I'm an engineer and have designed several parts that are made on metal lathes. I'm wondering why metal lathes aren't used for turning. In particular I'm wondering why woodturners don't use a tool holder on a stage or tools that resemble metal lathe tools. What is the advantage to manually holding your tools and using a tool rest? Without knowing much about woodturning I would assume that it's next to impossible to get repeatable/super accurate results (or perhaps better stated I'd assume it must take years of experience to get the hand eye coordination down to get good results). Perhaps the "one of a kind" nature of the process is part of the appeal?
I'm wondering why metal lathes aren't used for [wood]turning.
In my mind, it comes down to 3 Cs: cost, convenience, and creativity.
Cost: From what I've seen metal lathes cost more than wood lathes when you consider similar capacities and features. This is not surprising given the additional tooling necessary to work a metal lathe. One might argue that buying woodturning tools can also be expensive compared to buying HSS or cobalt metal lathe keys. However, the metal keys still need to be ground to different shapes to achieve different profiles, so there is still plenty of cost associated with the metal lathe tools.
Convenience: Setting up a metal lathe can be a pain, especially compared to a wood lathe. You have to get your tooling set up all in the right places and adjust your tools to feed in the proper direction, at the proper rate. Wood lathes, you just chuck your piece, set the tool rest, and go.
Creativity: I challenge any metal lathe user to create a Windsor chair leg more efficiently than an experienced turner. Sure, the metal lathe user could make the first one and set up a template for his keys to follow, but I suspect that doing so would result in a fairly rough finish with a lot of woods. Good turners can make a chair leg that's finish-ready (no sanding) with just a skew chisel.
Having manual control of the cutting tool means that you can feel when the wood is having issues cutting, which could be a sign that it's about to fail, that your edge is dull, or that you're just using the wrong cutting angle. You're more intimate with the actual piece and can better feel what's going on, instead of plugging it into a machine.
Also, with the cutting tool in your hands, you can much more easily make changes to your design on the fly just by flicking your wrist (this is an exaggeration, don't flick your wrist when turning or you'll suffer a catch). I know the way I work a lathe, having to have a metal lathe tooling setup have to play catchup to my brain's creativity would be incredibly frustrating.
It's worth noting that one may combine both worlds in a way. I've made a couple Oland Tools (image below) that make use of metal lathe keys. It's nifty since you can change out the tool heads easily and have quite a few profiles open to you. Basically, you drill a hole in the end of a metal rod large enough to fit an HSS key and hold it in place with a set screw.
Also, this is not to say that a metal lathe can't be used. Lots of smoking pipe makers use them due to the greater precision.
The "organic" nature of woodturning may be part of it -- its a kind or powered carving, after all, and many folks like the feel of wood responding to their hands.
The fact that wood is softer than most metals may also have a lot to do with it.
So may the fact that tolerances are completely different. Even when an "exact fit" is desired, it's not down to mils as metal often is.
There are jigs for doing more precise woodturning. They're mostly for special cases or large-scale production.
Two factors are :
Speed : metal lathes tend to run at lower speeds, often 1000rpm and lower. This is less prominent with modern lathes and carbide tooling, but with HSS and carbon steel tooling you have to limit the cutting speed to avoid overheating and softening the tool. (It's fun when that titanium billet work-hardens and your drill bit suddenly starts glowing red...) I understand woodturning works better at higher speeds, giving a finer finish.
Wood dust contains abrasive silicates : you don't want that stuff grinding away the bed on your precision metal lathe, if you want to achieve 0.0001 inch accuracy on metal afterwards!
Incidentally it's not completely unknown to turn metal freehand, with a toolrest and gravers ... watchmakers lathes are set up for this. Metal removal rate is minuscule, but that doesn't matter because the workpieces are tiny to start with.
Brendan Stemp an Australian Woodturner uses Metal work Lathe to produce recorders due to the accuracy needed, from my understanding he uses a fairly dense or hardwood for these. His Youtube Channel https://www.youtube.com/channel/UCYJ4e0XACGJQjJvfmVUcE-Q
To start I'm pretty sure that metal lathes and wood lathes have different speeds they tend to use. The two materials are very different.
Metals can take a level of accuracy impossible to achieve in wood. Just sanding a piece on the lathe can take off mm to give it a nice finish. So metal working lathe tools are going to be extreme overkill for your average wood project. On top of that wood swells and contracts and bends depending on it's environment, so it is just as much an art form as it is science. I've actually had bowls warping on the lathe just leaving it sit for an hour while I ate lunch.
Also there are duplicators out there that can be mounted on a lathe for making pieces "identical" to each other. they do exist. It also means finding pieces that are similar and dried correctly to the right moisture content to help keep them stable.
The main difference that I have seen is the material removal rates of manual metal lathes are much less by design than can be accomplished by hand on a wood lathe. Like try getting a 100+ IPM traverse rate on a manual lathe compared with holding the tool in your hand.
Same can be said with a manual mill and handheld router.
Though for CNC lathes there are much less differences between CNC metal and CNC wood lathes, machine rigidity requirements are probably the biggest difference there.
I'm wondering why metal lathes aren't used for turning. In particular I'm wondering why woodturners don't use a tool holder on a stage or tools that resemble metal lathe tools.
Sometimes they do, when precision is important. There are various specialized tools for forming balls or cutting several nested bowl blanks out of a single piece of stock, for example. There are also pattern lathes that cut one or more identical parts using tools that are guided by a pattern, and that's often what you'd find in a factory environment. And sometimes people even cut wood on a (gasp!) metal lathe, either because that's what they have at hand, or because they want the precision and features (e.g. thread cutting) that that machine affords.
But more often than not, wooden turned objects don't depend on the precision of a metal lathe. Do you care if your bowl is too large or small by 0.125"? If the legs on your table are imperceptibly different? If the tops of two turned boxes aren't interchangeable?
What is the advantage to manually holding your tools and using a tool rest?
Speed: Watch some videos of professional wood turners and you might be impressed at how quickly they turn a square blank into a finished object.
Adaptability: Wood isn't homogeneous to the degree that metal is. Wood has grain direction and figure, and often defects too, and turners take those into account as they decide how to position the tool to get the best result. Turners often use a single gouge for most of their work, but they'll turn it this way or that to use different parts of the edge in different orientations and from different angles. Watch somebody turn a small bowl and count the number of times they change the orientation of the tool; doing that on a metal lathe would take forever.
Freedom: Part of the fun of wood turning is that you can create the form as you work. Turning metal is often a matter of creating a part to exactly specifications. Turning wood can be more like sculpting, where you don't necessarily know exactly what you're going to end up with until you're done.
Without knowing much about woodturning I would assume that it's next to impossible to get repeatable/super accurate results
That's not necessarily true. Wood turners might not ever get into the kind of precision that machinists do, but you can get fairly repeatable results just by checking your results against a pattern, or by test fitting. Box lids often need to fit tightly, and with a little practice it's not hard to fine-tune either the lid or the box so that the fit is just right. Another example: wood turners often make "jam chucks" in which a portion of a wooden chuck is turned to a friction fit that'll hold a part tightly enough that it can be turned.