Kevin, Do you know what you are restarting? You may have missed the earlier corrections from George to anyone that posed the suggestion that higher speeds can produce better finishes. I agree with you, but then I have never turned with tools sharpened to the edge of a carver's chisel, like George. Mike Darlow's books provide a good alternative to those that lack any serous attempt to delve into details of what is going on as you cut. Remember, look both ways before turning...or is that only with crossing? Alan
Don't agree. It's pure baloney. Note that "strength" on its own is a meaningless term, because it does not specify in which aspect the wood is loaded. What is fairly obvious is that wood is "softer" in all loaded directions wet than dry, which is why friction heating would result in a tradeoff in increased strength in any load direction as the wood began to dry from EMC as it warmed. This would continue to the boiling point of water, where evaporative cooling would cease, and the wood approach zero MC. See FPL pubs.
Not to mention the obvious - that metal in the tool draws and transmits heat far better than wood - check the K - and the way to increase friction heating is to increase friction, which, if you look at your equations, is a result of the coefficient of friction times the pressure exerted (mass, in static applications). Speed is not mentioned, but it would certainly begin to trade off in convective cooling from the increased airflow over the rotating surface even as you attempted to heat the wood by pressing down. Of course, the wood will have already been cut by the time the bevel generates any heat to dry it.
Pressing on the wood messes up the cutting, too. Causes the tool to follow the differences in hardness in the wood, instead of making things circular.
If we are talking about woodturning, isn't it "obvious" the strength to which he refers would be that of the fibres to be cut by the tool? It is unlikely to be the tensile or compressive forces the blank can withstand or its hardness to a punch test is it?
What is fairly obvious is that wood is "softer" in all loaded
If the temperature continues to increase, any evaporative cooling would seem to be insufficient to prevent the temperature rise...wouldn't it? So what is the point of mentioning it? The moisture loss would be a surface phenomenon and I'd expect it to soon move back to the EMC. If the temperature is elevated, would not moisture movement be increased as well (more energy present)? Not that I can see how this impacts upon the cutting speed used...which was what initiated your reply.
A statement of fact, but why do you mention it? The heat is generated at the junction of the two materials and surface temp of the wood will reach something of an equilibrium in "normal" conditions where turning is regular/continuous; let us say planing cuts (unless you are attempting to scorch the wood, but even here both surfaces in contact woudl be at similar temps) The metal tool will conduct some heat away from the contact point more efficiently than the wood, but the temperature at the contact points of both materials would be pretty close. If anything was hotter I'd suspect it to be the wood, but the metal should absorb any excess heat so the cycle continues about an equilibrium point...doesn't it?
- check the K - and the way to increase friction
Increased speed will increase heat generation and I doubt anyone that turns would agree that convective cooling would be significant whilst friction generating the heat was present. As an example, would you say heat build up when sanding is faster at high revs or slow? That is why it is suggested to sand at a low speed; to minimise surface heating that may result in surface checking.
Only if you start from 0 revs or mean the cut during one revolution. Once you start cutting the bevel is in contact with the wood immediately adjacent to the cut and if any significant heat is generated it would surely spread away from the cutting point.
I'd agree, but only in a case with a grain orientation that permitted the tool to cut for more than an instant in a growth ring; I would not expect it with spindle turning (unless the speed is very slow), but perhaps when facing a horizontal trunk section where the rings are concentric to the lathe axis.
An interesting debate, but more of academic (?) interest to a most I'd suspect.
No, strength is measured as deformation to failure under load. The point of _cutting_ is to sever with minimal deformation. Unless, of course you're talking about beating the fibers into submission with a dull or poorly presented tool.
Seems that's what you're proposing. Though the proponents of toothed "gripping" keep trying to say that there's no damage done on their punch test.
Moisture evolved from the object will cool and lubricate the bevel following the cut. It _is_ independent of speed, as I contend and you seem to acknowledge.
The heat is generated
Two words - Heat Sink. If you look them up, you'll figure it out. The one with greater conduction cools the less.
You want to look up the basic friction equation. Once the piece is moving, friction is the product of the coefficient of friction and the mass or applied force keeping the surfaces in contact. Press too hard with sandpaper and you risk _eventual_ drying and checking of the surface. Of course, if it were being cut, it would already be gone. Perhaps that's why you don't see checks behind a friction-heated tool whose hot nose your fingers can't tolerate. Of course, some people fool themselves when cutting as with sanding by producing a smooth-looking burnished, hardened surface which becomes a mess once moisture is re-applied. Of note, it is the continued pressure of the bevel _behind_ the cut which compresses and heats the very surface of the wood.
You need to learn more about your material. Consider the relative closeness of the late wood rings as a harder face (quarter grain), the spreading area where it transitions to a face of purely soft early wood as a softer face (face grain). A tool which spans one and a half year's growth or less will react accordingly as if it's cutting harder or softer material. Thus it applies to spindle turning as well. Anchor the tool and let the material cut itself, don't push and chase it, or you will loose circularity.
No debate, just an attempt to get you on board with the knowledge required to generate one. So far you don't seem to have hit the books. Curiosity can be contagious. I'm not the kind to see things happen without seeking to discover how. That "be a student if you wish to be a teacher" belief. Kids used to tease me about teaching Physics in the shop, but there were days when a good part of the room was still debating/working on equations long after the ending bell. Understanding the principles makes the specifics a lot easier to master.
Hope the bell for recess rings before we are assigned homework or a term paper.
Charlie may be too busy taking notes to fear for his bearings.
Likely, anyone who understands this thread has progressed beyond 'learning to turn' and is now 'turning to learn'. IOW, they can find out for themselves by practicing, whether 'speed kills' or 'sloth is sinful' or like most of us, find both to be useful speeds without much deep thinking.
'The devil is in the proprioception'. The bell hasn't rung, so as an assignment add that to your palimpsest and discuss. 'G'
Can you explain how tensile strength is tested now? It certainly was a destructive test some time ago (not that this has anything to do with turning).
The point of
I know it is, that is why the force must be sufficient to cleanly sever the fibre. Not all fibres are of equal strength. it would seem to follow that for a given sharp tool, the force must be sufficient to cut before deformation. I have a question...would you say that a firm fibre would be potentially easier to cut than a softed fibre that may deform more easily?
I'm not trying to bring that into it at all, I was more trying to limit the discussion to factors appropriate to woodturning rather than construction. I have no idea what the toothed "gripping" is to which you refer.
Why must you try to talk down to people? My point was that the difference would be minimal. Please tell me what the difference in temperature will be between the timber and the tool tip. Also, I'd like to refer to the source of this information if you can provide it please.
I'm sorry, if I want to apply a wax finish...heat builds up faster at a higher speed when pressure is much the same for all speeds used. This seems to be a commonly expressed opinion and I can't fault it in "use".
Press too hard with
.... and generates the subsurface damage that was disussed before the guy that started this thread had used the chuck.
I know you tend to turn at slower speeds than most (from your continued mention of flying bits of wood (or similar)), but in spindle turning I find it imposible to accept that the tool could be moving in and out as you turn to an extent that results in a visible (or of interest) loss of circularity. I am sure I would not be turning items out of round in spindle work. Just work out what the tool's reciprocation rate would need to be for a spindle turning. I find it hard enough to accept this as a significant issue with bowl turning.
I don't want to burst any ballons, but you seem to be saying you know everything...a dangerous position to adopt. You may have noted I don't simply accept everything and this includes your frequent broad brush strokes about how you've looked at the physics and the equations..Similarly, attacking someone that challenges your lecture does not impress me. You make many statements, and intimidate in order to have them accepted. You commented on the heat sink effect of the tool, again I ask, what the difference in temperature will be between the timber and the tool tip. Also, I'd like to refer to the source of this information if you can provide it please.
Oh, if I didn't want to explore the ideas further, do you think I'd bother responding at all....Catch up again in a day I guess.
The methods of test are described in the wood handbook (FPL) in the appropriately-named chapter. It's the one I keep referencing in hope that we can agree on it's being common ground.
I have a question...would you say that a
Basic turning principle that the fiber to be cut should be supported by its fellows. Thus "turning down hill" and turning down grain. Means the softer cuts easier.
A heat sink in the form of the tool, resistance to heat flow in the wood means the tool is heated well, the wood, hardly. That plus the undeniable, that the tool remains in contact with wood, while any section of wood is only briefly - and less briefly if you follow the faster is hotter theorem - in contact with the metal, should be enough to understand that the wood doesn't heat in normal cutting.
You're mistaking lack of cooling for friction heating. Look at the information in reply to heat sinking, and consider how much less time the _same_ wood has to cool when you're friction polishing.
Get yourself some calipers and try it. Simple enough experiment, really. Press to just the first point of squirm (caused by differential cutting resistance!) on one section, hold tool distance on another, then see what results you get. If you skew along the cut with an edge held like this (/ or \), you'll get a better "average" of grain differences, and so a more rounded piece. Oddly, that's the planing conformation of a skew chisel. Almost as if they knew how to get the best average, and thus the best round. Of course, other factors make it more significant in bowl turning across the grain. Greater centrifugal force distorts the bowl, greater resistance when grain is picked up rather than peeled, difference in the coefficient of friction on end versus long grain, jumping the hard latewood ridge between two areas of earlywood as you come across the bottom and so forth. That's what makes for chatter - the wood flexing / tool moving. It's a relative thing. The less tool force applied in the direction of the natural flex, or the less flex permitted, such as with a steady, the better. Pressing with the gouge is one reason why folks resort to scrapers to even surface and circularity. Some increase speed, of course, but unless they back off the radial force, that just makes smaller chatter patterns.
Now who's talking down? You have demonstrated a lack of understanding which is not conducive to good turning, and I am attempting to ease your way by getting you to think. Things won't be as much of a mystery to you if you understand the principles which limit what can or cannot happen.
You may have noted I
I'll get the lab right on it. Let me know how near to the edge and how insulated from the tool I'll have to get to satisfy you. Of course, if you can demonstrate that wood conducts or acquires heat anywhere near the rate of HSS, I won't have to. You'll find information on the thermal properties of wood in the same source I keep referencing.
Thanks for the ad hominem. As they are generally just a fig leaf for the ego, designed to avoid concession, perhaps you've gained some understanding, as uncomfortable as the source may have made you. I will continue to answer the questions you pose to the best of my ability and knowledge, even when they're not questions at all, but foundationless assertions.
here's an easy experiment to prove that cutting heats the wood - I won't waste time arguing about the mechanism
put a 24 inch wet bowl blank on your lathe, take a 3/4 inch bowl gouge, and hollow agressively, so that the shavings are about 1/4 inch thick by 1/2 inch wide - make a nice long cut so you have a good pile of fresh shavings - Oh, Look!! they are steaming - Gee, they must be getting hot!! Amazing isn't it. Oh, feel the tool - not very hot.
Congratulations, you've just demonstrated that water absorbs heat faster than steel. I'm sure it's not news to most of us, since we use water to cool steel beginning with blooming in the mill, to grinding at the wheel. Now, would you care to tell us what it has to the question at hand, which is whether hotter wood is softer than cooler?
Of course, I'd prefer to save my elbow and hollow cleverly, rather than aggressively so I don't build up all kinds of heat.
Sorry George, The FPL meant nothing to me in your earlier post, though perhaps in the USA it is a well known abbreviation to describe the USDA publication (I'm in Australia). I have located it online and am downloading the current edition. I'll take a look later.
I understand the "down hill" fully, but can't see how this practice means softer cuts easier. I wondered whether you found firmer fibres, which would likely have firmer support cut easier (giving good finish) than softer. I expect you can follow my thought here, being that a fibre that is better supported will cut more cleanly and the resulting surface be better.
I agree that the tool is in constant contact and hence should increase in temp faster (and this would occur in a steady cut until an equilibrium temp was reached where the friction generated heat "balanced" the conductive heat up the tool. I cannot accept that the wood doesn't heat up. If it does, and each small area of the surface is only in contact with the tool for a brief time, cooling would be slowed due to the poor thermal conductivity of the air. Cut shavings would remove a quantity of the heated wood mass, however.
Are you suggesting that a speed change, when other factors remain constant, will not affect the rate of heat generation (no matter how the friction is created, tool , paper, cloth,). Heat either is, or isn't, generated more quickly as a result of incresing speed (extra cooling would only be appropriate if additional heat was present to be cooled. Or do you believe that incresed speed increases cooling and therefore lowers the surface temperature?).
I think you are saying that the wood does heat up, but it cools faster when heated during the cutting, rather than polishing. This seems to follow. As heat is created by friction and you agree it is the product of a coefficient of friction (surface) and the pressure applied, the more surface passes the area under pressure, the greated the generation of heat must be...so heat temperature will increase faster at faster speeds.
You made the statement, I simply asked for you to justify it. Thermal conductivity is not a measure of whether a material CAN get hot, it is a measure of how well it conducts heat. Once hot, do not poor conductors cool slower? If the friction generates heat, you contend that only the tool gets hot and that heat is conducted along the tool. I cannot see where you demonstrate that the wood will not be very close to the temperature of the tool tip. Examining a table in a reference won't tell anyone the answer.
If the tool tip is at 50 temp units it can't be conducting heat from a surface at 30 temp units can it? I am actually attempting to follow your explanations, but as I said, I don't simply accept them.
Of course, if you
Perhaps you can check the meaning of that yourself as there seem to be several references to "my understanding" when I question or present an alternate case. I haven't put myself forward as having all the knowledge, just one of the few readers interested enough to pursue the discussion.
As they are generally just a fig leaf for the
You make it clear you are the one with the knowledge and you want to impart it. Great, I hope more join you, but please take a look at the way you do it. If challenged, you rely on references to a table of properties which have questionnable relevance to turning a fruit bowl or finial rather than taking teh time to simply explain it aand provide the proof. Rather than stating easily followed procedures to turn safely and enjoyably, you delve to levels most here have no interest in following. Woodturning can be performed safely and to good result without overburdening oneself with a physics or materials science degree. YES, it is interesting to discuss now and again, but don't feel most here care to delve to such depths. If they do, and there are few of them, little is gained by dismissing their opinions. Hell, one day you may find that someone else was actually be right ...
Turn safely and don't stand under a large flock of geese...well I suppose you could in some cases... if they are travelling at 1500 feet, say 15 miles per hour, no wind...does goose faeces falll with minimal air resistance? (safest to work it out with, and without, I suppose)...standing outside just got tougher. Maybe just carry and open umbrella.
Back to the sink concept. Water takes a lot of heat to raise one degree, as you know. Means it stays relatively cooler than what it contacts for a longer heating period. Difference in heat translates to faster heat flow.
Of course, it takes a lot longer to air cool ten grams of water than ten grams of steel, too. Now, if there is sufficient evaporative cooling ....
Exactly, thought I said that was a basic turning principle, support. Hard may snap, but soft cuts great. Soak one piece of pasta, leave the other firm and see how it works supported on a cutting board.
I cannot accept that the
Can you accept that the wood which heats by the bevel is behind the cut? Unless you've got a dull tool with a high coefficient of friction and are pressing (perhaps speeding up the lathe!) to attempt to remove wood like Bill, that is. That's the definition of sharp, isn't it? Low friction.
I believe the oft-referenced equations for friction, where the only velocity is the acceleration of gravity. Use your favorite search engine. I can't make Greek characters with this ASCII set, but Friction = Coefficient of friction * Normal Force (weight). If you use the search, you can find enough illustration and explanation to internalize it to your own satisfaction.
You're supplying heat at a faster rate than it can sink into wood or cloth, therefore the wax melts. Interestingly, at that point, the temperature remains constant unless you heat the wax to ignition.
Can't say it often enough. See the Physics. Faster is _not_ a player in generation, but merely shortens the cooling interval where there is constant contact.
The best cut is defined by the minimum friction it generates, and any area of wood is so briefly in contact with the generator or reservior (tool) of that heat that it can't heat much. Wood has 4 times the specific heat of iron, so it gains slowly. Then there's the cooling provided by the moisture in the wood, the effects of which are shown in the table of heating data versus strength.
There's the rub. Your personal firewall. Look at the information given. Make the effort to understand. This means suspending yourself, accepting and evaluating the information, not whether you like George. Once you integrate the physics, wood technology, and your experience, you'll find yourself nodding your head in agreement. As you continue to follow the group, you'll also find yourself shaking it in disbelief as such things as "greater mass is good, but only if it's heaped on the stand. Don't make your lathe part of the earth itself by bolting it to the floor," or "serrations provide greater grip without altering the surface of the wood," to name but two, keep coming up again and again.
Water does it in complex transform rate to boiling. The boiling point of water is the key - it releases energy cools itself and grabs some more.
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