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I see a lack of CNC engine lathes? Is there a reason?

Discussion in 'CNC Lathes' started by Pyrex, Nov 7, 2018.

  1. Pyrex

    Pyrex New
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    Feel free to correct me on this, as my experience lies primarily with cartesian CNC routers. I recently upgraded my CNC and have a bunch of leftover NEMA 23 steppers, and I was trying to find a project to start on. I was curious if it is even possible to create a 3"x22" lathe (or somewhere around those dimensions) for use as a gunsmithing tool. It would primarily be used for threading and reprofiling barrels. I honestly do not know where to start on something like this, as I have very limited CNC lathe experience. I browsed some of the projects, and most of them appeared to either be wood lathes, or soft metal applications like aluminum. Any insight would be great where to start, just so I can see a basic layout of a lathe capable of mild/stainless steel work. My main questions are what kind of spindle motor would be necessary, and the general costs involved. Lastly, is the best approach to just retrofit an existing lathe or can you design one from the ground up affordable.. Thanks!
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Resident Builder Builder

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    What your seeking is a bit more hardcore than what goes on around here. I would suggest checking CNCZone. Aluminum framed systems are pretty much limited to cutting soft materials and as the common thread of this forum is aluminum framed systems, you're probably not going to find much to work with.
     
  3. Pyrex

    Pyrex New
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    Thanks Rick, that's great info. I did not know the nuances of the different forums.
     
  4. Rob Taylor

    Rob Taylor Veteran
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    I think I'm one of maybe only 2-3 people around here who are particularly interested in and/or work with "real" machines, it's definitely an aluminum-extrusion-heavy environment (as you'd expect, since that's what they sell- the store came first, back in the day). But Mark was super helpful to me back in mid-2013 around the V-Slot Kickstarter which ended up kicking me fully into making/machining and my journey of learning machine-building, so this is basically where I've made my home! I occasionally have ideas for extrusion machines too, as a lightweight, rigid material it's great.

    CNCZone is a great repository of information, though I think it's a slightly less friendly environment overall than here, but maybe that's just my perception. That, Practical Machinist and IndustryArena are where I get the majority of my information from, but I like bringing it back here because there's so much that can be learned and applied to hobbyist machines.

    I'm getting ready to convert my mini mill here- Grizzly G0758 Benchtop Mill Conversion - and I'm seriously thinking about doing the lathe as well once that's finished. I have a good idea of what I'd need to do in order to convert it, and it's a lot, which is what's giving me pause, but the result would be fantastic.

    So basically you want a machine that'll cut steel. So you need a big, heavy, rigid machine made from either cast iron, bolted hot-rolled steel, or mineral epoxy (or some combination thereof). If it's really just for simple reprofiling, maybe some fluting, and threading each end, you don't really have a ton of requirements, but it'll still get expensive fast, so I hope you have a way to monetise it (or a really huge rifle barrel collection...). There are two main options to start off with:

    1) Buy an off-the-shelf manual lathe. Probably a Grizzly, maybe a Precision Matthews. For 22" of useable space, make sure it's at least 30-36" long, because you need to factor in the chuck, tailstock, and width of the carriage. Might be worth trying to play with one locally to get a feel for the travel if this is a really hard limit. "Just buy one" is going to be fairly expensive- probably in the region of $3-4000 for a new one- and for CNC conversion, you probably need a new one- but you won't need to do very much precision work on it; the spindle, ways and cross slide are all set up for you. That's a pretty big deal. A 36" lathe is gonna take up a big chunk of floor space and might need a dedicated concrete pad for some machines, perhaps even a new power line run. Real machines aren't just more expensive for the unit itself, but everything that goes with it.

    2) Build something perfectly suited to your needs. I'd consider building it all on top of a granite surface plate with linear rails if you need real precision- and I imagine the threads have to be pretty concentric on a gun barrel.You couldn't get away with an 18x24" plate for 22" of travel, and going above that gets super expensive- not to mention, all that wasted width, because they're all a similar aspect ratio and straight-edges have the wrong sides flat. But it's doable. Alternatively, a steel bolt-up chassis with epoxy (or surface-ground steel, if you have a grinding company nearby) for precision surfaces and setting mating planes. This would require the most experience, but probably be the cheapest, especially if you have a local steel supplier. You're also gonna need to run linear rails for either option, which are super not-cheap, though you can almost always get away with one or two sizes down from what most people do, they have crazy load ratings. Basically, this option requires you to go in with a fair bit of fundamental knowledge pre-baked, because there's a TON to learn about how and why machines are built as they are. Done right, you should still be able to dial in tenths and actually run at faster speeds because you're on rails, not ways. It also has the possibility of running substantially over the budget of the off-the-shelf unit, though you have the possibility of buying parts a bit at a time.

    The screws will be ballscrews, there's just no question there. Much cheaper than they used to be though, so probably not that big a deal. At this point I'd put ballscrews on literally any machine, they're so inexpensive.

    If you buy a lathe (and I'd strongly recommend it if you don't already have at least a couple years' experience doing traditional "proper" machining, because it's a whole different world), then you can either a) replace the compound slide with a solid block toolholder, and then run a QCTP or some kind of built-in toolholding, b) figure out a way to do gang tooling, but this probably isn't the ideal solution on this type of work (better for short, small parts), or c) make some kind of turret, like the real CNC turning centers have. Obviously A will get you running the fastest and require the least time, knowledge and effort. You would also be able to turn and drill the cube to pretty high tolerances right on the machine itself in a 4-jaw.

    Then you're 1) ripping out the leadscrews and replacing them with a pair of ballscrews (don't need a separate threading screw or power cross-slide transmission), 2) adding an encoder to the spindle so that you have positional feedback for threading, and 3) dumping all the extraneous gearbox bits, pulleys, etc. The actual machine conversion isn't super difficult, it's the workholding and potential automation that makes lathes so tricky.

    Basically for 22" of travel, I'd make sure to budget around $5000. This lathe is under three grand and looks like a solid potential base for conversion: 12" x 36" Gear-Head, Cam Lock Spindle, Gap Bed Lathe | Grizzly Industrial - with a 12" swing, you could easily even throw linear rails screwed right into the ground ways on there and keep your 3" of swing with high travel speeds, if production speed was ever a concern. 1.5" thru-bore is pretty nice too.

    Add in tooling, electronics, screws, coolant and oil systems, whatever else, you're probably looking at $4500. A fairly reasonable price for the capabilities.

    Alternatively, start looking up particle packing in epoxy, torsional and flexural rigidity of steels in various sections, heat treatment and aging, tool pressure, spindle grinding, bearing preload, vibratory damping, etc etc etc... It's a deep dive.
     
    Rachel Holley and MaryD like this.
  5. stephen cia

    stephen cia Veteran
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    Good read. Been chasing the rabit lathe for a while now. I'm thinking of just get the package Taig and be done. But then I remember opebuilds! And thought hmm maybe I can start another project... At the moment still undecided like you said the dinn is deep on this one... But it's going to be a heck of a dive for sure :)
     
    Peter Van Der Walt likes this.

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