# Steppers vs Servos



## gary545 (Sep 11, 2009)

Hi my name is Gary I am trying to decide on a CNC router to buy. I will be going with a 4 x 8 with a 5 HP spindle and ATC. At first I thought that closed loop digital ac servos was the way to go but now I am told that the steps are 3/10,000ths and there is no difference in the end result unless the bit hits a nail or something which I see as unlikely. Can anyone tell when and why I would need to speen the extra bucks on cl servos? Also is there any reason with the use I have i mind to go with a welded steel frame over an aluminum frame? Thanks! Gary


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## Dr.Zook (Sep 10, 2004)

Welcome to the RouterForums Gary.


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## kolias (Dec 26, 2008)

Welcome to the forum Gary

For a 4x8 table with a 5 HP spindle I will definetly go with servos. Here is an extract from the Gecko site that should help you.

*Choosing stepper or servo:*
To determine which drive is best suited to your machine and motor, you must first decide between a stepper or a servo motor system. Consider using a stepper motor if less than 100 watts is required, a servo if 200 watts or more are required and either type between 100 watts and 200 watts. To determine the amount of watts needed, you must first determine several factors about your machine:
•	The weight of the heaviest object being moved: This will typically be your gantry and, for this example, we will use a gantry with the weight of 40 pounds. 
•	Inches per minute desired: This is a number that will make you deliriously happy with the performance of your machine and, for this example we will use 1000 IPM.
Now multiply IPM and pounds together and divide that number by the normalizing constant of 531. The answer will be the total number of watts needed for your machine. Below is the formula in basic form:
(Heaviest object * IPM) / 531 = Watts required
And solved for the numbers above:
(40 * 1000) / 531 = 75.329W
Because this number is below 100W, you will definitely want to stay in stepper territory. 

You don’t say the use you have in mind but a properly constructed aluminum frame should be fine.


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## xplorx4 (Dec 1, 2008)

Greetings Gary and welcome to the router forum.


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## gary545 (Sep 11, 2009)

Nicolas,

Thanks for the reply I hadn't realized the motors had different power output I thought it was just how smoothly they moved the gantry. I will be doing 3D carving of wood, foam, plastics and nfm, not planning any work for Nasa anytime soon. Thanks Gary


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## kolias (Dec 26, 2008)

gary545 said:


> Nicolas,
> 
> Thanks for the reply I hadn't realized the motors had different power output I thought it was just how smoothly they moved the gantry. I will be doing 3D carving of wood, foam, plastics and nfm, not planning any work for Nasa anytime soon. Thanks Gary


Unless money is not an object I wouldn't even consider aluminum for the frame. Considering the use, I will go with 3/4" MDF or plywood frame for the table support and six well braced 4"X4" for the legs.


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