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NeoTech:
I need to pick your brains, and interest i guess.

So that 5.5kW motor on top of the head is weighing in at 45kg, the spindle is 35kg, the head is about 35kg, and the baseplate is around 20kg. So about 135kg.
The pitch on the lead screw is 5mm, its a 32mm screw and the head runs on 30mm heavy load linear bearings.. heres is the kicker..
The servo motor is a 1500W motor, that can hold 6Nm, and accelerate to about 18Nm, in my application the real acceleration is about 11Nm.

The question is.. would my servo motor be able to move that head without a counter weight. Or should i start thinking of redesigning the column in such a manner that counterweights could be added.

Im considering going with a 4kW motor instead and save 15kg, but - that would make me loose about 2Hp. I have decided i will go with a ABB motor tho - not just sure what size yet.

NeoTech:
So i had some issues with the idea of 130kg milling head. But i got this formula off of a friend.
6Nm / (0.005/2/pi) * 0.8 = 6032N

That basicly means that in the lowend of the servo motors i can accelerate 600kgs with somewhat ease. the motors is 2600rpm, but lets say you have friction some strain and other issues, so it drops to around 2400rpm. Means if i work within the 6Nm holding power range of the motor, that 600kgs would be able to accelerate with 12m/s in theory. My motors can accelerate 18Nm, but with Amperage being an issue there im prob. limited to about 11Nm.

Still i dont think 130kg of milling head will be an issue. Not even sure i would need a counterweight actually. But i should prob. gear it down with some form of beltdrive to 2:1 ratio so i can actually utilize the theoretical acceleration. *drawing new table mounts*

Brass_Machine:
You could always put in your design an easy way to add a counter weight if needed. Even if it is only to allow a spot of cables to connect and an easy way to mount the weight. Better to design for it now than have to hodge podge it later.

PekkaNF:
I would design in gear or timing belt, least that gives you an option and there is always a possibilyt for a bigger servo.

But you definately need a brake that is spring operated and activates when power goes off. You might calculate how much the ball screw will hold when motor is off. My guess is that head weight would drive the motor and smash head down if fuse blows or something else would deactivate the servo motor.

If I understand it right couterweight is an option for slow machine. Counterweight adds mass, which you need to accelerate.

That might work out out, but if you need speed, you have a risk. If you have chains or cables AND you push the head down first (and shoot weight up) and then change direcion really fast you might hit it hard on rebound. Saw that happen once when masses were someting like 5t and and little smaller counterweight. Really big chains snapped and both parts crashed down.

Pekka

NeoTech:
Ah well i will prob. never go for a larger servo. The servos is 1.5kW in a 130mm frame. check my blog or earlier pictures i think i have them compared to a monster can of energy drink.
And im at the max for the servo drivers i bought.

But with that said, there is some deisng hinkys with putting a counter weight into this atm, for one i think i need to make the column wel mirrored, the outtake in the back atm, would be two outtakes on the sides instead and then i could prob. design some plate, gears, and chains.

The other way i think is to use wires and pulleys.. Where the weight hangs from a single pulley, and one end of that wire is stationary and the other end is connected to the faceplate for the head.
That could prob. be a hodge podge solution tho..

And as Pekka mentioned having 260kg accelerated will make for some funny behaviors when doing rapids. A friend of mine solve that by mounting the weight to a rail and using a brake on the slide, with no bearings, so when head is moving down it has some extra force needed for moving it down. guessing this could be achieved by making the weights between the head and counter different, but in reality you want to be able to move up faster than you wanna move down - at least in my head.

The third option i have been thinking of is some kind of "air spring" by utilizing like a fire extinguisher tank pressure lined and some form of piston, one could in theory create an airspring, but how practical is that that is the question i guess. =)

I need to take me a ponder... :doh:

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