A New Way To Cut Gummy Metals

Planning to machine some nasty aluminium alloy or some copper?

Coat it with Pritt or mark it up with a sharpie first!

phys.org/news/2018-07-metal-gummy-sharpie-science.html

I haven't tried this yet, but i will have a go next time.

Neil

Interesting Imremeber machining L34 alloy like cutting putty ! Terrible stuff to machine, paraffin was the best lube. Not likely to machine it now but worth noting for copper etc wonder how long before we see a commercial product !

If it really works it'll be remarkable. Will give it a try.

Regards

Richard.

It's a frustratingly poor article. Partly due to the rather garbled description and partly due to the researchers appearing to have made no obvious effort to explain or even speculate on the mechanism at play.

If this is somehow due to the behaviour (crumpling?) of the external surface where the solvent etc is applied, it's hard to see why it would differ from other liquids such as oils or emulsions (for instance) applied to the surface.

It's impossible to apply any form of cutting fluid to the actual shear face (at the cutting edge) although of course, the coating on modern inserts has this effect when up to temperature.

I expect somebody will play with this over the weekend?

Murray

Agree with Murray that it was a very poor article. Looks like PhD fodder pseudo research to me given the unrealistically slow speed of the demonstration cut and the absence of any video evidence of the crumpled chip effect. I'd have been much more impressed if they had shown the badly behaved crumpled cut off the untreated surface beside the well behaved cut from the treated surface.

I'd guess the postulated mechanism for the improvement is that the pritt stick, sharpie or whatever coating strengthens the chip so it flows straight up and away whilst the uncoated chip is so weak that it crumples against the cutter like the bellows on a concertina. Crumpled chip has lots of edges grabbing against the cutter surface so much more force is needed to evacuate the chip. This force also "reflects" down onto the crack driving process at the actual cut producing rough surface. The smooth chip from the treated material flow nicely over the cutter reducing force and eliminating "reflection". Nowt novel there. We have all seen this when a parting tool gets grabby just before the chip jams and both tool and work are wrecked unless super reflexed miraculous recovery is managed.

One of those set-up to demonstrate an effect things that really only apply over a very restricted range of conditions. Maybe if you re using a hand shaper or planer? I seriously doubt it has any relevance in the real world. The maths needed should be accessible. I'd start digging around the models for thin plastic film, paper flow onto big printing machine rolls and similar.

Lot of that sort of work about unfortunately.

Clive.

Maybe some top rake on the "cutter" shown would help reduce the crumpled chips in the first place, I don't think any of us would choose a zero rake tool for gummy metals.

Also if the DOC is right those crumpled cuts will break at the bends resulting on the metal coming off in chips rather than long strings, industry does not want long strinny swarf getting wrapped around their multi axis/tool CNC machines.

Edited By JasonB on 21/07/2018 07:00:20

Well, I use permanent marker for marking out and have never noticed any improvement in the cutting. It will be interesting to see the results of a controlled test.

Russell

We mustn't condemn the research because of the way it's reported. The abstract gives more detail;

Soft and highly-strain-hardening metals such as iron, aluminum, and tantalum, often called “gummy,” are notoriously difficult to cut. This is due to their tendency to exhibit redundant, unsteady plastic flow with large-amplitude folding, which results also in macroscale defects on the cut surface and large energy dissipation. In this work, we demonstrate that this difficulty can be overcome by merely coating the initial metal surface with common adhesive chemical media such as glues and inks. Using high-speed in situ imaging, we show that the media act by coupling unsteady surface-plastic-flow modes with interface energetics—a mechanochemical action—thereby effecting a ductile-to-brittle transition, locally. Consequently, the unsteady plastic flow with folding transitions to a periodic segmentation-type flow in the presence of the surface media, with near absence of defects on the cut surface and significantly lower energy dissipation (a reduction of up to 80%). This mechanochemical effect is controllable and not material specific, with the chemical media demonstrating comparable efficacy across different metal systems. This makes it quite distinct from other well-known mechanochemical effects, such as liquid-metal embrittlement and stress-corrosion cracking, that are both highly material specific and catastrophic. An analytic model incorporating local flow dynamics, stability of dislocation emission, and surface-media energetics is found to correctly predict the onset of the plastic-flow transition. The benign nature and simplicity of the media suggest wide-ranging opportunities for improving the performance of cutting and deformation processes for metals and alloys in practical settings.

"Scanning electron microscopy images of chip morphology in $\mathrm{Cu}$. (a) Characteristic mushroom-type structures on the chip free surface, a signature of sinuous flow, arise due to individual folds (yellow arrows) collapsing onto each other. (b) In the presence of an SA medium (glue 1), the flow transitions from sinuous to segmentation type, characterized by minor folding events in each segment (yellow arrows) and separated by periodic fracture surfaces (red arrows). The morphologies span the entire width of the chips."

Visit the link below, click on the images as the captions tell much:

journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.10.014009

The video shows a transition from treated to untreated, it also states that they are using high-speed imaging i.e. it's slowed down.

I think we need to try it in the real world before deciding it's bunkum.

In particular, I think their test is very relevant to tapping and die threading with soft metals.

Edited By Neil Wyatt on 21/07/2018 10:57:30

Nobody declaimed it as bunkum but you'd have to agree the original article made a poor job of presenting the work. It's a pity this is another one of those papers that is hidden behind one of these "pay walls". I'm a believer in open access to research papers. Bizarrely, the APS claims to believe in open access - but then charges for the privilege. Go figure, as they like to say.

It certainly seems to relate to the behaviour of the material surface in the "vee" where the original material is shearing into the compressed chip. If that is the case, you'd expect the effect to be more marked for shallow depths of cut - for heavy cuts, what happens on the surface will surely have less impact on the behaviour deeper within the shear zone.

Murray

Unless the cranks rapidly propagate beyond the surface?

Interesting stuff.

Neil

Would this mean applying glue or other medium to the surface and waiting for it to dry before each pass?

Ed.

Difficult to see that happening - it would require continuous cracks / openings to occur all the way across the shear zone. I've not heard any such suggestion previously. This would need to happen at supersonic speeds (literally in some cases) and there doesn't seem to be a requirement for the stuff to be mobile beforehand.

Mysterious!

So you would have to apply a new layer each pass of the tool. Not so bad with a PritStick, but other types would be a bit inconvenient!

Dont understand most of that Neil,but I sure learned a few new words. Not sure what they mean,but they sound great !!. Can,t wait to try them out !!

Looks like they made a shaper out of a Tormach 770.

I must confess that I find the pair of images incomprehensible:

The caption states:

Fixing the chips. A microscope image (about 400 micrometers wide) shows that the free side of the chip shaved off of a bare copper block by a cutting blade has a folded, “sinuous” texture (left). But if the block’s surface is coated with ink, the chip shears and cracks as it comes away from the surface, making cutting smoother and requiring less force (right). (See video below.) [Credit: A. Udupa/Purdue Univ.]

... but the right hand image is not easily comparable with the left.

[ hopefully this will be clarified if I can get to see the text ]

The video, however, is a very impressive piece of work and clearly demonstrates the effect.

.

If this work is credible, then there is a rather worrying implication that the use of "innocuous" markers and glues causes local embrittlement of metals .

.... Such effects are well known for plastics, but what if we risk inducing 'stress corrosion' by innocently labelling a metal pipe ?

MichaelG.

.

P.S.  ... Yes, I am aware of the statement, quoted by Neil, that : This makes it quite distinct from other well-known mechanochemical effects, such as liquid-metal embrittlement and stress-corrosion cracking, that are both highly material specific and catastrophic. ... but I fail to understand that.

Edited By Michael Gilligan on 22/07/2018 09:03:28

I have in the past used a piece of beeswax rubbed along the cutting line, works on bandsaws fine. Also applied to the blade helps. Sometimes a little smoke appears. when cutting thicker steel (1/4 inch)

Clive

No mention of cut depth in the article?
I might be led to conclude that the research is being carried out by operatives with no experience and limited knowledge of machining outside what they've read in other research.

I think it will work perfectly when the DOC is so small as to be of no use to any engineer, anywhere. :D

The article is probably not written by experience machinists.

If you follow the links to the abstracts one graph shows they are using cutting forces up to about 500 newtons, that's 100 lbf which is a pretty decent cut in a soft metal.

Neil

There is a dimension of 0.1 to 0.2mm mentioned on the page [linked at the top of the 'Abstract'] that I failed to specifically reference: **LINK**

https://physics.aps.org/articles/v11/72

[quote]

In the presence of a strongly interacting coating, the shape of the chip is quite different from that for the bare metal. Instead of a sinuous, folded surface, the chip fragments repeatedly into slab-like segments 0.1-0.2 mm thick. In other words, the metal deforms in a brittle, rather than ductile, fashion.

[/quote]

It should be possible to roughly scale everything from that.

MichaelG.