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Snapped Chisels!

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Jelly

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First time this has happened to me, and an unwelcome first at that, nevertheless.

Following on me from extolling the virtues of both safety glasses, and learning from your mistakes yesterday; I have had cause to take both of my own bits of advice....


Cutting some 19mm mortices through 100mm redwood for new garage doors, I rather hastily laid everything out and neglected to notice a large knot.

Anyway, having got down to the knot on the third stile I was mortising, and it really was having none of it, so being committed, I got my "big mallet" out (standard joiners mallet in proportions, but solid Lignum Vitea) and gave it a bit more wellie.

BANG!

Snapped a firmer chisel clean in two, from what I could tell the knot had pushed the blade causing a twist, whilst the mortice already cut held the top straight and true, add a bit more force and the steel was having none of it. It was an old battered chisel I save for doing hard jobs with, irritating but no fuss, I'll grind a new bevel and save the other bit for use in something else.

After 15 minutes of swearing with a pair of long-nosed vice grips, the bottom of the blade was retrieved, and I concluded I'd have to chop down the sides of the mortice to cut the knot, and then take more gentle passes, which worked just fine.

Fast forward to stile №4, check for no knots under my layout, off we go... Bam! Same thing happens again, this time with curved grain deflecting the chisel, which was one of my slightly nicer battered chisels, still no Biggie, but a bit more irritated with myself for doing it twice in quick succession.

Probably my own fault for rushing all the way through:
  • Should have rejected the two boards in question (and a couple of others) for having obvious defects I'd specifically asked to be excluded when I rang the order in, but didn't have time to check when I collected it so accepted the whole ¾ cube pack and had them pop it on the car with the sideloader.
  • Should have taken more care laying out the mortices to avoid defects, and make life easier.
  • Should not have attempted to just power through using more force.
I also had the realisation that after spending a long time working with metal rather than wood, my instincts are now to naturally swing with full force, which combined with using a mallet that weighs 2½lb and was originally intended to be a force multiplier to avoid having to swing as hard... Is probably resulting in too much force all round.
 

Just4Fun

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Did your failures show signs of any problem in the steel or is it consistent all across the breakage?

With money I got for my 13th birthday in 1972 I bought a set of 4 chisels. I don't know/remember what brand they were but they had no maker's name and just plain blue plastic handles. I could afford them so they must have been cheap. They were still quite new when I had the same problem, cutting a deep mortice with the 3/4" chisel. There was clearly a defect in the steel. I took the chisel back to the shop and they replaced it, no problem. I still use that set of chisels a lot and I regard them as my best & favourite chisels. They take a nice edge and are nice to work with. The one that failed must have been a non-representative sub-par example.
 

Ttrees

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Hello
I would look out for some "pig sticker" mortise chisels, if you need a real strong chisel.
You can really give them brute force, and they can be bought for less than a tenner on the bay or the likes of tooltique or similar tool dealers.

Edit quite a lot of them on the bay, for cheap.

Tom
 
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Jelly

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Did your failures show signs of any problem in the steel or is it consistent all across the breakage?
The second one to snap had a tiny corrosion pit on the corner of the blade which appears to be where the fracture propagated from, first one had no obvious defects, and in both cases the steel is uniform with the grain structure i'd expect consistently across the fracture.

Which is what lead me to conclude I was being too aggressive, and not adequately considering and working with the natural properties of the timber.


I would look out for some "pig sticker" mortise chisels, if you need a real strong chisel.
Funny you should mention this as I've got pig stickers for ⅛"-½" sizes; really rate them!

But, I've never seen them in larger sizes like ¾" or 1" and was idly contemplating making some from some EN8 bar stock with the carbon steel from my broken chisels welded in to form the edge.

Really at ¾" and up, boring the meat of a mortise out with an auger then cleaning up with chisels is probably a more sensible choice.
 

Jelly

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Did you drill out the majority of the wood before starting to square out the mortise
No, but I will be for the remaining 2 stiles!

Being that I'm used to using pig-stickers it never really occurred to me to do this, until just now when I was thinking about why ¾" pig-sticker chisels don't seem to be a thing.
 

D_W

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A couple of thoughts, as I've broken two chisels over the years:
1) a japanese mortise chisel that had a bad profile. IT was expensive and I figured it must've been made right, but it stuck hard in mortises and broke at the lamination
2) a large timber mortiser that bent loose at the lamination line

I don't think laminated or fully hardened really matters - I think a thick cross section, actually, of fully hardened steel, is probably a better chisel for mortising because there won't be a tendency to bend or break at the lamination like both of mine did.

But, when you're hammering a large amount of varying hardness material, one part of the chisel is probably trying to go one way, and on the other side of the cut, the "bad parts" of the wood are bending a small amount of the chisel drastically, and pop. Assuming that's what occurred.

I have mortised a lot of trash material and never really had that issue, as well as a lot of better material where I was dealing in a deeper mortise with a chisel that had no side relief (1 above) and too deep, too rough with #2

If you have to do more to get a chisel out than kind of shoulder it up a little bit or lever it forward like you're shifting gears on a piece of machinery (takes some to do it, but not all of the force you can generate), then the cut needs to be thinner per slice.

pigstickers aren't really a thing for most people because they have a long bevel and tall cross section. They don't really come into their own until you're working in deep mortises (and then the tall cross section and shallower primary angle makes them really easy to manipulate deep in, and less likely to get stuck). For cabinet mortises, quite often the mortise is getting to depth just at the point that a pigsticker is becoming useful. A lot of times you'll find one that's had the front of the chisel ground to a flat angle of 35 degrees or so, and for those, you end up with a lot of friction due to the large facet, and the loveliness of how the chisel works deep in a mortise with a longer primary bevel and rounded over back is lost.
 

D_W

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a smaller cabinet mortise chisel with tapered sides is nice, though - firmers with dead flat sides do tend to get stuck a little in grippy wood. if you can feel the chisel bending a little when you're trying to get it out, you're close to its limits.
 

Jelly

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But, when you're hammering a large amount of varying hardness material, one part of the chisel is probably trying to go one way, and on the other side of the cut, the "bad parts" of the wood are bending a small amount of the chisel drastically, and pop. Assuming that's what occurred.
That's wholly consistent with what I saw digging the broken blades out, and before that if I'm honest. A less tired, more switched on Jelly would likely have spotted it before it happened, rather than doing the same stupid thing twice...

dealing in a deeper mortise with a chisel that had no side relief (1 above) and too deep, too rough with #2

If you have to do more to get a chisel out than kind of shoulder it up a little bit or lever it forward like you're shifting gears on a piece of machinery (takes some to do it, but not all of the force you can generate), then the cut needs to be thinner per slice.
I think you've summed up exactly the issues here, I was taking too big a chunk per slice, and compensating by using more force, driving the chisel too deep with each blow, then having to really work to lever the chisel out.

More haste definitely resulted in less speed as a result.
 

Ttrees

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Not sure what sizes they do, but Crown tools make massive mortise chisels
that are kind of unwieldy long, I wouldn't be surprised if they made some 3/4" ones.
Not as tall, or not as much meat as on the pig stickers of old, but they might do the job if available.
I picked a new one up for a ten euros at a tool shop, too cheap not to.
Never used it though, so apologies that I cannot comment on it's performance.

Tom
 
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