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it is not really power you need but rigidity. my g0704 can machine ss easily wth a g540 setup. titanium is probably a little easier but you have to hit it hard as it work hardens really badly.
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Hi all,
I've used some sherline tools, and want to build my own desktop machine now, with a little more power.
If I want to cut stainless steel, or even titanium, at reasonable speeds and my ballscrews are 5mm pitch, what stepper torque do I need? 600 oz/in? 1600oz/in?
My milling bed/table is ~20lbs, solid steel.
it is not really power you need but rigidity. my g0704 can machine ss easily wth a g540 setup. titanium is probably a little easier but you have to hit it hard as it work hardens really badly.
What BoB, drivers and power supply will you be using, some of these come into play. Voltage has an effect on speed, I believe current has an effect on torque.
20lbs of steel is basically a tuning fork if you're doing anything other than drilling. Good luck with that.Originally Posted by PowermetalX
You want cast iron, because it will dampen vibrations. 40 pounds would be better, and 4000 would be best. My X2 table is probably closer to 40 and works OK, though.
Most of the small bench mills you see here use .100" or 2.54mm ball screws and ~400oz motors. People have used 270oz motors with success on well-tuned machines. So my guess is that 5mm pitch would work with the 425oz-class motors.
One thing to bear in mind is that as the motors get bigger, they tend to get slower--that is, it gets harder to spin them fast, so you end up spending more on higher-voltage drivers and all, to maybe end up at the same place as slower-pitch screws and lighter, faster motors which require less voltage. I tend to think the .100" pitch is something of a sweet spot for the little benchtop mills, meaning, anything weighing under a few hundred pounds.
"Most of the small bench mills you see here use .100" or 2.54mm ball screws" - Don't think so.... By far the most common ball-screw pitch is 0.200"/turn (5-pitch) for Imperial, and 5mm/turn for metric. 10-pitch ballscrews are actually relatively rare. Stepper motor top speed should really not be a serious issue until you get into larger motors, typically NEMA34s or even 42s. Any decent NEMA23 motor will probably reach more or less the same top speed as any other, within reasonable limits. Using a "slower pitch screw" and a "lighter faster motor" is almost guaranteed to result in a machine with significantly reduced performance as compared to one with a faster screw and larger motor (keeping the screw diameter and motor frame size the same). And the motors top speed capability is irrelevant if the motor does not have enough torque to push the load at that speed. Motors should be chosen based on the torque needed to meet the design requirements - velocity, acceleration, and thrust, taking into account the load, expected friction and other losses, plus some hefty margin. Every stepper motor has an optimal supply voltage to get maximum performance, and operating at a lower voltage is simply throwing away performance needlessly.
Regards,
Ray L.
D'oh! Ray, I stand corrected and agree on the .200, total brain fart moment. A 5mm screw is going to be effectively the same thing. That pretty much wipes out my point about slower/faster screws.
I do however stand by my basic assertion that torque ratings are not the end-all, but I could have probably stated that more clearly. A high-inductance, high-torque motor may very well underperform a low-inductance, lower-torque motor. At one extreme, I remember seeing an inexpensive 600oz-in motor/driver set on eBay where the motors had an inductance of something like 16mH versus a value of 2.8 for the Keling 381oz motor that is very popular these days. All other things being equal the 381oz motors may well give you faster rapids, for whatever those are really worth (not a ton, IMHO). Either motor will likely work just fine at the 10-20IPM cutting speeds typically seen on bench machines.
Agreed. 16mH is a real outlier!
Regards,
Ray L.
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