How complex would this DIY Swiss style lathe conversion be to set up hal/ini?

Going to try and explain the project in a way that y'all can imagine as I feel like it should not be too difficult to program in linuxcnc but I wanted to hear the community's inputs

project: Regular Taig CNC lathe with the 5c headstock. I have already converted the 5c headstock to a guide-bushing-esque headstock. My plan is simple: add a second linear "Z axis" that holds a servo that spins the stock and can also move forward and back to feed the stock through the guide bushing headstock. This guide bushing 5c headstock is thus unpowered.

My thought: Could I just have this new axis be the official Z axis, and have the native tag Z axis be another regular linear axis where it simply acts as one of the offset parameters when setting tool offsets? So the only movement the regular taig Z axis would do is move when switching to a different tool such as drills or boring bars of various lengths, while the newly added Z axis behind the headstock acts as the primary Z axis.

I have all of the build details down so I don't think that will be a problem, but I want to know if you guys suspect it will be hard to code in linuxcnc. I do have experience setting up regular lathes & 3/4 axis cnc machines.

This sounds interesting, but I can see a number of challenges.

It'd be easy to define the original Z as "W" - which is interpreted by LCNC as another linear axis along Z.

Based on your description, you'd be using the new W to reposition the carriage for tool changes like a gang-tool lathe?

Assuming that's the case, one slightly tricky part - depending on your coding/cnc skill - will be setting up tool change macros.  You'll probably need to remap "T" to a macro/ngc file that moves X & W for each tool.  Idea being you never command the W-axis directly in a program - only during tool touch-offs.

You might also want to reconsider leaving the original spindle/head unpowered, presuming it still has the OEM spindle bearings and you've basically just stuffed a bushing in the spindle bore.

Without a rotating guide-bushing you will be limited to surface speeds way below desired for carbide tooling - unless you add a serious oil flow system to the guide bushing.  With the price of servos so cheap these days, consider powering the guide bushing with a small servo slaved to the main spindle motor.  That should reduce the lubrication requirements as the bushing will be rotating the same speed as the stock.

The biggest problem I see with the arrangement is that you have no Y-axis, and thus no traditional X/Y 'frame' like that used on a Swiss lathe.  Your tools will all be on the same X-axis plane so your stock/part will have to pass between the tools - again, like a gang-tool lathe.

One practical problem will be getting your tools on center.  You'll have to use shims (annoying), or adjustable-height tool mounts ($$) for all your tools insead of the Y-axis like on a regular Swiss.  Or home-made holders that you bore in-situ for round-shank tooling.

Related problem... do you have enough height above the current tool slide to pass stock over it and between tools?  I can't imagine that little thing being able to turn more than about 3/4" stock, but tool center height is something to consider.  It'll be significantly less frustrating if your center height is a 'standard' height and you can use off-the-shelf holders/fixturing.

Last hurdle... how to you plan to advance more stock once you've parted off?  Traditional Swiss have an auto collet closer in the sliding head, and a bar feeder that can positively locate & move the stock forward (and back).  Unclamp head collet, head slides back while bar-feeder stays put, head re-clamps.  At the end of the bar the collet opens, the bar-feeder pulls the remnant out, dumps it, grabs a new bar and away it goes.

Or are you planning to run one part at a time and advance the stock by hand?  If so, what's the point of the lathe?  If you want some of the benefits of a Swiss (guide bushing support for floppy parts) a traveling steady rest like Andy Pugh built would get you there with rather less work:



Not trying to rain on your parade here - just some food for thought.  Love the challenge, fingers crossed and all that.

Hey thanks for your detailed response and definitely gives me some food for thought, I wanted to respond by point:

1. Yes plan is to remap T and use a W axis, and agreed that W would only be called during tool changes
2. My initial plan was to have a bushing with radial set screws to give enough friction to have the unpowered lathe headstock spin by friction, however, I agree I think it would probably be better to just slave two servos together so I will probably do that
3. I do plan to add a Y axis with two vertical axis at each end of the X axis cross slide but that is still a work in progress to design
4. I currently do have adjustable height tool holders and will use those temporarily
5. I do think I have space to slide the stock in - my maximum stock diameter will be 1/4 inch so around 6mm
6. As far as advancing the stock - I am not sure I am picturing what you are suggesting correctly, the new Z axis will be 12 or so inches so I will have about a foot of stock loaded in at a time, most of my parts are small so each bar of stock will be plenty of parts and I will just manually enter a new 12 inch bar of stock by retracting the new Z axis and unloading it. I don't mind this part being inconvenient for my use case as I only do very small volume parts
7. MOST IMPORTANT: so I have tried a traveling steady rest - the problem is, all of the tools have slightly different Z offsets (imagine a R handed 55 deg insert tool vs a parting blade - I guess I could solve this by having even more customized tool holders such that the actual cutting point of the insert of each tool type are all exactly lined up to the guide bushing, but I found this hard to do in my previous project which is why I was trying to pursue the W axis method (in addition to it being useful once I create the Y axis

ffffrf wrote:

7. MOST IMPORTANT: so I have tried a traveling steady rest - the problem is, all of the tools have slightly different Z offsets (imagine a R handed 55 deg insert tool vs a parting blade - I guess I could solve this by having even more customized tool holders such that the actual cutting point of the insert of each tool type are all exactly lined up to the guide bushing, but I found this hard to do in my previous project which is why I was trying to pursue the W axis method (in addition to it being useful once I create the Y axis
 

I don't think you're gonna be able to get every tool exactly on the same Z point using separate tool holders.  But you can certainly set a Z-offset for each tool even without the additional W-axis.  You mount the tools as close as you can to some Z-point, and then set a Z-offset for each one to get them spot on.

The only thing a separate W-axis + fixed location bushing would help with would be getting clearance for tool changes without having to pull the partly-machined part back through the bushing.  Although now that I think about it, that'd be a pretty good reason to have one...

My ramblings about loading the stock was meant to get you thinking about the stock remnant.  Even if your Z has 12" of travel, you won't be able to cut whatever stock is inside the headstock/bushing once the new Z spindle nose is up against the back of the headstock.

Even if the bushing is only 1"-2" long, if it's mounted in the old headstock you can't cut whatever length the headstock is.  The Taig headstock looks to be about 4" long.  Add ~1" for a pulley to power the rotating bushing... so 5" long.

That's 5" of waste for every 12" piece.  If you use 18" stock it hurts less, but about 1/3 your stock simply can't be machined because the sliding headstock runs in to the back of the fixed headstock.

I suggested a traveling steady because you could build it very, very short, but still have a powered bushing.  The steady frame gets bored in place (as in Andy's vid) for a couple of AC bearings, and you make a bushing carrier 'spindle' which is just barely longer than the bushing.  The frame could even have a mount for a small servo built in.  If you get the whole thing down to 2" long, that's WAY less waste per piece of stock.

If you aren't keen on a traveling steady mounted to the X/W carriage, making a new - really short - fixed headstock would cut way down on stock waste.