Re: Cutting forces and Acetal nuts

See comments below:

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "deruida" <deruida@y...>
wrote:

> Arvid
>
> A couple of years ago I built my own 24" x 24" x 8" gantry style
> router. Always keep one thing in mind...RIGIDITY IS EVERYTHING if
> you are going to cut aluminum! My machine will plow through
> hardwoods using a 1/2" diameter cutter cutting 3/4" deep without

any

> problem, however, I cannot cut aluminum. It does not take too much
> in terms of deflection to give you terrible results in cutting the
> aluminum. I've even tried to take just .005" deep cuts without

much

> success.

This is interesting. I have been doing some very light milling in
aluminium on a desktop vertical milling machine, so I can see your
point here. I will keep this in mind - thanks for pointing it out!


You will want to design your gantry's backbone as a boxed

> or tubular cross section of the largest dimensions that you can use
> to insure rigidity.

Yes, I was thinking of using 19 mm (3/4") MDF and building U
profiles, some 150 mm "wide" and 100 mm "deep" (6" x 4") for
the "legs" of the bridge, and building the top part of the bridge
itself as a tube. Also, there will be kind of a tube shape underneath
the table to rigidly connect the legs/bridge to the rest of the
machine. I don't know if this will be enough, but it's what I am able
to build and can afford at the moment.


I can appreciate your budget restraints because

> I had the same restraints while building my machine. I used 1/2"
> diameter acme lead screws with a .100 lead ...

What kind of nut did you use? I'm worried that the anti-backlas
Acetal nuts might not hold up to the cutting forces of light
aluminium work. This is actually my biggest problem right now - I
don't know what kind of forces to design for!


... and I don't recommend

> this lead because it is waaaaay too slow at about 23ipm rapids. I
> too said that speed didn't matter, but that changes soon :) Your
> accuracy goals are very generous ... *snip*

I will be using servo motors and I don't think speed will be a
problem here - on the contrary, most servo motors seems to go up to
at least 3000 rpm with almost 100% of rated torque available. I will
probably need some kind of gearing - and since I had planned to use a
toothed belt to connect motor to lead screw, this will not be a
problem.

I'm not so sure my accuracy goals are that generous - remember, we
are talking accuracy, not resolution. 1 mm of absolute accuracy over
600 mm is an error of less than 0.2%! The *resolution* of an axis
with a 4 mm lead screw, a 2:1 speed reduction, and a servo motor with
a 4000 counts/rev encoder would be 0.5 microns! This will be good
when I need to keep the motor at a slow, steady speed, but is WAY
more than needed for accuracy (or DRO) reasons.

I'd like to say also that all of this is coming from someone almost
completely without experience in this field, so please correct me if
I'm wrong!

> Hope this helps. Dan

Absolutely!

// Arvid