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Using a Posilock Chuck

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Chris Farbrace04/09/2010 22:09:58
17 forum posts
Hello,can any of the forum experts explain whats the correct procedure to fit a  mill bit into a posilock chuck.I have a nice metric Vertex chuck and collets set and screwed mills and am not at all sure of the order of assembly and tightening,any help would be appreciated thanks, Chris.
John Stevenson05/09/2010 12:17:36
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5068 forum posts
3 photos
MEW 140 page 38 has the official tightening instructions on it.
 
It's always been a argument point in engineering.
 
Clarkson say fit the collet into the sleeve nut and tighten by hand until the flange touches the body, now screw the cutter in until it becomes tight and give a final tighten with the spanner.
 
I was taught to leave a very small gap to allow the nut to tighten up tight because under heavy cutting forces the cutter does tighten up slightly and this affects the depth of cut, especially nowadays when you need to set tool length offsets for CNC machines.
 
John S.
Andrew Johnston05/09/2010 22:38:36
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7061 forum posts
719 photos
I prefer the Clarkson method. The only time I left a small gap between the sleeve nut and chuck body I shattered the endmill, at the screwed end. I assume this was because, as the sleeve nut wasn't fully home, the collet was not at the top of the endmill. As the cutter rotated slightly under the cutting loads the centre in the chuck split the top of the endmill as there was no support from the collet to resist the forces.

If the cutter is properly home the depth of cut shouldn't change as the cutter tightens slightly. The sequence as I see it is as follows. If the cutter turns (tightens) slightly under cutting loads it cannot move upwards because of the centre in the chuck. Instead the thread causes the collet to move down slightly into the taper on the sleeve body. Hence the collet grips the cutter more tightly.


As a proviso, and I might be talking nonsense, I've never used the old Clarkson 'C' style chucks, only the newer 'S' types. However I did recently make a new hardened and ground centre for a Clarkson 'C' style chuck for a friend, so I've had a good look at them.

Regards,

Andrew
KWIL06/09/2010 14:20:17
3681 forum posts
70 photos
I prefer and use John S'  version. You only leave a small amount to tighten, after you have wound up the cutter to touch the centre.  If you leave to much on a small stem cutter, say 6mm or 1/4, you can split the cutter's centre on the chuck, Don't ask. As for it tightening itself, try that with a 1mm slot drill and see where you get!
Andrew Johnston06/09/2010 20:08:33
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7061 forum posts
719 photos
Can't say I've used a 1mm slot drill in a Clarkson autolock chuck, but using a 1/16" slot drill worked fine with the 'Clarkson' method.
 
Granted the low cutting forces on such a small cutter are unlikely to tighten it much, but at least they are in the tightening direction. But then with such low forces one is unlikely to be worried about the cutter becoming loose either.
 
In general I don't bother using an Autolock style chuck with small cutters now, I use a plain shank cutter and an ER collet chuck.
 
Regards,
 
Andrew
John Stevenson06/09/2010 20:57:10
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5068 forum posts
3 photos
I have to use my Clarkson chuck quite a lot as it's built into a 4:1 speed increaser.
 
I run with a 16mm collet holding a straight shank ER11 collet chuck so I can hold small drills directly in the ER11.
 
Some jobs have over 600 2.5mm holes in them and over a run can easily add up to 12,000 or so holes.
 
John S.
KWIL06/09/2010 20:57:21
3681 forum posts
70 photos
So you have to check the z axis each time you change a cutter and when you go back to a previously use one?
Chris Farbrace09/09/2010 22:30:40
17 forum posts
Thanks for the replies guys,obviously a little controversy over the method to use but gives me a better understanding of what happens when its all tightened up.Chris

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