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Model Forum / General / Models / May 2008Servo motors
Can anybody give me some links to possible sources for servo motors suitable for use with smaller CNC machines. Apart from the 'refer to sales office' links the best I've come up with so far is a site offering motors at £1200 up to £3500. Somehow that seems a little excessive when the mill only cost £650. I know we will be looking at a price premium over the closed loop stepper option using 0.2Nm motors, but there must be something suitable to use with the Gecko servo drive at a reasonable price? I presume I am looking for something ideally with a Nema23 mount and I think 0.4Nm seems practical in that form factor? But with an assembly somewhat longer than the 0.2Nm stepper with encoder?  SignatureLester Caine - G8HFL ----------------------------- Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://home.lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php >Can anybody give me some links to possible sources for servo motors suitable >for use with smaller CNC machines. Apart from the 'refer to sales office' [quoted text clipped - 7 lines] >that form factor? But with an assembly somewhat longer than the 0.2Nm stepper >with encoder? Lester - Are you sure that you are talking about 0.2 Nm?? That's approximately sod all - or 28 oz-in in real money. Regards, Tony >> Can anybody give me some links to possible sources for servo motors suitable >> for use with smaller CNC machines. Apart from the 'refer to sales office' [quoted text clipped - 12 lines] > Are you sure that you are talking about 0.2 Nm?? That's approximately > sod all - or 28 oz-in in real money. You are right - I'm trying to compare different units and getting it wrong :) I'm looking at the MAC drives http://www.jvl.dk/files/pdf/Ld043gb.pdf But it would seem that I need a gear box to get the same power as the equivalent stepper? I understood servos had a lot more power? Where am I going wrong .... SignatureLester Caine - G8HFL ----------------------------- Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://home.lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php >>> Can anybody give me some links to possible sources for servo motors suitable >>> for use with smaller CNC machines. Apart from the 'refer to sales office' [quoted text clipped - 20 lines] >equivalent stepper? I understood servos had a lot more power? >Where am I going wrong .... You're going wrong by confusing power and torque. Power is torque times RPM. These (servo) motors are high revving (4000 RPM is very fast indeed for a stepper), and develop their highest power output at high RPM; by the same token, they generate relatively little torque. So yes, to get sensible performance out of them when driving a small mill, both in terms of generating sufficient torque and in terms of keeping the slew rates within bounds, you would need to gear them down (probably by a factor of around 4:1 I suspect in this case). In doing so you will up the available torque by the gearing ratio (less any frictional losses in the gearing). Regards, Tony >>>> Can anybody give me some links to possible sources for servo motors suitable >>>> for use with smaller CNC machines. Apart from the 'refer to sales office' [quoted text clipped - 32 lines] > the available torque by the gearing ratio (less any frictional losses > in the gearing). I think I had just about got there ... Bottom line - I don't see any advantage in trying to add servo's to a Taig size mill when compared with the closed loop stepper motor set-up. But I'm waiting on prices from a couple of sources at the moment anyway and given the longer motor length so needing to wrap the motor around the back using a timing belt, 5 to 1 reduction would be practical anyway. SignatureLester Caine - G8HFL ----------------------------- Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://home.lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php >when compared with the closed loop stepper motor set-up. Just being a little bit picky here:- A closed loop stepper motor setup would be quite rare and probably only be used by people called Gareth :-0 Everyone else would use them open loop. Mark Rand RTFM >>when compared with the closed loop stepper motor set-up. > [quoted text clipped - 7 lines] >Mark Rand >RTFM Mark - Actually, Taig offer a closed loop stepper setup. Regards, Tony >>>when compared with the closed loop stepper motor set-up. >> [quoted text clipped - 14 lines] > Regards, > Tony Now when I asked about closed loop set ups using steppers, the response was that steppers are upto it and you had to use servos to implement closed loop... So much for "Power of the crowd"! Steve >>>> when compared with the closed loop stepper motor set-up. >>> [quoted text clipped - 12 lines] > > So much for "Power of the crowd"! I am supplying currently both open loop and closed loop stepper motor systems using the Taig/Microproto controller. The closed loop give a three fold improvement in speed, but since it is implemented in the controller and only gives feedback to Mach3 when there is an error condition, I suspect Mach3 is getting ahead of the game from time to time. So current plan is an improved link with Mach3. Brian at Newfangled is due to send me a plugin for Mach3 which will run this improved arrangement. The reason for looking at servo motors was to compare prices, but I am finding that at this size of motor the stepper does seem to be preferable? Once you can monitor it's position ;) Now the next step is perhaps glass scales on the bed in place of the encoders direct to the motor? SignatureLester Caine - G8HFL ----------------------------- Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://home.lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php > The reason for looking at servo motors was to compare prices, but I am > finding that at this size of motor the stepper does seem to be > preferable? Once you can monitor it's position ;) Oh well - from £400 for a similar sized servo motor and I'm not sure if that includes in integral drive as against £75 for the stepper motor option complete - per channel ... SignatureLester Caine - G8HFL ----------------------------- Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact L.S.Caine Electronic Services - http://home.lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk// Firebird - http://www.firebirdsql.org/index.php >> The reason for looking at servo motors was to compare prices, but I am >> finding that at this size of motor the stepper does seem to be [quoted text clipped - 3 lines] >includes in integral drive as against £75 for the stepper motor option >complete - per channel ... I hate to suggest it but... http://www.cadcamcadcam.com/index.asp?PageAction=VIEWPROD&ProdID=62 Regards, Tony > The reason for looking at servo motors was to compare prices, but I am > finding that at this size of motor the stepper does seem to be > preferable? Once you can monitor it's position ;) > > Now the next step is perhaps glass scales on the bed in place of the > encoders direct to the motor? Lester. Take a look at the Italian motors supplied by motion control products in Bournemouth. For small motors they can be quite competitive. I use some 300w motors of this type on my Hardinge. It's not really useful to compare steppers holding torque against a servos peak torque. In my experience with larger machinery a servo with much less peak torque than a steppers holding torque will have the stepper for breakfast, dinner and tea! MCP also supply the leadshine dc servo drive which is no longer a gecko copy. It's better in fact. Wayne..... >>>>when compared with the closed loop stepper motor set-up. >>> [quoted text clipped - 22 lines] > >Steve See: http://en.wikipedia.org/wiki/Servo Quote: "Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback." So the Taig system does indeed use servos. But with stepper motors. Next question? Regards, Tony >>>when compared with the closed loop stepper motor set-up. >> [quoted text clipped - 11 lines] > > Actually, Taig offer a closed loop stepper setup. There has also been mention of a Servo Stepper driver on the Gecko forum at cnczone. I think a rough time scale for availability is later this year. >You're going wrong by confusing power and torque. > >Power is torque times RPM. >Regards, >Tony That's sort of right conceptually, but just in case you are tempted to actually try to calculate a 'real' power Power (W) = torque (Nm) * omega (speed in rads/sec or RPM*Pi/30) Richard >>You're going wrong by confusing power and torque. >> [quoted text clipped - 9 lines] > >Richard ...So I didn't say what the units were <G> Regards, Tony > Can anybody give me some links to possible sources for servo motors > suitable for use with smaller CNC machines. Apart from the 'refer to sales [quoted text clipped - 8 lines] > practical in that form factor? But with an assembly somewhat longer than > the 0.2Nm stepper with encoder? I have 2 motors stuck under the bench at work, I'll have a look and send the details from the label. Bob |
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