Advice on toolholders

[Rabs]

Hi everyone,
I have a mill which, frustratingly, does not have a drawbar. At present it uses the colchester easychange system:

(not to scale)
Dark Green=Mill body, Blue=spindle, Light Green = easychange chuck, Grey = clamping bolt, Red=toolholder

The problems with this setup are:

• Not very stiff - the height of the easychange chuck means that the tools end up very long way out from the spindle nose
• The easychange chuck I've got is either poor quality, damaged or worn and there is noticeable play between all the toolholders and the chuck
• The length restricts the working height of the mill
• It would be hard to implement automatic tool changes (not a high priority)

Does anyone have any comments on the arrangement below?


In this I put a thread directly on the toolholder and spin it into the taper on the spindle (it has the same, non-standard 20degree taper). I'd tighten by using a spanner on the top (pink) of the spindle (which has flats) and flats machined on the toolholder.

My concern is that this means that I will be rotating the taper faces together, rather than applying an axial force, as a normal drawbar would. Would this be a problem? It's obviously fine to screw onto a register with a lathe chuck, does the fact that it's a taper make any difference?
Any other comments on the plan?

[m_c]

It's not a good idea. 20deg should be self releasing, but it'll also tighten as the machine runs, which is likely to give issues getting tool holders back out.

I'd look at getting the quickchange system repaired, or drilling out the spindle so a proper drawbar can be fitted.

[Rabs]

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]

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?

[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.

[Rabs]

Ok, thanks. I'm still not quite with you though - sorry! If the toolholder screws in then it has to rotate, so how can I have a keyway? I suppose I could screw in the toolholder get it to register on the taper and then push a loose key in afterwards? But to give good alignment on the taper doesn't it have to be tight to the taper? In which case, anything that stopped the screw from tightening would also stop the taper from doing it's job properly to keep the tool aligned. Have I misunderstood?

[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.