[journeyonline]

All you experienced builders please comment!

I'm in the process of building a low cost 4'x4' CNC router based machine to work mostly with plywood. I planned on using ball screws originally by home shop cnc for the x and y axis. But I keep wondering if I should just add the extra 4' and make the machine 4'x8'. Home shop only sells a 6 foot length in thier ball screws. I could use a mixed environment that uses racks or a long lead screw to keep cost down. But is this a bad idea to mix ball screws, lead screws, and racks? So now I wonder what are my options since my budget is limited about $500? I don't need super accuracy but its always a plus.

[NC Cams]

Check the prices on you lead/acme/ball screw options and your budget will ultimately decide what you can use as an available option for the accuracy you want to achieve.

Following are GENERALIZATIONS:

High class machine tool makers use ball screws (VMC's, Lathes, mills, etc) where accuracy is paramount. Much higher relative cost and accuracy.

The mill/drill/imported hobby lathes use cut acme's as they are trying to manage cost to a price sensitive market. Much lower relative cost and accuracy.

Ground acme's and rolled thread screws fall inbetween (cost and accuracy)

The acme (ground vs cut vs rolled) versus ball screw (rolled vs ground) issue has been discussed quite extensively in other/prior posts. Use the search engine and it should take you there for a review of the discussion.

An "inexpensive ground" anyting is an oxymoron and rolling friction is always going to be lower than sliding friction (IE: ball versus acme).