Why are chips hot?

[strantor]

Chips are hot, we all know that, but (scientifically speaking), why are they so hot? The question is like "why is the sky blue?" and the answer is probably equally above my head but I'd still like to hear it if anyone knows the answer.

I know there is some friction involved but I don't think that's the whole story. There is shearing action too which generates heat (friction again?) But that doesn't feel like an adequate explanation either. I've never had a paper burst into flames from being cut with scissors. Have you ever cut metal with large shear press? Did it get skin-melting hot?

It seems to me that chips are hotter than they should be, at least as far as can be explained by the physics that I currently understand.

I'm pondering this as I look at the gnarly brand in the palm of my hand filling up with burn juice. Earlier today I was using a step drill to drill a hole into an electrical panel from the bottom up, when a wicked chip fell down and ended up between my hand and the drill grip. This is an exceptionally sharp step drill, very fast; it makes chips 1/16"-1/8" thick, that make a solid "ting ting" sound when they hit ground, sounds like framing nails falling on the ground. The chips are hard to bend by hand. Chips from lesser step drills are not this hot. Chips from normal twist drills are not this hot. This contradicts my expectation, which is that a sharper, more efficient drill does much more cutting than rubbing, and therefore the chip should be cooler. Something is going on here that I don't understand.

 

[MikeInOr]

Are the chips not as hot when you use friction reducing cutting fluid?

 

[MrCrankyface]

There's vast amounts of deformation going on when cutting metals.
The curling generates heat, the friction between surfaces generates heat. The thickness of the chip has an effect on how hot the chip gets(same energy but small mass of chip = higher temp) and so on.
Cutting paper with a scissor you have a near perfect shearing action through a material with weak bonding strength, not much energy gets converted into heat.
Shearing metals generates much more heat than paper but is still a planar deformation where you're forcing "layers" of crystals to dislodge at once instead of continiously deforming to create a chip.
Imagesearch "metal slip plane shear" and you get a lot of visualizations of the process.

Compare to something like this: https://blog.enerpac.com/wp-content/uploads/2020/09/built-up-edge-formation-min.png
Where you're trying to shear, curl and cut the metal at the same time, a lot more disturbances in the crystalline structures = heat.

I'm sure there's other factors at play as well depending on the material. Surely ductility plays a role in how much heat is generated from deformation etc.

A possible explanation for your stepdrill conundrum could be that it's simply doing so much work in a short amount of time hence still generates a lot of heat. If you were to run it at much slower rpm and maybe even take a break every 10 seconds I'm sure the chips would be cooler.
Judging by the thick chips they're probably creating a lot of heat from internal energy as a result of deformation.

Edit: If you're into reading, look into "Materials science and engineering" by David Rethwisch and William Callister. First couple of chapters is pretty much only about the internal structures and what's going on. I'd say 80% of the book is great if your goal is to understand metals better.

 

[cathead]

Positive rake cutting tools generate less heat than negative rake tooling.

 

[RJSakowski]

There were a couple of videos a while back that showed what actually happens in a shearing action when cutting metal. The chip actually goes through some impressive deformation. For lathe tools, it is common to grind a chip breaker into the tool. The chip breaker forces the chip into a tight curl so the chip is stressed beyond the breaking point and breaks into small chips instead of a ribbon.

Another way to look at it is in conservation of energy. We are putting a considerable amount of electrical energy into the cutting process and all that energy is ending up as heat. Some of it is in the tool and some is in the motor but the rest will end up in the chips.

What is interesting to me is that by proper design of a cutting edge, you can actually steer the heat into the chip instead of the cutting tool. I have to say that I don't fully understand the physics of it but it is impressive.

 

[markba633csi]

You are pushing and tearing molecules apart= heat
Just thought of something:
How much heat is generated in a typical car crash? How to measure that? LOL

 

[mmcmdl]

Obvious ! Chips are hot to let you know your top shirt buttons are not buttoned up . We were always taught that the heat should be in the chips and not the metal being machined . I guess all of the above are true .

 

[RaisedByWolves]

It’s called plastic deformation and is a state the metal goes through as part of the cutting process.

Imagine pushing clay with your hand and how it would pile up in front of it before curling up and flowing away in some direction.

ETA: Found a good video n this.

 

[John281]

Nearly all of the energy provided by the motor ends up in the chips.

 

[mksj]

General heat transfer when turning is that about 70-75% of the heat generated is transferred to the chip, 10% to the cutter and the remainder to the material being cut. When you factor in the size/mass for the heat transfer, that is a lot of energy transferred to the chip, thus it is stinking hot and then cools. Reason why it turns blue and then turns straw color as it cools. The temperatures 300-900C (energy transfer) as the metal is being sheared is quite high, and as the temperature increases the tool live is decreased. If the heat did not transfer to the chip (i.e. wrong cutting parameters) the heat would end up t in the cutting tip and it would fail quicker. Often when I can feel/see the cutting performance decrease the cutter interface can be red hot, and one is deforming/pushing the metal as opposed to shearing the metal. Coolant is the key to transferring the heat away as well as clearing the chips, but carbide cutting tools do not do well if the coolant isn't continuous die to the thermal shock if used intermittently.