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Offline dresda

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What size motors?
« on: April 18, 2009, 12:14:53 AM »
Hi, can somebody tell me what size motors I should use for a standard Bridgeport size mill.
Steppers or sevos, should the steppers be Nema 34 and around 1000oz-inch?
4-6-8 wire? How many ppr? Bi-polar or?
This is all new to me, only ever worked with Ac servo's and Fanuc, but now going over to the dark side with mach3... :-\
Ray.

Offline simpson36

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Re: What size motors?
« Reply #1 on: April 18, 2009, 05:09:14 AM »
Take a read on the www.geckodrives.com site. Lots of good info and formulai

Make sure you do your own price checking in whatever you are considering. Also note the dates on any forum postings you pick up with a search engine.

I can't tell you how many times I read about servos being 2 or more times as expensive as steppers. My own research into pricing showed a $15 difference per axis. Insignificant.

As a newbee, I followed the advice de jour and went with steppers, albeit the most powerfull I could get in NEMA 23. Steppers make sense in some applications, but if I had it to do over . . . . 

My new servo motor should be here shortly and I will take a walk down that path in preparation for retrofitting another larger mill.

BClemens

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Re: What size motors?
« Reply #2 on: April 18, 2009, 06:52:09 AM »
There are benefits and pitfalls with both type drive motors. Servos are more powerful at higher speeds where steppers begin to decline but the most prevalent benefit to stepper motors is that they don't require tuning via their drive. Tuning can be a problem then it must be done at a later date with servo motors again - and again. I would direct drive that Bridgeport with about 1500 Oz In stepper motors with appropriate drives from Gecko and forget about -  tuning, belt adjustments and wear, and all the 'bracketry' required for servo motors and their belt reduction and adjustment fixtures. I converted a similar mill and used servo motors.....And BTW: servo motors are very much more expensive than equivalent stepper motors and servo motors require an encoder to give position feedback to the drive where steppers are where they are told to be. Use size 34's.



2.25 cents worth at least!

Bill C.
Re: What size motors?
« Reply #3 on: April 18, 2009, 11:30:22 AM »
There are benefits and pitfalls with both type drive motors. Servos are more powerful at higher speeds where steppers begin to decline but the most prevalent benefit to stepper motors is that they don't require tuning via their drive. Tuning can be a problem then it must be done at a later date with servo motors again - and again. I would direct drive that Bridgeport with about 1500 Oz In stepper motors with appropriate drives from Gecko and forget about -  tuning, belt adjustments and wear, and all the 'bracketry' required for servo motors and their belt reduction and adjustment fixtures. I converted a similar mill and used servo motors.....And BTW: servo motors are very much more expensive than equivalent stepper motors and servo motors require an encoder to give position feedback to the drive where steppers are where they are told to be. Use size 34's.



2.25 cents worth at least!

Bill C.

"servo motors are very much more expensive than equivalent stepper motors" - That is simply not true!  The difference is perhaps very, very, very worst case $100 more per axis, which is almost lost in the noise compared to the overall cost of a decent conversion on a knee mill.  And the difference in performance can be very significant indeed.  There are plenty of valid reasons for preferring steppers over servos in some applications, but cost is not one of them, unless you're really pinching pennies.

I used 850 oz-in servos from www.homeshopcnc.com, with 2.5:1 GT2 belt reducers, and Gecko G320s.  My machine can get up to 400 IPM rapids on X and Y, and it absolutely un-stoppable at machining speeds.  I've snapped off 1/2" endmills without losing position.  Try to get that kind of performance from steppers.

Regards,
Ray L.
Regards,
Ray L.
Re: What size motors?
« Reply #4 on: April 18, 2009, 12:20:35 PM »
Quote
I would direct drive that Bridgeport with about 1500 Oz In stepper motors with appropriate drives from Gecko and forget about -  tuning, belt adjustments and wear, and all the 'bracketry' required for servo motors and their belt reduction and adjustment fixtures.
Hi Bill,
  Can you NOT direct drive with servos ?  What is the limitation here ? Please explain.
Thanks,
RC

Offline Hood

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Re: What size motors?
« Reply #5 on: April 18, 2009, 12:33:22 PM »
All depends what you want out of your machine, I have steppers on my Bridgeport and they work well, at the time I did that there was a BIG difference between servos and steppers because the cheap Chinese DC Servos were not readily available at that time.
 It works well and has never given me problems but it is slow, 2500mm/min rapids (100IPM) and accel is not lighning fast either but as I said it works well  and its fine if you are not in a real hurry.

 Never tried DC servos  with the likes of the Geckos so cant comment on them.

 On the Lathe and Beaver Mill I use AC Servos and they are blazingly fast both in rapids and accel but you need decent sized ones to do that, you could probably use 0.75 to 1Kw with some gearing but for me 1:1 gives the full benefits of the AC Servos so I have  1.5Kw on the Beaver axis  (bit bigger than a Bridgeport) and 7.5KW on the spindle, the lathe being a big machine needs 3KW on the axis and 12.5Kw on the spindle
 AC Servos are not cheap but the drives give you many more options than the standard  DC drives such as Geckos/Rutex etc.

All boils down to what you want out of your machine and how much you have to spend.

Hood

Offline dresda

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Re: What size motors?
« Reply #6 on: April 18, 2009, 01:57:39 PM »
Thanks for the prompt replies. I have the mach3, have fanuc Ac servo's on the machine at the moment and  ordered DSPMC module from Vital systems but I would like to also try steppers or cheap servo's.
I am still  looking at Camsoft but can't justify the cost at the moment with the economy the way it is.
One big advantage for me with Camsoft is the fact that I don't have to learn VB or C++.
Ray.

Offline Hood

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Re: What size motors?
« Reply #7 on: April 18, 2009, 02:34:17 PM »
If you have the Fanuc servos and you are getting the DSPMC then you are all set. As for needing to learn VB or C++ why would you need that? Custom macros require VB but will you need them?
Hood

BClemens

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Re: What size motors?
« Reply #8 on: April 18, 2009, 04:00:40 PM »
Should have qualified that statement about the price difference between good quality DC servo motors and stepper motors.... The DC servo motors that I used on this Bridgeport type machine are now about $600.00 each (with encoders) and similar sized (power wise) stepper motors are about a third of that. AC servo motors are probably worth checking out due to the increase in the price of DC motors - good quality.

Yes, servos motors can direct drive but low speed torque for a DC servo motor is probably about three times what a stepper requires - attempts to provide that low end torque have been in the form of spiral stacked armatures and the like which makes servo motors all the more expensive. The basic form in terms of manufacture of a stepper motor makes them cheaper than a servo motor as well - example - there are no brushes to contend with in a stepper. The small DC starters for outboard motors are a good example of the increase in price of that sort of DC motor - similar to servos for torque for size.

I'm generalizing by passing on my experiences with this. Didn't intend to ruffle any feathers. That seems easily done here....I guess I should just keep my experiences to myself, meaning that I'm not welcomed here?

Fine

Bill C.

Offline Hood

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Re: What size motors?
« Reply #9 on: April 18, 2009, 04:05:58 PM »
Bill you are more than welcome here, if you ruffle feathers then so be it, just :)
Hood