Re: Cutting forces and Acetal nuts

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. 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. 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 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 and if you are going to be using
stepper motors then you need to remember that they have the most
torque when stalled and they quickly loose their torque as speed
increases. I once read that steppers are limited to 1000rpm, but I
have never come even close to getting my motors to do that speed
even while not in the machine. If you take my 23ipm rapid speed
using my lead screws that equate to just 228rpm. Above that speed
there is just not enough torque to move the gantry. my steppers are
rated at 100-130 in-oz but since I am driving them using a 2 amp
bipolar chopper driver using a 24V supply, I'm not sure just how
much torque that they output. All I know is that I could easily
double my lead since I cannot stop the leadscrews by hand if the
machine is moving at around 10ipm. The leadscrews do tolerate the
dirty conditions much better than I had anticipated. Since my
drivers operate in half step mode, I get 400 steps per revolution
that gives me .00025" per step, so you see I could easily double my
lead and keep the accuracy very tight for wood, composits, etc.
Hope this helps. Dan


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "bitnick78"
<arvidpublic@h...> wrote:

> Hi all,
>
> I've been reading this group now for a while and I am impressed

with

> all the knowledge here! It's great!
>
> I want to build a CNC router/milling machine, bridge type, 600 mm

x

> 420 mm work space (that's about 24"x17"). I'm aiming for
> repeatability better than 0.1 mm (that's about 0.004") and total
> mechanical accuracy, well, something like 1 mm (0.04") over the

whole

> working area.
>
> I want to be able to cut wood, MDF, Delrin, perhaps make my own

PCBs,

> and preferably also cut aluminium panels and the like. I'd also

like

> the total cost (for lead screws, servo motors and MDF for the

frame)

> to be <$1000. Cutting speed is not important; this is a hobby
> project. I will try to build servo amplifiers myself since I'm

into

> electronics.
>
> So, does this sound reasonable?
>
> ... and if so, what kind of lead screw do I need?
>
> I'm leaning towards BS&A's 16 mm dia, 4 mm lead leadscrews
> with "Advanced Anti-Backlash Supernuts". (I can get them locally,

in

> Sweden, for about $300 - that is for 3 pieces, 35"+23"+11" long,

and

> three "supernuts". Add to this the cost of angular contact ball
> bearings and end treatment of the screws.) What I am worried

about,

> and have not been able to find much information about, is what

kind

> of cutting forces that can be expected from the kind of work I

want

> to do. This particular screw (or nut, actually) has a "design

load"

> of 175 lbs (or about 775 N if you prefer). Will this be enough, or

am

> I going to destroy it milling aluminium?
>
> Thanks for your help
>
> // Arvid