plasma design 8' x 4'

[irving2008]

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I have a couple of questions, I am genuinely interested. Having worked hard on getting rack and pinion / V bearings right, why did you decide to switch to Ballscrew / linear guide for the Z axis?

Also on the X carriage, the ratio of the distance between the wheels left/right to front/back is high. i.e. Its a wide, short carriage. In the past I have had problems with linear rails that stick when the ratio is too high, like the runners in a chest of drawers. Have you ever had anything like this? (probably not a problem because you drive from both sides)
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There is a rule, known as the 2 to 1 rule, which says that the point at which force is applied should never be further away than twice the distance between the support bearings. So if your bearings are 50mm apart the maximum height of your workpiece is 100mm (assuming the top of the gantry is not constrained). See the video here (if it asks you to register just put in any old rubbish for name and email, it doesnt check). A similar rule applies horizontally, but where both sides are running on rails and the two sides are connected its 4:1 or more depending on rigidity of the connecting piece, so for bearings 50mm apart the maxmimum width between rails is typically around 200 - 250mm. If you look for example at the Igus dryLin carriage plates, they always stick to a ratio between 3 and 4:1