Parting Off MEW225

[Anonymous]

Let us look at some numbers for the forces involved. I'll take my lathe as an example; it has a 3hp motor. That equates to 2238W available at the motor shaft. The lathe has a V-belt drive to the headstock, and then, I think, two sets of gears to the spindle. I am going to take the efficiency of the belt drive as 95% and that of each pair of gears as 98%. So at the spindle we get an available power of 2042W.

Typically I would part off in 2" plus diameter low carbon steel at 370rpm; equating to 38.746 radians/second. We know that power is equal to torque times angular velocity. Re-arranging, we get a torque of 52.7Nm. Let us suppose we are parting a 50mm diameter bar, so the radius is 25mm, or 0.025m. This gives a force, at a 25mm radius, of 2108N. If we convert to kgF, we get a maximum of 215kgF acting on the tool. I must admit that seems quite high to me, but I cannot see the boo-boo in my arithmetic?

Looked at another way, I use a 3mm (0.118" )  wide carbide insert and normally set a feed of 0.006" per rev. So at 370rpm, on the outside of a 2" bar that is a removal rate of 1.65 in³/min. A rough rule of thumb says that 1hp will remove 1 cubic inch of material per minute in low carbon steel. So we're looking at about 1.65hp, roughly half of what the lathe can provide. So in terms of power probably rather less onerous than normal roughing.

If we believe these numbers I know which way I would prefer the forces to be acting, ie, down through the slides and saddle to the bed, rather than trying to lift everything off.

I have always assumed that chatter is a result of an interaction between tool and work, and the cutting forces involved therein. I would not have thought that well adjusted headstock bearings would have much, if any, influence. My experience of chatter is it always occurs on light cuts, normally finishing ones.

Andrew

Edit: to get rid the of the **** annoying, and unwanted, smiley!

Edited By Andrew Johnston on 08/02/2015 19:07:09

[Neil WyattModerator @neilwyatt]

It seems Denford used to produce a toolholder similar to Reg Metrryweather's for the Denford Viceroy:

Neil

[Neil WyattModerator @neilwyatt]

And possibly the strangest design ever is here Practical Mechanics

Neil

[JasonBModerator @jasonb]

Posted by Neil Wyatt on 08/02/2015 19:10:02:

And possibly the strangest design ever is here Practical Mechanics

Neil

Looks a nice rigid setup

[Anonymous]

Posted by JasonB on 08/02/2015 19:45:50:

Looks a nice rigid setup

Should work then, as we seem to be saying that rear parting works because it lifts the cross slide when a dig in is imminent. Surely the sloppier the better?

Andrew

Edit: does that count as an innovation over and above the JohnS flip up tooling?

Edited By Andrew Johnston on 08/02/2015 19:56:48

[Neil WyattModerator @neilwyatt]

Back to my pet theory of eitehr rigid as possible OR spring out of the work.

Neil

P.S. Just discovered that <ctrl>-U gives underlines

[Neil WyattModerator @neilwyatt]

Michael G. will like THIS sprung toolholder.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 08/02/2015 20:50:16:

Michael G. will like THIS sprung toolholder.

.

Nice one, Neil …

You could probably make one for the Mega Adept, out of a 1/2" Whitworth nut & bolt.

MichaelG.

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Edit: here is another interesting one.

•  and, back to Parting Tools; I just found this patent.

Edited By Michael Gilligan on 08/02/2015 21:59:37

Edited By Michael Gilligan on 08/02/2015 22:19:31

[Chris TriceParticipant @christrice43267]

I don't follow that?

If I assume a front mounted tool and start feeding the cross slide towards the lathe centreline, and then push on the cross slide it moves even closer to the centreline by the amount of play in the screw. So far so good. If I now assume a rear mounted tool and start moving the cross slide towards the lathe centreline (in the opposite direction to before) what happens when I pull on the cross slide, ie, in towards the work? On my lathe it moves closer to the lathe centreline by the amount of play in the screw.

Surely the rotating work is always trying to drag the tool inwards towards the centreline, irrespective of where the tool is?

Andrew

Because as the load builds up on the front mounted blade tip, the blade/toolpost combination are flexed into the work suddenly increasing the load enormously to the point where the cut stops and the work rides up and over the tool (or if you prefer, the tool is dragged under). Any slop in the feedscrew can only allow this inward grab to increase unless your gib strips are so tight, the friction overcomes the force. With a rear mounted upside down tool being pulled into the work, the more force you apply, the more the tool wants to flex away from the workpiece. If it's easier, imagine you have a small tree with a single branch sticking out at ninety degrees (the equivalent of our front mounted tool). If you pull down on the branch, the whole tree tends to flex towards you. If you lift the branch (rear mounted tool), the tree tends to flex away from you. Ypu have to assume that there is a tiny component of flex in the toolpost, the way in which it bolts to the cross slide and the cross slide itself. It may not be much but not much makes a difference when parting off which is already a heavy cut. And yes, technically, rear mounted tools of all sorts would work better if you can anchor the cross slide sufficiently to the bed. The whole system would in effect be the same as a swan neck tool.

[blowlampParticipant @blowlamp]

So would a front-mounted swan-neck be as satisfying as a rigid rear-mount : ) ??

 

Martin.

Edited By blowlamp on 08/02/2015 23:50:46

[HopperParticipant @hopper]

Dunno about the cross slide lifting theory. My old banger parts off much better with the rear tool post when the carriage is locked up nice and solid to the bed ways and the cross slide gib screws nice and firm. Leave the carriage unlocked or the gib screws loosish and the rear parting tool does a merry dance.

So I'd vote for rigid as possible.

Swan-neck sprung tools for general turning were popular a hundred years ago but there's probably a reason they have all but disappeared.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Hopper on 09/02/2015 08:21:18:

Swan-neck sprung tools for general turning were popular a hundred years ago but there's probably a reason they have all but disappeared.

.

… there's probably a reason …

It may be the [arrogant?] assumption that Machine Tools had become so much better that such a 'workaround' for chatter was no longer appropriate.

MichaelG.

[jason udallParticipant @jasonudall57142]

Unlike many other “religious” debate..
This one can be solved …at least on an individual basis.

Try it.
Use what works best for YOU. ..
I would like know if there is a definitive answer…
But since production has not come up with a SINGLE answer..I doubt there is a single answer.
So once again try it.

There is not this level of debate over general turning…we just accept that different approaches apply to different situations. .

[Michael GilliganParticipant @michaelgilligan61133]

Posted by jason udall on 09/02/2015 08:57:08:
Unlike many other "religious" debate..
This one can be solved …at least on an individual basis.

.

Fair point, Jason

… but it does rather go against the purpose of a forum.

MichaelG.

[jason udallParticipant @jasonudall57142]

Ahh Michael…
The difference between discussion and argument…

I agree that a discussion of this ( ..any..) subject has merit
I am willing to argue the subject. ( either of about 4 sides at last count).. (whether I am PREPARED for such an argument .not so sure)..
..my assertion is.
This is unlikely to have a single answer.

A) single direction spindle.
A1) ..with frount mounted tooling only
A2) rear mounted tool

B) reversible spindle
B1) forward and front
B2) forward and rear
B3) reverse frount
B4) reverse and rear..

Then we have the tooling itself. ..

The point I am trying to make. . Is one solution isn’t going to be it..for a given lathe one solution will not be it.

In production. .we might use dedicated slides…
We might use turret mounted tools..
We might use different widths of blade/insert
HSS blades of different X section. .
Inserts of various rake and nose shape..
Differing surface speeds whether constant spindle speed or constant surface speed

In all that there is an overriding requirement..to part off..
Ideally with the pip where you can best tolerate either on stock or part..
Added is finish on part and stock ( saves on facing don’t you know)..

.but hey thats production. .move along nothing to see here..

Choice of solution in many of our model making machines is limited to tooling not tool mounting front or rear..

Maybe we should share what works for us.

I suspect thouse who can use rear mounted ..do so..and not least because of frustration with front mounting.
Those who use front mounting do so as the only option.
Me?.. front mounted. .
Carbide..HSS blades HSS ground tools..and when all else fails hacksaw and face…but most of all correct tool height…corrected for whatever movement happens under load….so lets say a tiny bit high..

[blowlampParticipant @blowlamp]

If an inverted rear tool works by using compliance in the system to back off when a dig-in starts, then what is the mechanism which triggers the dig-in?

Martin.

[jason udallParticipant @jasonudall57142]

..the mechanism…
Stuff bends…but if it bends so as to reduce the bending STRESS then it reaches an equilibrium.

A dig in would occur when such bend (strain)..tends to increase the bend (stress )..which increases the bend (strain)..
..its called feedback..in this case positive. .digin causes more digin..

Edited By jason udall on 09/02/2015 10:02:11

[jason udallParticipant @jasonudall57142]

And ..triggers digin…
Load such as movement in the tool presentation changes..

[CabengParticipant @cabeng]

Can I offer a somewhat different view of the parting off situation, arrived at after many years of expensive research – a.k.a. playing about and writing off parting tool tips? But first, some caveats:

1) I haven’t read the MEW article (I don’t buy the comics)

2) the following comments are based on the S7 & Connoisseur lathes, which I've used for umpty-one years, you'll have to make up your own minds re their applicability to your machine, if different.

3) This will take some time and space, I can't upload it as a single posting, and chopping it up, adding diagrams, etc. is a pain in the parting tool with this system.

4) I fully accept that I risk being accused of heresy by disciples of GHT, and hence being condemned to eternal damnation – after being burnt at the stake, of course!

But first, a response to Blowlamp's last posting – compliance isn't the solution, it's the problem! Think about a beefy CNC machine chewing metal off at high rates, parting like a knife going through butter – works perfectly, but it's very rigid, no compliance.

Kiwi Bloke1 wrote:

“The late, lamented George Thomas wrote pretty much all anyone could ever need to know about parting off (and many other topics) in his many comprehensive Model Engineer articles and his books.”

He did indeed write a lot of good stuff, but I certainly wouldn’t extend that accolade to his analysis of the front versus back tool post argument! As far as I know, he never actually claimed that his explanation was rock solid, e.g. in Model Engineer’s Workshop, p61 he writes:

“…I imagine (my underlining) that the main reason is that a tool subjected to a downward pressure tends to lean forwards and so dig in whereas upward pressure will cause the tool to move back and out of cut.”

So he’s not all that sure about it, then. Also on p61 is his sketch (Fig. 5.1) in support of the explanation:

He does say that the diagram is an over-simplification… well, dunno about that, I’d say it’s plain wrong! Firstly the directions of the forces on the tool tip are wrong, they should look like this:

That on its own is sufficient to get rid of the 'down & in' versus 'up and out' argument, but additionally GHT's choice of pivot points for the tool posts is entirely arbitrary, and bears no relation to reality! Unfortunately, if one starts from an incorrect assumption, one ends up with at worst an incorrect answer, and at best a misunderstanding of the problem – so given the above errors in the ‘explanation’ it is indeed unfortunate that GHT’s efforts have resulted in the imprinting of the ‘down and in’ versus ‘up and out’ argument into the genetic code of countless model engineers.

Muzzer wrote:

“We love taking simplistic views of problems. When we talk about the tool "springing", we are actually referring to the movement of the point of the tool relative to the point on the work piece where it acts. In between them is a whole series of slides, screws, bodies, joints etc – as well as the workpiece itself. The whole system is a combination of linear (sprung) and non-linear (backlash etc) elements, often with quite different characteristics in different directions”.

Absolutely right Muzzer, particularly the bit I’ve underlined, which hits the parting nail (and indeed any machining exercise on lathe or mill) smack on the head. What matters is the ability of the complete system to support the tool tip in a stable position relative to the workpiece, when the tip is subjected to the forces resulting from the metal cutting operation. So let’s look at that, having removed our GHT-coloured blinkers!

But first, can we agree that the problem of parting off can be parted off into two problems (sorry about that!), viz. chatter, and jamming?

Better stop there for this posting, or it's going to tell me it's too long!

[MuzzerParticipant @muzzer]

Posted by Neil Wyatt on 08/02/2015 20:50:16:

Michael G. will like THIS sprung toolholder.

Neil

I noticed the article on the previous page which explains how to tan muskrat skins – surely a valuable skill for any self respecting amateur mechanic. Apparently, rubbing them with butter or bathing them in gasoline makes them nice and soft. Must smell heavenly!!

Merry

[frank brownParticipant @frankbrown22225]

Parting off is no big deal to me, but I am not overly ambitious, every 1/2" or so in depth I retract the tool and make the slot wider.

I use a 3/8" X 3/32" tool with up to 1 1/2" sticking out of the tool holder. I do relieve the sides of it with a stone, don't know how much .002" per side? so the cutting edge is the widest part. The curious thing that I have seen, is that I always pull the tool post backward to remove all slack before I start, but when I stop piling on the cross feed, the tool does not stop cutting, rather still cuts at .001/revolution. I have never let it run for more then a minute so i don't know if its pulling the backlash back out of the cross slide or just an indication of some thing terrible to happen.

I wonder if some of the problems are associated with no side relief and the cross slide action is not central to the forces actually on the tool cutting edge is actually causing the tool to twist slightly allowing swarf to get down the side of it and causing the jam. I think I am suggesting that the "sharpness" of the cutting edge varies across its width.

The rear tool post only has the actual advantage of gravity pulling the chips out of the groove. The cross slide could pivot on its front , back or intermediate positon. The digging in is a natural knock on for a work  piece that is turning in the one direction. if you reversed the direction of the lathe and inverted the tool, a front mounted tool would not dig in as much as a rear one.

Frank

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Edited By frank brown on 09/02/2015 14:41:27

[Martin KyteParticipant @martinkyte99762]

Referring to the Merryweather Article it strikes me that the tool holder would work best bolted to the cross slide at the back with the cut taking place on the front and the lathe running in the normal direction. That way the forces on the bedways are the same (directionally) as normal turning and the pressure angle is still less than 90 degrees. This ensures that there is the same negative feedback from the cut in that it tends to push the toolpost away from the work and there is no tendency to dig in.

regards Martin

[Martin KyteParticipant @martinkyte99762]

. . . . and another thing. Most of this discussion assumes that the only movement occurs on the part of the tool holder setup. Whatever approach you use the work will move to a greater or lesser extent as it tries to ride up over the tool. Headstock bearings, spring in the work and probably most significantly the condition of the chuck and the number of jaws.

regards Martin

[Neil WyattModerator @neilwyatt]

Do read it properly Muzzer! You rub in the butter then wash it back out with gasoline! I've read large number of pages now – two recurring topics are fur trapping and mosquito control. Several things that look like ideas that were never tested in practice.

Neil

[MuzzerParticipant @muzzer]

The stomach churns. I wonder if the stench of rancid pelts had anything to do with the mosquito problem!

[Author]

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