Parting Off MEW225

[Anonymous]

The deflection of the tool versus the applied force will not be linear, particularly where parts of the lathe, as opposed to the tool, are concerned. If we take the topslide as an example it is not a simple pivot point. As the down force on the tool increases then the topslide may start to pivot. But as it does so two effects come into play. First the toe of the front dovetail (nearest the lathe centre) will tend to contact the non-moving part of the dovetail. Second, on the rear dovetail, once any play has been taken up, the contact is metal to metal, so the force needed for a given deflection will be much larger.

Pushing down hard on the parting tool insert (about 30kg) when extended 20mm I got less than 0.01mm of movement on the topslide with the gib screws set so that the top slide moves freely but without the topslide being locked. With the parting tool extended 50mm the deflection increases to 0.03mm. I don't think these deflections are significant. I normally operate with the topslide set at 90° with the gib screws done up tight, and locked as well.

Andrew

[Neil WyattModerator @neilwyatt]

> No variation is the exact opposite of the requirement – to maintain consistent cutting you really need CNC control or a very skilled operator

In thought that was my point?

Neil

[Chris TriceParticipant @christrice43267]

It was brought up earlier but Blowlamp is right in that flex in the WHOLE system, including spindle, chuck and workpiece all contribute to degrading the parting off situation. I have to say that as rigid as quick change tool posts are with normal turning, anyone who has suffered a dig in with a parting blade held in one will see for themselves the flex possible when they 'unwind' it.

 

Edited By Chris Trice on 27/02/2015 18:20:59

[Neil WyattModerator @neilwyatt]

> can't feel what's happening in the cut so that he can vary the feed,

<facepalm>

You said the feed won't vary… now you're saying a skilled operator can vary it. All I'm observing is that NO human can feed with the consistency of auto feed ALL the time, and that operator error may well be the cause of some dig ins.

Neil

[blowlampParticipant @blowlamp]

I'm still not seeing the explanation of how my puny Mini-Lathe is parting off from the front, with me holding the camera in one hand whilst I sporadically infeed without jamming with the other.

Martin.

[CabengParticipant @cabeng]

My apoligies Neil, I misinterpeted your comment.

I was trying to make the point that if a tool cut without diging in, there was no reason why it should do that at somee point during the cut. Martin pointed out that variation in feed would have some degree of 'automatic compensation' that would avoid dig in, which is correct. In theory, at least, so after your last posting I tried it out.

800, 1200 & 1500 rpm, 22mm bar, close to the jaws of a near-mint chuck, 2mm tip (the same one that I've been using since the beginning of this exercise), parting from the top slide. Hand feeding, varying the rate of feed in a sensible/realistic manner didn't have any effect. Then tried whipping the handle half way round as fast as I could to push it in 0.040". There was a limit to how fast I could do that, due to feedback on the handle. Didn't dig in, carried on cutting after the initial grunt.

Then set the gear box to 24 tpi (0.040"/rev) and fed into the bar at 800 rpm, equivalent to turning the handle once in 0.15 seconds – a bit extreme, I think. It managed two rotations whilst slowing down, then stalled. The earth moved on this one. Was it a dig in, or just the motor pushing round with it's stall torque to make the work climb on the tool? I don't know.

Conclusion is that variation in hand feed shouldn't cause a dig-in, unless one does something really stupid, like putting the gear box on the screw cutting range!

[Mark CParticipant @markc]

Out of interest, does everyone part small stuff (less than an inch) or am I on my own parting bigger diameters? I always thought the problems with parting tended to be on larger diameters up above 2 inch or more or on really small machines which are correspondingly less stiff.

Mark

[Michael HornerParticipant @michaelhorner54327]

Posted by Chris Trice on 27/02/2015 15:45:20:

I still think it's about the tool tipping forward into the work as the load on it suddenly takes it past the point it can shear the metal away which dramatically tilts it in even further into the work and it digs in. To return again to the toolholder in the article that started this thread off, if it works, we'll know, or at least be nearer a conclusion.

Edited By Chris Trice on 27/02/2015 15:45:49

Hi Chris

If the tool is on centre height how does the tip bite into more metal? If the tip is deflected down with a pivot point somewhere near the toolpost bolt it will be swinging out of the metal, which is how I envisage the swanneck tool to work.

Cheers Michael.

[Chris TriceParticipant @christrice43267]

For the same reason that an upside down rear toolpost flex's away. Dip back into the thread rather than having to repeat it all.

[Chris TriceParticipant @christrice43267]

The principle of the toolpost in the article which started the thread is based on flexing the tool away rather than inwards. Someone somewhere obviously believes it happens otherwise there would be no reason for this design. Emphasis is always put on rigidity for successful parting off which does indicate something moves if it's not present.

[blowlampParticipant @blowlamp]

If a jam does happen, then isn't it just as likely for the workpiece to lift and try and ride over the top of the tool?

Martin.

[Mark CParticipant @markc]

Martin,

I doubt if it is likely, more of a certainty when you think about what is going on and how everything is arranged.

Mark

[KWILParticipant @kwil]

Mark,

I will part every all the sizes I work in up to 100mm dia PB/MS, after that I am running out of blade!

[Mark CParticipant @markc]

Yes, me too – I tend to open the cut up a bit on big stuff to save any problems (2x blade width)

I mainly have trouble with aluminum over a couple of inch – the 2011 grade 4 inch bar I was using the other week was a pain and needed the speed altering as the cut progressed (i have inverter drive which helps).

Mark

[Anonymous]

Posted by Mark C on 28/02/2015 00:52:02:

Out of interest, does everyone part small stuff (less than an inch) or am I on my own parting bigger diameters? I always thought the problems with parting tended to be on larger diameters up above 2 inch or more or on really small machines which are correspondingly less stiff.

Mark

I part off all sizes up to my tool limit of about 4" depending upon the job. A lot of smaller diameters are already cut to rough length anyway as I don't have the room to have 2+ metres of stock hanging out the end of the headstock.

Andrew

[Mark CParticipant @markc]

Andrew,

I generally cut my bar stock into 1/3 length bits – this will then fit in the lathe if I need something long and is easier to store. Bigger stock (2 inch and over) has to be shorter so I can move it about (a 2 foot lump of 2 inch bar is plenty heavy enough when you are trying to be careful).

Mark

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