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Parting off on Myford lathes

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NJH18/02/2011 16:11:22
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2314 forum posts
139 photos
Hi Chris
 
How does this work then?
 
I had always imagined ( never having - yet!- been in a position of needing to grind the chuck jaws) that the way to proceed would be to cut a narrow ring and open the jaws such that the outer steps on the jaws tightened on the inside of the ring thus holding them firm. Please tell me why this wouldn't work. ( I hope that I don't need this information for some time!)
 
Regards
 
Norman
 
 
Stephen Leacock18/02/2011 16:27:31
4 forum posts
Again i still don`t understand guys If as you say the chuck jaws are bell mouthed surely i would get the same chatter in both forwards and reverse setup ( this is certainly not the case as there is a difference of night and day in my forward and reverse setups for parting )
David Clark 118/02/2011 16:31:12
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3357 forum posts
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Hi NJH
If you tighten the jaws up outwards onto a ring, you are pushing the bell mouthing the wrong way.You would pull the jaws in at the front rather than push them out. This means they would be like \ / rather than like / \.
regards David
Tony Jeffree18/02/2011 16:33:41
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569 forum posts
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Posted by NJH on 18/02/2011 16:11:22:
Hi Chris
 
How does this work then?
 
I had always imagined ( never having - yet!- been in a position of needing to grind the chuck jaws) that the way to proceed would be to cut a narrow ring and open the jaws such that the outer steps on the jaws tightened on the inside of the ring thus holding them firm. Please tell me why this wouldn't work. ( I hope that I don't need this information for some time!)
 
Norman -
 
The problem with constraining the jaws from the outside as you describe is that you want the jaws to be forced inwards as they would be when clamping a piece of bar. So, if there is any play in the jaws, your method will cause the resultant ground jaws not to be parallel when clamping a bar. Which kinda defeats the object.
 
On my Taig lathe, which has soft Aluminium jaws, they recommend using a small washer, clamped as far into the chuck as it will go, and then using a boring bar to skim the inner faces of the jaws. You can then take off the small pip left where the washer was with a file. The nice thing about that method is that if you chose a washer the size of the stock you want to machine, the chuck will grip the stock with minimal runout.
 
Regards,
Tony

NJH18/02/2011 17:31:01
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2314 forum posts
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David and Tony -
 
Thanks - that makes perfect sense. I will file the information away in my memory banks until required. (Although if that's longer than a couple of weeks I'm not so sure I will be able to retrieve it!)
 
Regards
 
Norman
chris stephens18/02/2011 17:40:10
1049 forum posts
1 photos
Hi Guys,
Not much point in me saying anything now, you lot beat me to it, again.
chriStephens
chris stephens18/02/2011 18:01:14
1049 forum posts
1 photos
Hi Stephen,
I quite understand you question. What I think this shows is that there is more than one reason for problems with parting. A sloppy chuck is just one of the causes, there are many others that work singularly or in tandem to cause you to have a bad day. As an instance, there is very common fault on mini-lathes in that the top slide gib dovetails are badly machined and by parting from the rear it might put the load on the good one. This however is not the answer to the problem just a way around it, the answer here would be to file off the sharp corner of the dovetails thus allowing the whole of the jib to come into contact, not just a line contact.
If a lathe is in good order (and of a reputable make) and a decent quality parting tool is used, parting from the front should not be a problem. If parting from the rear does make life easier and you don't notice any other problems I am not suggesting you go looking for some, but if say your gibs are loose, you might find that you will get a better finish when turning by sorting them.
chriStephens
mgj18/02/2011 18:22:51
1017 forum posts
14 photos
Graham - I don't know your tips, obviously, but your saying that your tips had about 7 deg of rake surprised me.
 
So I went out and took a look at mine, whch are Sandvik 3mm and Zenit GTN2 clones (same as the Seco 2mm ones for the Glanze parting tools)
 
Yes, they do have a steeply dropped front, which could easily be 7 deg, BUT the actual cutting edge is more or less at zero rake. (The ramp may be there for chip clearance?, but it is not at the same angle as the edge, by no means, and its the edge which defines the rake angle and the forces and vectors generated.)
 
Then, if you install that set up at centre height and walk into a hunk of metal and apply a download at the tip, of course all will give a bit, as it must, and one is actually cutting below centre and with negative rake, or more or less zero rake. Which is one reason why that design of tip is so chatter and jam free.
 
In a rear toolpost the same thing happens exept the tool lifts a smidgen, so again one is using the same "below" centre/shallow rake geometry.
 
Your tips may not be precisely like that, but most parting indexable tips are. (There are also very specific negative rake parting tips too of course)
-----------------
Its also one of these great falsehoods about parting on centre that many come up with. Being set on centre is not the same as cutting on centre. Setting on centre with the tool unloaded means cutting below and with a reduced effective rake angle. Obviously, since nothing is 100% rigid. If one were using an Oil Country sized lathe and tooling to part a small bar that statement might be marginal, but its true enough for our ME purposes
 
 
 

 

John Coates18/02/2011 18:42:37
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558 forum posts
28 photos
Posted by Stephen Leacock on 17/02/2011 22:12:42:
the lathe will part better with rear tool post or in my case running in reverse with parting blade mounted upside down .

Bloody hell Stephen - an ephifinal moment there for me! Never thought of that and it's so flippin' obvious. Next parting off I do I'm going to try that

 
D'oh!
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