Ed not sure what rapids the Series II had on the knee but I would imagine it would be as fast if not faster than 100IPM. They use ballscrews and air assist which basically means the knee has very little weight. If I remember correctly you can adjust the air pressure for the workpiece load as well but maybe Chip can give you more reliable answers.
Hood

I converted a Jet knee mill and did the knee instead of the quill. I used a 1200 in-oz stepper and put it directly on the crank position. It has worked for a year, but I recently tried to cut a mold in aluminum that ran a couple hours and I had some loss of Z position. The funny part was it actually GAINED in Z, so I wound up deeper that it was supposed to be. I think it was because the Zup moves were G00, while the down moves were at a slow feed rate.

I had an AC servo on hand so I decided to try that. I removed the gears and was able to mount the motor inside the knee casting with a short belt to the top of the screw. But I can only get a 2:1 belt drive in there, so the servo cannot quite handle it. I am waiting for a gearbox that fits on the top of the servo.

I like having the quill free to do delicate drilling or tapping operations. I also like it for tool change- I use the Tormach tool holders, and I put a heavy spring under my drawbar. I built a simple bridge above the drawbar. To change tools I simply push the quill up to compress the spring and eject the collet.

There are some photos of my Jet conversion on my website plsntcov.8m.com

Interesting website Ron, very informative.
Thanks for the link,
RC

Maybe the true reality of the choice to cnc the quill or knee is that most of us are retro fitting manual machines and the quill was not designed (or not close tolerance enough) for milling and would need substantial rework to fit the purpose. This could also be true of manufacturing machines manual or cnc that have had their hay-day and are now on the second-hand market.

I have a Chinese mill (which was all I could afford at the time) which is similar in size to a small Bridgeport. I'm in the stages of converting it to cnc and have 3 axis working now, I chose the knee to get moving first over the quill as the machine has essentially a mill/drill geared head and the instruction manual clearly states to: Lock the quill before milling.

Also the knee is an easier prospect. I fitted it with an air assist ram 100mm dia (4") with an inlet that would let me fit 1/2" compression fittings I coupled this to a precision air regulator, the only one I could find (on eBay) had larger inlets and outlets so I had to fit reducers to fit 1/2" and left the exhaust without a reducer. It finely balances the knee @ approx 50psi.

It runs a 24v 375w dc motor/ Gecko driver and is geared 4-1 to a 16mm pitch 25mm dia ballscrew I had to fit a brake which is on the top end of the ballscrew. Encoder is currently 500 cpr but I'm looking to raise this to 1000 cpr to get the resolution closer to the other axis. Backlash is currently 0.08mm and I'm hoping this is all in the second-hand ballscrew, I have designed all the nuts on the axis's to accept another ballscrew nut to reduce any backlash. With Mach3 backlash setup, The knee has good repeatability.


Once up and running, with my inexperience at tuning Gecko's I was concerned about machine making a noisy vibration like the bass note of a cello which seemed to fill the whole workshop when moving the knee in the downward direction. This was cured by much head scratching and then retuning the gecko drive which cured the problem. The servo drive G320 is very tunable on this axis and I could see the springiness of the system on a dial-gauge as I tuned it in.

After many hours fiddling I was satisfied that all was good. It now achieves 4000mm per min (157 ipm) at 150mm/sec/sec without faulting the drive. I expect more fiddling and tuning could raise that some but I'm happy.

The main important thing with all this sliding up and down of the knee is lubrication and plenty of it. I've made a air assisted oiler for all the slides and x-y ballscrew the z ballscrew is greased.

So for most of us cnc'ing the knee is the way to go.

John

I'm with Hood.  I think the smart way to handle Z travel is as he proposes.  Use the knee for TLO's.
My Series II knee has huge box ways and an air assisted knee.  I am sure I could get the
knee to some serious rapids.  I have seen knee's driven with 40 in-lb servo's  (read huge), and
the servo's were driven by Gecko's.  These knees were able to drill holes in 3/4" steel plate
all day with perfect reliability. 
I think driving the knee is going to wear it out prematurely.  I have heard other state that this
is the case.  Looking at the wear on some older machines that never had a power knee
seems to indicate that this is indeed the case.  The tremendous weight on the dovetails
makes them prone to wear just under normal (handcranking) use. 

My series 1 cnc is a bit restrictive with the 5" of quill travel, but as Ed says, it is certainly still
capable of doing most jobs.

Scott

Looking at the wear on some older machines that never had a power knee
seems to indicate that this is indeed the case.

Most likely because of the manual labor that is involved in cranking those tables, they only get used when absolutely necessary. That's how it is in my case anyhow, but I'm a bit on the lazy side.

Hi, Ed V

Mine are Stepper Motor, Haven't used them in quite some time, No pic.'s, Yes the air assist is adjustable, Up to 1,000 lbs part wt.

Sorry no real speed reports, I do like the independent Z quill feature.

The servo powered one's went back to Z axis spindle and I'd think there faster in Z axis.

Chip

I just got done re-biasing the optos on my BOB, and my knee rapid speed has gone from 25 IPM, BOB-limited, to 60 IPM, motor limited.  Not bad!

Regards,
Ray L.

Driving the knee is a waste of time, rapids far too slow, much better to spend £15k on a proper VMC

(My BP with a 8Nm stepper driven knee conversion works fine, it won't win any speed competitions (neither will my car), but does juuuust fine).
When it does eventualy wear out I will buy another mill, pick the best bits from both machines and rebuild, but by then "star trek replicators will be available and I'll go from CAD to part without all the messy swarf.

Derek

Gee, we seem to have an awful lot of very happy knee-drive folk here, considering what a bad idea it is!  :-)

Now that I'm getting 60+IPM out of my knee, and 300+ IPM on X & Y, I'm a VERY happy camper!

Regards,
Ray L.