Re: Opinions on $250 cnc machine

--- 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