mortice blow out

[Eric Rathhaus]

I've attach a crude drawing to show my disaster. I was chopping a thru mortice across the grain in mahogany. The board is a full 3/4 thickness and the first wall of the mortice is about 1.5" from the end. As I made my way across, riding the bevel, I noticed a piece of the wood on the end side of the mortice bulging out at an angle from two splits. I'm pretty sure I didn't twist or rotate the chisel nor was I lever up the waste. I'm wondering what I did wrong to cause the splitting?mortice blow out.pdfmortice blow out.pdf

[glenn bradley]

Here I am chopping a square hole for a Greene and Greene style plug but the clamp being used for reinforcement could be a help.

GnG Low CoD (175).jpg

[Luke Dupont]

By blow out, you mean splitting?

Were you chopping with or across the grain?

[Tom M King]

I didn't see the pictures. I start in the middle, and work towards the ends. The cuts close to the ends are paring cuts with no levering.

[Jim Koepke]

The .pdf wasn't clear for me.

The conventional wisdom is to have spare wood well beyond a mortise when cutting and then remove the wood after the mortise is cut.

Some woods cut fine with out the spare beyond the mortise, some don't.

jtk

[Andrew More]

Maybe nothing? Could be the checking/crack was there originally and the chopping just loosen the wood holding it in place.

[Eric Rathhaus]

I'll try attaching here. So it didn't blow out passed the end of the mortice. It split in two place along the wall of one of the sides that's parallel to one of the ends of the board. mortice blow out.pdf

[Eric Rathhaus]

Okay in my defense I did say the drawing was crude. I was attempting to show the positioning of the mortice, which runs across the width of the board about 1.5 inches of the end. The grain is running along the length of the board. Two splits appeared starting on one side of the wall of the mortice and running to the end of the board so that the piece bulged out my the end. So I should clamp a backer on the end to prevent the splitting?

[John Kananis]

Adding a backer helps but Jim gave sound advice above. Leave your work pieces long and cut to length after the mortise is chopped.

[Luke Dupont]

Okay in my defense I did say the drawing was crude. I was attempting to show the positioning of the mortice, which runs across the width of the board about 1.5 inches of the end. The grain is running along the length of the board. Two splits appeared starting on one side of the wall of the mortice and running to the end of the board so that the piece bulged out my the end. So I should clamp a backer on the end to prevent the splitting?

When you held the chisel to chop the mortise, was the chisel severing across the fibers, or splitting between them?

Usually mortises are oriented such that the length of the mortise corresponds to the length of the piece, and therefore the chisel is oriented in such a way that it is severing the fibers. But it sounds like you were cutting a mortise in which you might have actually oriented the chisel the wrong way (assuming you just used, say, a 1/2" chisel to cut a 1/2" mortise rather than using the drill+pare method).

If you attempt to chop such that the chisel is not severing across the grain, but rather splitting with the grain, you will run a high risk of, well, splitting the wood like you appear to have done. A chisel acts as a wedge in this case.

[Prashun Patel]

I still don't understand the picture.

Chopping and paring are riving cuts that are intended to run out to the center. You have to create relief and work thin and buttress the weak side in order to accomplish that.

Wood wants to split with the grain.
Wood is encouraged to split along its grain if you place a blade parallel to the grain.
Wood is encouraged to split if the blade is close to the edge.


So, when your mortise is close to the edge, clamp the board across its grain, and chop with the blade perpendicular to the direction of the grain as much as possible (make parallel, paring cuts as thin as possible).

It's easy to want to chop parallel to the grain because the chisel goes in so easily. But IMHO this is exactly the opposite of what you want to with a mortise chisel.

The mortise chisel has that heavy bevel to provide relief for the wood to fall away from the wall of the cut. It's critical when chopping these edge mortises to create that relief from the center, out. Not doing so creates pressure to push in the other directions (either blowing out the edge, or driving a split down the wood).

[Eric Rathhaus]

Hi thanks for the replies and sorry I wasn't clear. This is what I'm trying to do. I'm making a meditation bench. I want to attach the legs with M&T joints. The outer walls of the mortices begins about 1.5 inches in from each end of the bench. The grains runs parallel to the length of the bench so that the mortice is perpendicular to the length and grain. While chopping the mortice the wood split along the grain from the outer wall of the mortice through the end of the bench so that a piece was pushed out.

Luke - I don't see how I could use a chisel parallel to length of the board. Are u suggestion I can't chop a mortice that runs across the grain?

John - I'm not sure how leaving more on the end of the board would have helped unless I left an extra foot on each end, which seems like a great waste of material.

Would making stub tenons instead of thru tenons help? Could I pin the stub tenons?

[Prashun Patel]

I see what your dilemma is. The long side of the mortise runs perpendicular to the grain I think. This would make it seem intuitive that you should chop with the blade parallel to the grain. If yes, then this is what is causing the split. Close enough to the end OR the side would create force enough to split to the end. In this case, I would clamp across the width of the bench when chopping. I would also in this case probably bore out the center with a drill then chop back to the base lines (chisel perpendicular to the grain) and finally pare.

If you wedge the tenon, it (they) should run parallel to the width of the bench (perpendicular to the grain)

[Luke Dupont]

Hi thanks for the replies and sorry I wasn't clear. This is what I'm trying to do. I'm making a meditation bench. I want to attach the legs with M&T joints. The outer walls of the mortices begins about 1.5 inches in from each end of the bench. The grains runs parallel to the length of the bench so that the mortice is perpendicular to the length and grain. While chopping the mortice the wood split along the grain from the outer wall of the mortice through the end of the bench so that a piece was pushed out.

Luke - I don't see how I could use a chisel parallel to length of the board. Are u suggestion I can't chop a mortice that runs across the grain?

John - I'm not sure how leaving more on the end of the board would have helped unless I left an extra foot on each end, which seems like a great waste of material.

Would making stub tenons instead of thru tenons help? Could I pin the stub tenons?

Correct, you can't chop a mortise (in the conventional manner) which runs across the grain (by which I mean the length of the mortise runs across the grain).

You'll need to drill and pare out the waste and use a wide chisel to pare down the walls, such that you're cutting across the grain.

Take any piece of wood, and a chisel, and try chopping near the end of the board with the chisel oriented in either direction (splitting, or cross cutting, the grain) and you'll quickly understand. The chisel is a wedge, and in the wrong orientation, it will split the wood just like an axe would.

[Warren Mickley]

Correct, you can't chop a mortise (in the conventional manner) which runs across the grain (by which I mean the length of the mortise runs across the grain).

You'll need to drill and pare out the waste and use a wide chisel to pare down the walls, such that you're cutting across the grain.

Take any piece of wood, and a chisel, and try chopping near the end of the board with the chisel oriented in either direction (splitting, or cross cutting, the grain) and you'll quickly understand. The chisel is a wedge, and in the wrong orientation, it will split the wood just like an axe would.

Mortise chisels were not designed to work across the grain. All of our joinery is designed with reference to wood being weak in one direction, strong in another. Even if you had completed the joinery the way you imagined, leverage on the foot could break out the mortise.

A traditional design would have the foot at least four or eight inches from the end. There are usually twin tenons about 7/8X 1 1/2 or 2, through tenons wedged. Additionally there is a skirt on one or both sides or some kind of bracing to keep the foot from racking.