I've been looking to either build or retro-fit a lathe for CNC.
Looking at something like cnc'ing a Hardinge is very appealing but the stumbling block of almost 2000 pounds is hard to get over.
During my quest I ran across a lathe made by a group member using what looks like Thompson rails.
If a person were thinking of going that route.. what is a good material for the bed of the lathe?
At my disposal I've got various metals such as a large I beam of 100+ pounds and around 24" length, I'm guessing over 1/2" thick. Some steel plate in various thicknesses from 1/2" to 1 1/2".
I envision having it decked to make it flat and having the mounting holes bored at the same time.
bed weight is very imporant in a lathe consider your forces of a part spining and a tool cutting a lite weight lathe will transfer vibration from bed ,tool and part to each other that why hardinge's bed are one piece casting
There are other tricks that you can do to "deaden" a welded or tube steel frame. The best is to start with thick walled steel tube, finish a side to mount your rails, and fill it with lead shot or sand (make sure there is no salt in the sand!). The loose material inside acts to absorb vibration, and also makes for a firm base. Plus, by adding the weight later, you can position your machine where you like, then add the mass. Of course solid cast iron, composite granite, or real granite are still the best, but this approach is probbably the second best.
The stress on a lathe bed I would expect to be quite low for general use, you could use some mild steel plate then have it ground flat and parallel. On a conversion I am working on I purchased a 1x12x36 plate of 1018, cost was about $175 and you can expect about another $175 to grind it reasonably true, it looks good but does have some twist in it so using is as is won't work.
To me NEATmans suggestion of good quality cast iron would be the best bet but in trying hard to keep cost down I chose another route. Even now the use of the 1018 is on hold for another product that a friend showed me which is a type of epoxy, it is heavy, stone hard, strong, good specs on rigidity, machinable with carbide but likes diamond, and should absorb vibration well, cost for the above size is about $80 delivered. My friend who has yet to come through with anything but a promise is supposed to get me a section on his next job....... still waiting after almost 6 weeks so we will see. If I was sure it would work as hoped I would ask him to just order new and get it over with.
If you can afford or willing to spend the bucks, cast iron is the best bet, but as you can see there are alternatives, which one? it is all about $
the 2000 lbs is why a hardinge's a hardinge - its an advantage not a detriment. the lead shot/sand dampening works, wood lathe guys use it extensively, problem is it only dampens, doesn't provide any structural strengh. read up on stuff like harcrete polymer concrete filled steel structures, numerous posts here and on google - imo that's the way to go for a diy machine tool structure
The problem with making it from solid, whatever the material, is you loose the engineered approach (webbing, torsion box etc) that maximizes the rigidity and strength for the weight involved.
The thread herein on converting one of the smaller hardinge's is a good idea - don't try to reinvent the wheel on a robust bed casting.
then again it always comes down to what you want to do. my first lathe was a unimat, the old ones with the round bar ways (still have it). although useless compared to what i have now, it did cut metal and didn't stop me from having some fun with it and learning a bunch
I was looking over the choices I had using the advice I've received on this thread.
I tapped the 1.5" plate with a cresent wrench and it rang like a bell.
I see what you mean about using sand or some other form of deadening.
I would love to have a Hardinge and they are in my price range (on ebay) but I have no way of moving 1500 to 2000 pounds of machine.
I'm pretty sure my shop floor would hold the weight tho. It's a converted sandwich shop from many years ago. Very solid.
1" or 1-1/2" steel plate is fine for a bed - you may want to weld it onto a square tube to bring the height up and if you leave an inch or so of plate width on each side sticking out you have box ways started already. I'm looking to do this to my 7x in a few months, I need about 10-12" more of turning length and so I need to whip up a new bed.
I would't get too hung up on the vibration deadening properties of CI, steel is a fine material for machine tools as it's much stiffer than CI. While it's true that a single piece of steel will ring, a machine has lots of interfaces between the tool and the work (bearings, bolted on members, ways) that make the vibration deading issue more or less moot - there's so much more dampening elsewhere that it doesn't matter if you made the bed out of lead.
The far easier way to go is rent a forklift and move your new Hardinge inside. Unless you have to go down a flight of stairs or something you're all set. I did this for the gantry pick and place I recently bought, it's about 2500 lbs, but $100 for the forklift and done. I think it took longer to get the forklift started then to actually move the machine.
Thick steel tube with ribs welded to it. I think this is a good way to go. I'd imagine that a cylinder is less prone to vibration than a square tube. Any large flat surface is prone to vibration. The rips on top of the cylinder help even more.
The guys who turn on those big woodlathes often chuck up "natural edge" blanks, meaning pieces with bark still on them... totally imbalanced, and they can weigh quite a bit. A green log section can easily weigh over a hundred pounds.