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Posted by Lester Caine on May 12, 2008, 11:24 am
Please log in for more thread options Tony Jeffree wrote:
> wrote:
>
>> Tony Jeffree wrote:
>>> wrote:
>>>
>>>> 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?
>>> Lester -
>>>
>>> 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 ....
>
> 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).
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.
--
Lester 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
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>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?