Z Axis CNC: Quill vs Knee

[HimyKabibble]

I've spent the last two evenings reading all the many threads discussing CNC'ing the quill vs CNC'ing the knee. From all I've read, it seems to me the discussions all boil down to a number of simple trade-offs:

Quill Pros:
Relatively simple mechanically
Relatively low loads, so requires only a small-ish motor, ballscrew, etc.
Quill Cons:
May be less precise, if there is radial "slop" with quill extended
Limited travel

Knee Pros:
Greater travel
Knee Cons:
Requires a *big* motor, ballscrew, etc. to get good performance
Requires counter-balance or air springs to offset weight of knee
Possibly more wear on the knee ways?
Possible loss of precision since knee cannot be locked while cutting

Seems to me, if your quill is in good shape, the radial slop should not be a big issue, nor should the limited travel, in most cases, unless you have a tool changer. If your doing manual tool changes, you can zero the knee for each tool, and the 5" of quill travel should be adequate for the vast majority of jobs (obviously there are exceptions to this).

So, seems to me, a reasonable compromise for a non-auto-toolchange machine is a quill drive. If you do have an auto-toolchanger, maybe the quill drive is still a good way to go, and a knee drive could be used for tool length compensation only during tool changes, for which high rapid speeds would not necessarily be required, allowing use of a smaller motor, etc.

Does this seem reasonable? Am I missing anything?

Flame suit on, ducking under my desk now.... :-)

Regards,
Ray L.

[greg b]

hi ray dont worry about ducking under the desk ! we all have questions to ask .This is quite a interesting subject, I my self have been quite interested in converting my Knee feed on my bridgeport over to 4th axis for deep boring only.But i had no intentions of changing my knee lead screw over to a ball screw ,because its not going to move quick enough or has to be super accurate although backlash comp should take most of that out.I would imagine the size of the motor will have to be reasonably powerful the only thing thats going to be a little bit tricky is sorting out the 4th axis.I have asked members if they have done that mod yet but no one seems to have done it.regards greg b