New screws

[wayne_h]

I'm new to the forum and new (8 months) to owning a 36" Bridgeport. I came to the realization I needed to adjust the backlash on the X+Y (backlash on both wass close to 100) so I watched the video and gave it a shot. The screws could barely turn, but I did tighten them as much as possible. No real improvement, and certainly not right. I removed the Y screw and clearly it is worn. I measured about 100 thousandths on the crest where it was not worn (on the ends) and 0.065 on the crest in the center. So, I have a new set of screws and nuts on the way. Most expensive screws I ever bought!

To change the nuts, is it possible to slide the table to the left without removing it and replace the nuts? Otherwise it looks like it weights over 250 pounds, so I need to rig an engine support it when I slide it off.

I've also read no need to worry about backlash with a DRO, but it is so bad when I machine something it "hops" or "jumps" even when I am going up or down hill. So I'm assuming I need to fix the play in table?

Thanks in advance for any thoughts.

Wayne

 

[Cadillac]

One easier way to remove the table is get yourself a cart that can handle the weight. Then roll the cart to the side you decide to take it off. Move the table to cart height via the knee crank. Slide table on top of cart. You could also build yourself a platform and do the same process. It would give you the opportunity to check ways and gibb also.
When you say the table hops when cutting. You should take the backlash out of the movement before the cut and not reverse movement “in” the cut. Which would introduce the backlash of the screw giving the jump. You could be seeing hop from a loose gibb adjustment. Has that been checked? Either way I think you’ll like a new screw and nut. Should be silky smooth as long as the ways arnt as worn.

 

[cathead]

Wayne, welcome to HM.


The "hops and jumps" occur only when climb milling. What happens is the cutter is advancing the table up to the amount of the backlash.
If you stick to conventional milling, this wont occur.

 

[RJSakowski]

Although the DRO will give true table position and you don't have to compensate for any backlash, it is still highly desirable to remove as much backlash as possible.

You can determine if the lead screws are worn and how badly by measuring backlash at the middle of travel and at either end of travel. The extreme ends of travel see relatively little use and are the closest to new condition. If you have a large variation in backlash between the middle and the extremes, the lead screw is worn. On the other hand, if the backlash is fairly consistent over the whole range of travel but is excessive, then the lead screw nut is worn. Most likely both are factors.

Gib adjustment can be a factor in backlash. If the gibs are too tight, an additional amount of force is required to move the table which winds the whole drive train up and increases backlash on that axis. On the other hand, if gibs are too loose, the table can move in the perpendicular direction (loose x axis gibs creates play in y axis) giving rise to measure backlash on that axis.

 

[Dynahoe Dave]

If the lead screws are not badly worn, look up how to cut the nuts also. This gives a lot more range of adjustment for the backlash. If the nuts are really worn, might as well get new ones.
I had never heard of climb milling until poking around here and milling machine how to videos on youtube. Tried it. It makes a much nicer finish on the part when you are cutting with the side of an end mill. I've been using conventional to get close, then take a .005" pass climbing to get the nice finish.