Advice on toolholders

[m_c]

Personally, I wouldn't even test it. If the taper does lock, then short of drilling the threaded stud off, you won't be screwing the tool holder back out.

[Rabs]

Personally, I wouldn't even test it. If the taper does lock, then short of drilling the threaded stud off, you won't be screwing the tool holder back out.

OK thanks, I'll look into boring it out. Out of interest though, and please excuse my ignorance, why would it be worse than a screw on lathe chuck for getting stuck?

[m_c]

OK thanks, I'll look into boring it out. Out of interest though, and please excuse my ignorance, why would it be worse than a screw on lathe chuck for getting stuck?

Big lathes, the tapers never turn against each other. They're secured using either bolts, or a retaining nut.

The only small lathes I've seen with screw on chucks, use locating registers, not tapers.

Once a taper locks, you need to physically force them apart. They generally will not spin against each other, especially if you still have pressure holding them together.

By all means try it, however I wouldn't be surprised if they seize together, and you have to machine the holder back out.

[lucan07]

If the key locates after insertion it should not overtighten locking the mechanism beyond the point you leave it, I pesume it has been designed to act in this manner, presumable the chuck operated the taper when working and threads etc would have been worked to locate and lock if a professional design

[m_c]

Lucan, no offence, but your suggestion won't work.
Tapers rely on being firmly held together. With your suggestion, you would either have to ensure the tapers never fully tightened, meaning the toolholder will be loose and rattle in the spindle, or risk never being able to get the key in.

[lucan07]

If the key is sprung it would be designed to act at a point to stop overtightening as well as aligning the thread to start at a specific it would only engage at a point beyond just beyond correct torque when milling, presumably you release it to with the chuck to undo otherwise it would engage and lock once on each rotation.

[Rabs]

If the key is sprung it would be designed to act at a point to stop overtightening as well as aligning the thread to start at a specific it would only engage at a point beyond just beyond correct torque when milling, presumably you release it to with the chuck to undo otherwise it would engage and lock once on each rotation.

I think m_c's point is that the toolholder can't be tight against the key and the taper at the same time - it has to hit one of them first. If you want that to the key to stop over tightening, then itwon't be tight on the taper. If it isn't tight on the taper then the tool will wobble. Thanks for taking the time to help though - there might well be something there related to keyways. I'll give it some thought.

Big lathes, the tapers never turn against each other. They're secured using either bolts, or a retaining nut.

The only small lathes I've seen with screw on chucks, use locating registers, not tapers.

Ok, that makes sense. I suppose the smaller angle the taper the more prone it will be to locking because the larger the radial force is for a given axial bolt tension. That also explains why a lathe chuck with a flat register (which I suppose can be considered as a 180 degree 'taper') is much less prone to locking.

If it makes any difference this is a small machine with a 1/2hp spindle motor.

[lucan07]

If you wanted to go ahead with this I would look at getting some sort of keyway set up in holder, should be possible.

[Rabs]

Sorry, I'm not sure I understand. Do you mean I should add a keyway if I bore out the spindle, or if I use a screw in toolholder?
There is a keyway on the spindle taper at the moment. A ball bearing on the outside of the coventry chuck engages with it but it doesn't appear to be designed to take the cutting loads but rather to help you align the chuck the same way each time (I guess that the final grinding on the internal taper on the chuck may have been machined in-situ so you'd want to replace it in the same orientation each time to minimise runout)

[lucan07]

If you use screw in toolholder, not so much for the cutting loads just to prevent locking in any tools.