Chopping overlapping mortices without breakout

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Forum topic by DavidNorth posted 01-18-2013 09:16 AM 1206 views 0 times favorited 4 replies Add to Favorites Watch
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2 posts in 1923 days

01-18-2013 09:16 AM

Topic tags/keywords: chisel mortice marking gauage shop made tools

I’m in the process of building the Seaton Marking gauge form this article:

What I found to be a problem is, after chopping a nice mortice for the wedge according to the instructions, when I am chopping the arm mortice which encounters the other across part of its width, I end up with breakout because there’s material that isn’t supported due to this overlap.

I don’t think this is going to affect the function of this gauge, and won’t be visible on the outside of course, but I’m not too happy with having done a messy job of it on the inside. I’d do it again if I knew how to approach it better. Does any one have any tips?

Cheers, David

4 replies so far

View PaulsenbyHand's profile


12 posts in 1957 days

#1 posted 01-18-2013 12:32 PM

Hi David, it could be a solution to put in a temporary, snug fitting, wedge of some scrap wood (pine etc.) that way, the walls would be supported as you chop from the other direction. And using for example pine as the scrap wood, it would be a breeze to chop through.
I have made this gauge once and I love it.

-- David, Copenhagen, Denmark

View ChuckV's profile


3111 posts in 3495 days

#2 posted 01-18-2013 01:05 PM


I built a standing desk from Roy Underhill’s book “The Woodright’s Apprentice”. There are intersecting mortices in the legs. Roy gives this advice:

When chopping the intersecting mortices through the posts, make the first mortice only as deep as the intersection. That way, the chisel cutting the second mortice will not break out into thin air and splinter away the insides.

I have used this method on several projects and cut the mortices with a mortice machine and it works great.

I hope you can visualize what is meant. If you are making two mortices that are 1” deep and 1/4” wide, then make the first mortice only 3/4” (1” – 1/4”) deep. Then when you cut the second mortice to the full 1”, it will be in solid wood all the way down.

-- “Big man, pig man, ha ha, charade you are.” ― R. Waters

View DavidNorth's profile


2 posts in 1923 days

#3 posted 01-18-2013 06:59 PM

@PaulsenbyHand, that’s a great solution, seems obvious now I think about it.
@ChuckV, I wondered if something like this might work but the nature of the overlap where one mortice partially crosses the other at 90 degrees would make that tricky, but this seems like a great tip for the joinery in tables and the like though. I need to get that book, I’m currently working my way through all the old shows on the pop woodworking site.

View BigJimAK's profile


30 posts in 3258 days

#4 posted 02-08-2013 11:27 PM


Let me throw out a way to think of it that might help:

Think of the total excavated area as being 3 parts:

  • The volume only in the first mortise (A),
  • The volume only in the second mortise (B) and
  • the overlap©.

We normally think of doing it by:

First mortising to the depth that it excavates the “only first” mortise (A) + the overlap©,
then mortising the “only second area” (B), which leads to the end of the second cut being unsupported.

Instead we are going to mortise the “only the first” (A),
then mortising the “only the second area” (B) + the overlap©.

With two intersecting mortises to cut, you will need to decide which one is best being (A) and which (B) but the process remains the same.

Using this technique will permit you to have fully supported mortises for both cuts.

Jim in Alaska

-- Jim in Alaska

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