I've had this project on the backburner for what seems like forever, just waiting to get all my ducks in a row. Money, time, tools and space always seem to somehow conspire against my efforts but rather than build something with partial dedication, I've waited for the planets to align. Still a few more months to go before I have access to some tooling and can delve into construction but in anticipation I've been hammering out the final design. I'd like to share some ideas and ask questions.
The goal is to end up with a medium sized bench aluminum cutting cnc with some ferrous capability as well. Travels will be X 18" Y 11" Z 13" and rapids of roughly 400 ipm. Spindle is a modified R8 mini mill with a 2 HP treadmill motor. I've already cut down the casting and welded the mounting tabs. Motion control will be with Mach running Zeta4 stepper drives. It's to be seen whether the parallel port will be sufficient or a USB interface of some sort. X and Z steppers are 1200 oz-in nema 34's wired up in a T connection, which will provide performance half that between parallel and series. I got dyno results from Lin Engineering and used them to model the acceleration characteristics of the X and Z. Not too bad for moving 260 lbs! Also assumes the Z axis will be counterbalanced, probably with a pair of gas springs. Will probably limit speeds to 400 ipm and I doubt I can program these acceleration characteristics into mach, and will probably have to settle for a constant acceleration which will result in a linear speed curve with slope equivalent to optimal speed curve at the highest feedrate i want to use. Still should be plenty fast
I don't have any data on the Y motor. Just a round frame nema 34 which I've been told it's around 300 oz-in, but that should be ok for moving the relatively light head along the Y axis.
Linear bearings are 25mm for the X and 15mm for the Y and Z. Plan on ordering precision rolled ballscrews from the Chinese ebay seller linearmotionbearings2008. 20mm for the X and Z, 15 for the Y.
As for the structure of the machine, the bed is an 8x28 replacement bench mill table. The two vertical columns are 2.5" square pre-ground steel bars. the rest of the fabrication is surplus bought 3/4" flat ground stock. I plan on waterjetting this stock on a 5 axis machine to get zero taper on all the reference edges with non critical edges getting a low quality cut to save machining time and cost. The couple local shops I called said this should be feasible but I'd like to know what some of ya'll think. Also would like to know if it's possible to have the hole locations "punchmarked" by the waterjet to be finished on a drillpress later (I don't have a mill anymore and would like to avoid using one as much as possible).
In the end I'd like to have the whole thing mounted on an enclosed bench with flood cooling containment and have the controls mounted under the table.
Well that's probably enough for now, I'll leave the questions to be answered or asked. I'm pretty confident about the design so far, but feel free to fill my head with doubt