Mortice chisel grinding angles?

[Racers]

What have I done! 8-[

I'll get burnt at the stake at this rate :lol:

Don't criticize St Roy.

:wink:

Pete

 

[Eric The Viking]

What have I done! 8-[

I'll get burnt at the stake at this rate :lol:

Nah. But Roy's probably the only person in the Western world still making shows that way (outside of YouTube). Possibly a few cookery programmes ("here's one I made earlier..."), but even then it'll usually be a shorter sequence in one show, rather than the whole thing.

Sticking my neck out, it's possible they get two done in a day, too. Rather depends on the complexity and the amount of rehearsal they need. Each show will be different.

 

[bugbear]

What have I done! 8-[

Bought a knife to a gun fight!

BugBear

 

[matthewwh]

Very tricky to do craft videos that show the creative process in any other way:
"Could you just saw the end off that again?"
"No, it's already on the floor."

Anyway, mortice chisels...

35 is a good angle, but hollow ground is a bad idea.

If you look at one of the cutaway videos of a mortice chisel working (there are several on YouTube) you will see that the chisel rides on the bevel and takes diagonal slices. Flat will work, convex will work (you just make scooping cuts rather than straight ones) but hollow ground will leave a weaker edge, which will then try to dig itself in, giving premature edge failure.

Letting the tool slide slowly backwards and forwards on the toolrest as you grind is all you need to do to produce a gently convex edge.

It is usually worth working through every other possibility before re-doing heat treatment. Unless you know exactly what the composition of the steel is you are kinda fumbling in the dark, spark testing might give you a pointer but it's still a lottery.

Laminated blades would have been hardened throughout - the bits that need to be soft were made from unhardenable wrought so there would be no advantage in not hardening all of the edge steel. If a previous custodian has cooked a chisel on a grinder the damage rarely extends back more than 1/8" or so.

 

[David C]

I am surprised no one has mentioned grinding at 25 degrees.

This gives excellent penetration.

Of course the tip would be honed at roughly 35 to 40 degrees according to timber. Possibly even 45 for a Japanese chisel in hard exotics.

The rounding of the heel, mentioned by Leonard Lee in his excellent book Sharpening , is a valuable tip.

best wishes,
David

 

[Derek Cohen (Perth Oz)]

David, my oval bolstered mortice chisels have a flat 20 degree primary bevel, and a rounded 35 degree secondary bevel. This provides easy penetration and a durable edge.

I have found that the easiest way to add the secondary bevel is to freehand a rounded bevel (ala Paul Sellers). On these chisels, any grinding is done to the primary bevel on a belt sander, since it is important to keep this flat. Hollow grinding is used elsewhere, not here.

Regards from Perth

Derek

 

[Corneel]

20 or 25 degrees. Not enough difference to loose any sleep about.

I was watching a picture of mortise chisels in a book about 18th century tools. They all had pretty steep bevels ( should have meassured them but I was lazy on my back on the couch...). Some rather flat, some convex. It didn't look like they had much of a secundairy bevel.

That made me thinking.



This is a diagram of the mortising action. You make clearance in front of the chisel as you go. The chisel slides into the wood at an angle, indicated by the red arrow. Now, I think it slides at the angle of the secundairy bevel, usually about 35 degrees or so. So, in this case it doesn't really matter if the primairy is 20 or 35 degrees.

When you reach the end of your intended mortise you hopefully have a hole to the required depth in the middle, and two ramps upwards, towards the ends of the mortise. Now you shop straight down into these two ramps to remove them. Here I can see how a less steep allows more penetration, while it didn't make a difference in the first step.

Of course, all this is just my theory. I am curious to comments about this. Anyone have experience with steeper primairy bevels, just like you find them all the time on ebay?

 

[swagman]

Its should be of no surprise why the subject on sharpening and honing angles has gotten so confusing over recent years.

The point is to pick one angle and stick with it. For years I kept my tools at all sorts of different angles. My set of chisels used three different angles. I used 35° for the small chisels that were for light chopping, 30° for the middle-sized tools that were for chopping or paring and 27° for the wide chisels that were for paring only. I had steep angles for mortising chisels and plane irons used for figured woods.

Perhaps I’m just a simpleton, but keeping up with the angles was an annoyance. And so one day I threw away all my jigs for setting these different angles and made one for 35°. And I slowly converted every tool in my chest to that angle.

— Christopher Schwarz

[/u] http://www.popularwoodworking.com/woodw ... love-of-35

 

[bugbear]

If you look at one of the cutaway videos of a mortice chisel working (there are several on YouTube) you will see that the chisel rides on the bevel and takes diagonal slices.

For the sake of completeness, that's only one style of mortising (sometimes called Maynard).

Some people also use a sequence of vertical strokes (like an old fashioned hand powered mortise machine), with the back of the blade facing the waste-yet-to-be-removed.

BugBear

 

[bugbear]

This is a diagram of the mortising action. You make clearance in front of the chisel as you go. The chisel slides into the wood at an angle, indicated by the red arrow. Now, I think it slides at the angle of the secundairy bevel, usually about 35 degrees or so. So, in this case it doesn't really matter if the primairy is 20 or 35 degrees.

I think this depends on the size of the secondary. There must surely come a point where a tiny secondary cannot generate enough force, at which point the primary will take over.

I don't want to think about a fully convex bevel.

BugBear

 

[Phil Pascoe]

You do ... you know you do, really.

 

[Corneel]

Ha! I think I should do some experiments, but not in the imediate future, because I'm leaving tomorrow for a week in the Spanish Pyrenees.

 

[bugbear]

Ha! I think I should do some experiments, but not in the imediate future, because I'm leaving tomorrow for a week in the Spanish Pyrenees.

The big pointy things are called "Mountains"

BugBear (providing helpful tips to a Dutch person)

 

[Corneel]

Thanks :ho2

 

[Jacob]

Entertaining set of grinding angles on offer! Of course in reality they are ALL "correct"; it's the sharpness of the edge which matters much more, whatever the angle (within reason).
Though Dave's 25º wouldn't do for heavy use, ditto hollow ground. The convex rounded bevel also has a valuable function in aiding leverage in the corners of blind mortices but otherwise doesn't do anything much except making sharpening a lot easier. Good idea to have the working depth of the chisel polished up by honing during sharpening, as there is a lot of friction possible, but no need for obsessive flatness of faces etc - old ones are always slightly convex on the face due to sharpening being concentrated towards the pointy end. This has no effect on usage.

 

[D_W]

I set them up two ways:
* Oval bolstered chisels and other chisels with a really large cross section, I set them up so that they are shallow primary bevels (like 25 degrees) with a final bevel around 35 - intentionally rounded to that so that they're easy to sharpen freehand.
* Smaller chisels, I maintain a flat bevel on a stone and ride the bevel when cutting a mortise. Also easy to maintain. Someone said no hollow - which I don't think is a really big deal except that it creates a step in the bevel until the bevel gets into the wood.

I can see some advantages to both. I don't get much difference in terms of time with either way (presuming both chisels are decent).