What is recommended for ball screews? Bearings?

[vacpress]

hers something that i came up with after almost no great thought:

get together $150 and goto lumber yard. buy the strongest, straightest 4x4 lumber they have. its gonna be like $15 to $20 per 8' piece. thats find. get like 10 of these, and take them home..

build a large, strong wooden scaffolding about 12'x6'x5'. make it as strong and accurately as possible. it should be within a 16th! if your wood isnt straight, consider getting a planer.
when it looks strong enough - make it stronger. this thing has to be built like a deck that 100 people could stand on, only much more accurately.

within this frame, you can now build a system based around whatever crackerpot thigns you can think of. for starters, i would think about using something silly like rewound alternators acting as giant servo motors. use steel roller chain as your linear actuators. just make it all real strong. put a big mean tensioner on the thing. use gears from your old 10 speed as idlers. the 10' length is the real killer still.. getting something to move 10', with speed, with accuracy is hard. i couldnt even begin to take on such a task without thousands of dollars. i wouldnt want too. i mean.. i also bet you could get pretty good results with 4" iron pipe, the straightest ones available, and lots of rollerblade bearings....

the hard part about saving money by building yourself is building accurately.
i hope/bet noone reads this.

[chuckknigh]

i am not sure a garge door system would have the accuracy to use for cnc work at all. i remember tge garage door at my parents house - it rattled, screached, etc. the tracks the bearings ran in probably had 1/32 to 1/16" slop in all directions. all the components were made of bent sheet metal if i remember correctly.

True, but that doesn't mean that the rails weren't straight, and the screw/nut wasn't useful.

Think of it as a starting point.

I have 2 rails sitting out on my deck, right now, and more than a few ideas for it. They're pretty stiff, and should be able to be adjusted for reasonable precision...same thing with the screw.

It's just a fun idea. Considering that the walls of the building would provide the structure to keep it stiff, the problem then becomes the drive system.

-- Chuck Knight

[arvidb]

Originally posted by buscht *snip* I am not sure I understand +-10 mil. Is that +-10millimeters? (+-.397") If so, you could tie a rope to this and probably be accurate enough. If you mean +- 10mills (+-.003937") over a 10' bed, you had better rethink the whole idea of low cost, DIY. Routers like this would cost about $150,000+

I think 1 mil = 1/1000 of an inch (1 "milli-inch"). It's a common unit in circuitboard CAD programs. The pins on a PDIP chip is 2.54 mm, or 100 mils, apart. The bigger SMT parts (forgot what they're called) have 50 mil (1.27 mm) pin spacing.

But to keep this accuracy, or even repeatability, over 5'10'3' work space, will be very expensive and/or difficult:

1) If you're only going to cut foam and balsa then maybe the machine frame won't be extremely difficult. I still think it'll be the most difficult part, though.

2) My guess is timing belts would be your best option for driving the thing.

Arvid

[crazyman]

Sorry I haven't replied lately. I've be sick. Great responses guys. keep em coming.
My main reason for making the table so big was to do full size automotive panel molds not the actual finished part and also some woodworking of cabinet doors and other artsy fartsy stuff.
Even if this turns out to be greater than my budget and I have to downsize the table I think the discussion we are having is pushing the envelop on ideas.
Let make it 10'X10'X10' now...lol

Regarding using wood. I though of that but the thing would warp quite a bit as the humidity of the wood changes. Maybe some more dimensionally stable type of wood/laminate products? Maybe silent floor product? Not sure if these are much more stable either?

I have seen a large table use raised stationary I-beams as a raised gantry and the spindle moved on top of the I-Beams in the say Y direction, but then you may run into a heavier spindle assemble to contol both the X and z Directions.

[crazyman]

The +/- 10mil mentioned means +/- .010"