Servos or Steppers..... and other gecko related questions
Hi everyone,
I'm quite new to the world of CNC machining and am planning to build my own 3 axis CNC router.
To begin with I will be mainly machining plastics such as polycarbonate and polyethlyene but I would also like the machine to have enough power to machine soft metals such as ali and brass.
The only CNC router i've ever had access to cuts at 3 inch/sec and struggles with anything more than 1mm per pass in soft plastics. Milling soft ali even on the lower speed settings was difficult and painfully slow - the manufacturers claim of being able to mill stainless seems like a joke.
This experience has put me right off stepper motors, I want something that’s both powerful and reasonably fast and this stepper motor driven machine is neither which makes me think servos would be a better option.
Still, I’m no expert so I’d appreciate some opinions on this.
I phoned up gecko drive recently and explained what I was trying to achieve and although the person I spoke to did try his best to explain it I still don’t fully understand what I need.
I was told if I required 100W mechanical or less to use steppers or if it is 200W or more use servos and in between, either will do. Unfortunately that doesn’t mean much to me. I know how that translates into HP, but what I don’t know is how that translates into actual machining ability. Would I need a 200w or more motor
for milling ali or would 100 be ok ? And how greatly does motor wattage affect speed ?
Another thing which I'm still confused about is which gecko to get. Assuming I went for servo motors would I need the G320 or the G340? Again, the guy at gecko drive did try and explain but either because it was late and I was tired (I phoned at 12am uk time) or because I was concerned about running up a rapidly increasing 50p/minute international phone bill or simply cause I'm thick :o I'm still none the wiser to which one I actually need.
As far as I'm aware, a servo motor is just a standard DC motor with an encoder attached. Can someone recommend somewhere to buy these from ? Also, would I get better accuracy if I bought a servo motor that already had the encoder attached properly rather than trying to fit one myself to a motor that wasn’t designed for it ?
Can anyone recommend some places that sell servo motors that are compatible with the geckos ? Going by what I’ve read in other threads on this forum it seems they are quite fussy about which encoders will work properly.
What’s this USB G2002 I've heard mentioned. Would I be better off waiting for this instead of buying one of the 'older' gecko drives ?
Finally, I know I might get a slightly biased answer here, posting in a gecko forum but are there any reasons not to get one? All the comments I've read seem to be very positive but I just wondered if threes anything big which I might need that a gecko cant do but another product might ? - or is it the software used with the drive which makes the biggest difference to the outcome?
Once again, I apologise for my ignorance and the large number of questions but who knows ... one day I might even have learnt enough to help others on here with their CNC problems! :D
Regards
Dominic
http://www.ukrobotics.com
Re: Servos or Steppers..... and other gecko related questions
Quote:
Originally posted by UKRobotics
*snip* I was told if I required 100W mechanical or less to use steppers or if it is 200W or more use servos and in between, either will do. Unfortunately that doesn’t mean much to me. I know how that translates into HP, but what I don’t know is how that translates into actual machining ability. Would I need a 200w or more motor
for milling ali or would 100 be ok ? And how greatly does motor wattage affect speed ? *snip*
Power (designation P, unit watt [W] - and therefore sometimes called "wattage") is equal to speed times the force developed while moving.
For linear movement this is quite easy: power simply equals speed times force at that speed (in metric units: P [W] = v [m/s] * F [N]).
For rotary motion, it's really the same thing, but
1) you have to measure motor speed not in RPMs but in radians per second. This is called the "angular velocity" of the motor, designation theta. 1 RPM = 2*pi/60 [rad/s]. So 1000 RPM equals about 104.7 rad/s.
and 2) you have to measure motor strength in torque (designation M, I think, and metric unit newton meter [Nm]).
Then power is simply (in metric units again): P [W] = theta [rad/s] * M [Nm].
So a 100 W motor could be either 3000 RPM and 0.32 Nm (3000 RPM => about 314 rad/s * 0.32 Nm = 100 W), or 6000 RPM and 0.16 Nm, for example.
Observe that you cannot calculate the power of a stepper from its holding torque data. The holding torque is the torque at 0 RPM, and since power is torque times speed, you would end up with zero watts!
Quote:
Originally posted by UKRobotics
*snip* As far as I'm aware, a servo motor is just a standard DC motor with an encoder attached. *snip*
This is essentially correct. Servo motors are usually made to have a small rotary inertia (compared to "normal" motors), to be able to start and stop quickly. But this is mostly important for a production machine where every split second counts.
Hope this helps!
Arvid
