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Old Sat 27 August 2011, 00:23
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ball screws

I just saw some fairly low-cost ball screws on www.Automation4less.com that might be worth looking at. They have 16mm and 25mm screws with nuts preloaded with oversize balls to give backlash of less than 0.0006". The screws are available with endmounts and couplers at a very attractive price. The problem is that the maximum length being offered is 2,000mm (78-inches), so the ballscrews would not work on a full size machine.

A 10mm per turn 25mm diameter screw requires almost exactly as many steps per inch as a 10:1 geared motor driving a 1.25 pitch diameter spur gear.

Geckodrive has some 23 size motors for about $60 that can be run from their G250/G251/G540 stepper drivers. The motors are rated at 280 oz*in. At 10:1, they would produce 2,800 oz*in of holding torque.

Since a 10:1 ratio would require about 5,100 steps per inch, Mach 3 running at 45,000 pulses per second might be able to jog an axis at about 8" per second; however, ballscrews whip if spun too quickly, so their maximum practical speed might be less than that.

Note that the backlash is equivalent to about 3 steps, so Gecko's stepper drivers with a 2:1 or 4:1 pulse multiplier might be suited if pulses per second are limited by your software or hardware.

I've always thought that using linear rails and ballscrews would give the highest accuracy. The products at www.automation4less.com are inexpensive enough that those making smaller machines might want to look.

(Automation4less is temporarily out of end mounts, so they're not showing the price of the mounts, but if I remember correctly, the price for both mounts is about $150 per screw.)

Last edited by Richards; Sat 27 August 2011 at 00:26..
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Old Sun 28 August 2011, 08:31
PEU
Just call me: Pablo
 
Buenos Aires
Argentina
Mike, these look exactly the same as the ones sold by Chai (http://stores.ebay.com/linearmotionbearings) I purchased several times from him and the prices are great, he even offers end machining.
Send him a quote request with the exact sizes you need he usually replies within 24hs. Last time I purchased I ordered each ballscrew with double nuts so I can remove the backslash via preload. Measured backslash was about 0.04mm
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