stepper size required

[bru]

(I was suggested to repost this becasue i had it in the wrong forum and didn't get any replies)


Hi. First i'd like to say what a great forum! I have been researching home built CNC routers for a while and only recently stumbled upon this community.

I am in the process of gathering all the components for my router. I have the ball screws and linear guides and am looking now for the stepper motors. The ball screws have the following lengths of travel to give a size perspective of the machine. X,Y,Z respectively: 73", 42", 18". As a side note, I do not intend to use all 18" of Z height--I got a great deal on the screw and will cut it down to fit the design if necessary.

I will be planning to cut mostly wood, ren, and ocassionally aluminum. The projects will generally be furiture related: end tables, coffee tables book cases, etc.

My main question is in regards to the size of the stepper motors needed. From looking at past threads it seems most people are in the 200-400 oz-in range, but it also seems most peoples machines have a smaller foot print. The motors i have had my eyes on are 990 oz-in. Is this complete overkill for my project? I had planed to couple them directly to the ball screw shafts (direct drive). I would guess the Y-Z gantry will weigh about 100-150lbs when complete.

Any input or suggestions would be appreciated, thanks.

[CJL5585]

The weight of the gantry in itself cannot be used to determine a stepper motor size or in/oz torque rating.

What does come into play is how many inch pounds it takes to pull or push the gantry on the rails that are used. This can be determined by taking the completed gantry mounted on the rails, attaching a pull type weight scale, and reading the pound value it takes to move the gantry load.

Another factor is the tightness of the leadscrew nut on the leadscrew, or how much torque does it take to turn the leadscrew. The same method is used with an arm 12 inches long attached to the leadscrew, with the scale attached at the end of the 12 inch arm opposite the leadscrew end.

Just remember that big steppers cannot be driven by low end drives due to the voltage and current requirements of the large steppers. You will need a drive that can supply between 5 and 7 amps of current for each stepper.

Estimating a value for the steppers can be a hit or miss proposition. With large steppers you most likely will not be able to have high feedrates due to the torque curves of steppers at higher stepping rates. The power supply voltage also is a factor in the feedrates.

Now, if I were building a system of this dimension and using quality components such as linear ball bearing rails, ground ballscrews etc., I would use a stepper of at least 500 in/oz.

The materials used in construction also play an important role in motor size. If one uses pipe rails or v-groove rollers, threaded rod, etc. then those have to be taken into consideration and the motor torque may have to be increased to compensate for same.

Hope this helps in your decision.