Cutting a tapered mortise

Need some guidance/suggestions on the easiest/best way to cut the taper on a thru mortise in a post (8 x 8) that will ultimately house a dovetail tenon on the end of a cross beam. I have tried to cut a few with a framing chisel but have not mastered that technique, and I am not happy with the results. I do have a chain mortiser but is it safe to cut a taper with that? Any set up tips or techniques would be appreciated. As always, thanks for your time and sharing your knowledge with a beginner.

One thing, when cutting a taper always cut from the narrower end toward the wider end on a sloped face. This way you are cutting across the grain and not into it. If you try to cut from big to little you will get a lot of tearout and it will resist you a whole lot

What brand of chain mortiser? The Makita will allow an angled cut. The cross cut types, however do not.

In using hand tools, I have used a boring machine set to the angle and bored the end hole at the angle. I have also used a hand rip saw to cut each side of the mortice and finished it with corner and regular chisels. I have also used the Stihl method.

Perform most of the work from the wide end of the mortice, try not to push the last bit out the other side. Have good crisp ends marked on the mortice ends. I work the narrow end first and only an inch or less into the mortice, to lessen the chance of flipping the face out. Use the framing square to ensure both ends of the slope are true. It should be as true as you can make it, for sure no humps in the slope. I use a 1-1/2" chisel on 2" mortices almost all the time now. It is much easier to push/pound.

Is the cross beam housed into the 8x8 post? If so use a haunched shoulder not a full square housing. A square housing makes for difficult assembly.

I like half dove tails. But it could be reasonable to just use a through tenon with a couple heavy pegs. Span and loads would impact this decision.

Are you using a wedge driven from the inside or outside? I drive them wicked hard from the outside and then put a fastener in the wedge, screw or nail. I don't want it to move after things dry.

DL, I think you have that backwards. Cut from the big to the little, down the slope, the hole has to get smaller.

My mistake, you are correct. Thanks for catching that!

But also, sometimes it isn't so straightforward as this either. The wood grain can sometimes do funky things which meakes cutting a little more complicated than all this. But in general yes, big to little. Sometimes my mind works backwards when I type things out!

guys, thanks for the suggestions. The chain mortiser I have is a makita 7104L. Any tips or safety concerns with the set up and cutting the taper with this tool?

"Is the cross beam housed into the 8x8 post? If so use a haunched shoulder not a full square housing" I'm not sure of the terminology here. My posts have a 1" shoulder cut into the inside face of the post that will support the bottom of the cross beam. So I think this is a haunched shoulder?

I do plan on using a wedge from the outside of the post...good tip to anchor it with a mechanical fastener.

Thanks again for your time.

Yes, Skip, it looks like the shoulder is haunched, as you describe it. If you reduced the whole housing to 7" you would have an issue.

When using the morticer, it has three different angles while boring, just stop the last bore on an angle. I have very little experience in using one of these but with the use I have I don't see any issues.

You can cut an angled mortise with a cross cut mortiser. If you use the guide that references the face parallel to the depth of the mortise then you take your first plunge on the angle of the taper... mark the angle to the offset of your guide. If the mortiser references the face perpendicular to the depth of the mortise then you can cut a wedge to plunge the tapered angle. It is similar to using a drill guide from your drops for a piece of threaded rod at a rafter to kingpin to rafter connection.

If the wood is green you can always put the wedge for the dovetail on the inside to then tighten after shrinkage. I prefer it from the outside, if any force pulls on it will pull the wedge into the joint. If you are enclosing the structure from the outside there is no need to nail the wedge... the enclosure should hold the wedge in place.

With the makita mortiser I would recommend using a wedge on the high side of your mortiser... don't just rely on the clamp to hold the mortiser in place. I like to cute a few triangles made from 3/4 MDF (what was on hand in the shop at the time).

This is one of the best tension joints that can be used in wood joinery in tension. It has been pegged in history... I don't see the need other than to hold it in place during the erection. The wedge should hold it in place for the rest of time after the erection.

William Denton IV

You can cut a angled mortise with a cross-grain mortiser, by putting a wedge of the same slope underneath. It's pretty easy.

Definitely do the diminished haunch as Tim mentioned.

Laying out slopes on a timber that's not perfectly square can get pretty weird. With that wedged half-lap you have three sloped surfaces--the top of the wedge, the bottom of the tenon, and the top of the housing. What I do is lay out everything on the primary reference face (where the wedge goes in) first. Then draw your slopes on the two side faces. Then connect where those three slopes hit the inner non-reference face (arris to arris). You can only square them across that face if the timber is perfectly square. If the timber is not square, then the those lines will angle across that face. This will be most obvious on the diminished haunch, because it's the steepest angle. If that's utterly confusing, I can draw something up in SU.

Hi Brad, I have only had my hands on a cross grain mortiser for a matter of minutes. Shims are great.

Layout on square timber that are not square begs for a new topic, some time in the future. Rafter feet get complicated, too.

I was feeling motivated, so I drew it up. Here's an 8x8, that is way of out square (it measure 9.5" on on corner) to exaggerate things. The tenon slopes 1.5" over 8", the wedge slopes 3/4" of 8", and it is haunched to 7".

[IMG]http://i167.photobucket.com/albums/u122/timberwrestler/Halfwedgeddovetail1.jpg[/IMG]

This shot describes things a little bit. The haunch would be cut out after the mortise is complete.

[IMG]http://i167.photobucket.com/albums/u122/timberwrestler/Halfwedgeddovetail2.jpg[/IMG]

And here's a shot straight on at the inside of the post showing those 3 angled surfaces:

[IMG]http://i167.photobucket.com/albums/u122/timberwrestler/Halfwedgeddovetail3.jpg[/IMG]

It's weird, but it works. Maybe I should make a video...

As a blanket statement, all of my out of square timber are diamond shaped, they don't change in size. I use a slightly different approach. The two opposite sides are parallel and I only drop down the next best face, for the most part, and connect lines on the opposite side of the next best face. This could change if I choose to make the side of the timber the best face, which I don't usually do.

Hello everyone tonight

Great topic

I have been following the conversation with interest because I had the opportunity of examining an early truss in a 1865 lutheran church's attic

This truss's bottom chord was 12" square hand hewn, and the bridge truss that supported half the weight of the spire had legs that were fastened to the chord with half dovetails held in place with wedges driven from the top

with shrinkage over the years the dovetails have moved upwards about .5" out of their seatings but still holding tight--great joint

NH