Physics of lathe

[Bill Kahn]

On a simple lathe cut, what happens to the energy? Energy being force time distance and both are happening.

Being neither a physicist nor a machinist I am puzzled.

I guess some energy must go to shearing. But, that energy would be the same for a 1mil cut as a 10 mil cut. I have no idea how tightly metal atoms stick to each other or how many such atoms one needs to pull apart. Maybe though we are really pulling apart the loosely bound atoms at crystal boundaries?

But clearly 10mil takes way more energy to make than a 1mil though the number of atoms pulled apart from each other is the same.

I guess the chip then bends (all my chips are bendy). So, is most of the energy actually used to bend the chip? I would guess that bending a piece of metal is probably proportional to like the square of the thickness. So, is that why deep cuts are so much harder to make?

So, energy is used to shear, to bend, and the chips to break. (And sound and kinetic energy (translational and rotational) of the chips.) And in shearing and bending heat is produced, but I have the sense that some energy is left behind in the bend, separate from the heat.

Clearly fuzzy language--I just don't enough physical science to state precisely.

I don't think this question really has much to do with hobby machining--maybe better for a physics list? But hobbies are funny--so many things about machining are interesting.

-Bill

 

[682bear]

It is mostly converted to heat.

-Bear

 

[RJSakowski]

Quite a bit of the energy goes into heat created in the cutting process largely through deformation of the metal. Heavy cuts on steel will produce blue chips, indicating a temperature of around 300ºC. After prolonged cutting, the stock will also get quite hot.

A certain amount of energy is used in just turning the motor and drive train. Most of this will also be converted to heat from friction, air resistance, and resistive losses in the motor. A small amount will be converted to sound.

 

[Dave Paine]

If you want to experience the energy of cutting material, clamp a piece of wood on a bench and use a hand plane starting with a very thin shaving, then increase the depth of the blade and keep making cuts.

Cutting wood does not generate a lot of heat but you will experience the energy to cut thicker shavings, especially if the wood is dense.

 

[Wreck™Wreck]

You have entirely to much free time.

 

[rzbill]

Nice one Wreck.

Bill, FYI bending will be a function of thickness cubed, not squared.

 

[RandyM]

You have entirely to much free time.

I know you were trying to be humorous, and it is easy to make fun of someone, especially someone trying to learn something that you have no interest in. I commend Bill for wanting to understand some fundamentals of his machine that most of us don't even consider.

Bill, this was not a dumb question, and we thank you for asking and help us give thought to something that few of us even consider.

 

[Tozguy]

Oh Wreck, it took me 10 minutes to stop my laughing to tears.

 

[magu]

I'll have to see if I can still find it, but somewhere I have some reference material on power required to make cuts with respect to different variables (speed, feed, material, tool geometry etc). While it won't directly answer your question, it may make for an interesting tangent to those interested in that sort of thing. It may take a bit though, that's from way back when I was in engineering school which was longer ago than I care to admit.

 

[Dabbler]

Bill, carbide manufacturers have studied this extensively... Too bad they don't publish much on the subject. The energy does get converted to heat and the residual vleocity of the chips. How the heat is generated is quite interesting... I think your analysis is very good, so here's a few things to add:

Some of it is generated in the shearing force of the chip, the frictional force on the chip with the top of the cutter, and the frictional force on the rotating piece. The force is different if you have a neutral, positive or negative rake tools.

Positive rake tools need the least horsepower, thus generate the least heat. The trade off is that on some materials, the cutter is prone to oscillate on the surface, giving an uneven finish. Positive rake carbide tools tend to be more fragile and don't last as long as negative rake tools. The chip puts less pressure on the cutter, which is a very big advantage on small hobbyist lathes. They leave a little more heat in the workpiece than negative rake tooling.

Neutral rake tools can give a better finish on troublesome materials but need more horsepower. Neutral rake tools usually leave more of the generated heat into the work piece. They are more durable than positive rake tools. Form tools, such as threading tools are almost always neutral rake.

Negative rake tools take a lot more horsepower than the other types, as well as needing more rigidity in the tool holding system, all the way down to the lathe bed. The positive is that the majority of that extra horsepower usually is transferred into the chip and less into the work. (I wish I knew the physics of that). Negative rake tools last a lot longer, have more tolerance for interrupted cuts and are favoured in production shops and CNC equipment.

Bill knowing these things can help you to get shortcuts to a better finish and longer tool life!