# Stuck gib in mill knee



## fillister (Nov 7, 2020)

I thought I would post my mistake so hopefully others could avoid this issue.
I was almost finished installing the Align servo on the z axis of my mill and was in the process of setting the position of the limit switches. I hand cranked the knee to the lowest position as I wanted the maximum clearance available, but when I tried to move the knee up with the hand crank it immediately jammed and would no longer move. 
I could see the gib extending about an inch below the base of the knee, but unfortunately there was not enough room to use a hammer to tap on the bottom of the gib. I tried using a 36" pry bar but the gib would not budge.
A web search reveled that the only real solution would be to disassemble the mill, lay the base casting on it's back, drill a hole in the base to allow for a drift to be used directly on the bottom of the gib, fortunately heating the knee casting was not necessary in my case.
I drilled a 1" hole below the gib using a hole saw, I then wedged a wood 4 x 4  between the knee and the 3 x 3 x 1/4 steel tubes bolted in place of the turret. I used a 3/4 inch x 12" bronze bar as a drift punch, , a good friend bravely held the punch in contact with the gib while I gave it six strong blows with a 5 pound sledge hammer and on the sixth blow it came loose and with no damage to the tapered gib. 

I'm a lucky happy man today,


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## leeko (Nov 7, 2020)

Glad you got it fixed!

I may be being dumb, but I don't see what mistake was made?

Lee

Sent from my SM-N960U using Tapatalk


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## fillister (Nov 7, 2020)

leeko said:


> Glad you got it fixed!
> 
> I may be being dumb, but I don't see what mistake was made?
> 
> ...


Leeko, I call it a mistake, but is was reallly a lack of knowledge I did not realize that it is a design flaw and even happens to bridgeports  if you drop the knee to its lower limit and that part of the dovetail is dirty the gib can stick.

Lesson learned for me, keep all dovetails and ways super clean, but I'll never take the knee that low again.


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## leeko (Nov 7, 2020)

Gotcha! That's definitely good info for us newbies to know - thanks again

Lee

Sent from my SM-N960U using Tapatalk


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## brino (Nov 7, 2020)

fillister said:


> I'm a lucky happy man today



........and your brave friend is still your friend?

Thanks for the warning!
-brino


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## fillister (Nov 7, 2020)

brino said:


> ........and your brave friend is still your friend?
> 
> Thanks for the warning!
> -brino


Yes, I never missed a swing and he was never injured, I could never have done this without his help..


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## Ulma Doctor (Nov 7, 2020)

nice save @fillister!

i don't usually have anyone brave enough to hold my drifts,(although i'm a surgeon with a hammer )
so i have devised many methods to perch or hold my bang sticks (drifts)
1: Vise Grips- hit them as hard as you want with a bad swing- they'll forgive you
2: Channel Locks- same as 1 but not as forgiving, reduced grip in comparison.
3 : Car Jack- perches the bang stick at variable height, so you can swing from behind the ear and move stuck things
4: Jack Stand - same idea as #3, not as adjustable but, still a strong contender
5: Cribbing- alternate stacked wood at the desired height to hold the bang stick

i call drifts bang sticks to this day.
an English as a second language worker at a facility i was performing rough service work in,
heard me hitting a 2" bronze rod with a sledge hammer to separate a stuck shaft in an industrial grinder.

apparently he was impressed how hard i was hitting the bang stick, 
the shaft flew about 6 feet out of the machine when it finally let go after about the dozen swings it took.
he said "Good Bang Stick" in broken English.
i laugh every time i use a drift


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## Flyinfool (Nov 8, 2020)

Had that same scenario at work. If you never go all the way to the bottom of travel with the knee, make sure to clean the dovetails and ways to within an inch of their lives before you do.
We had to do major disassembly of the machine, and lay it on its back, drill the hole for the drift, get out the BFH and still had to use a lot of heat to finally get it to pop. Heating was done with a 2 inch rose bud tip on the oxy/act torch and took a good half hour to heat up enough to pop. We had that center up to around 500°F and it took over night forall that cast iron to cool back to room temp, and we still ended up needing a new gib.

If you are ever lowering the knee and feel the resistance in the force needed to turn the crank change STOP and go back up before you jame things tight. Surprising ly the change felt will usually be the the crank will turn easier, once it gets back to turning harder is when you are really jaming things up hard. It gets easier because of the weight of the knee, as the tight gib friction builds it will start to bear the weight of the knee and the crank will turn free as it takes up the backlash in the knee drive, it will then get tight again as you are now power driving the knee further into lockup.


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## fillister (Nov 8, 2020)

Flyinfool said:


> Had that same scenario at work. If you never go all the way to the bottom of travel with the knee, make sure to clean the dovetails and ways to within an inch of their lives before you do.
> We had to do major disassembly of the machine, and lay it on its back, drill the hole for the drift, get out the BFH and still had to use a lot of heat to finally get it to pop. Heating was done with a 2 inch rose bud tip on the oxy/act torch and took a good half hour to heat up enough to pop. We had that center up to around 500°F and it took over night forall that cast iron to cool back to room temp, and we still ended up needing a new gib.
> 
> If you are ever lowering the knee and feel the resistance in the force needed to turn the crank change STOP and go back up before you jame things tight. Surprising ly the change felt will usually be the the crank will turn easier, once it gets back to turning harder is when you are really jaming things up hard. It gets easier because of the weight of the knee, as the tight gib friction builds it will start to bear the weight of the knee and the crank will turn free as it takes up the backlash in the knee drive, it will then get tight again as you are now power driving the knee further into lockup.


Flyinfool,
What you describe is exactly what happened, thank goodness that I at least stopped when it jammed and didn't continue to force it. I was dreading the thought of having to heat the casting or having a broken gib, if either of the last two items happened the mill probably would have been junk.


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## gonzo (Nov 8, 2020)

A mill where Ii worked had a tapered wedge in the ways that was adjustable to take out any slack. It had a notch in it that allowed a screw to move it in and out. 
 One day the edge of the notch broke off allowing the wedge to be forced ever tighter along the ways and thus rendered the table un-movable. 
We used the biggest guy around to smack the end of the table with a sledge hammer ( used a piece of wood to protect the table) to no avail. The table was stuck tight.
The maintenance supervisor ( me ) was summoned to come fix it. Alas!
I made a cardboard wall around the table, filled up with dry ice, and left it for a couple of hours. 
It worked, the table moved freely, and I was greatly relieved as I had no alternate solution.


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## fillister (Jul 9, 2021)

All its been 9 long months since this catastrophe happened, so I thought I would give a follow up.

Once I was able to get the knee gib removed, I thought that it was about time to put wheels on the mill and during this process I discovered that the rear of the casting was cracked in the same general location on each side. After pondering this for sometime the friend that helped me remove the gib had a friend that contracts with the city to perform magnaflux inspections and he kindly offered to magnaflux the areas in question for free.
Following his detailed inspection we were able to locate the ends of the cracks and I then drilled a .25" pilot hole followed by a .5" through holes at the ends of the cracks. 

Not one to let a simple repair remain simple I decided to install 2 - .375" thick 1018 hr plates on the inside of the casting to span the cracked areas. 
I drilled 11 - 5/16 holes in each plate. Clamped the plates in place and proceeded to use the plates as a drill jig to drill the casting for 3/16 allen set screws. I through tapped the casting which ranged from 1/2 to 5/8 thick, thoroughly cleaned and rough ground the inside of the casting and applied a structural epoxy to bed the plate and installed the fasteners but leaving then shy of the outer face by an 1/8". The outer face of the tapped holes and the hole that was drilled for the removal of the gib were patched with loctite steel epoxy and ground smooth.

Following this work I decided that the base casting needed to be repainted and the nicks in the filler material smoothed out with new body filler. Unfortunately no matter how many times I washed the the casting with acetone or lacquer thinner the new filler would not stick as the old filler seemed to be oil soaked. Again a simple task becomes more complicated, I had to completely strip off all of the filler using 3M clean and strip wheels on my 7" side grinder. After a through cleaning I reapplied new filler and sanded the casing smooth. Applied 2 coats of PPG epoxy primer and top coated with PPG Urethane enamel. 

For the wheel assemblies I cut 2 pieces of 3/8 HR  plate to fit  the contour of the base casting and bent the side support wings in my press break.  
I drilled the base plates to mirror the location of the base casting mounting holes as these will later be tapped to 1/2-13 and used for jacking screws with swivel feet. I then proceeded to fab up the mounts for the 700 lb rated casters from 3/16 CR plate and tig welded the wheel mounts to the base plates, this assembly was then primed and panted at the same time as the base casting.

Finally the mill is assembled, the only open item is to fit up the limit switch for the powered z axis. And this takes me back to what started this project, lowering the knee to the maximum to set the lower limit but I'll be much more careful this time. 

Thanks and keep your ways clean and lubed.


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## hman (Jul 9, 2021)

Wow!  Quite the journey, but t looks like you got there OK.


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