So if you're showing up to evaluate a used mill, what are you looking for? What kind of test cuts would you run to be able to verify whether the table was loose or sloppy? What makes something a "knee mill"?
Apologies but it looks we're going to bury you in questions before you get back to the proper write up... 
No apologies necessary, everyone that has asked questions or commented on this build thread thus far has helped me with my car in the past.
With that said, allow me to break down most of what you need to know to buy one of these things:
Basics:A knee mill is a machine that allows the table to be moved separately from the spindle. The table moves up and down 12-15" and the spindle will travel 5.5" on most BP's. This way you can accommodate taller work pieces without added stress on the spindle.
BP's are most common and preferential to most people. However, lots of very good clones exist and I wouldn't hesitate to buy most of them. Some of these include: Supermax, Lagun, Wells-index, Acer, and quite a few others...I'd suggest researching any clones you look at. With that said, I would stick with BP's if possible.
Targeting a machine:1) Decide if you want a Series I machine or a Series II. Ill make this simple for you, a series I is about 2k lbs, a series II is double that. The extra mass is nice for heavy jobs but it amplifies the inability to move the machine tenfold.
2) you're going to want a "J head" equipped machine, the older M-heads are good machines but lack the support that J-heads get and they aren't nearly as robust. The 2J head is also available on machines newer than 1975-ish, this would give you variable speed without changing belts (BIG ADVANTAGE). Mine is a basic J-head machine.
3) you want a Dovetail column, basically the column that the head is mounted to can move in and out (in the Y direction with respect to the table axis). The other option is a round ram machine and they aren't nearly as stable. Also, you'll probably never move it anyway to the beefier the better!
4) Y-axis travel, older machines had 9 inches, newer machines have 12. By newer I mean 1972 or so and newer. This is easily distinguishable by the serial number which is stamped on the Y-axis way cover. Plenty of people don't even realize it exists because you have to move the table all the way back in the Y direction to see it. These number always start with BR, a 12" travel machine will have BR/12 in the beginning (its actually BR over 12, like a fraction, not a forward slash lol)
EDIT: I'm sorry, its 12/BR, and here's a pic for reference:
[attachimg=3]
5) They're available with 32",36",42",46" tables. The bigger the better, BUT the 46" table puts a lot of stress on the ways when they're extended all the way and they can beat up on the gibs. Mine is a 42", just for size comparison.
6) Nice things to looks for: X-axis power feed. One shot oiling system, DIGITAL READOUTS!, CNC setup
, etc
Inspection:Once you've found what your looking for and you've decided to take a look:
1) Head-
Run the machine and listen for any grinding noises, a little humming is fine from the spindle bearings. These machines have little oil caps on the right side of the head (when your facing the front) lift them up and check for oil, if it has oil someone was clearly taking care of it. Run the spindle down while the machine is running and make sure the noise never changes.
Also check the auto-feed feature. On the left side of the head (again, when your facing it) are the controls for auto down feed. This is for tapping or drilling cycles and is super useful. Unfortunately, a lot of people are rough on the system and mangle the gears that run it, make sure you get a demonstration of how it works and THAT it works.
2) Table/Ways-
Like Freeskier said, shake the table back and fourth...that's always a good start. The table runs in ways and the ways have gibs pressing on them, the gibs are adjustable but they can be used to fool you when your purchasing. What you're looking for are good scrape marks all the way up and down all the ways like shown here:
[attachimg=2]
If you can find a machine with chrome ways that's even better, but not a deal breaker. Move the table to all the extents of its travel and look for scrape marks. Remember, these machines are usually used right in the middle of the travel area. If you move the table and it starts getting harder to move toward the ends of travel that's likely because the center is worn and someone adjusted the gibs to compensate, the ends aren't worn so the gibs get too tight when you move away from the worn area. As far as the knee movement, they're almost all worn. The knee is heavy and beats up on the ways for it pretty hard, you wont be using the knee to get z-axis movement while machine 99 times our of 100 anyway, so its no big deal...just move it to where you need it and lock it down. BUT, certainly move the knee all the way up and down to make sure it travels okay...might way to save your arm strength for that, you've got to spin the handle around about 50,000 times to get full travel haha.
The table itself is a non issue, if its rusty then stone it down and wipe it with some wd40. Just make sure no chunks are missing from where some asshat tightened something down wayyyy to much. If its got nice scrape marks like mine, hooray for you!, that's probably a well kept machine.
Check for any signs of GREASE on the ways or around the zirks. Those look like grease zirks but they are for way oil (Vactra 2 is what I use), not grease.
3) Leadscrews-
ALWAYS check the backlash on the machine, most people who own these have dial indicators and a magnetic base for machining work but if they don't then you should bring one and measure the backlash. Just stick the base to the table, zero out the indicator on the spindle and move the handle some increment (.050, for example) and check to see how much the indicator actually moves. Some backlash is fine, but it gets out of control sometimes. If the machine has Digital readouts (DRO's), then screw it, you'll never machine off the handle increments anyway. If you can find one with ballscrews (usually only on CNC machines) then that's cool because they'll have no backlash, but keep in mind that with no backlash the tool pressure can sometimes cause the axis to walk a bit unless you lock down the axis or hold the handle the whole time.
That's all I can think of right now, I'll probably come up with a couple more things later. If Anyone on this forum has questions about BP's or needs some assistance buying one please feel free to PM me or post here.
