Twist in a Bridgeport table, Y axis

here is the problem. A few weeks back I started having a bit of torquing twist in a bridgeport table in the Y axis. By this I mean that moving the cutter in a slot going Y positive, the slot would machine well. Returning the cutter, moving it .005 offset to climb mill in return, the slot would develop a .013 taper over one inch.

I put two magnetic indicators, 1 inch travel, on the ways and measured the table movement. Moving Y positive (cutter), the table would remain at "0" on both sides of the table. Returning, the table would flex, thus the right indocator would stay at zero on the return over 1 inch travel, but the left side indicator would be .013 behind. This indicated a gib issue, the gib being on the left.

I tried adjusting the gib, but itt only tightened the machine operation. I got it to .003 doing this, but the machine was so tight it acted like it was "locked".

The next step was to take out the gib, clean all things out extremely good and proper on the machine and the gib. The turcite is undamaged from my indication. Tightened the gib,it traveled well beyond the back wiper plate. Thus a .003 shim behind the entire length of the gib (plastic shim) behind the gib. Got it to .001 difference, but still tight. I took another gib out from a similar year /make machine, the measurements are standard between them, maybe .003 difference on the large end.

This issue occured quite suddenly. Two of were teaching in the shop at the time, so I am not sure if the machine was abused during the time the other teacher was there, but I tend not to believe this as he is extremely trust worthy. My other bridgeports (I have six total) measure maybe .001 to .0015" twist/torque.

I also checked the screw mount all around. It is not loose as far as i can check it without pulling the table completely off.

I am in a hiatus where i can really spend some time on the machine fixing it now. I am open to ideas.

Rotation around the Y (yaw) axis is a classic product of yaw tracking error. The obvious fix is to adjust the saddle gib to take out excess clearance.

If your machine has any wear on it you'll first have to consider the "hour glass" in the saddle dovetail. I would adjust the saddle to about 2" short of full travel, remove the felt wiper guards (good time to work over/replace them) and tighten the saddle gib snug (remove, clean and replace is also good) then back off the jib 1/16" (corresponds to about 0.0013" or so).

Crank the saddle full travel. If it snugs up a little at the ends of the travle you did it just about right for a worn machine. Center the saddle in its travel. Set a couple of indicators to sense saddle rotation and try to muscle the table clockwise and counter clockwise. Note the indicator readings for clockwise Vs counter-clockwise. Separate out parallel movement. The difference in +/- readings indicates yaw tracking error.

If your adjustment results in this reading being 0.002" or less, rejoice. If it's more then your machine is showing significant wear and you need to consider expedients or a re-scrape.

Among the expedients is riding the saddle clamp to maintain a consistent clearance. You'll get slightly rainbow fratures but usually you can live with them.

Liberally oil and replace the way wipers.

You should consider working over the table and the saddle gibs too. It's a once a year maintence thing.

you da man forrest

Don't forget to check and adjust the gib for wear on the 'x' axis also. Both can contribute, and maybe you're getting that last little bit of taper that you can't seem to eliminate from the 'x' axis, not the 'y'.

Edit: Oops, looks like Forrest mentioned that at the end.

forget about the gibs

take out the y-axis screw and check all the bolts to be in good condition and then threads where ther bolts go in as well

then reassemble using good screws and threads

i take it you know the rest on how to do it

the problem is/could be a lose screw in the spindle assembly

if the spindle can move even a little bit, the nut will act like a washer on a rod and cause all kinds of problems

if only one of six machines has developed this problem
take a look at you oiler