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I'm building a nearly 4x8 foot CNC. I'm figuring to use double ballscrews on the Y axis, but how bad a mistake would it be to use a single in the center instead? Would I get a lot of racking? Or is it just too much work on the one motor and ballscrew? I'm using NEMA 34 with 20mm rods.
 

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David - Machinist in wood
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I think one that size should have two. Our 2x4 with 125 lb. gantry has two and I can't see one as large as yours getting by with only one motor. Racking issues aside, I just think it needs two.

David
 

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Mike
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I would worry about screw whip even using 25mm screws at that length. I agree with Gerry on using Rack and Pinion instead of the screws for the Y axis.
 
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David - Machinist in wood
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Thanks for this insight! So I guess the next question is: Where can I find a rack and pinion that size?
They don't have to be that long. You can butt shorter sections against each other to form one continuous rack. Just make sure they're straight.

David
 

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I bought my rack and pinion from McMaster Carr. 20 pitch, 20° Pressure angle It’s available in 6’ length. I have seen a 4’ x 8’ with a singe center screw, a DMS Patriot machine. The screw was probably 3” in diameter and the servo driving it was the size of my tablesaw motor. DMS makes nice machines in Colorado Springs.
 
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