Help with motor sizing

[flinty]

Hi Guy's.
I am in the process of designing a new CNC 3 axix router for wood/plastics etc, I have given the spec to a large german servo company and they have come back with motor sizes I don't agree with( and a price to match $28K maybe nervous of the project ) here is the basic layout information given to them, and I was wondering if one of you could verify servo motor sizes/torques for the 3 axis.


Project is a CNC wood/plastic Router.
Table working area size is 3,040mm X 1600 mm Z axis movement of approximately 350mm.
Style of build is a traveling gantry ( single drive in the centre )fixed table.
X axis gantry will be driven by a single central ball screw, mounted under the table with a diameter of 40mm with a pitch of 20mm maximum speed of the ball screw is around 1016 rpm )
The weight of the gantry complete with the router should not exceed 150kg and will be supported by two linear rails 25mm with 2 bearing blocks per side.
Acceleration I would assume to be around 0.5sec about 0.1g
We are intending to mount the servo motor on the gantry cross brace under the table and drive the ball screw with a tooth timiming belt ratio + - 3 :1
Cross Y axix will be a 25mm ball screw with a pitch of 10mm speed of maximum 1110 rpm. ( torque can also be reduced with a toothed belt ).
The Z axix will be a 16mm ball screw with a pitch of 5mm required to rotate at 2500 rpm max.
The Router motor I would like to use will be in the order of 7.5Kw to a speed of 20,000 rpm and the size of cutter will be a maximum of 20mm dia ( feeds will be in the order of 5 meters per minute.
with this size cutter and up to 25 meters per minute for the small 6mm cutters ). (PS maybe someone can offer a router motor Culombo have an agent here who will not sell me a unit, thy build routers).

Kind regards
Chris Flint

[WoodSnarfer]

I just looked at the x axis, as an example.

With a screw pitch of 20mm, at a screw rotational speed of 1016 RPM, you are moving the gantry at a rate of 34 cm per second (!)

That is a pretty fast moving gantry...are you sure that's the spec you want? You've got a pretty large table, so maybe that is in fact a good number...

Your acceleration seems a bit on the slow side, at 500 milliseconds (that means it will take a little over one full second to stop and reverse direction).

At 500 milliseconds, for a 150Kg gantry, through a 1:3 belt/pulley and a 20mm screw...I come up with needing a motor that produces at least 65 oz-in of continuous torque, or about 150 watts of mechanical power.

I would have set the accleration at 100 milliseconds, which would have given me a motor spec of around 275 oz-in continous torque, or 625 watts of mechanical power (which is a pretty good sized servo motor). Depending on the motor efficiency, that could be around 10 amps at 90VDC. Pretty big.

In all these calculations, I'm coming up with the 'bare minimum' value, then (more or less) doubling it, to give enough headroom.

What did the servo company state? $28K just for three servo motors sounds enormous -- more like something you could use to drive a ski lift instead of a wood carving machine

[flinty]

Thanks for the reply so far I tend to agree with you, what they came up with is 2 off 8 Nm motors at 3000 rpm and 1 off 6 Nm @3000 rpm for the Z axis ( which I believe is an overkill on Z ).
The speed that I am trying to get is around 600 ipm that is so I can position fast or go to tool change fast, and is the maximum I can go without stress on the 40mm ball screw, perhaps I can live with a slower unit, I want to build it to machine up custom doors in hardwood.
The Siemans quote includes PCU servo drives and a handwheel, I think they are about 15K too high looking at some of the sites for servo motors, the biggest 5inch units 10Nm with drivers at around $1400 US each
The reason I am keen on the servo instead of steppers is I am a little nervous of the torque drop off in the steppers, perhaps this is not a real problem as the maximum force is on acceleration.
I am pleased you highlighted the rate of acceleration to 100ms I fully agree with you.
Please keep that help coming
Kind regards
Chris

[WoodSnarfer]

Yes, for this machine you definitely ought to be looking at servos - especially at the cutting and jogging speeds you are talking about.

When you say an 8Nm motor -- wow, if that is constant torque then it is really big. Did they tell you the electrical power requirements for the motor? Are they DC servos or AC servos?

I did neglect one piece of data in my calculations, and that is the rotational acceleration of the drive screw you are using. That is a pretty large screw (I understand why you want it, because of the speed you are running). So, there will be force required to accelerate the screw itself. I had about 2x headroom in my calculation without the screw, so even with the mass and acceleration of the screw added, I still bet you need no more than a 3.5Nm motor.

I would shop for another quotation, and also challenge the servo company to explain to you how they did their calculations.

Out of curiosity, do you know how much the x-axis drive screw will weigh?

-Chris

[ViperTX]

flinty,

Without the info on what they really quoted you, we are basically lost.

9K per axis is what I would expect from a commercial company providing you with servos and drivers for each axis based on your specifications and their desire to insure you don't wind up with underpowered axis drive.

[flinty]

Here is the actual quote I recieved edited to remove the suppier .
The weight of the Y axix will be about 60 Kg at worst and the Z axis will be about 25kg
More info to come later lads and so far thanks a heap
chris