I finally got round to stripping down the old Harrison knee mill I've had sitting doing nothing for the past couple of years today. And I'm now trying to work out how to power the knee.

I'm going to be putting on a pneumatic cylinder to act as a counter balance, as my rough guestimate is the knee + table weighs around 100kg (about 220lbs for thos not versed in modern weighing systems ;)). That'll be the easy bit (famous last words!)

My problem at the moment is how to mount the ballscrew. I've got some rough ideas, but can't decide if the servo should be mounted on the knee itself, or on the base.
Just now mounting the servo/belt drive on the base is looking like the better option, but I'd like to see some pictures of other peoples solutions.
I've searched the forum archives, and google but haven't managed to turn up anything. The only place I can remember seeing details of a knee being retroffited was on IJ's infamous thread, but his posts are long gone.

Anybody got any suitable links/pics/general comments?

There was one I posted in the link you refer to, This was on an Excello that the knee had been retrofitted for positioning only, Using a ball screw jack.
Personally I wouldn't use the knee for Z axis control.
It sounds like you are using belt drive? What ratio?
Old link here http://www.cnczone.com/forums/showthread.php?t=4327&page=4&pp=15&highlight=ballscrew+jack
Mine has a pneumatic cylinder either side of the knee for counter balance.
Al.

Thanks for that Al.
I didn't bother re-reading the thread, as all the photos had been deleted, but from your description it sounds like the motor/drive has been fitted in place of the manual handle. Whatever way I decide, I'll be either driving the ballscrew, or the nut directly via a belt.

My only option is the knee, as a quill feed was never an option on the Harrison mills. (It's essentially a horizontal mill, with an optional vertical milling head).

Current thoughts are heading towards a 25mmx5mm ballscrew, with motor/ratio still to be decided. It'll be my first trip into the land of CNC, so I'm taking it all one step at a time.

Good news!!!
Just found IJs photos on my computer :)

But the knee on his mill was big enough to fit the complete motor/belt assembly inside the knee.

I've attached a general phot of my mill, aswell as the photo of IJ's mill showing his knee drive.

I was talking to an old machinist a while back and he was telling me that he would use the knee of the mill instead of the quill for Z motion just because the quill was better off being bolted in place, in his opinion. But I guess its all preference.

I was talking to an old machinist a while back and he was telling me that he would use the knee of the mill instead of the quill for Z motion just because the quill was better off being bolted in place, in his opinion. But I guess its all preference.

The main problem, and I think IJ found this out, is that there is huge inertia involved when moving the mass of the table, knee and weight of the part, when executing rapid moves such as peck drilling etc.
Al.

This is true Al but if you think about it most of the medium sized CNC machines have more mass in the z-axis, than a typical series 1 BP Knee.

Just get a bigger motor! err err err err! WE ARE HE-MEN!

MORE Power! MORE Power! MORE Power! MORE Power! :D

The inertia required to move the knee is large, but it is going to be counter balanced, so that should remove a fair bit of the load.

Having looked at the machine again, the easiest option is going to be mounting the servo on the base, and spinning the nut.
I'll just have to make sure there's a good guard over/around the servo/drive to keep any stray coolant away.

The Bridgeport series II cnc mill has the Z axis on the knee straight from the factory. When you put 60psi in the counterbalence air piston it works quite nicely. Al is correct about drilling though. Don't be expecting any 100ipm rapids in your cycles.

100ipm????
I'll be happy if I get 10ipm.

I'm currently sitting looking at the various ball screw options, and trying to figure out a design to spin the nut...

I get 20ipm with a wimpy 18lb-in servo driving the .2" ballscrew with 4:1 reduction.