Re: Knee for z-axis control?

Eric,

You might want to think about going with a pneumatic cylinder, an
accumulator, and a regulator instead of gas springs.

It gives the advantage of being adjustable (varying table load) and
if the accumulator/cylinder ratio is high enough it's fairly constant
lift force.

Unlike gas springs which force becomes exponential with compression.

Incase you don't know the math:
Radius ^2 @Pi (3.14) * PSI = Lift force

On a 4" bore cylinder that works out to:
2^2 = 4 *3.14 = 12.56 Square Inches of piston area. To hold up 400
LBS you would need about 32PSI.

You could make a little chart and mount it to the side of the mill.

32 PSI empty table
38 PSI 1 Vise
44 PSI 2 Vise

This way loading on the axis motor would be more or less constant,
plus you could use a smaller motor.

Check on eBay cylinders are cheap.

Good luck on the knee. You'll love the travel.

Thanks
Aaron Moss
www.IndustrialHobbies.com






--- In CAD_CAM_EDM_DRO@yahoogroups.com, "pondindustrial"
<pondindustrial@y...> wrote:

> 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