Re: [CAD_CAM_EDM_DRO] Gecko + Servos + Taig Mill

lasernerd@... wrote:

> Hello all,
>
> I've been using a Haas VF-2 and an MHP CNC mill at work, and I've
> been bitten by the CNC bug - bad. I have a Taig mill in my basement.
> It's a manual machine. I'm thinking about retrofitting it for 3 axis
> CNC capability.
>
> I'm new to servos, so I'm not sure if they're what I'm looking for. I
> have a 3 axis CNC controller (steppers) right now, but it doesn't
> support very much power, and I'd like to up the amps on the mill. I
> do understand that I should not expect such high feed rates using the
> Taig, but it would be nice to have enough power under the hood for
> whatever I want to make.
>
> So, without further ado, here's my question: How much power should I
> be looking for in the servos, and where should I be looking? I have
> some Vexta 125 oz steppers and they seem pretty powerful, but I've
> heard the Taig likes 200+ oz. How does this transfer to servos?

The difference is that servos only take power when actually needed, so
instead of drawing maximum power at rest, they draw no power at rest,
and the servo amp supplies power as needed to make the demanded
feedrate. This means that servo motors can be run well above their
continuous duty rating for short bursts. So, a 50 Oz-In cont. motor
might be able to deliver 400+ Oz-In for a few seconds when needed.
What you need to do is figure out the worst case cutting force, based
on cutter size and motor HP. Assuming the worst case, ie. all motor
torque is turned into linear force from the tool to the work, and then
onto the table. Once you have this figured, you then can back-calculate
linear force to leadscrew torque, given the leadscrew pitch. (A great
simplification with ballscrews is to assume the screw is like a drum
with a string wound on it, where the circumference of the drum equals
the screw pitch.) Now, with the torque needed at the leadscrew to
hold against the worst-case cutting force, you can compute required
motor torque, given the belt reduction ratio you plan to use to get
reasonable speed matching between the motor and leadscrew.

I used 1000 Lbs linear force as the baseline on my Bridgeport
conversion. The X axis drive can knock the head out of tram, so it
is enough!

Jon

>
> I'd like to change the Taig over to ballscrews, so I suppose this
> should be included to get a good answer.
>
> I'll be cutting everything from plastic to stainless steel on the
> mill. I'd like decent rapid feed rates (is 100ipm too much to ask
> for?).
>
> Sorry about the simple question. I'm a newbie to all this super-sexy
> CNC stuff :)
>
> Regards,
>
> Robin
>
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