Servos or Steppers..... and other gecko related questions

[UKRobotics]

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 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!


Regards


Dominic

http://www.ukrobotics.com

[snaggletto]

Hey,
I'm no expert here, but I've been doing some research on building a servo based system.

Geckos, G320/G340 run 'brushed' DC servos only. Brushless DC servos and AC servos are incompatable due to differences in how they are driven. Brushed motors are cheaper and easier to drive so are the most common in hobby based cnc. Anoter company, Rutex,

http://www.rutex.com/us/catalog

has brushed and brushless drives. They also have higher voltage capabilities 100V vs. 80V for Geckos, and a model with 40A capability vs. 20A for Geckos. They will also run with TTL and differential encoders, as Geckos only run with TTL (without modification). That said, the Gecko's are powerful enough to run large metalworking mills, plus the end user support is vastly superior to Rutex. So I recommend sticking with Geckos.


Ebay is a GREAT place to buy this stuff;
This would be a nice ready made CNC Gecko based control with power supply and motors included (powerful motors, should be overkill for your needs) if you have the $;


http://cgi.ebay.com/ws/eBayISAPI.dll...9&category=633

If you would rather build yourself, here are some large/powerful motors that again would be plenty beefy;

http://cgi.ebay.com/ws/eBayISAPI.dll...category=50924

These would need encoders added as mentioned in the description.

These already have encoders and would be quite nice;

http://cgi.ebay.com/ws/eBayISAPI.dll...category=42920

Your next thing would be a DC power supply. For high power systems, the power fluctuation can be murder on standard regulated/swithched power supplies, although some have had luck. Most, go for 'homebuilt' unregulated power supplies. You will have to determine the motor specs; volts and amps to properly size your power supply (Geckos take a maximum 80V and 20A per axis/motor). Check out these links for nice power supplies;

http://www.industrialhobbies.com/

under the 'Estore';

Also, Mr. Doug Fortune has some nice PS's at www.cnckits.com. However, either my address is bad or his site is down right now.




The G2002 is a new Gecko product as yet to be released. I don't know when. It should have some great features. However, if you buy G320's and decide to upgrade when G2002 comes out, you will be able to sell the G320's no prob.


As for software, check out the windows 2k/XP based MACH2 at

http://www.artofcnc.ca

It is a fully featured program that works great with Geckos.

Good luck.


http://www.cnckits.com/

which seems to be temporarily down as of this writing.

[turmite]

Hi UK.

The guy at Gecko is named Mariss and is truley a brillant mind when it comes to electronics and motors. What he meant by the 100/200 watt is actually quite simple when you get to understand Mariss. 100 watts or less does not need the extra cost involved of servos! When it boils down to money I always look for the most efficent means to accomplish my goals.

Servos will give you quite a bit more torque across the speed range of the motor,,,,,,but at a higher cost, and that is the reason Mariss uses the 100/200 analogy. He deals with many diy folks and understands that cost is always important.

The G2002/2003 is basically a very high speed step generator. You will still have to have the drivers such as the 210 or the 320 which both use the step multipliers. The 2002 will not have encoder feedback capability but the 2003 will. I also believe if I remember correctly what Mariss told me is that the 2003 will also be capable of plc duties as well. You will also need a GUI or graphical user interface and the one I will use is Mach2. Mach2 is being written right now to work with the G200x series and when finished will be the finest package for diy on the market. Uh...that is in my humble opinion of course!

Mike

[arvidb]

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!

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

[UKRobotics]

Thanks everyone for the help so far.

Can somone please explain the difference between the G320 and the G340 ?

The Geckodrive site says the G340 has a "PLL step pulse multiplier" so if somone could translate that into english for me, that would be great!


Regards

Dominic

http://www.ukrobotics.com

[Al_The_Man]

PLL "phase Locked Loop" This is an electronic circuit for precise control of an oscillator. It started life in the early 30's by British Scientists, It came in to its own later when used for oscillator control in TV sets.
Al

[UKRobotics]

Sorry... I didnt write that question very well.

I wasnt looking for a definition of PLL, but more of an explanation of how it would be usefull in a CNC system. Ovbiously I'd prefer to buy the G320 because its quite a lot cheaper but if the G340 is a lot better then I'd be willing to spend the extra money for a better product. I suppose the question should have been, Do I really need the G340 or will the G320 do a good enough job ?. Does the G340 control the servo motors with greater precision, or am I missing the point completely ?

Regards

Dominic


http://www.ukrobotics.com

[Mariss Freimanis]

The G340 is marginally more useful than a screen door on a submarine.

Its only function is to turn your fine 1,000 line encoder into a 100, 200, or 500 line encoder. Why would you want want something like that? Only if you are already stuck with a high resolution encoder, a wimpy step pulse source and you need to go fast.

Break any part of that chain and you are far better off with a G320. It costs a lot less, in fact, just about the cost of an encoder from www.usdigital.com .

Its only advantage is sevro stiffness. A 1,000 line encoder will have a +/- 0.045 degree dither while all moves will be in 0.9 degree increments.

It's a specialized drive meant to meet specialized needs. Don't waste your money on it unless you meet the requirements listed above.

How do I know? I designed and manufacture the drives.

Mariss

[UKRobotics]

So would a G320 and a encoder with fewer lines still have the ability to mill out things such as gears for radio controlled cars which require a very high degree of accuracy?


Regards

Dominic


http://www.ukrobotics.com

[HomeCNC]

Yes, I use the G320 exclusively. The drives and motors are not the major problem with accuracy. It's the machine you attach the servos to. Don't skimp on the quality of the parts when building your CNC machine. Backlash will kill you for accuracy (or maybe that's repeatablity) OH NO NOT THAT AGAIN!!!!

[arvidb]

Originally posted by HomeCNC
*snip* Backlash will kill you for accuracy (or maybe that's repeatablity) OH NO NOT THAT AGAIN!!!!

*LOL*

Backlash and low rigidity is not only bad for the reasons above; it also is bad for quality of cut, tool life etc. At least for harder materials.

Arvid

[lenitech]

I am looking at using the master5 or mach2 software with the gecko g320's to run my bridgeport series 1 cnc mill. I want to use the original stepper motors and then add linear encoders so that I always know where the table is and I don't have to worry about backlash. I need to know how to set up the software. i.e. do I put the actual steps/inch of the motor or the encoder into the software. Does what I am trying to do make sense?