Re: Opinions on $250 cnc machine

--- In CAD_CAM_EDM_DRO@yahoogroups.com, John Dammeyer <johnd@a...> wrote:

>
> Hi John,
>
> I've built up an XY table using MDF and 1/2" DEC printer rails. For the
> leadscrew it uses standard 1/4-20 allthread and tapped teflon

inserts. A

> more complete description is on message 289 with photos posted in

the photo

> section at EDMHomeBuilders.
>
> I built this to hold a frame of 12 circuit board so I could move

each board

> under a programming head and program rfPIC micros and then test them
> automatically. It's got enough power that I had to add braces to

the end

> plate after I inadvertently ran the stepper up against it and broke

the glue

> joint. The Teflon nut has zero backlash (for now) and very low

friction.

>
> But: The moving table is heavy and when I move to one end it flexes the
> rails on the bottom. A lighter bent sheet aluminium arrangement

would have

> been better. (or maybe your poly).
>
> As a pick and place type unit or circuit board drilling system or

perhaps

> even for engraving, it may work fine. But having also built a Gingery
> Lathe, I can tell you there is no substitute for mass mass mass in a

mill or

> a lathe. If you want precision you can't use allthread. Just put two
> pieces beside each other and see if all the threads interlock. They

won't.

> It's what a friend calls the drunken thread syndrome and it's a side

effect

> of how they are manufactured.
>
> A decent set of threaded leadscrews will end up covering most of

your costs.

> Just look at the Sherline. Mostly extruded aluminium with lots of CNC
> operations to reduce human involvement in manufacturing. I've seen

the work

> done people using one of those and it's nothing short of outstanding but
> enlarging a machine usually means you have to cube the mass to get

the same

> level of performance.
>
> Regards,
>
> John Dammeyer
>
> > >
> > > Yahoo! Groups Links
> > >
> >
> > I've
> > been using easily replacable "snap-in" polyethylene nuts -no backlash
> > on the only one I've had to use...It's just a 1" cube drilled &
> > tapped.
> >
> > Before I get in any trouble let me say that I'm here to share
> > and get experienced opinions on something I'm positive will
> > benefit all. I haven't seen this stuff used and it should be.
> >
> > I built a disc finisher (polishing machine) 10 years ago out of it
> > and have used it 15 hrs a week every since.
> >
> > My first lathe was cast on the end of an old blower motor.
> > I made actual usable roller bearings out of the stuff.
> >
> > I,d like to make a sortof "snap-together" customizable
> > cnc kit and I have.
> >
> > I do want my idea critiqued, critisized, and taken apart
> > because the thing HAS to work.
> >
> > Thanks -John
>

Thank you and you are right.

Threaded rod is not the best thing but I do wish I could get
pics of the detail it will do. It is amazing provided everything
fits well on the machine.

I'm making a machine that would come Very usable but if one wanted
precision rod or screws it would be a simple thing to add those
better parts. It's modular. You can see the x-z assembly in it's rough
form. Mass -like you said is much better than one might guess.
A block of poly is maybe half what aluminum is.

There' a "trick" to using this stuff and that "trick" is make it 1.5
times as thick as you think it should be.

Rigid as can be.

I just think it would be great it a person could have a
good,reliable,solid, tabletop
machine for less than $500 for the whole works.

and you can.


thanks -John