My CNC questions thread - keep an eye on this one...

[steamer]

My experience with stepper were for some OEM equipment running open loop with home grown (corporate mind you...so not necessarily cheap) drivers running open loop....the only way to get back in sync was to home the axis...painful.

An encoder and feedback would help! still don't like steppers!

Dave

 

[blighty]

steamer......

i take it you don't like steppers then? did you once get beaten up by a gang of stepper's when you where walking home from the pub late one night?

i think servos and steppers have the place. i have seen home cnc stuff run very well with both setups, but any industrial stuff would be servos.

Dieselpilot......

see what you mean, as in 10mm +- .01. at the mo i get .01 and some times .00mm (not being pedantic). did a job for a mate and one of his dimensions was 8.26mm. i said "you what?????" he said "8.26mm!!! end of!!" i made 30 of them, all came out 8.26mm ;D......... the rest of the part was s**t, but that's not the point :big:

also going by what you said, if i have 960oz stepper, 5mm pitch and micro stepping 10. what formula would i use to work out how (sorry loss of a better word) "push" does my table have. never really looked in to this stuff., i just got a bigger steeper than my mate had :big: job done.

 

[mu38&Bg#]

Figuring axis force is just a matter of taking into account all the reduction ratios and using the formula for the torque reduction in the steppers. Any large company manufacturing steppers or servos will have a guide. I looked at one from Vexta yesterday.

http://www.orientalmotor.com/products/pdfs/2012-2013/G/usa_tech_calculation.pdf

If you're machine can hold an acceptable tolerance, then it's doing well. The resolution really comes in when you make a small change in tool radius compensation (or tool offset in the lathe) and you can see the result in the parts.

Greg

 

[RonGinger]

I wondered how long it would take to get into the stepper vs servo war. It always does, on every forum topic on CNC.

First note there are millions of commercial machines, from printers to CNC to whatever, that run on steppers, doing their job reliably day in day out. The simple fact is if a stepper is properly loaded it will NEVER miss a step. If you overload it it will loose steps, and it doesnt loose 'a step' it goes full bonkers and winds up way off, with a horrible, rock crushing sound.

It is also an undisputed fact that a stepper system will always be cheaper- no encoder needed, and simpler electronics.

It is also true steppers never need to be tuned like servos.

Another fact is that steppers are not made above about 1/3hp, so if you are building a bridgeport size machine or larger then servos are the only game in town. If you are doing an X2 size, or maybe a Sherline size machine steppers will do a great job.

A few years ago a totally new hybrid magnetic technology came to steppers. They doubled or trippled the power of steppers. They can be identified by the square bodies. Never take anold round body stepper even if its free. Why work with old junk when new motors are cheap.

Thats my gal of gas (petrol) ;D on the stepper vs servo war.

 

[steamer]

It is also true steppers never need to be tuned like servos.

Depending on how you drive them....not necessarily true....too much current for the inertia and torque ripple will get ya......Have eatin that dirt before... ;D

But generally...I agree.....but I still don't like steppers!..... 8)

Dave

 

[steamer]

And no I'm not endorsing one or the other!....safely extracting myself from the center of the fire fight while "stepping" down off the podium... :

Dave

 

[Dinkum]

Thanks for all the input so far. It is interesting that this servo vs stepper war has come up because I was wondering myself about using servos for an eventual CNC mill conversion.

My machine is basically one of these (a bit newer I think) : http://www.homemodelenginemachinist.com/index.php?topic=18347.msg189665#msg189665

Would steppers do for something this size?

 

[RonGinger]

I have a Jet knee mill that I converted with steppers that is about that size or just a bit larger. I used a stepper to elevate the knee instead of operate the quill. I figured if it worked, great, if not Id consider it a knee power feed and then do the quill.

Well, it works great and I really like having the quill free. That lets me do a very easy Z zero set, I just move the table to about the right height, then lower the tool until it touches the top of the work, lock the quill and set zero.

Actually, I first did this one with servos, but they caused me more trouble than I wanted so I switched to steppers.

Here is a link to the conversion report http://plsntcov.8m.com/JetMill/Jet.html

 

[blighty]

hi dinkum

would you mill look more like this one http://www.tphmachinetools.co.uk/pr...pecode=35&sizecode=30&uniqcode=5806&machine=n

if so your in luck as a mate converted one many moons ago. hes using 640oz steppers on X,Y,Z. only bad thing about it is the quill is driven by the stepper, so he only has about 60mm of day light. plus with the quill fully extended it tends to slop around a bit. ok the sloppy quill may be down to his mill, but your one may be better. with this mill i would go the way of RonGinger and cnc the knee. if you are going to cnc the knee with a stepper, you will need a LOT more than 640oz. you could always counter balance the knee to give the stepper a hand.

 

[kvom]

My mill uses steppers and is pretty accurate. However, experience has taught me that any time I think it "might" have lost a step (e.g., part moves slightly) I need to start the program over.

 

[Dinkum]

Hi,

Thanks for the input everyone.

Blighty, that mill in the link is similar to mine. Counterbalancing is an interesting idea.

Ron, thanks for the link. I'm sure it will prove helpful when (hopefully) I eventually do a conversion. The knee idea seems like a good one.


ATM I am sticking to building a CNC dividing head - currently I have bought the electronics - I just need the motor to complete the electrical side of things. So I'll be back soon with a flood of technical questions...


Once again, thanks for the advice. Forums are amazing - expert advice, no charge. Thm: Thm: Thm: