# Physics of lathe



## Bill Kahn (Dec 7, 2017)

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 (Dec 7, 2017)

It is mostly converted to heat.

-Bear


----------



## RJSakowski (Dec 7, 2017)

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 (Dec 7, 2017)

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 (Dec 7, 2017)

You have entirely to much free time.


----------



## rzbill (Dec 8, 2017)

Nice one Wreck. 

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


----------



## RandyM (Dec 8, 2017)

Wreck™Wreck said:


> 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 (Dec 8, 2017)

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


----------



## magu (Dec 8, 2017)

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 (Dec 8, 2017)

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!


----------



## higgite (Dec 8, 2017)

Energy vs work is always a fun adventure. Google is your friend. 

Tom


----------



## Vacuum (Dec 8, 2017)

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


----------



## Uglydog (Dec 8, 2017)

Other than the link which Vacuum posted before I did, I don't have any insight on this thread.
Regardless, I will be following the thread closely.

Daryl
MN


----------



## owl (Dec 8, 2017)

Funny you should bring this up.  If you look at the history of thermodynamics, Count Rumford (Benjamin Thompson) originally developed the concept of the calorie while watching cannon barrels being bored during the Napoleonic wars.  So this issue has been studied from the very beginning of the science.


----------



## CluelessNewB (Dec 8, 2017)

Slow Motion Youtube Video of Chip Formation.


----------



## tq60 (Dec 8, 2017)

There is a fair amount of data on this subject all over the place.

Look in areas of old where discussion of HSS tooling is involved send forming the tool shape.

Seem to recall much discussion on Relationships between angles and energy etc.


----------



## Dave Paine (Dec 8, 2017)

Interesting video.   Watching the metal in the chip being swaged it is easy to appreciate why there is heat generated in the cutting action.


----------



## Cactus Farmer (Dec 8, 2017)

CluelessNewB said:


> Slow Motion Youtube Video of Chip Formation.


I would like very much to have a printed copy of this explanation. It is very interesting as I have had simular thoughts but to a lesser degree of sufistacation.  Now it's time to reoil my framulator buchings!


----------



## RJSakowski (Dec 8, 2017)

Dave Paine said:


> 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.


Actually, it does generate heat, just not enough to  be noticed.

For example, if you were planing an oak board, using 20 lbs of force and moving at a half a foot per second, power required would be 20lbs x .5 ft/sec or .018 hp or 13 watts.  For a 1 ft. pass, this would be 26 watt-sec. or 26 j.  Oak has a specific heat of about 2 j./g-ºC so we would expect a temperature rise of 13 ºC/g.  A  1ft by 1/16" by 2" shaving has a volume of 1.5 of cu. in. or 25 cc. and, at a density of .7g/cc., a mass of 17g.  This would be less than 1ºC temperature rise in the shaving.  Also, at least part of the energy would go to increasing the temperature of the board.

I know from experience, the shavings coming off my power planer, my router,  and my power hand planer are warm when they hit my skin, implying a minimum temperature rise of around 10ºC.


----------



## Cactus Farmer (Dec 8, 2017)

OK, I do understand some of this but please explain how 12L14 machines like cheese and 4140 is tough and I can't take anywhere the cut I do with `12L14.  I know it's leaded but it's almost as strong and and it case hardens as well as anything.  Then there is 1144 or Stressproof, no welding but the chips behave themselves.  ???


----------



## Dave Paine (Dec 8, 2017)

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 (Dec 8, 2017)

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 (Dec 8, 2017)

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


----------



## Dabbler (Dec 8, 2017)

*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 (Dec 8, 2017)

JPMacG said:


> 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 (Dec 8, 2017)

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 (Dec 8, 2017)

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


----------



## ch2co (Dec 8, 2017)

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


----------



## Wreck™Wreck (Dec 8, 2017)

RandyM said:


> 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 (Dec 8, 2017)

Vacuum said:


> 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.


----------



## Downunder Bob (Dec 8, 2017)

Dabbler said:


> *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.



The lead acts as a lubricant, as well as lowering the shear strength.


----------



## whitmore (Dec 8, 2017)

Bill Kahn said:


> On a simple lathe cut, what happens to the energy? Energy being force time distance and both are happening.
> ...
> I guess some energy must go to shearing.



That's true; you have (at least) to break the chemical bond that holds the material together,
and the amount of energy must be at least the area of the newly created surface
times the energy per square inch that is the 'shear strength' of the material.
The Charpy shear test is intended to determine this energy.
There's also reformation of the bonds near the surface (cold working) especially
if the tool is dull... and plows rather than shearing.

That surface energy is why heavy cuts and coarse grinding is preferred for fast
material removal: it's more energy efficient, makes the least new surface.

On really tough materials, any fracture, rather than parting cleanly, takes a forked
path (forced sideways by inclusions like glass in fiberglass, or by vanadium
carbide needle crystals in tool steel) so that much additional energy can be
absorbed.   Deformation like that causes local heating, so sharp hard tools
that slice clean are energy-savers; sharp abrasive grains are very effective.  Accidental adhesion
with the tool causes friction heating, so cutting 'lubricant' also lessens the
energy need (it poisons the surface sites that would form weld-like bonds
with the cutting tool).    The TiN coatings also deter bonding with some
sticky metals (aluminum for one).   The cutting tool relief angle, and any chip-breaker
grooves, lessen the energy cost by reducing such adhesion, too.

Some alloys and some tools are just TOO weld-prone, or worse.   That's why
diamond wheels are less effective than CBN on steel.

Chemistry, and physics, are complex enough that it's easier to cut-and-try than to predict.
For well-understood physics and quick cuts, use shaped-charge explosives.  There, 
you want prediction rather than trial-and-error.


----------



## RandyM (Dec 9, 2017)

Wreck™Wreck said:


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



Well, obviously this topic is of no interest to you. Then maybe it best you refrain from posting until you have something of value to add.


----------



## higgite (Dec 9, 2017)

The Turbo Encabulator post is the most “liked” post of any in this thread by two-fold. Maybe I'm wrong, but it struck me as some friendly humor for us non-physicists and physicists alike, nothing more. Let's keep the Friendly Forum friendly. Just saying.

Tom


----------



## DHarris (Dec 9, 2017)

Whitmore, excellent explanation! Thank you


----------



## Bill Kahn (Dec 9, 2017)

whitmore said:


> That's true; you have (at least) to break the chemical bond that holds the material together,
> and the amount of energy must be at least the area of the newly created surface
> times the energy per square inch that is the 'shear strength' of the material.
> ...


Thank you Whitmore.  You made part of the puzzle simple.  The energy goes, in part, to shearing and is proportional to a constant k1 (dependent on the cutter and the material), a constant k2 (dependent on the material alone), and the surface area created.  Lots of other mechanisms explained in other posts on how the energy ends up as heat.  And, in addition, energy is used to bend the chips.  One poster said this was proportional to the cube of their thickness.

So, I now have a more refined question.  If you look at the actual force on the cutter (conceptually easily measured with a force gauge, but I have now clearly learned ain't nothing easy in practice), and the linear distance there is a total amount of mechanical work done (e.g. energy expended) in some fixed time (yes, work per time is power, but I'll just think of work right now).  Through multiple mechanisms much of that work becomes heat.  Even the sound produced ultimately become heat.  But, there seem to be two (at least) receivers of the energy which do not end up as heat:
1) The shearing used energy.  (Yes, there is heating too, but the shearing itself uses energy independent of the heat). And 2) bending the chips.  (Again, yes, the chips get hot, but simply bending a rod take energy independent of the rod getting warm). 
So, a refined physics question...

How much of the mechanical work put into the workpiece ends up a heat?  How much is in the shear?  How much is in the bending?

I almost wonder if chip breakage is actually important separate from ugly surface-finish-damage-inducing sharf.  If the chip didn't have to bend at all the I could cut 10 mil as easily as 1 mil.  I have the sense that the radius of curvature at the end of a curly chip is less than in the middle.  So, a set of broken chips of equal total length to a single long one I think would have less total curvature.  And so less total energy.

Might it be the case the core mechanism enabling deeper cuts (all else being equal) is getting the chips to break so you don't have to bend them as much?  Could bending the chips actually be a major use of the input mechanical energy?  

Am loving this hobby--even while I muse on the physics, today I made my first internal threads (1" 20TPI).  How absolutely satisfying to have made my own coupler and threaded rod.

-Bill


----------



## Boswell (Dec 9, 2017)

I have looked at these videos several times and they are facinating. This time I noticed something I had not before. The chip is being compressed. If you look at the speed the metal is moving past the cutter, it looks to be much slower by at least half than the non-chip metal, that to me implies that it is being compressed and thus another energy sink.


----------



## whitmore (Dec 10, 2017)

Bill Kahn said:


> How much of the mechanical work put into the workpiece ends up a heat?  How much is in the shear?  How much is in the bending?
> 
> I almost wonder if chip breakage is actually important separate from ugly surface-finish-damage-inducing sharf.l



We can look at the motor power at idle, and under heavy cutting load, and get a fairly good idea
of the energy use, and that will relate somewhat to the sheared area (which we can tell from
examining the swarf).   What one learns, becomes part of a SFM recommendation...
so looking at tabulated surface feet/minute for the material ought to be a starting
point (and refine the edge and coolant/lube if that recommendation isn't working).

Chip breakage may be VERY important, as it determines if the
swarf was a spring compressed on the cutting edge, or if the cold-work damage to the swarf
was high enough to produce an occasional brittle fracture. Broken chips means less friction, and probably
a cooler edge.

It isn't clear how to interpret chip breakage (or temperature, or color), though.
Chips fracture easily with a non-ductile or low tensile strength workpiece (cast iron).   
In a sense, steel wool and bronze wool are useful spring-tempered products made on a lathe
in the absence of chip breakage.
So, if the chip character changes, probably consider resharpening.   Otherwise, I dunno.


----------



## john.oliver35 (Dec 10, 2017)

I have a comment related to the where all of the energy ultimately ends up - this is probably not helpful to machining, but the engineer in me just had to blurt it out  I believe that at the end of the day all of the energy has been dissipated as heat:

* Deformation of metal raises the temperature of atoms as they slide past each other
* flying chips hit the floor and transfer all of their kinetic energy to deformation and vibration of the colliding surfaces
* Bent chips either spring back to lower energy state or are permanently deformed, dissipating the bending energy imparted onto them as heat
* Audible noise and vibration are transferred to walls, floor, and air where they heat those bodies

In the end, every watt of electrical power that came from the outlet ends up heating the room.  I guess one tiny exception would be the energy retained in residual stresses within a part.  This energy would be stored as potential energy until released by a future machining operation, or as heat as the part warps!


----------



## KBeitz (Sep 17, 2018)

Some of the energy is converted into light. Not all is in the band we can see. some is in the inferred range.


----------



## RJSakowski (Sep 17, 2018)

Some of the energy will be stored as internal stress in the chips.  This is potential energy which can later be released.  On a larger scale, machining CR steel or aluminum extrusions can relieve some of the stress, creating an imbalance and subsequent warping of the machined part.

I don't recall that anyone mentioned sound energy either. At least in some of my machining, it seems like that may be a major issue.


----------



## silence dogood (Sep 17, 2018)

Bill has brought a really good question.  In high school machine shop, I had to turn a cylinder to a certain diameter.  I immediately measured it with a micrometer. It was right on, or so I thought. Took it to the instructor and he refuse to measure it because it was too hot.  Imagine to my surprise  after it cooled the part was under sized.


----------

