Through Mortice: Alignment and Tear-out

[GarethR]

I'm in the process of building Christian Becksvoort's Shaker Stool which requires the tenoned legs to sit in dados on the underside of the stool top. I've cut the dados and now need to cut the through mortice for each tenon. The instructions call for cutting the mortice's from both sides, however I'm wondering if I'm going to able to align the bottom and top of each mortice, especially given that the bottom is dadoed so I'd need to be fairly accurate when cutting from the top. Does anyone have suggestions on how to best handle this task? I was thinking of cutting the bulk of the waste out with a forstner bit from the underside and then approaching the edges slowly from the same side.

ShakerStool.jpg

Any suggestions appreciated.

Thanks,

Gareth

[elanjacobs]

Your drill-and-clean-up method sounds good to me. Clamping a square block to the piece on the side you're cutting from will give you a nice square guide for the chisel to keep you on track. Might be worth scoring across the grain with a knife on the other side so you don't get breakout from your chiselling.

Just had another thought; if the dado is deep enough, you might be able to run a flush cut router bit off it.

[Kuffy]

Your problem is the same that is suffered when cleaning out the waste from hand cut dovetails. Mark out your mortises on the top using a cutting guage set to the same distance from edge as the dado. This gives u a nice clean crisp line for which u can register your chisel into. Drill out the bulk waste and pair down from the top with chisels. It needs to be a good structural fit, but the only critical aestetic part is the top of the tenon/mortise so that needs to be very close to perfect.

[derekcohen]

Mark both sides. Drill out the centre. Score the perimeter with a knife (both sides). Pare half-way from each side, meeting at the centre.

Regards from Perth

Derek

[GarciaJ]

I'm in the process of building Christian Becksvoort's Shaker Stool which requires the tenoned legs to sit in dados on the underside of the stool top. I've cut the dados and now need to cut the through mortice for each tenon. The instructions call for cutting the mortice's from both sides, however I'm wondering if I'm going to able to align the bottom and top of each mortice, especially given that the bottom is dadoed so I'd need to be fairly accurate when cutting from the top. Does anyone have suggestions on how to best handle this task? I was thinking of cutting the bulk of the waste out with a forstner bit from the underside and then approaching the edges slowly from the same side.

ShakerStool.jpg

Any suggestions appreciated.

Thanks,

Gareth

Do you really HAVE to have through tenons? That stool would look really nice and be just as functional and strong with normal tenons.

Jorge

[GarethR]

Thanks all for your feedback.

Elan, each dado is ~3mm deep, I might be able to get a flush trim bit to work from the top down. I'd need to make sure I kept within the limits of the ends of the mortise either by eye (?) or using a hard guide on the underside.

Derek and Kuffy, the biggest issue with markup (for me) from the top is getting the edges aligned with the edges of the underside dado, I'm not overly confident they're going to meet well enough to be dead aligned with the dado edge.

Jorge, the through tenons are largely aesthetic. They have contrasting darker wedges towards either end of the tenons. You're right though, it's not essential to have through tenons so I might forego them for simplicity.

Thanks again all.

Gareth

[Lplates]

Thanks all for your feedback.

Elan, each dado is ~3mm deep, I might be able to get a flush trim bit to work from the top down. I'd need to make sure I kept within the limits of the ends of the mortise either by eye (?) or using a hard guide on the underside.

Derek and Kuffy, the biggest issue with markup (for me) from the top is getting the edges aligned with the edges of the underside dado, I'm not overly confident they're going to meet well enough to be dead aligned with the dado edge.

Jorge, the through tenons are largely aesthetic. They have contrasting darker wedges towards either end of the tenons. You're right though, it's not essential to have through tenons so I might forego them for simplicity.

Thanks again all.

Gareth

Hi Gareth,

I've done a similar design step stool with through tenons out of eastern red cedar (just had a single plank of the stuff). It was just a quick project to keep me occupied and try out some handwork techniques. I cut the mortises by hand relatively carefully. As you know though chisels have a tendency to push back due to the beveled cutting edge. IN any case if i had another go i would cut the tenons a bit smaller and then sneak up on the fit by shaving off small amounts until it fit. However since that stool was one of my first hand mortise attempts it was a bit rough. The way i got around this was to cut a groove into the tenon and then use contrasting wedges to push the tenon apart to make it fit more snug. The tenon was basically cut in half for it's whole length with the cut being approx 2-3mm wide. Then the wedges were carefully shaped using an inverted belt sander and tapped into place during assembly. It worked a treat.

If you use this approach the thing to keep in mind is that some soft woods can split if the wedges are too aggressive, so the fit has to be pretty close already. I had a similar experience with a three legged stool made from Oregon. IN any case this seems like a relatively small project so if you make a mistake you can always make another one, it's the best way to learn in my opinion. Measure things carefully and make the mortise cut a bit smaller, then you can remove small amounts carefully with a sharp chisel until it fits nicely.

Good Luck and please post some pictures when you're done.
Mat

[IanW]

Laying out your mortises in a straight, regular board like that stool top shouldn't be too hard to manage. The only way to get confidence is to do it. A few pointers:

Make sure your trysquare is accurate; easily tested by the scribe & flip method. If it isn't, chuck it now (unless it's a re-calibratable type) and get one that is. Using an inaccurate square is the quickest road to insanity, imo.

Use either a very sharp pencil or a scribing knife to mark out. The knife is more accurate, but you may not want to have layout marks on your work (or spend time planing them out). A common problem is using too thick a pencil, and allowing placing errors with the square to add up as you go around. Some recommend using a 0.5mm propelling pencil, but I prefer to just sharpen an ordinary HB for non-permanent marking.

If the surfaces of your board are parallel, extending your layout lines from the top to the bottom should ensure they coincide. I always extend the lines right around the piece I'm working on. If they meet up precisely, everything is fine, if they don't something is out of whack & needs to be identified & fixed.

If the ends of the top board are dead square (across as well as up & down), you could use a mortising or marking gauge to set out the edges of the mortises, but you'll need one with a decent beam and a wide fence, as the distance in from the edge is getting to the limit of most standard gauges, judging from the pic. To avoid having deep lines where you don't want them, measure out & mark the mortise edges (or preferably, scribe them from the pre-cut tenons on your legs) and stop the gauge short of these lines.

Concentrate on having the mortises matching the tenons neatly on the top side, where they will be most visible. It doesn't matter if the sides of the mortise are a bit wide, in fact you want them a bit wider at the top so the wedges will spread the tenons & tighten them securely. It only takes a gap of about 1.5mm or less, sloped off to about half the depth of the mortise, to lock them very firmly.

When chopping mortises, I always first deepen the scribe line with a firm tap on a very sharp chisel. Not too hard, because the bevel will drive the chisel slightly back into the wood, enlarging the mortise by a teeny fraction. Go ahead & remove the waste as suggested above, leaving about 0.5mm or slightly more wood inside those chisel lines. When you've broken through, carefully pare away that last little bit of wood back to the line, working from each side to the middle.

With good setting out, a sharp chisel, and a bit of care, you'll surprise yourself how easy it is...

Cheers,

[RoyG]

Pick your method choice from the many potential solutions mentioned above. Then grab a bit f scrap. Route the 3mm dado across the piece of scrap. The make a trial through mortice using whatever method you've chosen. If your chosen method doesn't work to your satisfaction, the do another for practice, or try a different method to see if you get a more acceptable result.

RoyG

[GarciaJ]

Jorge, the through tenons are largely aesthetic. They have contrasting darker wedges towards either end of the tenons. You're right though, it's not essential to have through tenons so I might forego them for simplicity.

I made a Morris style reclining chair with bent arm rests. The plans called for through tenons with a sort of pyramid shape protruding through the flat horizontal part at the front of the arm rests. I modified the design and used a normal mortise and tenon joint. see picture

Best move I've ever made ... where else was I going to rest my glass?

(7) detail bent arm rest.jpg

In your case, if you are going to use a nicely grained timber, the through tenon may actually ruin the look of the stool. Having said that, I am a strong believer that there is no such think as a bad decision... whatever you decide will be fine... Regarding your question: I would drill the required holes right through using a drill press and scrap on the bottom of the seat to eliminate the possibility of grain tearing out. You can then clean up the mortice using sharp chisels and a file, or sand paper.

Regards

Jorge

[GarethR]

Thanks everyone for your feedback and suggestions.

In the end I used a combination of marking and clearing out the waste with a chisel and router:

• marked out the tenons on the dado (underside) and transferred these measurements to the topside
• drilled out the bulk of the waste using a forstner bit from the underside just until the center-spur on the bit poked through, flipped and completed the hole from the topside.
• cleared down about 1/4-1/3 from the underside using a chisel leaving enough of an edge for a flush-trim router bit
• trimmed from the top down using the router
• cleared out the corners with a chisel

In all happy with the end result, here's one of the ends:

ThroughTenons_Side#1.jpg

And here's both ends dry assembled (unfinished, still some way to go):

LegsTop_Assembly1.jpg

Thanks again,

Gareth

[IanW]

Looks like a pretty tolerable result to me, Gareth!

Cheers,

[GarciaJ]

Thanks everyone for your feedback and suggestions.

In the end I used a combination of marking and clearing out the waste with a chisel and router:

• marked out the tenons on the dado (underside) and transferred these measurements to the topside
• drilled out the bulk of the waste using a forstner bit from the underside just until the center-spur on the bit poked through, flipped and completed the hole from the topside.
• cleared down about 1/4-1/3 from the underside using a chisel leaving enough of an edge for a flush-trim router bit
• trimmed from the top down using the router
• cleared out the corners with a chisel

In all happy with the end result, here's one of the ends:



And here's both ends dry assembled (unfinished, still some way to go):



Thanks again,

Gareth

Looks clean and very precisely done... I would be very proud of that!

Jorge