Re: Opinions on $250 cnc machine

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "whelenremington"
<wperun1@r...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "kz1927" <kz1927@y...> wrote:
> >
> > Hello All,
> >
> > If someone made a desktop cnc machine (without motors or controller)
> > with a cutting area of about 10" x 10" that was solid and accurate,
> > and could be sold for less than $250.
> > would there be an interest in such a machine?
> >
> > Thanks -John
> >
>
> John, I've been thinking of just such a machine concept. I believe
> that you are using the polyurethane for the bed or base. From what I
> can see most large displacement mills are of the gantry design, and
> their frames do not have the stifness to support themselves on what
> ever surface they are placed on without racking. So I've thought of
> using a cast iron surface plate (18"x18", 90 pounds, $450 + shipping)
> or a granite surface plate (18"x18"x3", 150 pounds, $55 + shipping).
> The x axis with the y axis on top of it could be attached to the cast
> iron surface plate, but not the granite surface plate. Yesterday I
> thought of CORIAN, it seems stable, but when I researched it the
> sheets are only 3/4" thick. They must glue the CORIAN to plywood or
> something.
>
> As far as mechanical properties, I have reviewed my Mechanical
> Engineering equations from 20 years ago, and have come up with the
> following results. The x axis moves 16 inches, so I modeled a 1 inch
> diameter steel shaft, 16 inches long, simply supported. Assuming 100
> pounds of load in the center, I got 0.006 inches deflection. The
> angle at the ends of the simply supported shaft, after deflection, is
> 0.064 degrees, which is well within the 0.5 degree max for Thompson
> ball bushings.
>
> The spring constant of this simply supported shaft is 16,681 lbs/inch.
> The natural frequency is 214 Hz. Now the natural frequency is around
> the frequency of the stepping motors, but the spring constant is
> enormous.
>
> Another thing that I've been thinking of is why spend all that money
> on Thompson ball bushings. Just use oilite bushings on Thompson
> hardened and ground "Quick Shafts".
>
> So, to answer your question, YES there would be a demand for a CNC
> mill with 10" x 10" displacements for $250. Do you have these for
> sale? Can you make one for 16" x 12", with the y axis on top of the
> x axis? What are you using for bushings to ride the rails, and what
> are you using to drive the tables, Acme lead screw with nut?
>
> Whelen
>

Thanks for your interest!

You are exactly right. There's not need for expensive parts when
you could simply make parts on demand as you need them. -Cheap!

With this kit If you want acme then simply replace threaded rod
easily. You know it could be a first class little machine with fine
parts, but all that adds to cost. It works great on Hardware store
parts. If you think about it Rigidity and closeness of fit are
what counts.

An Oilite bushing is less than $2.00 and if it had to be replaced
every year it would be 15 years worth of Oilite for the cost
of One fine linear bearing.

Thanks -John