OK. Real world scenario. At work I run a Biesse Rover 335 CNC PTP router (a cross between a point-to-point borer and a CNC router). The gantry design is cantilevered and depends on having the base of its two supporting rails vertically spaced about 47cm (18 1/2in) apart along the back of the machine. The rails support a gantry which carries 24 vertical / 8 horizontal drilling heads as well as 3 x 7.5HP HSD high speed spindles. In use this cantilevered "arm" is stable and doesn't dip towards the table at the furthest reach of Y-axis (950mm), probably because the two side members are 9 x 3-1/4in channel section steel sitting on a cast-steel Z-axis carraige . The Y-axis carraige baerings are approximately 43 cm (17 in) apart in the Z-axis direction and 64 cm (25 in) apart in the Y axis direction. The Z-axis slides are a two part affair, pneumatically driven (to drop the head to its "zero" position) and servo driven (from the "zero" position to cutting depth). The spindles are bolted onto aluminium face plates some 10mm thick and these in turn have THK bearings running on linear tracks (two of them) with the ball screw in between. Our unit is supporting considerably more weight than a small unit like yours and bearing wear isn't an issue. Lubrication, however, is - we relube twice a week, religiously, with the correct red EP grease and NEVER change manufacturer. I posted this info to give you an idea how full-size machines tackle this problem in the real world.
Having owned a ShopBot in the past I was sceptical when we bought this machine that it would be ridgid enough (ShopBots flex not a little, I believe because the Y-axis gantry is just not ridgid enough). Incidentally, there was a CNC manufacturer here in the UK a few years ago who used to build machines with a "wide base gantry" (rather like the ShopBot) which used a two rail carraige to hold its spindle heads. The motor was bolted onto a chassis which was sort of U-section with a guide rail and bearing sets at each end of the "U".
Hope the above info is of some assistance.
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Scrit
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