Advice on toolholders

**Rabs** · 23-03-2016

Thank you, that's helpful. if the toolholder getting stuck is the biggest worry then I might try it once or twice to see how hard it is to remove them after they've been tightened up by some heavy cuts. I don't have to modify anything to test that. If you are right, as I suspect you will be, I'll look into boring out the spindle.
I wonder if that could be done in-situ with a series of drills the same diameter and different lengths? I'd prefer to not take the whole spindle assembly apart if I can avoid it.

**m_c** · 23-03-2016

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** · 23-03-2016

Originally Posted by 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.

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** · 23-03-2016

Originally Posted by Rabs

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** · 23-03-2016

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** · 23-03-2016

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** · 23-03-2016

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.

**lucan07** · 23-03-2016

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** · 23-03-2016

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)