Fortis vice

[andrewu]

I have an old Fortis vice which has been absolutely reliable for many years however it now ‘jumps’ when tightened and does not give any grip. It has a lever by the handle for quick adjustment which raises and lowers the locking jaw. When it jumps the lower locking jaw is forced down and then is not in contact with the main screw, hence the ‘jump’ when the two part.
I have taken it apart, cleaned and inspected, the screws do not look worn.

Does anyone know what could be causing this!

 

[dutchgray]

Check to see if the nut is hanging up and not dropping into the screw properly, if the nut is worn, if there is enough spring tension on the QR arm to pull the nut in, if the QR flat bar is twisted or bent so the nut isn't pulling in.

The buttres thread form that these use should lock together under load, but it needs to fit together well enough before you put it under load.

 

[andrewu]

Thanks. The nut is underneath and is pushed up by the spring loaded flat bar, but when pressure is applied to the jaws on tightening, the nut is forced down and separates from the screw?

 

[dutchgray]

When pressure is applied the nut is supposed to lock into the screw and transfer the load into the body of the vice.

It's been my experience that when these quick release vices start jumping out the cause is often the result of small amounts of wear over multiple parts of the mechanism.
The nut being worn is the most likely culprit if everything else operates like it is supposed to.

 

[RTM]

I have taken it apart, cleaned and inspected, the screws do not look worn.

Does anyone know what could be causing this!

well, there goes my first guess.

You mentioned a spring pushing the movable jaw up. Is it seating fully into the threads of the main screw? Without installing, do things look the same on the bench and mounted in place, the jaw into the main screw? (Are you getting sub optimal engagement?)

Last one of these I worked on was a woodworking vise, and it used gravity. When tested upside down, where things were easier to see, it would not function.

Is the movable quick release nut reversible, and installed correctly?

Some pix may help