Wikipedia:Reference desk/Archives/Science/2013 August 5

= August 5 =

CNC milling?
I've been running one of these 3D printers for a while now, making and selling various items, particularly decorative figurines and keyrings, and I'm wondering about trying this milling thing, opens up opportunities for all sorts of different materials. Thing is, I'm not too sure on what sort of machine I'd want, wondering at what the limits would be on these 4-axis machines, whether they would be able to carve the shapes I'd want or not. then again, would that not involve attaching the item to the machine in some way, leaving a clear mark on it? I realise I don't actually know how these machines work at all, would it be possible just to upload a 3D model file into one and leave it to do all the work, or are things more complicated with these?

213.104.128.16 (talk) 11:23, 5 August 2013 (UTC)


 * As far as leaving a mark where it's attached, yes, you would typically have some finishing work to do after it come off the machine, to grind/sand it down and polish it. StuRat (talk) 12:55, 5 August 2013 (UTC)


 * (I also have a 3D printer - and also a couple of laser cutters - and I'm about to get into 3D milling machines too - so I've done quite a bit of research into them).


 * There are basically two kinds of CNC milling machines - 3 DOF and 5/6 DOF. "DOF" means "degrees of freedom" and describes the number of ways in which the cutting head can move.


 * A 3 DOF machine can move in three axes just like your 3D printer can - but it can't create undercuts because it's removing material from above, not building it up from below.
 * A 5 DOF or 6 DOF machine can also tilt either the milling head or the workpiece (or sometimes both) to reach underneath overhangs...it can make some things that even 3D printers can't make...but even a 6 DOF machine can't make some things that a 3D printer can make because the cutting head is kinda large and can't make it into some small spaces. A 5/6 DOF machine is basically a robot arm holding an electric drill with a milling bit.


 * So to do most of what you can do with a 3D printer really requires a 5 DOF or 6 DOF milling machine...the snag is that those things cost an absolute fortune! Many tens or possibly hundreds of thousands of dollars.  3 DOF machines are vastly cheaper - and there are designs out there to let you make them for about the same cost as a 3D printer.


 * The term "4 axis" is a bit confusing. Sometimes it refers to a 3 DOF machine with the fourth axis being the rotation of the milling bit itself (which is cheating!)...rarely it means that this is a 4 DOF machine - which would be able to rotate the workpiece around one axis (like a lathe maybe) with a 3 DOF machine doing the cutting.  That wouldn't get you much further than a straight 3 DOF machine.  Almost certainly, if this is something costing under $10,000 dollars then "4 axis" really means "3 DOF".


 * To attach the workpiece to the machine generally involves clamping small pieces to the platform - which might not leave a mark - but does result in you having a block of material at the base of your object that can't be milled and may have to be sawed off by hand. For large sheet materials, some CNC machines have a vacuum table that holds the part in place.  Those only work well for relatively large parts.  Yet a third alternative is if you can drill a hole inside the base of the workpiece and thread a bolt into it.  You can then clamp the bolt, do the milling, then unscrew the bolt at the end.


 * In terms of working with these machines - they are very much like a 3D printer - you can upload a 3D model and (with appropriate software to plan the tool paths) just set the machine running.


 * To mentally visualize what a 3 DOF CNC machine can do, imagine you had a small electric drill mounted in your 3D printer in place of the hot-end extruder. Now imagine you lower the Z table all the way to the bottom (or raise the head all the way to the top - depending on how your 3D printer works) - then put a solid block of plastic into the machine.  Now, this hypothetical 3D printer could cut away material, one layer at a time by moving the head back and forth over the parts of the plastic that you don't want anymore - then raising the workpiece (or lowering the head) a fraction of an inch and doing it again.  The process is kinda like 3D printing in reverse.


 * If you have a website someplace with pictures of the things you sell, I could quickly tell you which of them a 3 DOF machine could make.


 * SteveBaker (talk) 14:07, 5 August 2013 (UTC)
 * You could try one of the milling services on the web, just like there are 3D printing services. That would give you a good feel for what you can get done and the prices are fairly reasonable. Have you tried sintered metal 3D printing? Dmcq (talk) 15:26, 5 August 2013 (UTC)
 * If you want a number of the same you could try casting in metal. and of course if they can be made from sheet metal there's all sorts of other options. Dmcq (talk) 15:35, 5 August 2013 (UTC)


 * 3D printers that can do sintered metal aren't cheap - they rely on high powered lasers, plasma arcs or something similarly expensive and dangerous to sinter the metal - and that's a very difficult problem for a cheap design. My two 100 watt (made-on-the-cheap) lasersaurs cost $8,000 each in parts alone - and 100 watts of laser power is nowhere near enough to do metal sintering.  So unless you're making a LOT of money doing this, you'll be stuck with paying someone else to make the parts on a machine you'll never be able to afford!


 * I know of one guy who made a cheap(ish) laser-sintering 3D printer - but his machine sinters wax laced with carbon...hardly metal...but you could use his wax objects in a lost-wax casting process.


 * But if you're thinking about making molds and casting objects in metal then another approach is to make a two-part 3D-printed mold and use that to cast metal objects. Obviously you have to have relatively low melting point metals and do your printing with the highest melting point plastic your printer can manage...sadly ABS and PLA (which are the two plastics that most 3D printers use) both melt at around 220C - which is unfortunate because metals like tin and lead melt at temperatures at least that high - so the mold would melt and be useless.  But there are a few metal mixtures that melt at much lower temperatures - yet at high enough temperature to be useful for things like jewellery and key-fobs - sadly, some of those are toxic, so you have to be careful. Field's metal might be a good choice - it melts at 62C - Woods_metal gives a list that includes all of the low-melting-point metals that I know of.


 * Another option is to make the object itself in plastic, then make a synthetic rubber mold from it and cast using that.


 * If you're interested in materials other than plastic - but would like to stick with 3D printing, this stuff should be of interest. It's made from wood-pulp and formed into a filament with a binder that most 3D printers can melt.  Printed objects look and feel like wood (although without any grain).  Some people are investigating using 3D printers with two heads - one loaded with light colored wood filament and the other with a darker version of the same material - and actually printing fake grain into the objects they make.


 * Fun stuff! SteveBaker (talk) 16:04, 5 August 2013 (UTC)
 * I was thinking of them sending off to a service doing sintered metal like for a milling service, sorry I should have made that clear. I'd have thought buying any machines would need to be carefully justified compared to sending off to a service which specializes in doing the work. Dmcq (talk) 20:22, 5 August 2013 (UTC)
 * Yes, I agree that using a service that can print in difficult materials is a good option - but from the point of view of someone who is attempting to manufacture things for profit, using a service like that will sharply limit how much money you can make - they definitely aren't cheap. For the laser-cut stuff that we do, it would be about 20 times more expensive to have the product made by a service than making it ourselves.  We could possibly use them for making prototypes - but unless a LARGE fraction of the final selling price is reflecting your design expertise, it's not a viable option. SteveBaker (talk) 16:35, 6 August 2013 (UTC)
 * Sounds good that wood material. Someone told me once they were going to use a wood effect paint and I thought they were joking - and they went ahead and got a nice wood effect with it! By the way for that wood material they can make it darker or lighter by using a higher or lower temperature when they melt it. Dmcq (talk) 20:38, 5 August 2013 (UTC)
 * Oooh! That's a really good idea.  (Am ordering wood filament for my machine now!)...do you happen to know what temperature range is needed to produce a substantial change in color? SteveBaker (talk) 16:35, 6 August 2013 (UTC)
 * It says on the page you pointed to, you can range between 180°C and 245°C, shows it is quite effective. Dmcq (talk) 17:45, 6 August 2013 (UTC)

Poison ivy for apes
Take a typical member of each great ape species and expose all of them to poison ivy. The human will get urushiol-induced contact dermatitis, but what about the others; are we the only species allergic to it, or will other apes also have problems? Neither article that I linked says anything about other species; I know that the plant doesn't cause problems for ruminants, but I know nothing about non-human primates. Nyttend (talk) 14:36, 5 August 2013 (UTC)


 * Having furry bodies helps them a lot. In order to get problems with poison ivy, it has to rub against your skin.  A thick layer of fur makes that rather difficult - so I would expect that other great apes would have much less problems with the stuff than we "naked apes" do.  The chemical involved (Urushiol) provokes an immune response that ultimately causes the symptoms. This article says "Deer and other animals can eat poison oak leaves without any apparent ill effect. Birds nest and find shelter in the plant. Only primates (humans, apes, chimpanzees, etc.) seem to have a reaction to poison oak." - so I guess you have an answer. SteveBaker (talk) 15:13, 5 August 2013 (UTC)

Drawback or limitation of modern synthetic theory of evolution or neo-Darwinism
What are some Drawback or limitation of modern synthetic theory of evolution or neo-Darwinism?

AmRit GhiMire &#39;Ranjit&#39; (talk) 14:48, 5 August 2013 (UTC)


 * So, to be clear, you're talking about Modern evolutionary synthesis (which is also called "neo-Darwinism" and which is the way that evolution is understood in the last 50 years) - which (according to our article) "is still, to a large extent, the current paradigm in evolutionary biology."


 * The most obvious unresolved issue for me is abiogenesis. How did the first self-replicating molecule come about?  Once such a thing existed, evolution became inevitable...but getting from "inert" chemistry to that is something of a leap.  There are plenty of possible explanations - most of them plausible - but we don't know exactly which of them is true.


 * Our article Objections to evolution covers a huge array of objections that have been raised - but few, if any, are thought to be problematic by the vast percentage of scientists in the field - they are generally trivially easy to debunk using easily obtained evidence.


 * Evolution is one of the most solidly well-understood and verifiable theories in science - there really aren't "drawbacks" or "limitations" in the theory.


 * SteveBaker (talk) 15:06, 5 August 2013 (UTC)


 * I have to agree, there aren't really any drawbacks. There are disputes over the interpretation of such things as kin selection and selfish genes, but these are actually metaphysical disagreements--no one disputes the facts.  Evolutionary psychology is ideologically controversial, but it's quite far removed from the modern synthesis itself. μηδείς (talk) 17:15, 5 August 2013 (UTC)
 * At least at the molecular level, the occasional impact of "Hopeful Monsters" shouldn't be dismissed entirely - things really do get abruptly reverse transcribed, recombined etc. to form dramatically new gene structures. Also, the theory doesn't really address the issue of heritable epigenetic change - the mechanisms making that possible are the products of evolution, but it means that the reaction norms of any given genotype are far more complex than previously imagined. Wnt (talk) 18:26, 5 August 2013 (UTC)

Who was Dr Price?
As a side issue to a question on the Language Desk, I found that the triple expansion engine (as used in the Titanic) seems to have been a British invention. According to History of British Shipping by Adam Kirkaldy, "Then Dr Price invented the triple expansion engine, effecting further economies in the consumption of fuel" (p.131). One was apparently fitted to SS Aberdeen (1881). Does anybody know anything about the ingenious Dr Price? His full name would be a start. Our Compound engine has a Spanish destroyer as the first in 1886, although it was built on Clydebank. Alansplodge (talk) 15:51, 5 August 2013 (UTC)


 * An article in The Quarterly Journal of Economics, "Substitution and Complementarity in Endogenous Innovation" (August 1993), discusses the rise of steam ships and cites:
 * Harley, Charles K., "The Shift from Sailing Ships to Steamships, 1850-1890: A Study in Technological Change and its Diffusion," in Donald N. McCloskey, ed., Essays on a Mature Economy: Britain after 1840 (Princeton, NJ: Princeton University Press, 1971).
 * Rosenberg, Nathan, Perspectives on Technology (Cambridge: Cambridge University Press, 1976). Ch. 11
 * I don't know whether either of these would mention Dr. Price, but they're the closest I come to any useful-seeming information so far. --some jerk on the Internet    (talk)  21:00, 5 August 2013 (UTC)


 * I found many texts over the decades which parroted the line "hen Dr Price invented the triple expansion engine.." without any details. But the actual designer of the first successful triple expansion engine, used on the Aberdeen (1881) for high speed London-Capetown-Melbourne runs was "Doctor" Alexander Carnegie Kirk, See Biographical Dictionary of the History of Technology (2013) - Page 694. Maybe Price was one of his associates. [[User:Edison|Edison] (talk) 03:52, 6 August 2013 (UTC)


 * Thank you both. I intend to put an entry on Timeline of steam power if nothing else. We already have an article, Alexander C. Kirk, which mentions the Aberdeen and her engines; how I didn't find it myself remains a mystery. One can imagine a muffled cry from the Aberdeen's engine room voice tube; "She cannae tak' much more Kirk!" ;-) Alansplodge (talk) 16:34, 6 August 2013 (UTC)

Epilepsy: is there a limit to how often seizures can be photoinduced?
The way I understand it, flashing lights, even TV episodes, can induce epilepsy - occasionally even first seizures in children who haven't previously experienced them. Yet epileptic seizures are typically followed by a postictal state where no seizure can be induced. What I'm wondering is, does the photoinduction actually increase the overall tendency to have seizures on a consistent basis, and if so how much, or does it just affect the timing of seizures? In other words, is there a sort of "fuel for epilepsy" that gets consumed during seizures, and the lights merely provide a "spark"? (I'm picturing a  hopefully  thought experiment where an epileptic al-Qaeda member is locked in subbasement 25 under Diego Garcia, in a cell with a stunning light show going day and night... by what factor are his seizures increased over the long haul?) Wnt (talk) 18:38, 5 August 2013 (UTC)
 * Yes, there is a sort of "fuel for epilepsy". Electrical activity in the brain depends on ion concentration differences between the interior and exterior of brain cells, and those differences run down during a seizure.  Afterward they are slowly reestablished by ion pumps.  And yes, seizures can increase the probability of future seizures, by means of a phenomenon known as kindling.  That doesn't always happen, but it can.  Seizures can be triggered in anybody by electrically stimulating the brain -- that's what happens in ECT -- but light will only trigger seizures in people whose brains are particularly vulnerable.  I don't know whether there is evidence that experiencing light-induced seizures increases the probability of future seizures. Looie496 (talk) 19:04, 5 August 2013 (UTC)
 * Very good point about the kindling model! I suppose the question actually breaks down into two separate conditions: (a) whether photoinduction can be used (perhaps together with isoniazid, thiosemicarbazide, harman, pentylenetetrazole, 3-mercaptopropionic acid, and/or strychnine) to help induce first seizures in non-epileptics and perhaps even to "kindle" long term epilepsy, and (b) what its effect is on those who already have longstanding epilepsy.  I was only thinking about the latter above, but of course, depending on the person either might be relevant. Wnt (talk) 19:27, 5 August 2013 (UTC)

Human decontamination
Hello everyone,

I was wondering what cleansing agents were used for human radioactive decontamination? Do they contain more tensioactive agents than normal soaps, or special chelating agents like EDTA to help remove metallic radionuclides? Or perhaps do they contain some abrasive agents (as suggests the term Abrasivstoff in Dekontafix) to help remove the dead layer of skin and along with it any surface contamination? Regards, 141.30.214.203 (talk) 19:15, 5 August 2013 (UTC)


 * While we didn't go into too much details during my basic training two decades ago, the mantra seemed to be lots of soap and lots of water. Granted, we were mostly taught about chemical decontamination - but no mention was made that nuclear decontamination would be done differently. WegianWarrior (talk) 19:24, 5 August 2013 (UTC)


 * A quick search turns up which seems to prefer dermabrasives (I actually didn't read much of this, but it looks useful). Wnt (talk) 19:31, 5 August 2013 (UTC)


 * Thank you for this paper; it looks like many solutions exist depending on the nature of the contaminant: EDTA for 99Tcm, soft abrasives, foams, detergents, etc. Always a pleasure to read the Ref Desk. Regards, 141.30.214.203 (talk) 19:49, 5 August 2013 (UTC)

Deep space travelers beware
What are some of the little known dangers of deep space travel? Are we attempting to solve them now or is that on the back-burner as we focus on inter-planetary travel closer to Earth? — Preceding unsigned comment added by 65.64.111.126 (talk) 19:21, 5 August 2013 (UTC)
 * Health threat from cosmic rays, IRWolfie- (talk) 19:53, 5 August 2013 (UTC)
 * Also the deterioration of the human body in low gravity, and the inherent dangers of travelling in an inhospitable vacuum vast distances from any help or hope of rescue or resupply, while completely dependent on high-technology devices for survival. -- The Anome (talk) 19:56, 5 August 2013 (UTC)
 * See also Effect of spaceflight on the human body and Space medicine. Dominus Vobisdu (talk) 19:58, 5 August 2013 (UTC)
 * And this is just for interplanetary travel. Deep space, in the sense of interstellar travel, also has the disadvantage that it is currently impossible -- and may well forever be impossible -- to traverse these distances in a human lifetime, or much vaster timescales in the case of intergalactic travel. Even travelling to the Oort Cloud, just a single light-year away in our immediate solar neighborhood, would present a major challenge. Other than the creation of faster than light travel or teleportation, possible hacks that conform to current physical knowledge which have been suggested to sort this out include generation ships, sleeper ships, "nearly as fast as light" ships which would exploit relativistic time dilation, and the use of von Neumann probes or starwisps to carry and re-create human life from stored information or biological material over the course of thousands, or millions, of years. Each of these may or may not be possible, and would in each case carry risks of their own. Unless something drastic happens to the human race in the next thousand years, I would expect someone's going to try at least one of these in that time. -- The Anome (talk)


 * Oh, and also don't forget the psychological effects of long-term isolation of tiny groups of people. -- The Anome (talk) 20:23, 5 August 2013 (UTC)


 * Ah, yes. The dreaded SPACE MADNESS! "I have had this ice cream bar since I was a child. People always trying to take it from me. Why won't they leave me ALONE!!!!" (Now I've got a hankering for some chocolate covered raisins. Popping out to the store... be right back!) Dominus Vobisdu (talk) 20:32, 5 August 2013 (UTC)


 * Yep. Space Madness (see here for TVTropes' take on this), a.k.a. the Pain of Space in "Scanners Live in Vain". -- The Anome (talk) 20:52, 5 August 2013 (UTC)


 * You asked for little-known problems, but most people responded with well-known problems. Here's some that might be a problem from years in space:


 * 1) Nutrition. We may not know about every vitamin and mineral humans need.  It's possible there's something vital that we run out of after years without eating real food.


 * 2) Exposure to chemicals. Materials in the ship might give off fumes as they age, which are not filtered out, causing long-term exposure hazards.  The same is true of packaging leaching chemicals into the water and food.


 * 3) Instead of isolation, people might have problems being stuck with the same group of people for years. For example, who is having sex with whom could cause some serious tensions.


 * 4) Lack of elderly care facilities. In a multi-generational ship, what do you do with people too old to care for themselves, especially those with dementia ?


 * 5) Too small of a gene pool. In a multi-generational ship, inbreeding will eventually be a problem unless, say, millions of frozen sperm and eggs are shipped with them to be used for reproduction.


 * 6) Freezer burn. If food, sperm, eggs, or even people are frozen, then slight changes in temperature, over time, can cause degradation. StuRat (talk) 21:26, 5 August 2013 (UTC)


 * Solution for (4) and maybe (1) too: Soylent Green. Clarityfiend (talk) 22:35, 5 August 2013 (UTC)


 * There may be social problems with a multi-generational ship, too.
 * In books this is always represented as a total breakdown of civilization and people forgetting that they're on a ship, but it wouldn't have to be that drastic. What if the people who arrive at the destination know full well they're on a ship, but don't want to abandon their home by going down to the planet? (Imagine giving up your home and roughing it on an unknown world because your great-great-great-grandfather agreed to be a pioneer!)
 * That's just one example. Just about any kind of social breakdown we have here on Earth would be worse if it happened on a generational ship. APL (talk) 08:51, 6 August 2013 (UTC)
 * Genuine inquiry here: I remember one novel detailing exactly the first scenario you describe above; who else touched upon it? Snow (talk) 09:11, 6 August 2013 (UTC)
 * Have a skim through this. I can think of a couple offhand that also fit the scenario (Brian Aldiss' Non-Stop, David Lake's Walkers on the Sky, but it's a popular trope, and I'm not at home where I can reflect further and skim my collection (I'm at work and really should be doing some :-) ). {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 13:01, 6 August 2013 (UTC)
 * Larry Niven's short story "The Long Way Home" is a variation of it. Clarityfiend (talk) 00:29, 7 August 2013 (UTC)
 * A risk I would worry about would be being passed by later, more advanced ships. This has been explored by a number of sci-fi writers, so maybe it's not all that "unknown", but it would sure suck to go through all that trouble only to get to the other planet and have people already there who tell you "Oh yea, We invented the Warp Drive about a week after you left." APL (talk) 08:51, 6 August 2013 (UTC)
 * Are you sure that's really the worst case scenario? Mightn't it be worse to hear that they invented it last week? Snow (talk) 09:50, 6 August 2013 (UTC)
 * One of the earlier treatments would be A. E. van Vogt's "Far Centaurus". Clarityfiend (talk) 00:31, 7 August 2013 (UTC)
 * Dust.  No, seriously, very much a problem and one of my favourite examples of how poorly understood the rigors of space-travel are and how inadequately the complexities are treated in popular media and amongst amateur enthusiasts.   There's a lot of matter out there and when you're traveling at the kinds of velocities that we would almost certainly be looking at for exploration beyond the solar system, it only takes a very, very small amount of it to annihilate a craft of any realistic proportions upon collision.  As to your second question, much thought has been given to how we might tackle the herculean task of interstellar travel, with most of the better-regarded proposals originating from the middle of the last century and having been debated (with regard to which is most feasible) by engineers and scientists from a wide-array of disciplines ever since.  However, as regards actual practical efforts (or indeed even efforts to generate general support for such a venture), your assumption is quite correct; the priority in terms of research an expenditure has been overwhelmingly on "local" space travel (and indeed, extremely local), and there's increasingly less support for even these efforts in terms of both public support in leading industrial nations and financial support from the governments of same.   You know, every time I've ever heard the query "If you could have a conversation with anyone, living or dead, who would it be?", one of the names that always comes to the top in my mind is Carl Sagan, but at this point I don't think I could look him in the eye, we've all collectively dropped the ball so poorly as regards vision for the future of our species outside of Earth. Snow (talk) 09:00, 6 August 2013 (UTC)

If you are a telephone sanitizer, the risks are well know, just not to you. See: []. Dominus Vobisdu (talk) 09:13, 6 August 2013 (UTC)
 * Which brings up the next little-known danger: forgetting where your towel is. - ¡Ouch! (hurt me / more pain) 09:36, 7 August 2013 (UTC)
 * There are also a possibly infinite number of unknown unknowns in Parts Unknown (might help to read the transcript in the description, but only maybe). InedibleHulk (talk) 09:51, 6 August 2013 (UTC)

On a generational ship, would there not also be a risk of a population explosion and therefore using up the resources too soon? 184.147.136.32 (talk) 11:20, 6 August 2013 (UTC)


 * Don't forget to pack all the microorganisms for your gut flora etc. 20.137.2.50 (talk) 12:57, 6 August 2013 (UTC)

Carl Sagan has pointed out that on the Earth you can find enough Deuterium to move the Earth itself all the way to Saturn's orbit. This can be done to deal with the Sun's gradual increase in brightness. Count Iblis (talk) 13:00, 6 August 2013 (UTC)
 * I fail to see what that has to do with the question which was asked. IRWolfie- (talk) 23:51, 6 August 2013 (UTC)
 * The implication (as I read it) was that the Earth itself could be a vessel capable of significant space travel, which affects some of e.g. StuRat's points. ManyQuestionsFewAnswers (talk) 22:04, 7 August 2013 (UTC)
 * The earth is already engaged in significant space travel. I suspect if it was forced out of its orbit, there could be catastrophic consequences for its inhabitants. So let's hope no one tries it anytime soon. ←Baseball Bugs What's up, Doc? carrots→ 02:02, 8 August 2013 (UTC)