I need to cut a 2x5 thru mortise through an 8x8 valley rafter for a spline. The mortise is at an angle (ie: the mortise is horizontal but the valley is at a 30 degree slope). Any tips on cutting this other than drilling and chiselling? I have a makita mortiser but I don't think it will help much on this.

Hi Dave,

I assume you mean to say that the through mortise is 30 degrees off of 90 degrees (right angle) correct?

If so, here's a suggestion. The idea is basically to create a Wedge Template that your chain mortiser can clamp to, ensuring the correct angle, and a secure surface for the mortiser to clamp to. To do this,

1. Take a scrap piece of 8x8 about 14 inches long and cut a 'normal' 2x5 mortise in the middle about 5 inches deep with your Makita.

2. Now cut it lengthwise at 30 degrees.

3. For a place to secure clamps, from the leftover piece (bottom) cross cut two pieces about 1.5 inches wide from the narrow end of the scrap. Glue these to the top (face) of the wedge at each end. Orient the small end of this piece opposite that of the small end of the wedge.

4. To use, clamp the wedge on your piece. Since the chain mortiser will now be offset at a 30 degree angle, it might be more comfortable (and safer) to raise one end, or lower one end of the timber. Then, clamp the mortiser onto the wedge and mortise as usual going as deep as you can. Turn the piece over, and come in from the other side. However, make the angle is correct. You may need to raise or lower the other side.

I really don't know if this will get you more than 50% through on each side, but it should get you enough that cleaning the rest out with drill bit and chisel is pretty easy.

Hope this helps,

-Steve

Disclaimer: This is just an idea. I haven't actually done this and may not work perfectly but it should get you close. Also, I'm not sure what potentially dangerous things can happen when mortising at an angle, and I'm sure that Makita doesn't recommend, endorse or warranty this operation. So, if something goes horribly wrong, please don't blame me.

Here's a scarey thought:

I saw a very accomplished timber framer do something very similar, FREEHAND, with a Mafel chain mortiser.

Joel

we were cutting angled mortices last winter and we screwed an angled block to the bottom of our makita... the angle was 2.5deg so the clamp still reached the timber. It worked fine, including the tilt function. I think Steve has a good idea, although freehanding the drill and chiseling may be faster if your chisel is sharp enough.

Don't know enough about the differences between the two machines to reccomend whether or not to perform the Mortise Wedge procedure above, but, I do know that if you drill a 4 x 4 with the right dill bit size, then wedge cut the wood to the correct angle, the wedge can be clampled to the valley and used as a guide to pre-drill the Mortise. It will eliminate a pile of clisel cleanup work. Also, it will allow you to drill from both sides, so the Mortise would be crisp edged.
Hope this helps.

The mortise is 60 degrees from perpendicular so I don't think 'wedging' the mortiser will work. I guess drilling and chiselling is my best option.