Mortising machine chisel blowout

[Jim Mackell]

I have a 20 year old manual mortising machine. You know, the one operated by Armstrong with the side lever! Working with some QSWO and making a 1/4 inch mortise. Cleared a pilot hole and then chopped about an inch wide and about an inch and a half deep. Then had the chisel blow out the side of the workpiece! Shut everything down and removed the chisel from the machine and found it had bent. Anyone ever had that happen to them?

Replacement chisels with a 5/8 shank are hard to find. Most machines and suppliers tend to be using a 3/4 shank. Guess I need to see if I can alter the housing.

[Jim Becker]

Did your drill bit get loose so you were forcing the chisel through the material? The chisels are only for cleaning the edges of the hole already created by the drill bit which typically pokes just below the end of the chisel as you know. If the bit stopped "doing the real work", I can see what you say happening. Or is this machine something where you predrill first and then use the machine? In that case, perhaps metal fatigue came into play...or an embedded knot or something deflected the tooling...or it's yet another episode of "2020".

[Alex Zeller]

I believe Penn State sells premium HM chisels that are 5/8" shanks as well as the Delta brand HM chisels are 5/8". In normal times finding one would be easy but we aren't in normal times. I just picked up a HM machine so I'm not an expert but when was the last time you sharpened the chisel? I would think working with oak you would need a very sharp chisel.

[Brian Holcombe]

It’s important to sharpen them, much like a bench chisel.

[Ken Fitzgerald]

I have a General International mortising machine and it works well when mortising white oak. In fact, I will be using it on red oak in the next day or so. As Brian said, I have found it necessary to make the chisels sharp, sharp, sharp! I have a two kits I use. One kit reams a new inner edge on the chisel and the other is a diamond hone. I only use the reamer when necessary which isn't often. The hone gets used, sometimes multiple times, before and during a project. One porch swing I made of white oak had somewhere between 60-80 mortises. I also flatten the outside of the chisel and sharpen the bit too.

[Jim Mackell]

It’s important to sharpen them, much like a bench chisel.


Well Brian, I thought it was sharp enough. Either it wasn't or as Jim Becker suggested, metal fatigue set in. And we were so close to making out of 2020 unscathed!