Persuade me not to go DMM servo

Rather glib title I know but the the thread is a serious one.


I've pretty much decided to go DMM servo (Nema 23 equivalent) when putting together a CNC setup I'm considering.

For all the reasons you might expect, speed, accuracy, smoothness, noise etc but reading the forum, there are some clever people here who squeeze as much as possible out of steppers despite their weaknesses, and get great accuracy, speed etc. So I was wondering if there is any gotcha to put me off DMM servos (or servos in general)


They will be driving a gantry from between 60-80kg,

Y (2x) 16x10mm BS 1330mm
X 16x10mm BS 1040mm
one motor each, both driven nut via belt 1.25x reduction, fixed ends
Z 16x5mm 1:1 ratio 190mm


Doing some rough calculations, Y inertia ratio will be between 2-4x. Possible rapids at 900-1200IPM.... which covers the Y in under 3 seconds, scary stuff.


I might add a laser at some point, so I think speed and accuracy would be advantageous but at the cost of at least £1500* (damn you import costs), vs 3x low inductance steppers, 3x good drivers, approx £250-£500, the servos better be amazing.
Envisaged material: wood, non ferrous.
(*both not including motion control)

Thanks all.

Quote:

---

Originally Posted by rvectors

Rather glib title I know but the the thread is a serious one.


I've pretty much decided to go DMM servo (Nema 23 equivalent) when putting together a CNC setup I'm considering.

For all the reasons you might expect, speed, accuracy, smoothness, noise etc but reading the forum, there are some clever people here who squeeze as much as possible out of steppers despite their weaknesses, and get great accuracy, speed etc. So I was wondering if there is any gotcha to put me off DMM servos (or servos in general)


They will be driving a gantry from between 60-80kg,

Y (2x) 16x10mm BS 1330mm
X 16x10mm BS 1040mm
one motor each, both driven nut via belt 1.25x reduction, fixed ends
Z 16x5mm 1:1 ratio 190mm


Doing some rough calculations, Y inertia ratio will be between 2-4x. Possible rapids at 900-1200IPM.... which covers the Y in under 3 seconds, scary stuff.


I might add a laser at some point, so I think speed and accuracy would be advantageous but at the cost of at least £1500* (damn you import costs), vs 3x low inductance steppers, 3x good drivers, approx £250-£500, the servos better be amazing.
Envisaged material: wood, non ferrous.
(*both not including motion control)

Thanks all.

---

You should not compare a Servo to a Stepper for accuracy there is no comparison even with the closed loop steppers, it's all about the Encoder 16 Bit and higher is what you want to be looking at if you want smoothness speed and accuracy

He's talking about machine accuracy not servo/stepper accuracy. They are two different things.

OP. If I am reading right you are looking for 900 IPM on 10mm pitch screws with rotating nuts spun at a motor/nut ratio of 1.25:1.
That means 900*25.4 = 22860 mm/min --> 22860/10 = 2,286 rpm of nut ---> 2286*1.25 = 2857 motor RPM. Ain't going to get that out of a stepper and have any kind of torque reserve.

Yes, you won't really see any difference in machine accuracy, but the servos will give you more resolution. I'd recommend servos over steppers once you get over about 500ipm, because with steppers, you need to trade resolution for speed.

With servos, I'd recommend 25mm pitch ballscrews, with a higher gear reduction to slow the rpm of the nut.

You might want to consider Delta servos instead of DMM, might save you some money on shipping.
https://www.ebay.com/itm/Delta-0-4KW...53.m1438.l2649

Thanks guys, and yes that's exactly what I meant


I had pretty much decided on servos for that very reason but spending 3 times the amount, eats into funds that could be used to kit out other items. Still, I'd much rather spend on getting it right from the outset.


I'll definitely check out the Delta servos. Speaking of purely stepper/servo accuracy, one thing I saw but was not sure of its real world significance... with DMM, the encoder is 16bit (65,536) but max accuracy is 14bit (4000). Whatever it is, it'll probably exceed the machine's inherent precision**.


As to the BS size. Unfortunately I'm not designing the CNC, its a design by German engineer (Hermann Möderl), who I'm guessing licensed the design to Sorotec. Its the EMS P3 (Large) Cobra. I spent a lot of time searching for the best on a limited budget, but which offers the widest future use as my skills and knowledge increased.
My admittedly newly acquired, and limited knowledge, did make me think something along those lines regarding the BS/gearing, as the design looks based around steppers. Price-wise 4800EUR (just the bare mechanical structure), the design concepts look solid, and definitely good up against some of the more well known brands in the same price bracket.

I'd be stuck with the BS size but I think I can ask for a slightly bigger gearing ratio. Not much, 1.36 rather than 1.25, without needing to adjust the design (and then cost). If i'm not mistaken, this also adds a little to resolution (accuracy, error spread over wider movement?) at the transmission side, and linear acceleration, which is what many overlook.



**
Repeat accuracy <0.02 mm.
Maximum backlash of the ball screw <0.02 mm
Accuracy taken across all axes and the entire milling surface about 0.30 mm


Edit
Those Deltas look interesting, 17 or 20bit incremental encoder vs absolute for DMM. The price does work out cheaper, especially if I don't need to build a Linear PSU. Not sure how quality (parts+software) compares with DMM but both have auto tune and both are manufactured in China.

Quote:

---

Originally Posted by rvectors

Thanks guys, and yes that's exactly what I meant


I had pretty much decided on servos for that very reason but spending 3 times the amount, eats into funds that could be used to kit out other items. Still, I'd much rather spend on getting it right from the outset.


I'll definitely check out the Delta servos. Speaking of purely stepper/servo accuracy, one thing I saw but was not sure of its real world significance... with DMM, the encoder is 16bit (65,536) but max accuracy is 14bit (4000). Whatever it is, it'll probably exceed the machine's inherent precision**.

---

Panasonic servos are also a good choice, and I would use Panasonic over Delta

That's a misconception everyone has the 14 bit accuracy has nothing to do with the Encoder Resolution and most don't offer this spec, as it can be confusing, it referrers to the spacing of the Encoders division's and has no affect on the true 16Bit (65,536 ) PPR

Most Encoders fall into this accuracy range, unless you want to pay mega amounts of money, but this has no affect on the encoder rated resolution of 16 Bit in this case