*Knee Mill Retrofit*

[IJ.]

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[Klox]

IJ, i think you did very well thus far! I know everything will work out just fine.

Klox

[IJ.]

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[IJ.]

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[HuFlungDung]

Good job, IJ

I'm waiting to hear how you like the performance of that Z axis Knee drive, whether it can stand the heavy use that I imagine for it. You don't mind being a guinea pig for research, do you?

[IJ.]

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[ty1295]

I had another guy tell me he made a 4" air piston.

Hooked it to knee, instant adjustable counterweight just add/decrease air pressure to get close to no load on knee. Seemed reasonable to me -vs- hanging 600lbs or so off back of mill.

I am also very curious on performance of knee, as I plan to do almost identical on mine providing yours works out well.

[IJ.]

ty1295: Now that's a cool idea !

[Al_The_Man]

I have an Excello Knee mill that is fitted with a ballscrew jack for operating the knee, with a 840 oz/inch step motor for operation , one turn of the motor moves the knee .050. The jack drive shaft comes out of the knee casting at right angles to the screw, so the motor is mounted horizontally to the side. There are two large 2" diam air cylinders used either side for counter weight control, they have an automatic pneumatic pressure release valve that keeps constant pressure whether raising or lowering.
The motor is fitted with a holding brake to prevent back feeding when not moving.
The knee can be jogged by pulse steps or run continuous.
This mill has a Z axis on the quill, so the knee control is just for initial positioning.
Al

[HuFlungDung]

Yes, the air cylinder counterbalance sounds like a good idea! If a person was going to be using it constantly (as IJ will), do you think it would work to plumb the cylinders to a large air tank "accumulator", so that the pressure would maintain a steady rate, no matter where the cylinder was on its stroke?

[Al_The_Man]

Hu, Essentially that is what this system does, it is a special constant pressure and relief valve, I think even if you had an accumulator you would still need the relief when on the downward stroke, Likewise you would need automatic top up as the cyclinder rises. This system works well. I don't know who makes the valve assembly, I think a spare came with the machine so I will try and look for a name.
Al

[ESjaavik]

If the accumulator have a volume much larger than the displacement, it can be a closed system. That would save having a compressor running. The piping between cylinder and accumulator should have ample dimensions, dependin on max Z feed rate. There is no need to make the knee absolutely "weightless" in all positions.

OTOH if you want air assistance of the acceleration, a two-way cylinder and active control would be needed. Probably will be cheaper to use a more powerful servo.

I've seen used long stroke gas cylinders like the ones used for tailgates. It's simple and maintenance free.