[mysterious]

I will be doing a CNC conversion on a Shoptask 3 in 1 machine. I know servos are supposed to be faster and more accurate than steppers, but I am doing this as a hobby and don't need super high speed. Also are servo systems harder to use as far as programming or are they about the same? I haven't heard this discussed too often, so is it not even an issue. I can get servo versus stepper for about $600 more, but is it really worth it? This is a mill lathe conversion and I think steppers would be fast enough, I don't think the machine can keep up with servos top speed anyways. Tell me what you guys think.

[ger21]

Steppers can be every bit or even more accurate than servos. Spend the $600 on other areas and go with the steppers. As long as the steppers can give you enough power.

Gerry

[HuFlungDung]

I would recommend that you use a closed loop system, whichever way you go. That is my main objection to the less expensive stepper systems, is that error can creep in and you will be totally unaware of it until it is too late. This means some kind of encoder feedback, and software that will make use of the information to keep the current commanded position correct.

[ger21]

While Hu raises a valid point, it all depends on your budget and what you're planning on doing. Are you planning on using Geckos to drive the steppers? What size are the steppers? And what software? You can use Geckos, steppers and Mach2 all for typically well under $1000. I'm assuming that since this is a hobby it would be more than adequate. Unless I'm mistaken, the software alone for a system like Hu is talking about can cost more than that. And the hardware even more. Care to elaborate on this Hu?

Gerry

[HuFlungDung]

I can't say that I know much about the detailed workings of the low budget systems. Its up to the buyer to try to find the most performance for the least money. But, part of the equation is knowing what you should be looking for, and knowing the ultimate limitations of the machine that you end up with. Machining without feedback to the controller is like driving blindfolded, IMO

I really don't believe that there is much of a difference between the hobbyist and the professional in wanting the machining job to be carried out successfully. A wide tolerance range is not the same as random errors due to mispositioning.

[Swede]

I tend to agree with Hu in that closing the loop via encoding ensures the accuracy of the program. Importantly, it also provides peace of mind. Consider, you are on machining step 6 of a complex, expensive part. Steps 1 through 5 went great, but during step six, you lose position, the part is scrap.

It is much the same in manual machining. I have had parts that were literally chucked for a week in a dividing head doing numberless operations. The closer I got to finishing the part, the more focused I became in not screwing it up. Scrapping some aluminum round you chucked 10 minutes ago doesn't hurt, but scrapping a radial engine crankcase, or ANY complex part with 3 weeks of work behind it, well let's just say "hurts" is an understatement.

It's all budget, of course, do what you can with what you've got. Best of luck!

[Stevie]

couldn't he use Rutex drivers with servos and then a low cost controler like Mach2 do just as well for him

[mysterious]

I am planning on going with geckos, 500 oz. steppers, and Mach 2. I thought only servos could give feedback and with steppers you just crossed your fingers and hoped that there are no mistakes. I don't hink I will be machining anything too complex, mostly some goped aluminum parts. Back to the feedback thing, so you can have steppers and get feedback on what they are doing? Very interesting. Tell me more on how you set this up. Just when I thought that I was beginning to understand something, I read something else. Also with servos I know you have to be careful with how you wire stuff because you can get cross feed and your coordinates would get lost, how do you guys avoid this?

[mysterious]

Oh also, from what I here geckos with good steppers being run by Mach2 rarely mis any steps. How true is this and if it is would it be more dependable than servos and having to worry about cross feed messing everything up.

[ger21]

With the exception of electrical interference, generally the only reason a stepper will lose steps is if it's not powerful enough for the application. Or, since steppers lose torque as speed increases, because you're going to fast.

[Stevie]

Hey Gerry; your pretty darn close to me; just a hop-skip and a jump over the big river

[ger21]

Actually, I'll be there tomorrow picking my wife up from the airport (she's in Halifax visiting a friend). You wouldn't know off hand which exit the airport is off, would you? I've only been there once before. Thanks.

[mysterious]

Yes, I don't think steppers are as sensitive to interference, but I thought that servos were a little more sensitive.

Hey Stevie are you trying to hijack my thread?

[Stevie]

No problem; just stay on the HYW 402 untill you see the Airport Road turn off or Modeland Rd and get onto #7 (it's called London Rd also) the airport is clearly signed

Or you could get off at Idian Road drive up Exmouth; onto Murphy Rd then turn into Fraser Ave in Twin Lakes and I'll make you a coffee; she can find her own way home (Joke)

[Stevie]

Try steppers with encoders

[ger21]

Yes, I don't think steppers are as sensitive to interference, but I thought that servos were a little more sensitive.

It's not the steppers or servos that can suffer interference. More like the wiring and/or drives. Interference with the step and/or dir lines can cause missed and/or gained steps with either servos, or steppers. If you use Geckos for either steppers or servos, I don't think makes any difference in regards to interference. With proper grounding and shielding, and the right size steppers, you shouldn't have any problems.

Back to the feedback thing, so you can have steppers and get feedback on what they are doing? Very interesting. Tell me more on how you set this up

You can't do this with Mach2 or Geckos. You need special (expensive) drivers and software to go with them (more expensive).

Gerry

[Stevie]

Gerry; on second thoughts just turn off on Modeland Rd; then follow the signs; I'm not sure about a direct turn into the airport; i think it's actually past and then a dubble back

[mysterious]

Sounds like I am going with steppers. Thanx again guys.

[Stevie]

http://www.aaaim.com/cgi-local/shop9...uy=1/item=1814

Stepper with encoder; not a bad price; might be just the ticket

[ynneb]

Im eight weeks into been an expert on the subject. Before that I had never heard of CNC.

The reason I chose to use servos was the uniform torque across the speed range, unlike steppers that lose their torque at higher speeds and therefore may loses steps. Servos been a DC motor have a smooth motion. Steppers move by a series of jumps. Although gearing down and micro stepping can reduce this to a very small amount. I read a post in this forum where this guy after building his machine regretted using steppers and wished he had used servos. That was enough to convince me. I am sure current stepper users will jump to thier defence though.
I guess if most of your cutting is going to be straight lines then steppers will be fine. I gather a servo will do nice rounds.

As I said before I am no expert and am just going off the readings in this forum. But the general consensus seems to be servos are better. With the occasional stepper user who will dissagree.