Mortise chisel keep blowing out the back

[ZedX]

I'm cutting mortises for a workbench build and every single one blows out at the back when I'm about 3/4 through. I'm using a 3/8" mortise chisel, chopping from both sides, but the exit is always a splintered mess. I've tried marking deeper lines, clamping scrap to the back, and even drilling relief holes first, but nope, still getting blowout. Is this just part of hand cut mortises that gets cleaned up later, or am I missing something obvious?

 

[HerrFixIt]

The issue is likely how the bevel is exiting the back face. To fix that, try flipping your chisel so the flat side is facing the finished exit wall. Instead of chopping with the bevel, use the flat side to pare or slice out the final waste, gently lifting it out. This makes the flat, clean reference face the one that exits the wood. Also double-check that your chisel is razor sharp. A dull edge will crush and tear the fibers, making blowout almost guaranteed.

 

[handfuldad]

You're close with the scrap backing, but try a sacrificial piece that's actually clamped tight....like really tight. Also, are you paring the final cuts or still chopping? should be paring those last few passes.
What kind of wood are you working with? Some species are just more prone to this than others.

 

[Alan]

@ZedX what you’re seeing is pretty common, but there are ways to reduce it. The key is controlling how the chisel exits the back. Make the flat side of the chisel face the finished wall on your exit, and pare the last bit instead of chopping. It lifts the fibers instead of smashing them. Also, clamp a sacrificial scrap tightly behind your mortise. It really helps stop splintering. And don’t forget: a razor-sharp chisel makes all the difference. What wood are you working with? Some species blow out way easier than others.

 

[chateaudechambord]

Yeah. The biggest help is to pare the last bit instead of chopping. Flip the chisel so the flat side faces the exit wall and lift the fibers out gently. Clamping a sacrificial scrap really tight behind the mortise also reduces blowout a lot. And yes, a razor-sharp chisel is essential. Dull edges just tear the wood.

 

[Alan]

Yeah. The biggest help is to pare the last bit instead of chopping. Flip the chisel so the flat side faces the exit wall and lift the fibers out gently. Clamping a sacrificial scrap really tight behind the mortise also reduces blowout a lot. And yes, a razor-sharp chisel is essential. Dull edges just tear the wood.

Thanks for reiterating the last bit. It's a very important tip.