A bid, a miss and some questions

[Pretorien]

Just have the results in hand for an auction that caught my eye - it was essentially the "chassis" of a Taig mill - base, saddle, table and column - no spindle or motor and needed at least nuts and an X-axis handle. It first showed up at a starting price of $49 - after a very useful exchange with Nick Carter, I had a number for what would be needed to turn these bare bones into a CNC ready mill with the Er16 spindle. I subtracted that number from the cost of a new unit with shipping, subtracted a "cushion" (what I was willing to risk on the unit needing more than I expected - $250 in this case), subtracted the estimated shipping and arrived at a bid which I submitted - and lost ( by almost 3X!)

But, while I was waiting for the auction to run its course, I spent some time looking and thinking about the motors and drivers that I would need.

Here's the application: ~ 3" X 6" sheet brass and aluminum - 0.030" - 0.125" thick, mostly through cuts with end mills no larger than 1/8" (I have an X2 with 3 axis DRO when I need to swing a bigger tool and, for really big drills, a 16" Walker-Turner drill press - how many are old enough to recall that name?)

So - my guess is that most of the load on the steppers will simply be moving the table with little contribution from cutting tool resistance - this is what it feels like when I am hand cranking my X2 - even at the less than optimum spindle speed, I can see the bit cutting but I can't "feel" it with these small cutters - That said, I will need to keep things snug because I need hold reasonable tolerances.

OK - the point of all this - There are a bunch of obvious choices around - Xylotex, Gecko, HobbyCNC Keiling etc (and one that I can't call to mind but saw in my wanderings - reasonably priced individual drive assemblies each of which had its own integrated fan).

I have also seen on several (auction and other) sites, some very cheap packages that typically use much smaller motors (150-175 oz/in vs the 250-280 oz/in motors found on typical packages) I have also noted the the "factory" Taig full CNC set-up uses 200 oz/in motors.

Are the larger motors (a) just right (b) overkill for my application (c) good insurance against missed steps?

In the distant past,I did a fair amount of electronic design and assembly (vacuum tube audio gear) and am leaning toward brewing my own linear power supply if I decide against one of the popular package assemblies (With a linear supply, if there is noise or ripple, I know where to look - don't really understand switching supplies)

Thanks in advance for suggestions and counsel.

Pretorien

[Stepper Monkey]

The reason for the smaller motors on some of the Taigs is that they actually perform better when using the budget drivers (Xylotex, etc.) Too big of a motor with the budget drivers and either a) you drive them at less than rated current and get less than rated power anyway, or b) they have high strength and low current ratings, so the inductance is high enough the coils never charge all the way before firing anyway, and again less than rated power at any real speed. You also get a lot of other less tangible gremlins using either of these ways that really kill performance.
Both of these options give less optimal performance than just using smaller, lower inductance motors with the budget driver in the first place.
Even one of the major dealers, who in order to just get machines to move has to play the "number game" and ship Taigs with 495/oz motors despite his advice, uses 200/oz motors on his own. So do I (about 230/oz for mine). Works great with a Xylotex actually. It's just that no one will believe they work better, they want the bigger hammer and end up getting a machine that is weak, and therefore think they need even bigger motors without ever thinking about how they match the driver. It is a vicious cycle.

That said, the bigger motors can obviously be of great use on a Taig, but only when coupled with the proper higher voltage and current drivers like the Gecko, the MCP, or the bigger Kelings.

Basically, just match the motors to the driver and you are fine either way. It ain't rocket surgery, just do the math, but there is always someone that finds it easier to try and flame those who give that advice off of a forum than to just pick up a calculator, so there is a lot of confusion out there still.

[Pretorien]

Thanks, Stepper Monkey - let's see if I understand your comment:

Just looked at a site picked at random from Google - compared 2 NEMA 23 motors:

175 oz/in, 2.5 V, 2.8A, 0.9 ohms, 2.5 mH

260 oz/in, 3.2 V, 2.8 A, 1.13 ohms, 3.6 mH

The numbers suggest that the second motor has more turns on the winding (higher torque= ampere turns, higher voltage for the same current (greater resistance) and more inductance.

The increased inductance will lead to slower response to changes in the input signal therefore, if the higher torque is not needed, the smaller motor will outperform the larger given the same drive function.

Is this a reasonable interpretation?

Pretorien

[Stepper Monkey]

It gets pretty complicated, so it is still a bit of an oversimplication, but yes, that is a pretty good summation. The bigger motor will actually deliver the greater torque at low speeds, but as speeds start to increase (time to charge coils decrease) the torque drops - really fast if it is high inductance. The smaller motor has less torque at slow speeds but has more of a flat curve.

In practice either one of those two could very nicely work with the Taig as they are both pretty low inductance, so even with a smaller driver both could work depending on your uses and desired speeds. A motor with say, 400 - 500 oz/in and 10 - 13 mH however, would underperform both in that same application.

[Robin Hewitt]

Basically there are two kinds of stepper motors in this world, fixed voltage and fixed current.

If your motor is designed to work at 12 Volts with no current limiting then it won't go very quick. If your motor is rated in Amps then you simply use more Volts to overcome the inductance while limiting the current. Low rotor inertia counts for squat if you are hanging 4 lbs of lead screw on the end. There is not necessarily much improvement connecting paralell rather than series. The numbers mean little without speed/torque graphs to back them up, you don't need a maths degree to see the speed at which the output torque starts to plummet.

Both these motors are obviously Amp orientated and will do fine.

I know the oz-in rating looks important, but it's only a guide because it's pull in power against speed that counts, not holding torque.

Have you considered size 34 motors? I think motor choice is a bit like hand guns, you can get phenominal power in smaller sizes but when things get sticky you want your caliber to start with a four