View Full Version : Bridgeport Knee binding

01-25-2012, 01:31 PM
Well out of the blue my brideport knee won't raise past a certain
point anymore.

And Murphy's law is in full effect: With all my strength I can get it
to within about 0.1" of the cutter -- with the spindle fully extended.

It's about 1/2 way up the column more or less.. I can lower it just fine..
when I raise to that certain point it starts to bind. Like the knee is
locked. If I reverse I can get maybe a full turn or two and then the
knee "drops" a bit.. like when you lower the table a hair and release the
lock you know that "thump"?

Before I tear into this any wagers what it might be?

I have way guards on it the ways look clear (ie no big chips in there) and
it always happens at the same spot.

Lock seems to work fine everywhere.

Could this be a broken gib? (tightening as I raise it?)
I really hope not. I have X and Y gibs (waiting for that special day
when I can break the BP down and refit them) -- but I never
bought a replacement knee gib.


01-25-2012, 01:46 PM
There could be a number of reasons......... first if you had a broken gib you would be able to lower the knee, but raising it from any point would cause it to bind as the broken piece of gib would instantly start to bind on the way up. I would check it anyway. Also check the lock pin. Did you make any adj. to the gib at a different point in the colume??? Sometimes adjusting the gib at a loose point will cause binding at point in the colume with less wear.
Lastly there coiuld be a piece of swarf stuck in the thread of the screw.
Also look for any external things, such as misc readout brackes etc.


01-25-2012, 01:55 PM
Thanks JoeLee, yeah I'll be digging through it tomorrow
and no, no recent adjustments, its been fine for 5 years now like
I said pretty much out of the blue. Lets hope its just a chip.


Forrest Addy
01-25-2012, 03:14 PM
Check the elevating screw. It's telescopic. One piece screws into the other. Ones in a great while, one piece will lock up and you have about 1/2 the travel you need.

Never ever apply "all your stregth" to an axis motoin. Any unexpected unexplainable resistance (clamp dragging etc) and that's your cue to stop and investigate.

01-25-2012, 03:32 PM
Thanks Forrest.. how do I go about checking that? I'll break out the drawings
to see if I can learn something there. Thought it was one piece.


01-25-2012, 03:46 PM
before you take the knee gib out ..make sure the whole of the knee is supported from the ram .


the adjustment screw ..the head of it is in a slot in the gib.


you can just see the slot to the far right of the picture


01-25-2012, 03:49 PM
after its out ..looks like this


i spent a couple of hours sawing bits of string up and down the gap ..whilst rinseing with kero .

while you are there, make sure the oiler holes are clear .

all the best.mark

Rich Carlstedt
01-25-2012, 03:53 PM
There is a screw in the column to prevent you from raising the knee too high.
It may have come out and hit the top of the knee, and you can't see it with way covers.


Forrest Addy
01-25-2012, 03:55 PM
Epsilon, The shop vac is your friend. It will suck up all those beards of chips as they emerge from the disassembly. .

01-25-2012, 04:00 PM
Never ever apply "all your stregth" to an axis motoin. Any unexpected unexplainable resistance (clamp dragging etc) and that's your cue to stop and investigate.

Agreed! For a good reason why, Try pushing on your table and use the handle to move the axis. Then realise that a light turn on the handle = way more force then you could ever exert. after disengaging the leadscrew I hardly even move my table against the resistance of the gibs by pushing on the table.

"All your strength" = Something will bend and/or snap and it won't be something cheap.

Don Young
01-25-2012, 10:29 PM
My main advice is to under no circumstances let the knee get all the way down when something is wrong. If the gib wedges tight it can generally be freed by lowering the knee. If it wedges with the knee at the bottom it is likely to be a major operation to get it loose. Like laying the mill on its back and cutting a hole thru the base to drive the gib out, or something similar!

My first step would be to remove the gib with the knee as high as possible without binding. Don't try to force it if the screw does not want to lift it. That is almost sure to break the notch out and make it much harder to remove.

01-25-2012, 10:35 PM
Check the elevating screw. It's telescopic. One piece screws into the other. Ones in a great while, one piece will lock up and you have about 1/2 the travel you need.

Never seen one of those on a BP. Just a single jack screw that has a top bearing to push the knee working against a brass nut on the bottom post.

Carefully check the gib... Pull it out or use a pin to drive from the bottom (knee raised a bit), and see if it's on one piece, or two.

01-26-2012, 12:38 AM
If youve never had the table off, now is a good time to pull it and clean out the knee. Personally, I would go through all parts to all three axes and clean/check them thoroughly. You can save your time/effort now, but you will pay greatly for it later.

01-26-2012, 09:04 AM
Broke the head off of the gib:



Not as bad as I'd feared, I suppose. I imagine I can just weld/braze this
back on for now?

The bigger problem is that the gib looks like it hasn't seen oil in quite
some time. The wear area looks to be about 0.020" or so.. at least by
feel anyway.


01-26-2012, 09:12 AM
lots of wear there ..you can clearly see it ...the same wear will be on the knee ways ..

if it has that sort of wear /...in what i think is a low wear part of the machine..

then the rest may be worse

i say put it back together ..and put it on Craig's list .

find i nicer one for hopefully the same price as yours sells for

another pic of mine ..

it is a little bit worn in that area ..but not as much as yours ..mines a 1965 machine


all the best.markj

01-26-2012, 11:15 AM
my dog walks a little funny but I wouldn't craigslist him.
mark I appreciate the frankness but this machine isn't going anywhere
for now. Its been doing good work (for me) up until the bum knee.

maybe my standards aren't so high but I'm having a good time and
its making parts that fit my needs.

i plan on (one day) scraping this machine back into shape but I'm not
there yet. slowly getting the skill and tool sets together but thats
some time off yet.

I wouldn't be scraping necessarily to bring a machine back from the
dead.. like I said, so far, knock on wood.. but I think I would enjoy
the experience and learn a little while I'm at it.

the only downsides is that I wouldn't have a mill probably for a solid
month if not more.. that, and I still haven't figured out how to handle
all the heavy parts.. or get her into shape without a 5'x'5 surface plate.

01-26-2012, 11:56 AM
They are cast iron gibs. I've never had much luck with those tabs holding after brazing. On three mills they broke off again. You might try to mill the end off flat and drill/tread/ bolt on a strong tab with a couple of small SHCS.

Any new gib you buy will be oversized and needs to be ground/scraped to size. Last year I did see a guy selling three different sized gibs - you measure and he supplies one that is "closer" than a generic replacement.

01-26-2012, 01:04 PM
Funny thing is I bought X and Y gibs this past summer from H&W.
Real nice folks, at least judging from the 2 or 3 phone calls. I only
asked for X and Y and they said "you sure you don't want all three"

"nah, don't think I'll need the knee gib for now"

I didn't expect to be even thinking about someday scraping the