Re: Knee for z-axis control?

Hi group,

Thanks to all of you for the input. Reading through this discussion
was reminiscent of a political debate ;), but the outcome was worth
it! I'm pretty much now sold on using the knee with the use of gas
springs; there are simply too many advantages to pass it up. I'll
also say that with the input from the `experienced' CNC-knee users,
none have suggested the knee couldn't hold tolerances or had
chattering problems that couldn't be compensated for; therefore again
I'm sold! My first attempt will utilize the existing acme screw in
hopes that it will provide sufficient accuracy and drive, if it
proves not worthy, I'll convert to a ball-screw and add a brake to
the drive.

I share the reluctance to drive the mass of knee up and down on a
regular basis, but I think with the proper drive and counterweight
the weight can be handled effectively. Besides, that's why they make
a variety of motor sizes, to match the load of the application at
hand! I'm not afraid of hard work, as long as I'm not the one doing
it :).

Thanks, Eric



--- In CAD_CAM_EDM_DRO@yahoogroups.com, Sven-Åke Larsson <sven@s...>
wrote:

> > I'm well aware of what Mazak and others do. I see very little
> > similarity between those type machines and Z-ing the knee of a
> > B'port type mill where a good percentage of the machine's mass is

in

> > the knee assembly.
>
>
> I'm been working with two machines with Z-knees, one Majo and one

Italian machine (can't remember the name though) and I can tell you
all that the mass moving up and down on a Bridgeport knee is
ridiculous small compared to these machines. I could lower the Z-axis
so much that I could mill directly in a injection mould without
disassembling it, and then we're talking abut 400kg extra weight.

> Yes, I also see that it's a lot easier to make the head moving, but

I think all the negative reactions here are too negative. I would go
for the knee. ;-)

>
> Regards,
> Sven