Bridgeport Series I Mill: Instability of x-axis while moving the knee.

Hello, everyone,

My name is Marc and I am a first time poster here. I'll get straight to the point.

I have been working on a Bridgeport Series I Mill (manuf. in 2000) with a 2J head for the last ~2 years. I do not own it, but attend an engineering college where it is housed. The mill has always turned out good parts (when a competent operator is using it of course ). However, recently, it has been doing something rather strange.

If I move the knee up or down, I will lose my location in the x-axis. For example, I will touch off an edge, set a zero, move over the radius of the edge finder, re-set my zero, and then will drop the knee to mount a drill chuck. When I drop the spindle with a bit in the chuck to check my location, it'll be offset from the edge by up to 1.5 inches!

This happens regardless of the table being locked or not. I do not believe it is an issue with the x-axis gib being worn, as I maintain stability in this direction while taking heavy cuts in either the X or Y direction (even while climb milling). I only lose my place when moving the knee.

What really stumps me about this is that the DRO and even the hand dial do not show that the table has traversed in X, despite where the center of the spindle ends up with respect to the work piece.

Like I said earlier, I do not own this mill. Regardless of this, I do my best job to take good care of it for several reasons. The first being that the school's technician responsible for this machine has been kind enough to allow me unrestricted access to it. I do not take this privilege for granted and am very grateful for the opportunity. Secondly, I see these machines as finely tuned works of mechanical art with astounding manufacturing capability, and appreciate them as such.

Despite my efforts to maintain the mill's health, I cannot always monitor it. Every now and again, the wrong person may find their way into the shop, and slam some worn out cutter into a piece of steel, throwing the head alignment into outer space. I am sure years of this have gotten to the poor thing, and some component somewhere has finally given out.

Hopefully someone has fixed a similar issue or can identify some component that is leading to this problem.

Thanks a ton for any advice that anyone may have!

My first thought (ok, it took a couple thoughts): The 4 bolts that let the ram rotate on the column are loose and the process of tightening the tool moves the entire head/ram assembly. I'm not sure why this wouldn't be an issue when milling in the X direction, but I'll start with it.

-You set a position, lower the knee, raise the knee, and the table has shifted in X (left or right) by up to 1.5 inches. Not 0.0015", but 1.5".
-When you say table locked do you mean X and Y?

Some options I don't think are it:
-For the handwheel to not move you either aren't actually moving relative to the saddle, or the nut/lead screw are stripped. In this case the DRO would still move, and I doubt the mill would cut in the X direction.

-The part you are measuring too, or the vise, etc. are not fixed to the table.

-I don't think there is 1.5" of X-axis movement in the saddle even with the Y-axis gib completely removed.

-When you say X, you do mean left to right yes?

-If you move it up and down without changing tools does it move? If so chuck up a long rod in a collet and see if it moving when you are lowering or raising. If not when you change tools without lifting it up and down does it move?

-

OldSpaghettiMike said:

it'll be offset from the edge by up to 1.5 inches!

Click to expand...

Stop right there before you get BURIED in irrelevant advice about minutia. Those fine details can come later.

What you need FIRST is simpler.

TWO sets of eyeballs on it while you are moving things about. Your and the tech you mention, for example.

A full INCH And A HALF is not just wear or gibs or leadscrews or wingnut politics or kinky sex.

Something BIG and UGLY is FUBAR. That much movement, the Mark One Eyeball should spot.

You need the second set of Eyeballs as an observer while you are cranking the vertical.

It should be obvious as sunshine or moonlight if only someone is LOOKING at it as it happens.

Find that first. Come back if the fix is not obvious.

You got a TURRET on that bugger that can swivel like a tank or a warship, yah?

Agree with Thermite. 1.5" is enough movement that a second set of eyes looking at the sliding surfaces (knee to column, saddle to knee, table to saddle) should easily spot something majorly wrong. That's a ton of misalignment!

But given that your post says that you swap from edge finder to chuck/bit I'm going to go out on a limb and say that what's changed is the length of your tool stick-out. And if the head of your mill is majorly out of tram (forward/back nod or left/right misalignment) that's going to show up as a major difference in where the end of different tools of varying depths intersect the table as you move it up and down.

As a quick check...extend your quill as far as it will go and check how square the quill is to the table at 3,6,9, and 12 O'clock. I'm guessing 1.5" of difference is going to show up really quick as the quill not being straight up and down.

Then tram your head.