Chopping Mortises out of Plumb

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Forum topic by Matt posted 02-18-2015 02:50 AM 836 views 0 times favorited 12 replies Add to Favorites Watch
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137 posts in 1303 days

02-18-2015 02:50 AM

i’m building a new frame for an old work table. It’s a basic frame, four legs each joined by a haunched mortise and tenon joint. If you’re familiar with “The Essential Woodworker,” by Robert Wearing, it’s the same style as the table described in that.

I’ve finished chopping the mortises for two legs and have noticed a problem: All four of my mortises are out of plumb, so now their respective tenons come out of the joint at an angle other than 90 degrees to the leg. I noticed this problem with the mortises on the workbench I built a few months ago, so this means I’m consistently doing something wrong.

I stand with my body parallel to the leg and the mortise I’m chopping, hold the chisel (a bevel edge, not a mortise chisel) with downward but relaxed pressure, and hit the chisel with my mallet with what I think is a straight-downward strike. When I hit the chisel, my mallet is roughly parallel to my body.

I’m guessing that my problem comes from either the chisel not being truly verticle to the wood, possibly because of how I’m strinking the chisel.

Any one have any ideas or pointers? I may experiment with some different stances to better see the angle of the chisel to the wood. That’s all I can guess would help right now.

12 replies so far

View knockknock's profile


332 posts in 1594 days

#1 posted 02-18-2015 06:17 AM

I usually line things up so that the mortise (and board) I am chopping is going straight away from me lengthwise. I also frequently sit down for a better sight angle. This makes it easier to see vertical, and I can usually use the length of the board as a reference. For clamping, I either use a side vice (small boards), or one of those twin screw wooden cabinet makers clamps clamped to the bench top (workmate in my case).

View waho6o9's profile


7119 posts in 1998 days

#2 posted 02-18-2015 02:14 PM

Clamp a block on the line and have the chisel follow the block.

View Tim's profile


3031 posts in 1382 days

#3 posted 02-18-2015 04:03 PM

What I’ve noticed when watching someone like Paul Sellers who can chop a plump mortise almost in his sleep is that every once in a while you see them set their chisel not quite plump and it’s really obvious on camera. It exaggerates the error. So if you have one you can set up to record you could use that to practice with until you can learn to chop plump. Another way would be to set up some mirrors so you can see it right away. Paul Also uses a block similar to what Waho posted for mortises that need to be perfect, but you need to eventually train yourself to chop more plumb by feel. You can do that with any of the above methods by trying them a little so you see what plumb feels like, then try again, check, rinse repeat. You can also check your mortise with a square by sticking the chisel in and putting the square up to it, but by the time you can do that you’re a ways in and may need to correct a lot. But at least that way you don’t finish a whole mortise out of plumb. And on a through mortise you can also go in half way and then go in half way from the other side. You reduce your error that way because it meets in the middle.

View DrDirt's profile


4136 posts in 3163 days

#4 posted 02-18-2015 04:19 PM

Like waho said
Some use a guide block which ensures the cuts are plumb

I don’t run into this because I cut with either a router, or a drill press, so am only cleaning up a ‘squarely’ drilled hole with a chisel.

-- 'Political correctness is fascism pretending to be manners' ~George Carlin

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Richard H

483 posts in 1101 days

#5 posted 02-18-2015 04:28 PM

Which dimension is coming out of of plumb? Is it the one you can adjust left to right or the one you adjust forward to backwards? I have a easy time with sighting the left to right one which I was taught to run down the length of the mortise. The Forward and back one ends up being the ends of the mortise which if you come in from both sides can curve out from the center of the mortise a bit and still not affect the strength much.

Or to put it another way.

Are you sighting down the length of the mortise or looking at it from the side?

Are you chopping from both sides of the boards towards the middle?

If you are having lot of issues try drilling out the center of it first and you might find it helps give the chisel somewhere to go and stay on line.

Hope this helps,

View Matt's profile


137 posts in 1303 days

#6 posted 02-18-2015 10:42 PM

Thanks everyone for the help.

Richard- The dimension coming out of plumb is the left-right one. For example, if I’m sighting down the length of the mortise and looking at the back of the chisel, the chisel is not straight up and down. Now that you mention it, I start from the left (with the mortise parallel to my shoulders and the length of my bench) and work about two thirds of the way through, then go from the right side.

I’ll try the block clamping method shown by Waho and Dr. Dirt, except I’ll clamp the block alongth the length of the mortise and register the side of my chisel against that.

View bobro's profile


308 posts in 731 days

#7 posted 02-18-2015 11:00 PM

People all over the world chopping mortises sit cross-legged, or squat on their haunches, or straddle a board, or have a knee or even a foot up on a low bench, holding the work down as they hunch over to chop. I have something of a funk hip, so I must vary my stance often, if I want to walk the next day.

From ancient Egyptian copper chisels which were shaped liked flattened standard screwdrivers, to the fish tail chisels still used in China, to modern chisels, all manner of chisels have been used.

There are heavy mallets, light mallets, hammers. Some guys smack like they’re killing a bison, some guys tap like they’re checking to hear if a melon is ripe, some do as as much carving and paring as hammering.

Given the myriad of possible ways to go about chopping a mortise, it does not seem likely to me that stance, tool, or technique per se is the problem.

I will bet that once you forget about how you’re standing, swinging the mallet, holding your chisel, and all that stuff, and concentrate on the mortise itself, you will be fine.

It’s like shooting- sure you take care of the weapon, your stance, breathing, and all that, but when you are engaged in the act of shooting, your concern is THE TARGET.

This is my opinion, hope it is helpful.

-- Lao Ma: You are so full of anger and hatred. Xena: Everybody's gotta be full of something.

View pintodeluxe's profile


4825 posts in 2234 days

#8 posted 02-18-2015 11:20 PM

Unless you are a hand-tool only type of woodworker, try a mortising machine. My first mortiser was a $100 benchtop version, but it did a fine job of cutting accurate mortises. There was once a time when mortisers were out of reach on a hobby budget, but they are widely available now.
Every mortise I cut by hand I have been disappointed with. Even the workpieces with accurate cuts were so beat up on the back side from hammering the chisel, that they were nearly unusable.

Good luck with the mortises.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View jdh122's profile


878 posts in 2238 days

#9 posted 02-18-2015 11:35 PM

It’s much easier to judge plumb in one direction than the other. By that I mean that if you hold your hand out in front of you, it’s easier to tell if your hand is tilted left or right than it is to tell if it’s tilted forward or back. When you stand perpendicular to the mortise you have to judge whether your chisel is tilted forward or back to get a plumb mortise. Try standing the other way.
When I chop mortises into round chair legs I set up two squares (or a square and a bevel square, if one angle is supposed to be other than 90) and use them basically like waho suggested.

-- Jeremy, in the Acadian forests

View djwong's profile


167 posts in 2640 days

#10 posted 02-19-2015 10:10 AM

I like to use 1-2-3 machinist blocks as paring guides for the endgrain cuts. They can be quite reasonably priced for the import ones. I rough mortise within my lines, and set the block for the final paring cuts. You can set the blocks easily by placing you chisel in your knife line, and bringing the block up to it. I use a small f-clamp to hold the block in place.

-- David W. Cupertino, CA

View Matt's profile


137 posts in 1303 days

#11 posted 02-19-2015 10:15 PM

I will bet that once you forget about how you re standing, swinging the mallet, holding your chisel, and all that stuff, and concentrate on the mortise itself, you will be fine.

It s like shooting- sure you take care of the weapon, your stance, breathing, and all that, but when you are engaged in the act of shooting, your concern is THE TARGET.

This is my opinion, hope it is helpful.
- bobro

It’s interesting that you mentioned that. When I was learning how to shoot trap, the man teaching me realized that I was completely overthinking the entire process and gave me some simple advice, like yours—“Don’t think.” It helped, and those words come back to me for all sorts of things now.

However, I haven’t really been overanalyzing my mortising til now; in fact, I had a pretty good groove going til I realized I was doing something to cause my mortises to not go straight down. Now I’m trying to figure out what the problem is so I can correct it.

It’s probably true that my chisel is consistently not straight as I chop, so I’ll try the blocks next time. Thanks again to everyone for the advice!

View waho6o9's profile


7119 posts in 1998 days

#12 posted 02-20-2015 12:28 AM

Maybe keep a stropping pad close to keep a keen edge
on your chisel.

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