Critique my mini epoxy granite lathe build?

[Nealieboyee]

Hello All!

I'm in the planning stages of a mini lathe made from epoxy granite. Nothing fancy and not a heavy duty machine.

I have a few INA 20mm linear carriages and rail and a couple of THK 25mm carriages and rail (one short piece really). I thought I might put them to good use. I will be cutting plastics, brass, aluminium and possibly the odd mild steel part. I'm not sure yet if I NEED a tailstock yet.

Body is epoxy granite. Headstock is built into the EG and will be using taper roller bearings or angular contact. Chuck will probably be 80mm on a 25mm shaft.

Here are screenshots of my crappy sketchup model showing what I'm thinking. Does anything stand out to you folks as being missing? There are no ballscrews for travel modelled yet.

https://live.staticflickr.com/65535/49097183292_62e92d9edb_c.jpg

https://live.staticflickr.com/65535/49096477993_61e495b047_z.jpg

https://live.staticflickr.com/65535/49096986706_5981dcb6b8_z.jpg

Thanks
Neal

[pippin88]

You have modelled a compound. You don't need one and it would reduce rigidty.

The compound exists on manual lathes to allow angles / tapers.

[Nealieboyee]

You have modelled a compound. You don't need one and it would reduce rigidty.

The compound exists on manual lathes to allow angles / tapers.

Thanks . Yes I was hoping to keep the compound as I would like to be able to turn tapers. This is a manual lathe.
What do you think of using linear bearings (20 and 25mm) in this application, as opposed to dovetail slides?

[pippin88]

Parts made for CNC (linear bearings and ball screws) are not suited for a manual machine.

You will likely get back driving / unintentional movement.

[Doddy]

What Pippin said, also, I'd review the bed length - you mention turning tapers which generally implies a component of "some" length - it looks like your bed is in the order of 300mm, with a compound slide and chuck somewhere in that space - there won't be much travel left.

[Nealieboyee]

What would you recommend replacing the linear carriages with for sliding?

[Doddy]

Linear rails and carriages will work - but you know that.

You're adding a lot of unnecessary height to the cross/top slides and the resultant increase in height of the spindle above the bed. I'm probably talking out of my arse here but the force on a cutter in the top slide lathe are a lot more predictable than a mill - typically with just about a single vector - are there no more appropriate systems than square rail for this application? That said, if you have the rail already....

If you want to cut tapers, and reduce your overall height - look at existing taper attachments for lathes (you need to be able to decouple the cross slide from the screw). You can lose the compound with this.

My main agreement with Pippen though is the amount of back-drive. Even on my somewhat light-weight ML7 you need a carriage lock to avoid back-drive on the original acme thread (I've yet to assess this on a ball/belt/stepper combo). My main, main issue though is bed length. If you know that this is sufficient for your need then go for it, but I can't help but think that you're going to be limited to something not much more that 100mm Z travel once you've finished.

EDIT: Cold light of day: back-drive - not with the spilt nut engaged (hence acme thread) - my bad, disregard - I had to lock the carriage to avoid back-drive against the rack-and-pinion of the carriage position wheel.

[routercnc]

The other thing to think about is how you will align the tool height to the spindle centre. If you are going very simple then the surface the tool sits on needs to be on the low side so you can shim up to suit each tool. You can't shim down.

More complex arrangements are a quick change tool holder with a threaded height adjuster per tool. I would have a look at this option as it makes tool changes quick without dialing them in each time or keeping a set of shims per tool.

[Nealieboyee]

Linear rails and carriages will work - but you know that.

You're adding a lot of unnecessary height to the cross/top slides and the resultant increase in height of the spindle above the bed. I'm probably talking out of my arse here but the force on a cutter in the top slide lathe are a lot more predictable than a mill - typically with just about a single vector - are there no more appropriate systems than square rail for this application? That said, if you have the rail already....

If you want to cut tapers, and reduce your overall height - look at existing taper attachments for lathes (you need to be able to decouple the cross slide from the screw). You can lose the compound with this.

My main agreement with Pippen though is the amount of back-drive. Even on my somewhat light-weight ML7 you need a carriage lock to avoid back-drive on the original acme thread (I've yet to assess this on a ball/belt/stepper combo). My main, main issue though is bed length. If you know that this is sufficient for your need then go for it, but I can't help but think that you're going to be limited to something not much more that 100mm Z travel once you've finished.

EDIT: Cold light of day: back-drive - not with the spilt nut engaged (hence acme thread) - my bad, disregard - I had to lock the carriage to avoid back-drive against the rack-and-pinion of the carriage position wheel.

All valid points. As you said, I already have the carriages and rail. I do have some acme thread I pulled out of a reclining couch a while ago. I can try to make some nuts for it. Either that or I can lock the ballscrew somehow to prevent backdrive.
I'll definitely lengthen the bed. I will lower the cross-slide and do away with tapers then. Perhaps design it to take a quick change tool holder.

This is just experimentation for me. I've always wanted a lathe and also to try my hand at epoxy granite. So why not. Not needing anything massive though.

Thanks

[Nealieboyee]

Right, changes have been made. No compound slide, bed is longer by 160mm, and QCTP added. The distance from bed to chuck center is now 125mm, around 25mm lower than before.

How we looking?

Cheers
Neal

https://live.staticflickr.com/65535/49106724778_5b579d3741_c.jpg

https://live.staticflickr.com/65535/49107426072_a91b9c0e08_z.jpg

https://live.staticflickr.com/65535/49106724758_845390f601.jpg

[Kitwn]

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

[Doddy]

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.

[Nealieboyee]

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

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.


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.

I'm actually building the spindle assembly myself.
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/65535/49110765207_9a8a43afa8_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.

[John McNamara]

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/5487/82f4c63376150991dd323d0ef15a7da2fee8.pdf

This is the actual machine designed in the paper! https://my.mech.utah.edu/~bamberg/research/StarToolGrinder/EberhardBamberg-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/UCCkSr3M8GXbS4txqPY7OMxQ/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/forums/postings.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