New project: CNC Lathe

[Com]

Hi guys,

I've decided I'm going to build a CNC lathe, I've got a little more time and money at the moment so I'm about ready to start this project.

I've put my mini mill cnc conversion on hold because the mounts are out of alignment anyway and it would be helpful if i had a lathe to build some new mounts, also I've got a few other projects planned using the lathe.

Anyway, heres the plan; The lathe needs to be able to turn a variety of metals and plastics. It needs to be able to turn a minimum of 14" in between centres. It must have an accuracy of atleast 0.03mm. It also must be a sealed unit as i plan to use coolant.

I'm going to try and make sure all axis's run on THK linear rails. And they must be driven by precision ground ball screws.

I've been thinking allot about the base today. Originally i wanted to pay professionals to cast me a base but I'm thinking this is going to be a little expensive and i dont think bolting the rails to the cast iron is going to help much. I've decided to puchase steel square bars, 2x1m 4x4cm 1.5cm thickness. I'm going to construct a frame with the square bars measuring 70cm x 30cm, although i might have to make it wider. Now i havnt decided yet but because i want this machine to be desktop based im thinking about mounting anti backlash/vibration rubber feet stops on the frame. I'm then going to mark out the holes for the linear rails, drill the holes then put a small bit of capping where each hole is on the inside of the square tubing. Then i'll fill the frame with cement. Thats all i've really figured out so far, I've already purchased a denford precision ground ball screw for the y axis, and i did purchase 2 ultract 2 servo motors but i don't really think they are suitable to im re selling them.

Does the frame sound like its going to be sturdy enough for this project? Obvoiusly i hav'nt built any of this yet as im still throwing ideas together.

[nervis1]

What are you going to do for a spindle?

I've been playing with the idea of making a cnc lathe for fun but finding a decent spindle has me stumped (without spending $5000 on one).

[Com]

Well thats one thing that I've thought about for a while, but I hav'nt yet looked at the back of a chuck so im not sure how it mounts, i plan on getting a 4 jaw 125mm chuck at some point and then going from there.

[Com]

Well I've looked about and managed to find some pictures of the back of a chuck, im thinking the best method is going to be a couple of linear bearings inside some sort of pillow block. Then a preicision rod mounted inside the bearings and fixed. Im still now totally sure how the lathe chuck mounts, i can only think of 2 ways, 1 being some sort of threaded rod or somthing simular screwed into the back of the chuck or 2 i think theres 3 holes in the back of the chuck for mounting to a plate?

[nervis1]

Cam lock, A series, bolt on with "tru-set", there are lots of types. I'd go with a 5c collet nose, probably be a lot more accurate for small parts in the end. The big question is how do you make a head stock and spindle? One guy here made one but he hasn't posted up any plans and was a little grey when it came to describing how he made his. At 6k rpm I don't think there is any room for error and thats what has really kept me from starting that project. I looked into buying one pre made but I could not find one under 5k. That did not include the motor or controller. In the end it may be cheaper and less hassle to just buy a used CNC lathe for 10-12k and retrofit or repair as needed.

Regardless keep us up to date on your machine.

BTW I don't think a tubing weldment is a solid base for a lathe, how about a nice big piece of blanchard ground steel, say 2" X 10 X 20 on a cement base? That's how I'd do it (for a home made job anyway).

[Com]

Oh well, i'll cross that bridge when i come to it, anyway i was actually thinking of putting a sheet of steel over or under the frame and filling that with concrete too.

[Mcgyver]

Com, I tabled the concrete idea here and was given the good advice to only consider a polymer concrete or better yet polymer and crushed granite as it the concrete will keep moving for years as it fully cures. subsequent reading and research supports this 100%. if it was mine, i'd also send any cold rolled or welded structure out to be normalized. regardless, I'd want the rails attached to something i could scrap in later if the darn thing did move - remember a little bit of twist in the bed will drive you crazy because the lathe will only turn tapers.

http://www.cnczone.com/forums/showthread.php?t=9035
http://yarchive.net/metal/machine_base_materials.html

cast iron has 2x of the vibration dampening of steel and polymer granite has 2x that of CI.

The spindle is a big turning job, but using a very light interference fit the accuracy of the spindle will depend on the bearings (which will btw by expensive) the final cam lock and 5c bits can be cut in afterwards. I definitely like the 5c idea, although I'd make the spindle 4.5 or 5MT and then use a 5C adapter)....but as pointed out it is not a small undertaking.

[Com]

Oh ****..i purshased some linear rails a while before i decided to start this project, origianlly i was going to make somthing much smaller but i then decided bigger will be better, the rails measure 2x5.5" and 4x4.5" each rail has 2 carriages on it and are made by THK. Anyway do you think there will be a problem with lining up the rails very carefully to make 2 larger rails?

Mcgyver - Any idea where it purchase polymer cement? I've been trying to find it on google but ive had no luck, does it come in a sort of powder in a bag or somthing?

[DieGuy]

How about using a piece of countertop granite for a base and adding your rails to that. Would be massive and solid for sure. You might have to shim the rails a bit but it should be easy to obtain and be pretty darn flat to begin with.

[Com]

Would bolting to a granite plate really work though?

[Mcgyver]

Originally Posted by Com Any idea where it purchase polymer cement?

no idea, but going on the granite countertop idea, why not look at one of the el cheapo composite granite surface plates? it would be a lot heavier than the countertop, already to a high standard of flatness and they aren't that much money.

[DieGuy]

Originally Posted by Com Would bolting to a granite plate really work though?

Sure it would, You just need to epoxy some steel liners with tapped holes in them. Just use a masonary drill to put say a 1/2 inch hole through the material and epoxy the steel into the hole then drill and tap the steel. If you really want to be sure you could make the liner with a head on them and place the head on the back side of the granite. Then put some feet on the granite a little thicker than the head. Just make the liners slightly below flush with the top surface of the granite, most rails are 20mm+ in wide and they whould cover your little steel liner trick up.

The other option is to just put a clearance hole through the granite and a nut on the back side. but personally I like the liner idea.

I am going to try it on my next project. I bet it will be the most solid and stable base one could want.