CNC Mill as Lathe - Part II

[Chris64]

I've asked this question before, but now with a twist.

We've been talking about adding a 4th axis for a while. After spending some time drooling over vids on Youtube watching these crazy high-end turning systems that can turn and mill (like those cheapo 3-in 1 harbor freight deals - except CNC and very good).

So my thought was to build a nice big 4th axis. On an encoder so you could use it as a 4th axis but be capable of higer rpm (lathe speeds). On my vertical mill I could mount lathe cutter mounts to the front portion where the Y axis handle is (since the lathe only really needs X & Y).

I could then in theory, turn a piece, cut it to shape. Then move it under the mill head and poke some holes in it or re-surface sides, keyways etc. Plus it becomes the ultimate 3 in 1.

It just seems so possible. Any thoughts?

[Chris64]

Here's an example - this one is very elaborate and even has another cutter on a different 90 degree to the main cutter. i wasn't thinking of going that far. But yes to a combo 4th axis/lathe spindle.
http://www.youtube.com/watch?v=sYqN90gHOZM

Also I went out and looked a the machine and I was thinking the cutter could be mounted to the V-bed (right name?) for the vertical position of the table. It would be a very rigid place to mount something and even with the table raised to hits max height it would still have room for a mount. The only problem with this is that the cutter would be backwards (compared to a traditional lathe). Would this present any other problems?

[Geof]

Originally Posted by Chris64 ......Also I went out and looked a the machine and I was thinking the cutter could be mounted to the V-bed (right name?) for the vertical position of the table. It would be a very rigid place to mount something and even with the table raised to hits max height it would still have room for a mount. The only problem with this is that the cutter would be backwards (compared to a traditional lathe). Would this present any other problems?

Do you mean the slide for the vertical axis? In other words the Z axis slide.

So your cutter is mounted behind the rotating work piece?

This is no problem, many CNC lathes have that configuration and the lathe rotates normally with the cutter upside down. Twist your point of view upside down and you will see that tool in front right way up is the same as tool at back upside down. You just have to remember that X minus is toward the center of rotation; do not think of the X travel in terms of toward you ar away from you, always use the centerline as the reference.

[Chris64]

Originally Posted by Geof Do you mean the slide for the vertical axis? In other words the Z axis slide.

Not the Z axis. On my knee mill it is the part that I can crank up or down depending on how thick the material is. I would leave the Z alone so that I could still use that for traditional milling.

Also, there might be enough room back there to throw in a turret so that it could change tooling. Might be a little overwhelming for a project, but very cool.

[Geof]

Originally Posted by Chris64 Not the Z axis. On my knee mill it is the part that I can crank up or down depending on how thick the material is....

In the same relative position with reference to the X and Y though; vertical and behind.

[snapman]

I was thinking about this earlier this year. Only in my version, I was thinking of mounting the tools on the spindle head. I actually made some plates with a threaded matrix in anticipation of trying it out but didn't thinka 4th axis rotary table would be able to spin fast enough? Or that another spindle without the worm gear would be able to still function as a 4th axis for milling work? I think I have a pic of the spindle head which may better explain what I am talking about- will post in a few if I find it.

[snapman]

Here are some pictures of the spindle head on the Prolight 2000 I use for prototyping my fastener designs. I also have a riveting attachment that mounts via 1/2 inch collet that basically turns the machine into a cnc orbital riveter- works great. I am trying to make this little mill as versatile as possible since I will only be protoyping and testing out ideas on it.

Anyway, on the one side is a 50k rpm NSK spindle for the really tiny endmills I often use and on the other side a similar threaded matrix (although I don't think the holes in the pics are threaded yet) that I was thinking lathe tools could be mounted in a row (gang style tooling, if I am correct?) for use on turning stock in a typical 4th axis type setup only spinning much faster?

Anyway, it's something I might mess around with later- I have a Prolight 3000 lathe but I don't have any software for it plus I would need OneCNC lathe- would rather spend the money on upgrading my current OneCNC to the latest version with 4th axis.

[Chris64]

Here's a quick scribble of what I was thinking. The bracket that would hold the tool is high lighted red. As shown, the motor would actually sit on the table (like a 4th axis). I thought about putting the tool on the Z axis but I would question it's rigidity. Plus then you couldn't incorporate milling all in one process. A steady rest would be easy too.

Snapman, I think I'm understanding your concern. If you use a worm drive your gear reduction might be too much for it to have the correct RPM. If you don't, will it be able to hold position as a 4th axis. My thought is that the encoder should keep it inline, but I'm sure this isn't right and would certainly but a tough load on the motor. I suppose you could activate a motor brake of some kind. (come on smart people...help me out with this one!)

I was thinking maybe something like a 1/2 Horsepower DC motor. geared up (so it's faster maybe 2 to 1), then feed into a 1/4 worm drive (for minimal feedback). This should allow for RPM's to be as high as 250RPM (maybe faster) depending on the motor. It's not blazing - but it'll turn. I'm not sure that it would be enough power though. Do you think DC motor would be the best choice though? I think if it's going to have an encoder on it there's really no choice.

[Chris64]

OK, so I went looking around at some industrial Surplus places looking to see what they had as far as motors and gears go. No luck in gears, but tons of motors.

I need some help with this though. I've been thinking about building this 4th axis with a two speed transmission. My first design was of a direct drive with a small gear reduction for lathe functions and a worm drive for traditional 4th axis milling type work. I like the concept but all the gears seem like they would cause slop (major problem). So I was also considering a belt drive and maybe just a very large gear reduction (for 4th axis milling). Do you think the motor would have enough holding power during milling operations without a worm drive or is this just completely unrealistic?

I don't want to require a seperate brake as I may want to be milling while the 4th axis is in motion.

Also, any idea's on Motors? AC are cheap but may not work well with encoders. 1/2 HP? Enough? Could a Gecko drive something this large? I'd like the "4th axis mill" to work just like a standard servo with a fixed Max RPM, but the "4th axis Lathe" should be able to support variable RPM from the controller ideally - I suppose a pully setup would probably be ok though (and easier).

Maybe the right answer is to have the transmission switch between two different motors instead of expecting a single motor to handle both tasks.