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Help! Twisted Knee/Z? Bizarre misaligned knee.

rpseguin

Stainless
Joined
Jun 28, 2006
Location
Napa, CA
I saw something bizarre today that I’ve never seen before and I’m curious what is going on or how it could happen.
I was looking at a Bridgeport series 1 machine and the Z/knee was visibly tilted to the left. It was clearly visible to the naked eye, so we put a level on it and it was stunningly out of square.

Shared album - Ralph Seguin - Google Photos

The mill was sitting on a measured level floor, so it has nothing to do with leveling.

We also put a level sitting on to of the y ways and it was clearly out of level/square.

Shared album - Ralph Seguin - Google Photos

The possible scenarios that I thought of were:

1. Someone reground the Z/knee or the Y axes and did a very, very bad job.

2. Z gibs are in wrong?!??

Anyone seen anything like this?
What is going on?

The knee moves up and down fine.
All axes move fine.
 
Thanks!
Good idea. I actually did try moving it with no apparent deflection, but I’m not sure how much force it would take.
I think it would have moved noticeably if there were no gib.
 
The knee, saddle, and table are probably 500-600lbs, should be reasonably balanced but still might take some force to rock. Won't hurt anything (I think) to put some muscle on it when trying to move it. Also check whether there's any gapping at the knee to column dovetail, if there's none it's likely not a missing gib.

If something has forced the knee over, there's a small possibility the Z elevating nut or screw is damaged, but probably not significantly.
 
The knee, saddle, and table are probably 500-600lbs, should be reasonably balanced but still might take some force to rock. Won't hurt anything (I think) to put some muscle on it when trying to move it. Also check whether there's any gapping at the knee to column dovetail, if there's none it's likely not a missing gib.

If something has forced the knee over, there's a small possibility the Z elevating nut or screw is damaged, but probably not significantly.

Thanks!
Yeah, I was looking at the dovetail between the knee and body casting/Z and it looked reasonably ok to me.
I will try your suggestions and put some force on it.
The shop where the mill is has a forklift, so they could sling the table on the left side and apply some more slow, controlled force. I wouldn’t have them lift it, just some force to see if the knee shifts over.
 
Sure, just be sure that the X leadscrew is protected and can't be damaged during the lift. Except for where the bearing anchors and hand cranks are, the X leadscrew is usually the lowest part of the table.
 
Shouldn't be need for any big amount of force, certainly not "fork lift" kinda force anyway. Probably do more harm than really tell you what the cause is. Remove all the way wipers and the problem should become pretty clear. If the knee gib is there, it's probably broken and only a short piece of it's left. I'd start by removing the gib.
 
Shouldn't be need for any big amount of force, certainly not "fork lift" kinda force anyway. Probably do more harm than really tell you what the cause is. Remove all the way wipers and the problem should become pretty clear. If the knee gib is there, it's probably broken and only a short piece of it's left. I'd start by removing the gib.

Thanks!
Yeah, I was thinking very limited and more controlled force with the forklift, but I’ll refrain. :-)
In any case, I’ve never removed or touched a knee gib before.
Do I need to remove the table and saddle, or is there, hopefully a way to look at it and replace it with the saddle and table on the mill?
 
You can certainly inspect it with the saddle still on, replacing it will be more difficult. Move the saddle forward, toward the operator. There should be way wipers installed on the top of the knee where it meets the column, remove them. The gib, if it's there, will be on the left side, facing the machine. Back out the adjusting screw and slide the gib up and out of the machine. You probably want to support the knee on wood blocks before you do this as it will be free to move side to side, as well as in and out, quite a bit and this movement may damage the elevating screw or bang up way surfaces.
I can't tell from your pics but are you sure the whole machine is not tilted, just the knee? The sides of the knee usually line up pretty well with the sides of the column. There is typically very little mismatch, over the entire length of the knee, even though they are both just cast surfaces. Looking at the sides of the machine, does the knee stick out on one side at the top or bottom?

On edit: If the gib is broken or missing, a new(or used) one must be FITTED to the machine. This is NOT an off the shelf part swap! The new gib needs fitting by scraping to match the way surfaces exactly. If the ways are significantly worn, it makes fitting even more difficult.
 
Inquiring minds want to know. Did you figure out what the problem was?

So, it was an optical illusion caused by the tops of the Z/knee dovetail being pretty far off from each other.
Plus, I think the base of the casting is pretty far from being level.
However, the Z and X and Y are all orthogonal/perpendicular to each other.

I brought a good sized right angle plate with me and the Z/knee dovetail is nice and square:

Shared album - Ralph Seguin - Google Photos

So, I bought the mill and am now cleaning it up and have questions about paints, cleanup, ...
Creating a new thread for that:


http://www.practicalmachinist.com/v...t-series-1-refurb-project-350887/#post3182464
 








 
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