Hand cut mortise tolerance

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Forum topic by aerialcopper posted 08-08-2019 08:06 PM 549 views 0 times favorited 5 replies Add to Favorites Watch
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26 posts in 820 days

08-08-2019 08:06 PM

Topic tags/keywords: mortise chisel chiseled through mortise hand cut mortise mortise walls

When cutting mortises by hand, what profile is desired on the internal faces? Specifically I’m wondering about through-mortises, where we want a tight fit on the tenon at the shoulder and the showing face. Does a skilled jointer strive for mortise walls that look like they’ve been planed? Or will they bulge the walls outward half a millimeter in interest of efficiency? Is it instead desirable to bulge the end grain walls inwards for a press-fit, like on a hammer head?

Just looking for some reference. Books and forum posts on the subject welcome. Thanks!

-- Dylan-

5 replies so far

View Bill_Steele's profile


714 posts in 2539 days

#1 posted 08-08-2019 08:47 PM

I would think that they should fit together smoothly with no slop—perhaps a “piston fit”. I have seen where you can cut the tenon a tiny bit thicker than the mortise and then compress the wood fibers of the tenon using a hammer or a vise. The wood fibers will expand and “lock” the tenon in the mortise.

If you plan to use glue make sure the joint is NOT so tight that it is starved of glue when assembled. I don’t think the tenon should be press-fit. You can hold the tenon with wedges or maybe a tusk.

With a through tenon, I think you only see the tenon extending beyond the mortise. You don’t see the mortise walls.

To make the shoulders fit tight you can undercut them if necessary.

View Mark's profile


1046 posts in 2782 days

#2 posted 08-08-2019 09:12 PM

Ive only done a few through tenons, but I’ve always tried to make them a clean snug fit, especially on the out side. remember to clean your glue from the visible portion of the mortice. It not a bad idea to add a wooden peg in the leg to pin the mortice and tenon together.

-- Mark

View bandit571's profile


26167 posts in 3490 days

#3 posted 08-08-2019 09:49 PM

Usually the thickness of a dollar bill. Inside the mortise does NOT need to be polished smooth, a little roughness inside is better grips for the glue.

Some will “flare” the narrow ends of a mortise, cut a couple kerfs in the end of the tenon….add a small wedge in the kerf, glue optional, and drive the tenon home. tenon flares out like a dovetail…that you can not pull back out.

-- A Planer? I'M the planer, this is what I use

View aerialcopper's profile


26 posts in 820 days

#4 posted 08-09-2019 03:36 AM

Thanks all for suggestions, sounds like a crisp mortise with flat walls is the goal.

I’m currently struggling with the final paring of end grain mortise walls. My chisel pulls into the wood (hard maple) no matter how much I fight it- I’ll mess with sharpening, method, chisel width. Intuition says a sharp enough chisel with a light enough cut cannot fail.

Thanks again!

-- Dylan-

View MPython's profile


280 posts in 619 days

#5 posted 08-09-2019 03:51 AM

Start chiseling the mortise in the center and work toward both ends. The finals cuts should be light paring cuts on each end of the mortise. As you said, a light cut with a sharp chisel shouldn’t deflect your chisel noticeably. If you start chiseling from the ends, you will almost certainly have deflection issues.

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