Knee Mill - Z Axis Options

Guys... just acquired a Bridgeport Style Knee Mill and I can't help thinking... this would be great to CNC... Well the X & Y Axis look easy enough to convert, but the Z Axis has me confused...

I could CNC the quill...but.. as with most mills.. when you lower the quill you also lose some accuracy becasue of spindle flex etc... according to a few machinists I've spoken to they all use the knee Z to alter depth of cut.

Converting the knee seems to make sense but I'm unsure of how to attach/setup the motors as there are no brackets or fixing points.

Like to hear your ideas and advice on this please...

Moondog

You may have seen this already. http://www.cnczone.com/forums/showthread.php?t=4327

I just looked at this thread quick now, it seems to have been changed (I had read the whole thing a few months back).

IJ (your fellow countryman) had a very huge thread on his complete knee mill conversion, including his trials & tribulations with his Z (knee drive).
Maybe you could get in touch with him.

Well, the net result of the knee mill experiment in the absentee thread was that moving the knee requires a big servo motor and a big drive amplifier to feed it. Another option is to somehow counterbalance the weight of the knee.

Having used retrofitted cnc mills with Z on the quill, I can say they are not too bad. The biggest pain is not having a high enough work envelope for some jobs. Mixing long drill bits with short stubby endmills in a job requiring 4" of Z depth just doesn't work :D

Moving the knee is not necessarily more accurate than moving the quill. This is because on a manual knee mill, the machinist locks the knee for rigidity. Locking is not going to be feasible for a knee Z drive.

Making a good connection to your quill for a quill Z drive can sometimes be a challenge too. Whether you have to machine some surfaces to make a mounting plate on the quill or the knee, it is almost mandatory that you have a shop nearby with a working mill that you can use to modify your own.

The ideal solution may be to do both.
Have the quill asigned as a Z axes and the knee as W axes.
The lighter cuts and drilling operations are best done with
the movement of the quill.
Heavy machining is best done with the quill fully retracted and
movement of the knee.
Depending on what kinds of work you do most you could do one
and then build the hardware needed to do the other later.
Good Luck