Help with My Jet Mortiser Machine

[ken gibbs]

All at once the lock bolt in my five year old dedicated Jet mortiseing machine will not bottom out and lock the square chisel. So when I try to cut a mortise the square chisel starts to rotate. The lock bolt does not appear to have any damage and the socket that you screw the bolt into is clean with obstructions. Does anybody have any ideas about what to try next? Thanks.

[Josiah Bartlett]

Does the bolt keep turning, or does it stop? Can you tell if it is making contact with the chisel? Rub the bottom of the lock bolt with a dark pencil, and then install it and take it back out, and pull out the chisel to see if it is contacting the chisel (should leave a mark).

Also, does the chisel have a groove worn into where the lock bolt bottoms out?

I like to sharpen the point or serrate the bottom of lock bolts like this so they grip better, but you have to make sure the chisel is right on before you tighten it down because it will make a divot, and may make it a bit more difficult to adjust later.

[Stephen Cherry]

Do you have a busing that reduces the diameter of the hole? From maybe 1 inch down to 3/4 or 5/8?

If so, maybe the hole in the bushing is no longer aligned with the threaded hole of the lock bolt.

My guess is that you need to rotate the bushing to align the hole with the lock bolt.

[ken gibbs]

Does the bolt keep turning, or does it stop? Can you tell if it is making contact with the chisel? Rub the bottom of the lock bolt with a dark pencil, and then install it and take it back out, and pull out the chisel to see if it is contacting the chisel (should leave a mark).

Also, does the chisel have a groove worn into where the lock bolt bottoms out?

I like to sharpen the point or serrate the bottom of lock bolts like this so they grip better, but you have to make sure the chisel is right on before you tighten it down because it will make a divot, and may make it a bit more difficult to adjust later.


The set bolt will not go all of the way to and through the bushing that holds the chisel. The bit spins freely when locked down but the chisel rotates.