Thread: Mortise Chisel Angle Question

Some say 30 deg some say 25 deg, mine are 25

I have found 25 to be too fragile for mortising. Cuts great for the first two hits then needs resharpening. I use 30 ish

I use 30-ish, I find 25 is more prone to bending over.

works well for me.

A mortice chisels needs an edge that is around 35 degrees. 30 degrees is right for a bench chisel that will be hit with a hammer. A mortice chisel tends to take thicker slices and also be expected to prize out chips.

Oval bolstered mortice chisels have a 20 degree primary bevel and a 35 degree secondary.

Japanese mortice chisels tend to have a 30-35 degree single bevel.

Note also that these mortice chisel bevels are never hollow ground.

Regards from Perth

Derek

Agree with derek, though I have hollow ground some mortise chisels before with no ill effect. I have only done it when setting them the first time if they come to me in a state of semi disrepair and need a drastic change.

Important thought on the bevel is whether or not you're going to ride it. If you are, and you want to do so accurately, a single bevel is probably a good idea. If the edge doesn't hold up, you can put the tiniest microbevel, rounded, on a more blunt single bevel without losing the ability to ride the bevel (tiny means a small fraction of a mm, a few strokes of a finish stone, etc, and no more.) the mini micro bevel is fairly good policy on any chisel that isn't holding an edge well, rather than steepening a larger bevel and increasing the resistance/wedging.

I have mine at around 30 degrees and they work great. I wouldn't change a thing about mine (Lie Nielsen brand)

I understand the reason for the line of thought that hollow grinding is bad. Hollow grinding reduces the amount of material supporting the edge, and it also reduces the amount of torque that the chisel can take, and arguably the main thing that a mortise chisel is made for which others are not is prying out wood.

Nonetheless, I disagree that hollow grinding should be avoided. Whether or not it was avoided historically... I can't really comment. But my thought is that if you're prying with a mortise chisel so hard that you break off the tip of the chisel, then you were prying too hard anyway. Same as if you're beating your chisels so hard it's breaking the handles (guilty). The solution is not to call up your friends in Buloke country to buy harder wood. The solution is to hit them less hard.

I vote for 30 degrees in old school, high carbon steel.

Good luck,
Luke

The strength argument is often brought up, but it's without merit. You'll not see a hollow ground chisel broken into the hollow unless the secant drawn through the arc is at an angle that also would've failed. It's fairly common to have the edge fail on a hollow ground chisel in the flat, but that's the part that's equivalent to any other.

The issue for mortise chisels is more that when you hollow grind the bevel, you make a step where you're riding it. You want it to be flat so that riding it is even with no step/lip to catch at the edge of the hollow.

This is starting to stray into the splitting hairs category that sometimes doesn’t help folks much.

For a mortise chisel 25 would usually be considered too shallow an angle as it doesn’t support the edge to resist the deforming/cupping that can occur when chopping mortises. This would be more of a issue with harder timbers and most likely irrelevant in white/radiata pine.


When I say 30-ish I mean more than 30 but not 40.

Mine are honed by hand across the entire surface, I don’t hone a secondary bevel.

There’s physics involved here: because I am chopping more than paring I want to keep a steeper angle and avoid “wedging” effects. I chop shallower with harder timbers and deeper with softer.

I also regularly use axes (and knives) that are honed with a convex edge to support chopping and edge retention. I’ve considered doing this with a mortise chisel on the “beveled” face but I really don’t want another set of sharpening equipment set up in my workspace.

A hollow grind is going the opposite way and is very unlikely to be helpful in mortise chisels.

Most times when furniture making or in joinery I’ll mark the outline and make a shallow incision with chisels and then hog out the majority of the mortise waste with an auger bit or drill and then chop the remainder with mortise and registered firmer chisels. However, I also use a twybil and more uncommonly a mortise axe when timber framing or in heavy work as an intermediate step before reaching for chisels.

I know I could cut a mortise just using chisels but it rarely presents itself as the most effective or efficient option.

I'm curious as to how much faster drilling holes and then paring or drilling and then chopping is vs. just chopping.

This is one of those things that gets kicked around on forums, but I can never make drilling faster than just chopping, so just curious.

The hollow grind in terms of creating a primary isn't intended to be an advantage, it's just to correct a chisel that's out of shape. Mortise chisels aren't something I've ever reground after that, just hone by hand, so I can't comment on long term use, but I haven't noticed any ill effect of not following up the primary grind by fully flattening the hollow (it'll eventually come out).

In terms of wedging, there isn't anything for a shallow-angle chisel to wedge against - it's different than a starting cut with an axe, but even in an axe, wedging is undesirable. The curve you're talking about on an axe carries through all the way up the cheeks on an axe, but they're not really similar except that we want an axe to be completely free of the wood as soon as the chip is severed (in a mortise, the tool is bound in the wood, we just have to be judicious enough to not bite of more than we should in each pass).

My comment about the hollow grind and strength is more related to a personal irk. The weakness of it is like an old wives tale. it sounds good because it seems like it could be the case conceptually. The benefit of it everywhere else other than mortise chisels (plane irons, bench chisels, etc) is that it's enormously fast and allows you to be extremely accurate with freehand honing. Newbies show up from time to time and someone pipes up that it creates a weak bevel, and every single time that's been said, I've requested pictures of an edge that has broken up into the hollow (not just chipped in the microbevel or flat, because that would've occurred regardless of the hollow). For ten years, I've asked this same question and I have never seen a single person even say that they've seen it occur, let alone find a picture. And, so, an old wives tale sends a lot of new-to-the-hobby folks to do the useless activity of sticking their chisels and plane irons in a guide so that they can waste time rubbing them on their honing stones to work the entire primary.

Of course, you can maintain a bevel freehand fairly easily with a crystolon stone, but not many beginners have one large enough to use. Use of one with a honing guide would become a waste of time because the friable grit ends up all over the wheel, as does the oil from the stone.

I think in discussions, what would be instructive for people to do (but nobody does it) is make a dozen mortises with chisels set up various ways (gradual primary and small rounded secondary as derek mentions, completely flat single bevel, etc, and perhaps intentionally put a hollow on a chisel and only remove it part of the way). Time the making of mortises with each one and find out what's what for real, make notes about quality or difficulty moving the chisel around (for example, if you were to use a sash mortise chisel with flat sides, you'd almost certainly notice much more resistance moving a chisel in a mortise than you would with an older oval bolstered chisel that had significant tapering in its thickness. If you go back far enough, you find that the mortise chisels were tapered both in thickness and width along their length. I have some bench chisels set up like that - it certainly eliminates any grip on the sides of the chisel compared to just about anything else, and the mortise is only marginally less neat.

We rarely have any of that objective look at anything, though.

I think the ability of the user to keep the hammer striking and keep the chisel unstuck is far more important than the type of bevel in general, though.