slot mortiser

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Loose Tenons

I’m a big fan of loose tenons.  Because I have a slot mortiser, they are usually quicker to create than traditional tenons.  I still use traditional tenons when the situation calls for it (haunched tenons, for example), but my first choice is loose tenons if everything else is equal.

In traditional tenon joinery, the mortise is typically made a little longer than the tenon is wide.  The terminology gets a little confusing here, since mortise length is the same as tenon width (and mortise width = tenon thickness, and mortise depth = tenon length).  The “slop” in traditional mortise-and-tenon joinery allows for wiggle room to get the aprons aligned with the tops of the legs.

Of course, the width of the mortise and the thickness of the tenon have to match, but I’ll get to that in a minute.

Here’s a photo of the mortise in the apron.  I have marked out the ends of the mortise using a combination square, referenced off the top edge of the apron, as described here.  Note that I have carefully cut the mortise right to the layout lines.

On the legs, things are a bit different.  The layout lines are in the same relative locations, but this time I cut about 1/32″ past the lines.  Why?  Because my tenons, which will be glued into the aprons in the next step, are cut to fit the apron mortises with no gaps, ensuring their precise location.  The 1/32″ overcut on the legs ensures that I will have the wiggle room I need later to align the aprons with the leg tops.

If both apron and leg mortises were overcut, the tenon may end up in the wrong location on the apron and not give me the adjustment necessary for alignment.  Here’s the leg mortise below.

And here’s a photo of the tenon in the apron:

The thickness of the tenon is, of course, very important to the strength of the joint.  A few thousandths thick or thin can be an issue.  What makes this tricky is that the mortiser bit cuts differently in end grain than it does in face grain.  The face grain mortise is almost always slightly larger than the end grain mortise.

To accommodate the difference, I mill the tenon material thickness to fit the face grain mortise (in the leg).  I aim for a snug fit, but not so snug that it won’t go together with glue.  I’ll surface the stock in my planer and use a handplane to fine tune the fit if necessary.  I then cut the tenon stock to final width, round the edges on my router table with a quarter-round bit, and cut to length on the table saw.  They are too thick, however, to fit correctly in the aprons.  I fix that by sanding half the face of the tenon on a MDF board that has a half sheet of 120-grit sandpaper glued to it (a handy thing to have around).  I also chamfer both ends of the tenon on the sandpaper.  The tenon now fits nicely in the apron and can be glued in place.  I know this sounds like a lot of steps, but it goes very quickly once you figure it out.

Here’s a photo of the setup for cutting the tenons to length.

This is a handy setup for cutting small parts to length.  It’s not safe to use a stop in this situation, as the cut parts would be trapped between the stop and the blade.  Instead, the length is indicated by the end of the 1/4″ MDF.  I just position the end of the MDF relative to the blade, clamp it in place, and locate the tenon material by eye.  If I want more accuracy, I can hold a temporary stop in place against the MDF end, locate the stock, remove the stop, and make the cut.  Easy-peasy.

Here’s a photo of all the apron and leg parts.  The #7 parts are the end aprons, which at the time had not been cut to length.  Those lengths could not be determined until the side aprons were located on the center apron assembly.

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I’m back at work on the dining table.  I didn’t get much work done during the fall semester, and even less blogging.  It’s time to get caught up on the blog and back to work on the table.

The outer apron assembly, which includes the legs, is guided along the inner apron assembly by maple guides.  The guides, about 5/8″ square, are attached to the inner face of the outer aprons.  Grooves routed along the outside of the inner aprons locate the guides.  If that’s clear as mud, perhaps these drawings will clear things up.

I was concerned that the outer apron assemblies might bind when being pulled out, so I added center guides to minimize this.  The center guides also act as stops to prevent the outer aprons from getting pulled off the table.  The stop feature will be accomplished by attaching a brass plate on the inner end of the guide.  If you click the upper picture you can just make out the stop in the enlarged pic.

Due to the relatively small size of the closed table and the width of the leaf, which has to store between the center stretchers, there wasn’t much room for everything to fit.  Some careful planning was required.  The end of the center guide, when the table is closed, barely clears the center stretchers.  When open, the stop actually moves past the inner face of the inner apron.  To make this work, I needed to rout a mortise into the inner apron surface.  A detail is shown below.

The depth of the mortise was determined using my SketchUp model.  When the table is closed, there is 1/4″ clearance between the stop and the outside face of the center stretchers.  When the table is open, there is 1/4″ clearance between the edges of the leaf and two halves of the top (not shown in the pics above).  Because of the tight clearances, I was very careful about getting the model right.

Cutting the mortise was straightforward, but I wanted to make sure both portions of the mortise were concentric.  I carefully laid out the cuts and then clamped the apron into my slot mortiser.  I used a 3/8″ bit to rout the through portion, then changed to a 3/4″ bit to rout the shallow part.  I set the stops on the mortiser to maintain a consistent length.  Here’s the setup for the second cut.

The remainder of the material was removed using a sled on the table saw.

I started out this process thinking I would use a hand-held router and guide bushings to cut the mortise.  After starting down that path, it quickly became apparent that it was far more complicated than necessary.  The slot mortiser was by far the better solution.

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The mortise and tenon joint could probably be considered the cornerstone of furniture making.  The dovetail is certainly prettier, but not nearly as common or helpful.  I can make tables, chairs, and lots of other pieces and never even think about a dovetail.

My preference for making a mortise and tenon joint is to use loose tenons.  I find them much easier and quicker to create, and the shoulders are always coplanar.  You have to make at least one mortise anyway; why not make two while your at it?  Then make the tenon using your planer, allowing you to sneak up on the fit.

I didn’t come to this decision without a lot of head-scratching.  I made lots of M&T’s before getting here, and have taught lots of students how to make them.  My thoughts on some other methods:

Mortise:  Hand chopped – Neanderthals only.  Drill press mortise, cleaned up with a chisel – too much work, but the simplest method in terms of machinery used (unless you don’t have a drill press).  Router jig mortise – not a bad choice if you have a decent jig, and a decent jig isn’t hard to make.  Hollow chisel mortiser – I have always had a philosophical objection to the “cutting” action of this tool, although I know they work well with a sharp chisel and bit.  Slot mortiser – the best method, but you have to own a slot mortiser, and the simplest ones are over $600.  Dedicated joinery machine (Leigh FMT, WoodRat, etc.) – not a bad way to go, but can be complex and pricey.

Tenon:  By hand – see previous paragraph.  Table saw/dado set/sled – works okay, but I don’t like the surface left behind by the dado set.  Table saw/tenon jig – works well, but multiple setups involved (although this is my second choice).  Slot mortiser – difficult to set up most of them to do this well.  Dedicated joinery machine – see previous paragraph.

And, let’s not forget about the square-end/round-end mortise/tenon issue.  I don’t typically make through mortises for Arts and Crafts style furniture, so this isn’t an issue for me.  Round-ended tenons work just fine; no one sees them anyway (not that I’m trying to hide anything…)

All that to say I like loose tenons.  Which means I own a slot mortiser.  Can’t do loose tenons easily without it.  Mine is an old Inca machine, originally intended to mount on the back side of an Inca model 259 (I think) tilting-table table saw.  That little saw was rock steady, with arbor bearings bigger than my Powermatic 66.  The problem with the mortiser attachment was the bit speed; it ran at the same speed as the saw blade, about 3400 rpm.  Very slow by router bit standards.  I purchased the trick Clico bits, which were supposed to work well at that speed, but the whole process was too slow…

Until I built a new jig around the mortiser that allowed me to use my router:

This thing rocks!  It cuts mortises accurately, quickly. and cleanly.  Messy and loud, but I can live with that.  I made the fixture of MDF, thinking it was a prototype and I’d make the “real thing” once I got the bugs worked out of it.  That was about six years ago…

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