it's a very precise joint, which is why there are so many after market devices to make it. ;)

[Robert Engel]

Its very possible either the M or T are not centered.

Even if this is the case, (not very easy to explain) but the tuning technique is to present the face of the tenon to the outside of the board on the side to which it will contact. When the outside surface of the rail is flush with the side face of the mortise, then the rail and stile will be flush when glued.

In this case I would plug the mortise and re-route paying very careful attention to the shoulder thicknesses. If they are off center, route the slot accordingly.

If the router bit is limiting your ability to do this, then glue a shim onto the tenon and re-cut.

[Ted Reischl]

To diagnose your issue more information is needed.

A) Are both pieces identical in terms of thickness?

B) Is the tenon centered? How did you measure it?

If you are using a scale to measure things, good luck. Scales are subject to interpretation by the user.

A good way to cut mortises precisely is to use a router table. Horizontal works best but it can be done on a vertical too, just a little more fiddling around in regards to adjusting the cut depths for each pass.

Jigs are probably the worst way to do it IMHO. A jig may be perfect for one job, but if the pieces in the next job are slightly thicker/thinner a new round of adjusting is required. There are too many factors with jigs. Bushing diameter, stock diameter, fence in relation to workpiece, etc.

Dedicated mortising machines are better but it also takes some fiddling to get them set up to center the mortise.

These days I cut almost all my mortises on a CNC machine. I can easily locate the face of the fixed jaw in the vise. Then it is just a matter of simple math.

The most important thing with mortise and tenon joints no matter how you cut them is getting stock thickness consistent, without that you are just wasting your efforts.

Recently I made two bath vanities and wall cabinets that used over 300 mortise and tenons for frame and louvered doors. All of the joints were cut on the cnc, and they all fit precisely.

[Brian Holcombe]

I cut most things with handtools, but the method does not matter much since good practice is good practice. Good practice varies depending on the number of variables you're dealing with, such as the accuracy of the stock and the machines, the safest approach is to work from reference faces (typing from the outside, show face). I use a marking gauge to layout my tenons, then saw them.

For my current project, which is a kitchen, I needed to speed things along so my tenons are cut by bandsaw, I saw next to the line then move the fence and saw the opposing side on the opposing side of the line. I work in batches and so the tenons are more or less identical from one to the next. I can then set my router fence to that same outside reference face, adjusting the cutter so that it lines up with the tenon. At that point I can cut the mortises and they'll line up spot on.