Hey Doug,
First off, this is just MY opinion, OK? Both configurations will work, obviously, but I'm not real happy with this mechanism on the LMS. As to the reason why:
When I first was learning at my friends gunsmithing business, he had BP clone. I was taught (and became accustomed to) bringing the table close to the cutter with the knee crank, touching the cutter to the work surface by bringing the quil down and locking it, setting the Z zero on the knee dial (he only had a two-axis DRO) and then bumping the Z handle down to clear the cutter from the work. This set my Z-axis reference, or zero. I'm sure the machinists here would describe a different way, but this worked well, and quickly, for the bulk of the machining we did in the shop.
My first little mill like this was back in 2000 and I bought it from a guy who advertised in the Home Shop Machinist. He basically took the cheap Chinese mill and modified it with a R-8 spindle (not available back then), class 7 bearings, a quill that went about 1.5", and a leadscrew for the Z axis very similar to the one on the PM machines. Add the solid column, and inch leadscrews and it was quite the little machine. He put so much work in this thing that he wasn't making much money on each one, and he still had to compete with the HF crowd that figured price was everything, even if the machine wasn't the same...
Anyway, the transition from the big mill to this little one was very minimal, with the primary difference being where the Z-axis knob was located.
Now I have this LMS machine that has no qilll and the slop on the Z-axis fine feed knob is about a full turn or more. Couple that with the lack of feel on the knob/rack-and-pinion interface, and I am not quite sure when the head actually starts to move. Which is why I bought the DRO, so I can better track the absolute movement of the Z-axis regardless of how much slop is in the mechanism. A rather expensive fix you might say, but I like having a readout instead of trying to count handle turns anyway. As I have said before, I'm not a machinist, I just like using some of their machines to make things.
The PM-25 has almost the exact configuration, albeit in a larger package, as the mill I had years ago. I didn't get it because I was afraid of the extra weight and my ability to manhandle it around by myself without any lifts or other mechanical assistance. I goofed, because I managed to muscle around my 300 lb South Bend without too much trouble (minor muscle pulls and my hip complaining doesn't count). Oh well, lesson learned.
With the air spring, the LMS mill has over 10" of Z-axis travel. The PM has 13", and the quill. The PM has more Y-axis travel too, which I will have to work around on the LMS machine. And a Z-axis leadscrew w/quill. I thought about trying to modify the LMS mill to approximate the one I had years ago, but if I disassemble my LMS mill, I have nothing to make the mods with...
Buyers remorse? Probably. More like not completely thinking this through before I jumped. I will eventually get the PM machine, and maybe relegate the LMS to precision drill press duty.
But keep in mind, this is just me. Others have and like the little LMS machine, and it does pretty decent work for me. It's just not quite what I want for the work I want to do or how I do it. I'm just expressing my experience. Not trying to influence your decision.
Bill
wrmiller, what don't you like about the rack & pinion? I'm just curious - I'm not married to LMS yet, and I could get the PM machine for roughly the same $$$ as the LMS package (PM says their shipping is free - can that be right? You mean they'll put a machine on a truck in Pennsylvania and drive all the way to Arizona, and then drive back - all for NOTHING?).
Another thing that attracted me to the LMS 3690 mill is that the head has about 10" drop, and the PM machine has about 2" of quill drop. Apparently they work completely differently - the PM's quill drops from the spindle while the head remains stationary, while the LMS machine's entire head drops while the quill remains stationary. Are you saying the Z axis rack/pinion yields some sort of undesirable action?
The 3-axis DRO pro looks pretty nice. Hope you like it.
