Physics of lathe

[Dave Paine]

I should have said cutting wood by hand plane does not generate much heat, at least when I have hand planed pieces.

Cutting wood on a wood lathe generates a lot of heat. High velocity, deep cuts, etc. In wet wood it is not uncommon to see steam coming from the wood if deep cuts. A lot of the physics is likely the same as metal, just different force needed for wood vs metal, although some dense woods are difficult to turn.

 

[JPMacG]

The OPs question is interesting.

Is energy consumed (or generated) at a chemical level or crystal structure or atomic level by breaking apart the structure of the metal? It would take energy to put the metal back together again, so it seems that energy should be released by cutting the metal apart.

Do all the joules of thermal energy released in machining equal the joules of electrical energy that went into the motor (minus losses)?

 

[DHarris]

Man, those slow motion shots looks like they are pushing a butter knife thru modeling clay!!

 

[Dabbler]

Cactus Farmer The amount to work it takes to cut 4140 versus 12L14 is almost completely related to the shear strength of the material. No matter how sharp a cutter is, just ahead of the cutter is a zone that is deformed by the process - As it deforms it shears. 12L14 has a lot lower shear strength (that is why lead is added to the steel).

BTW machining heat treated and drawn 4140 is a lot easier to machine and produces a better finish. It is referreed to as 'hard turning'. you take much shallower cuts to compensate for the greater tensile strength. I usually rough turn it with the typical poor finish, heat treat and draw it to RC40, then finish the part. Depending on the application, that is good enough, or I can reharden and redraw it to the preferred characteristic.

 

[RJSakowski]

The OPs question is interesting.

Is energy consumed (or generated) at a chemical level or crystal structure or atomic level by breaking apart the structure of the metal? It would take energy to put the metal back together again, so it seems that energy should be released by cutting the metal apart.

Do all the joules of thermal energy released in machining equal the joules of electrical energy that went into the motor (minus losses)?

Things that release energy when undergoing a change tend to be explosive. Chemical reactions can be either exothermic or endothermic. An exothermic reaction releases energy (think nitroglycerine) while an endothermic reaction requires additional energy to be added in order to complete. Most reactions require a certain amount of energy to initiate them. This is what makes the reactants stable.

Regarding machining materials, I don't believe that much energy would be imparted into the material itself. Energy would be required to make the deformation which separates the chip from the stock and alters its shape but that energy would be heat from the friction incurred during the cutting process. However, energy must be conserved so the energy and/or work input to a system must equal the energy increase and/or work output.

In the case of the lathe motor, electrical energy is input to the motor as amps x volts =watts. (watts is a measure of power which is the rate of energy input) A 120 volt 1 hp motor might consume 8 amps of current at full load. That would be 960 watts. The output is the rotating shaft doing work and is 1hp which is equal to 746 watts. The missing 214 watts is energy going into heating the motor due to electrical resistance and friction. There are additional frictional losses in the drive belts and pulleys and gears further reducing the available energy at the spindle. Finally the cutting tool is working on the stock absorbing kinetic energy from the spindle and transforming it into heat energy. There may be some additional energy transformations such as a screeching sound, glowing chips, and sometimes sparks bit these eventually end up as heat as well.

So, yes, it is fairly safe to assume that all that electrical energy went into heat. That works well in the northern climes during the winter, not so well down in South Texas in the middle of summer. It is also the reason tha flood coolant is used in high volume machining operations

 

[JPMacG]

Thank you RJ. Way back, in my less than fond memories of physical chemistry from 40 years ago, I recall something about plastics or ceramics having exothermic fractures. This was the basis of my (poorly worded) post. But I certainly agree that most of the heat generated during machining is caused by friction.

 

[ch2co]

This microscopy film is old, but I found it to be very explanatory .

 

[ch2co]

I really DON'T believe that CCl4 Carbon Tetrachloride should be used as a lubricant, however.

 

[Wreck™Wreck]

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.

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.

You have no idea what interests I may have, (aside from over the top comedic performances).

 

[Downunder Bob]

Not really on topic but I found this video very informative on what is occurring during machining with various steels and tools.
Video cannot be attached because it is not from a approved site. I will try to attach a link.
Cutting Steel At The Microscope
http://www.snotr.com/video/9463

An interesting video, I have seen one similar some time ago, but it realy shows the amount of deformation going on, which is where most of the energy is being used.