Two ballscrews on both X and Y axis?

[LoveLearn]

Johathan, it seems to me that you've just outlined the classic comparison between bed mills verses a gantry mills. For any specific machine frame's total mass, more well-selected boxing-in material allocations make more rigid structures than "open C-shapes" like bed mills. Extending ideal mass allocation concepts to their extreme, as Buckminster Fuller explained ad nausium, triangular enclosures are more rigid than 4-sided box structures. Yet when we examine the most widely selected industrial machines, open C-shape configurations are more popular than fully-boxed double-upright gantry designs. Open bed mills can provide wonderfully satisfying human work/play space experiences because they minimize components "in the way" blocking work space access. Just off hand, I don't know of any triangularly supported mills, despite the fact that vertical space is often available which could accommodate upward projecting triangle intersections.

Perhaps these comments will trigger some inventive souls to design even more rigid mill configurations for any selected total mass. I have not seen conversations about rigidity/mass ratios related to machine tools comparable to aircraft and human-powered vehicle design conversations. Yet that kind of elegance seems appropriate. Lessons we've learned about frame design strategy in every venue should be applied to every other venue, including mill frame design. If one's goals include minimizing cost/performance ratio, maximizing rigidity/mass ratio, fitting machines within available environmental space, traditional design paths probably have not yet been optimized. I'd love to see a community of clever souls sharing thoughts and developing insights which evolve into a new and better high-performance mill class. With effort comparable to what many spend working cross-word puzzles, which in 5 decades effort generates zero gain to the world, we might make a wonderful contribution toward making human populations more productive so they can lead better lives.

If a pole-supported tent's center joining fixture supports a couple dog-chains leading down to a swing seat, that tent's support structure's apparent rigidity can instantly be increased many fold by adding person's weight to that center-supported swing seat. Similarly, top loading a box-like supported mill could greatly improve its effective rigidity. That top loading material need not be expensive. Don't be constrained by popular model design strategies.

Since I first studied and compared various rotary to linear motion conversion designs, I have always remembered that classic screw drive's high internal friction converts most input energy into waste heat, so we see lots of 30%-efficient range screw drives. But that same comparatively high friction acts as an effective one-way clutch. You can drive all screws by rotating them, but the driven load can't drive the screw unless its super steeply tapered.

Compare stepper motors driving ball screws verses driving acme screws. Highly energy-efficient (often around 90%) recirculating ball drives can't perform as a one-way clutch, so the motor MUST resist load-induced driving forces, unless you add an electrically released spring-loaded brake. By comparison, a stepper motor driving an acme screw can't be pushed about by the load because that force can't spin the screw. More expensive stepper motor electric drivers commonly allow the person setting up the system to select one of several motor de-energizing routines triggered by motor command inactivity. When a motor is not being commanded to spin, one choice would be to hold motor driving electric current constant, wasting power and keeping motors hotter than the work requires. That's how low-priced drivers behave. Another choice is to taper driving current down to about 30% after 100 milliseconds inactivity. Another choice can be dropping driving current to zero when triggered by inactivity. But if you're using micro-stepping and don't happen to stop on a full step position, dropping current to zero would let the stepper motor's permanent magnets rotate it to the magnetically-closest full step position. Inactivity-triggered electric unloading enables stepper motor driven devices to become much more electrically efficient than low-end drivers cause. Motor heating functionally depends on average electric load. So if your driver reduces or disconnects power during inactive periods, you motors are running cooler so you can usually briefly over-drive them with higher than "rated" voltage during peak acceleration periods without turning motor windings into a melted fuse. If you're running recirculating ball drivers on your vertical Z axis, I'd definitely suggest adding one counter weight if its an open-C configuration, or two opposite-side counter weights if its a gantry configuration. The goal is to make net average vertical force "seen" by the ball screw(s) approach zero without introducing parallelogram-distorting forces.

Just some thoughts triggered by your inquiry.
Enjoy the "mind candy."
John