what are the signs to look for in a mill when its too far gone and worn out??
First off, I'm talking Bridgeport type mills here, which IMO is the type every home machinist should get if one can afford it and has the space. Many points I mention will also apply to other types of mills though.
When I walk up to a Bridgeport to determine if it's had a hard life or not, I first look for table damage, then wear on the two flat surfaces (see pic of my mill below) on top of the knee casting where the saddle sits. These bearing surfaces will most often have a swirly/criss-cross pattern on them. The pattern will generally be clear and sharp at the front & back sections because the saddle doesn't travel all the way across frequently during use. In the center area is where you want to look for wear; by moving the saddle all the way to front & rear so you can examine these surfaces. If the swirly pattern is completely gone in the middle, the machine has seen significant use. You also want to check for deep scrapes on these surfaces, indicating that steel chips have been trapped under the saddle and have caused accelerated wear of the bearing surfaces. Also, if when cranking the handle to move the saddle in and out it's very tight at the front & rear but loose in the center, the machine has seen better days.
Undue wear on these surfaces can also indicate that the machine's lube system, if so equipped, is not working properly.
Every older Bridgeport I've torn down has had at least one oil tube, fitting or gallery plugged with gunk. You also want to inspect for wear on the quill. It should be bright and clean. Bring it all the way down and look for deep scrapes. While in the down position, grab the end and try to move it. It shouldn't move
at all.
Stick your finger up into the spindle and feel all around where the collet seats. It should be smooth on the tapered area near the end of the spindle nose. If not, a collet has spun at some time and galled the surface.
You might check to see if the key for the collet is there, but most of them have been sheared off on older machines. That's not really a problem; the machine can work fine without it. It's easy to replace with a $5 part if you want to do so.
Raise the knee as high as it will go (make sure the knee clamp is free). Then, as fast as you can crank, lower it to the bottom of its travel. It should go down smoothly with no 'fits & starts.' If while cranking it you suddenly feel less resistance and it 'clunks' down, that's not good. Also, squealing while lowering the knee indicates that the screw/nut has not been lubricated properly, maybe for a very long time.
Also, I'd never buy a mill that wasn't hooked up to power so I could test function of all features that require it. You want to make sure that the motor runs fwd & rev both in high & low range, quill feed operates in all positions, brake stops the spindle, etc. Listen for funny noises from the head, especially in low range. Make sure that the DRO and power feed work.
If the machine has obviously been painted very recently, or the seller doesn't freely allow you to take your time inspecting it, look elsewhere (Think 'used car salesman' here).
