Re: Z axis

--- In CAD_CAM_EDM_DRO@y..., Peter <prenolds@s...> wrote:
> I drive the knee on my CNC mill (BP Clone) and can report good
results
> although it took a lot of fiddling to get things set up correctly.
I
> posted questions on alt.machines.cnc and received many of the same
> helpful suggestions described earlier - i.e. gas springs and/or
> emergency brake to stop the knee's weight backdriving the ball
nut. I
> really appreciate having 16" of vertical travel under computer
control.
>
> My machine started life with an acme screw for Z but when I drove
the
> knee up and down at my stepper's top speed (yielding 24 IPM on Z
axis)I
> got horrible gib vibrations which I couldn't really eliminate with
gib
> adjustment. (It would just return at different speed.) And I still
had
> a backlash problem if I ran at full speed - so I installed a ball
screw
> & nut to see if that would solve the problem.
>
> I also purchased two $14 gas springs (75lb each) from McMaster Carr
(p/n
> 9416K403 - mounting fittings are extra)and installed them at the
front
> of the knee. I'd probably suggest a pair of slightly stronger gas
> springs (say 125lb) but a combined 150lb of gas spring stops any
> movement of the knee - even with the stepper motor removed.
(Though I
> wouldn't stick my hand under it without wood blocks to support
it...)
>
> I can post a couple of pics tomorrow if anyone is interested.
>
> Cheers, Peter
>
> p.s. I've had an excellent experience dealing with Rockford Ball
Screw.
> They are very well organised.
>
> p.p.s. I've also been dealing with Flashcut for a couple of years
and
> love their little controllers (I have 2 cnc'd machines) and PC
software
> - they just don't break!
>
>
>
> Jon Elson wrote:
>
> > mjf462001 wrote:
> >
> >
> >>--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> >>
> >>>mjf462001 wrote:
> >>>
> >>>
> >>>>Hugh,
> >>>>
> >>>>Thank you for the link. I am still not sure weather
> >>>>I should drive the quill or the knee. When I get around to
making
> >>>>
> >>up
> >>
> >>>>my mind I will at least have some pics to reference to.
> >>>>
> >>>If you use the machine to drill holes, or if you have some slack
> >>>in the vertical ways for the knee, like mine, then I think you
> >>>
> >>should
> >>
> >>>go with the quill. My knee sags out, toward the operator, when
you
> >>>lower it. I can't tighten the gib because there is a worn spot
on
> >>>
> >>the
> >>
> >>>ways.
> >>>
> >>>Jon
> >>>
> >>John
> >>I saw a pic of Dan's setup. The way he drives the knee, I would
say
> >>it looks pretty rigid!
> >>
> >>Without having to do the math I wonder how much torque a guy
needs
> >>so he wouldn't have to gear down (slow down) the z axis drive.
> >>The mill is new and without any wear.
> >>
> >
> > I'm not saying it isn't rigid, just that you are moving a LOT of
iron to
> > poke
> > a 10 gram drill bit into a workpiece. On a Bridgeport Series-I
size
> > machine,
> > you are talking about moving about 1000 Lbs of iron up and down
every
> > time. It certainly will work, but it may cause much more rapid
wear
> on the
> > knee slide. You should note this is the most highly stressed
way on the
> > machine, as the weight is completely off to one side of the
slide.
> Worse,
> > it puts the load on the dovetail, rather than on the flat!
> >
> > I think it is unreasonable to try to CNC the knee without a
ballscrew
> > retrofit. And, after a ballscrew, you'd need a safety brake to
prevent
> > the table from dropping when you go to E-stop, or lose power. A
> > reliable counterbalance, either weights and pulleys or gas
springs,
> > would help out, but then you'd definitely need a ballscrew, or
the
> > backlash would show up, without the knee weight.
> >
> > Jon
> >
> >
> > Addresses:
> > FAQ: http://www.ktmarketing.com/faq.html
> > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> >
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