Why TWO motors for the Z axis ?

Indeed, all the classic designs I see use two motors, one per each lead screw instead of one plus timing belt for the two. This is risky in case a motor looses steps, the system will bind, considering the screw advantage may not even be noticed and may result in serious damage.

Edited 1 time(s). Last edit at 01/11/2016 11:15AM by MKSA.

Right on all counts. In machines where the X axis is lifted by two Z screws, the X axis alignment with the Y and Z axes is critically dependent on those screws staying in sync. If the axes are not orthogonal, circles print as elipses and rectangles print as rhombuses. Try getting eliptical gears to mesh or making a tightly fitting lid for a box with misaligned axes. It doesn't work.

The reason they use 2 motors instead of a closed loop belt with a single motor is that motors are very cheap and closed loop belts and pulleys are less cheap.

It seems that a lot of people are satisfied printing Yoda heads and tugboats so alignment of the axes doesn't matter much.

My printer has two Z screws driven by a belt and a single motor. You can see the design by clicking the link in my sig, below.

Edited 1 time(s). Last edit at 01/11/2016 11:30AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

OK, so it is one improvement I plan to do as soon as I get my Geetech Prussa printer.
I think I will have some fun finding all the design errors, quality control errors .... I bet stiffness, straightness, squareness, smoothness, flatness, LochNess will be major issues too.

The last time I looked for a closed loop belt I could only find 1500(odd)mm, when I looked last week I found these
[www.ebay.co.uk]

great for 400mm distance between axis with a little extra for an offset motor or belt tensioner.

You can order belts from Stock Drive Products if you're in the US. Not exactly cheap, but not outrageous. They're bound to have a size that you need. With ebay you're very lucky if you find one that's the right size. Plan on ordering a belt that is longer than needed and use an idler pulley (ball bearings on a bolt) to take up the slack, unless you can make your motor mount moveable. You want at least 90 degrees of belt wrap or at least 6 teeth engaged on a pulley.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

RS, Farnell, Conrad sells various timing belts. The GT2 ( Gates Timing belt) seems the de facto standard for "beginners" 3D printers but other could work here as we need to synchronize two shafts. Many scanners, printers also uses these belts (Engine too but not the same size ). They could be a source of quality belts, pulleys, guide rods, bearings, fasteners etc...

Edited 2 time(s). Last edit at 01/12/2016 01:56PM by MKSA.

Lots of closed belts around. Many places call them endless instead though. Like [www.robotdigg.com] which sells ok quality items.

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My Instructables - both total newbie instructables and some for intermediate users.
My Designs on Thingiverse
YouTube channel containing a few 3D printing videos - they are videos for my Instructables, and mostly not standalone.
Ultius / Tantillus Thingiverse Group