Re: What is the weak link in machine precision?

Jon,

I think I will be purchasing a new machine. Can't see any reason to
purchase a worn machine for a grand and fix any broken parts when a
new knee mill can be had for 4k and no problems. If I do have error
in the z-axis, how can that be taken out?

Any recommendations of machines that are better than others?

Thanks.

Tom.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> zipdrive2k wrote:
>
> >I am wanting to retro-fit a knee mill for prototyping aluminum and
> >some steel tooling dies. Many of the parts are complex surfaces
and
> >require 3 axis simutanious machining. I need to keep the machine
> >accurcy in the 0.001" to 0.003" range. Quill will be locked and
knee
> >will be moved for z-axis. Extreem speed is not a requirement.
> >
> First test: Put a dial test indicator in the spindle and something
> precisely
> vertical, such as the fixed vise jaw or an angle plate, on the
table.
> Now, crank the knee up and down. Does the dial indicator move
> abruptly when you change direction of the table? If so, that may
well
> be your limit on accuracy. Many machines, especially not brand new,
> have worn spots in the vertical knee ways that allow the whole
knee/saddle/
> table assembly to rock forward and back on the ways.
>
> >
> >I am assuming that the software will be running closed loop and
the
> >culmitave error will be less than the mechanical error of the
machine
> >elements. Yes / No?
> >
> There should be NO cumulative error whatsoever, with the possible
exception
> of thermal effects. (Cumulative meaning over time and motion, not
referring
> to linear scaling error, the farther you move the proportionally
greater
> the error.)
>
> > Replacing the acme threaded rods with ball
> >screws (0.0006"/ft) seems to be the first step.
> >
> What backlash do those screws provide? Is it guaranteed?
>
> > Z-axis ways - anyone
> >experence excessive wear or degration in performance by using the
> >knee in constant motion? I (again) assume that the ways need to
be
> >chromed to help resist wear, or do they not wear for the first
2000
> >hrs of use (or some other vage number)?
> >
> >
> Chromed ways are a great help. Chrome is fantastically hard if
done right.
>
> Jon