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:
18005
(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?
18006

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.

[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.

[John S]

Questions,
In Fig 2 what is the diameter to the thing spindle part, the bit that holds the pulley.
What is the biggest tool you would be satisfied using ? Be reasonable here.

Any pictures as opposed to blocky diagrams.

[Rabs]

Questions,
In Fig 2 what is the diameter to the thing spindle part, the bit that holds the pulley.
What is the biggest tool you would be satisfied using ? Be reasonable here.

Any pictures as opposed to blocky diagrams.

Here you go. The spindle block is enclosed, with all bearings and pulleys concealed. At the top there is only an 18mm diameter shaft protruding. Ignore the bit of aluminium extrusion, it's just a temporary mount for a spindle speed sensor.
1802218023

At the nose the spindle is 40mm in diameter
18024

Here is the easychange chuck. To secure it you push a bolt from below, through the hole in the narrow end of the chuck and into a thread on the spindle. (see original post)
180251802618027

Here is one of the easychange toolholders:
18028

And here's what the setup looks like as I received it:
18031

Here is a tooholder I've knocked up with a stud at the top and how it looks screwed directly into the spindle. I've not used this yet on the back of m_c's warning.
1802918030
I've made it with drive slots to fit the easychgange chuck but these could also be used for a spanner to release it from the spindle, with a normal spanner used on the flats at the top of the spindle.

[magicniner]

It's the Coventry Easy Change system, I have the same on my bench top CNC mill, it locks up nice and tight and works very well, check all the internal and external tapers for damage.
I rarely use tooling bigger than 10mm, carbide tooling and High Speed Machining feeds, speeds & DOC are the order of the day,

- Nick

[m_c]

Is it just the light/reflections, or is the taper on that toolholder look like somebody's sanded/filed it?

I've got the easy change system on bench top mill, and my holders all lock in that tight, I usually have to use a screwdriver to break them away from the taper to get them out.
Could it be the ratchet mechanisms not working correctly?

I'd perhaps give Coventry a phone, and see what they say. It could be as simple as the spindle adapter needing stripped and cleaned, although I have no idea how they separate.

Contact details can be found on the Coventry Engineering website - http://www.coveng.co.uk/group/coventry-toolholders/products/toolholders-for-manual-machines/easy-change.html

[Rabs]

Is it just the light/reflections, or is the taper on that toolholder look like somebody's sanded/filed it?

I've got the easy change system on bench top mill, and my holders all lock in that tight, I usually have to use a screwdriver to break them away from the taper to get them out.
Could it be the ratchet mechanisms not working correctly?

I'd perhaps give Coventry a phone, and see what they say. It could be as simple as the spindle adapter needing stripped and cleaned, although I have no idea how they separate.

Contact details can be found on the Coventry Engineering website - http://www.coveng.co.uk/group/coventry-toolholders/products/toolholders-for-manual-machines/easy-change.html

OK, thanks guys. I'll give it all a good clean and check over before doing anything drastic. Maybe the locking mechanism just has some swarf in in.

If you mean the toolholder with the threaded stud at the top. It looks rough because I've just rough turned it to check the principle. I was intending to grind and harden the taper before using it in anger.

[Rabs]

So, it turns out that the looseness wasn't actually in the easychange chuck or the toolholders but in the spindle itself. Thanks for your help everyone. I'll start a new thread with a question on what to do next.