Pointers on gantry design for 8" cube

[mwirth]

Folks,

Hope I'm not committing an RTFM error here

I want to build a 3D CNC router with roughly an 8" cube of swept space. I have my own steppers (#43?), over a dozen 3/8" dia. linear bearings and 3/8" precision shaft and my own machine shop (lathe and mill).

Can you point me at a suitable gantry design that I can use or modify. I plan to use it for multiple purposes (e.g., RepRap-style plastic extruder head, engraving metal scales and dials with a Dremel tool, air-powered spindle or somewhat heavier head). Thus I would like the structure to be as stiff as possible and capable of holding position in spite of vibration or head weight to within 0.001". (Backlash to be controlled by preloading, for example.)

Engineering analyses, real-life experience, etc., are a plus.

TIA,

Mike

[spoiledbrat]

MWirth,

Take a look at my entries in the gallery. It shows my 1st router, which was constructed of 1/2" aluminum plate. Main point from that design is, don't try to save space at the cost of rigidity. You will notice my gantry walls are approximately 6 inches wide. This prevents parallelogram movement left to right. Basically, I tried to get some serious triangulation going on in the direction where it might flex due to cutting load.

The X axis is driven by one ballscrew, which is not optimum for the 24" space. Ideally, I would have driven the gantry with two screws. If you are willing to lengthen your gantry base, and get those bearing spread out along the x-axis, it should present no problem. This would, of course, make your machine a lot bigger, but would provide better rigidity (no racking) while driving with one lead screw.

I would ditch the linear shafting and bearings, and go with linear guideways. Automation 4 less dot com has them for excellent prices, and you do not need to get large ones. 15mm or smaller should work great. They also have the old style (only one ball-track on each side of the guide and bearings) for a great discount. If you were to take that linear guide, and bolt it to something rigid, it should outperform that linear shafting by a long-shot.

Now, these blocks can be had for around 24 ea, and you would need 12 for a 3 axis machine. This might seem like a lot, but I would submit that the linear guides are the heart and soul of any machine. If you want rigidity, it all starts there.

Cheers, and good luck!

Rob