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Since the title of the thread includes the word 'critique'...
Have you thought of buying a lathe and then using it to help make the parts for something REALLY exciting? I can't help thinking you are going to be disappointing with the limits of this machine however much fun you have playing with the expensive epoxy granite.
Kit
Random thoughts. I'm guessing you're buying in the spindle assembly. Do you have an option of mounting other chucks or faceplates? A four-jaw is a serious alternative and a faceplate can cover the whacky jobs that you just can't chuck-up (I turned down a spigot on an external door handle - soft white metal, to get a smooth shank to then place a phosphor bronze sleeve over and re-cut a groove for a circlip ... stupid amount of effort for a low value item but which was otherwise impossible to replace with like for the mate I was doing this for) - could only be done on a faceplate with clamps.
To some extent that impacts the position of the rails for the bed - more need for how you currently have it with the rails all the way to the spindle, if you've a fixed chuck (plan B!!!) then you could save a little on the rail. Put another way, the rail might get in the way of the work piece, though with 125mm you may be okay with that anyway, if not you could consider a gap bed, but I think you're probably okay there.
Extra bed length?, until you run out of space for the lathe you'll never regret that. It brings in the option of a tail-stock and accurate drilling/reaming. Keep that in mind. and you might want to consider if there's any machinations required to support a tail-stock (thinking you'd likely need to overreach the rather wide cross slide - could be incorporated into the design of the tail, or by narrowing the cross slide. In amongst this consider the positioning for the feed screw - I'm guessing that you're thinking of the centre-line along the bed, under the cross-slide? (cast a slot in the granite to cater for the ball-nut), it's the logical place from a mill design but less so for a lathe where you do have to consider the thill-stock. Unless....
There's something bugging me about your design - it's simple and I can't help but think that there's scope to make the cross slide optional (harder to do if you choose to go ball-screw) - cast a couple of removable end blanks that allow you to mount short sections of rail to allow the cross slide to be slid onto and removed (think the principle of a railway turntable) - replaced with e.g. a drill/tail-stock to better exploit the available bed length without cluttering it. Of course this relies on being able to easily decouple the cross slide from any lead screw - easy with acme, less so for ball. Assuming that you're using a lead screw of course. You've not mentioned screw cutting but it's kind of the elephant in the room when it comes to lathe design. And I'm assuming that if you are considering this you're simply omitting this from the design at this stage for clarity.
I will say, however, this design is heading rapidly to a solution suited for adaptation to CNC. You have to think carefully if you want to go the route to having a manual lathe, and all of the constraints that imposes, versus a complete CNC solution which allows you to avoid some of the compromises, go full ball-screw and get a solid CNC solution.
EDIT:
Aha, didn't notice Kitwin's reply on page 2. I tend to agree.
Good point, but based on the fact that the rails and linear carriage were absolutely free, the cost of the epoxy isn't a big deal. I'm fine with having limits on the machine. I can work within them. I'm sure I could buy a lathe, but where is the fun in that? I'll take satisfaction of creating something myself over an off the shelf item any day. The amount of joy I got from building my CNC router far outweighed the outlay on parts. By the time I finished it, I had way more of an understanding of the machine inner workings than I would have just buying one.Quote:
Originally Posted by Kitwn
I'm actually building the spindle assembly myself.Quote:
Originally Posted by Doddy
I can move the rails away from the spindle a bit more to give it some room, but the closer I can bring the cross slide (see image), the more viable a tailstock becomes, as it wouldn't have to reach too far over my cross slide.
https://live.staticflickr.com/65535/...8a43afa8_o.png
I'd like to try using the acme thread I have lying around. 1. To reduce the cost of having to buy ballscrews and 2. Like you said, decoupling the cross slide for removal becomes a lot easier. I could just use a split nut and slide it off.
I'm staying away from screw cutting altogether. And as for CNC, maybe one day I'll add a 4th axis or something to my CNC. Maybe.
Simple is correct. I'm trying to keep it as simple as possible.
HI Nealieboyee
Have you considered making the headstock separate? That would make preparing the mounting surfaces for the bed rails somewhat simpler. It would also allow the headstock to aligned after it is completed. threaded steel sockets can be cast in the bed then threaded studs in slightly oversize holes in steel sleeves cast in the head stock can used to bolt and align the head stock initially with packing for testing later the void between the castings can be filled with epoxy. Stephen Gotteswinter shows this in a video. https://www.youtube.com/watch?v=U7Qs-J2swIc
The same method is cited here in this seminal CNC paper https://pdfs.semanticscholar.org/548...5a7da2fee8.pdf
This is the actual machine designed in the paper! https://my.mech.utah.edu/~bamberg/re...mberg-PhD.html
And Peter at Edge Precision has one..https://www.youtube.com/watch?v=akF7jvDRxH8
Check out the rest of his videos..https://www.youtube.com/channel/UCCk...PY7OMxQ/videos
No back to your lathe......
The bed rail supports I assume made from steel cast in position within the bed mold will need to be flat and parallel, While it is possible to scrape them to perfection having done it for a milling machine I am working on it involved a lot of time and physical effort. Your bed would fit nicely on a surface grinder or a mid size mill. A headstock cast in position might make this difficult.
FWIW you may find this link to my mill project interesting It is stalled at the moment due to work commitments. I hope to get back to it over the holidays. https://www.model-engineer.co.uk/for....asp?th=139042
CNC? Have you considered incorporating step motors to drive your screws. in your design? Some have a shaft at each end allowing you to attach a hand wheel to the back end of the motor. This would allow you to start with manual control and later convert the machine to CNC The manual control can be left operable.
The low friction linear bearings may create back driving issues. Without the "stiction" that the dovetails found on most lathes provide. Maybe if steppers were fitted the screws could be driven manually with a pulse generator instead of a hand wheel. I have seen small boards to do this on the net.
The natural progression from this would be to fit ball screws instead of Acme screws. Yes more expensive but thanks to Asia maybe affordable.
Regards
John