There are two adjustments on the vertical slide to compensate for wear.
There is a tapered gib that controls the slide "rock" from side to side. This gib adjusts the space of the slide between the box ways, and it is on the operators side of the inside of the boxes. You remove or grind the shims under the holding strap connected to the gib. This allows the gib to move up and makes it tighter. Before making this adjustment you should check the wear on the machine. Use a good tenths reading indicator and position it so that you can read the top outside edge of the vertical casting (not the table). Position the slide at the top of the vertical travel and run the table full right set the indicator to zero and move the table full left. Note the indicator movement. Move the table down on the vertical and repeat the test.
Do this in at least 3 places along the vertical ways...try to do one test where you normally use the machine most. You will likely get the lowest reading at full down and full up. Adjust the gib to get something around .12-.15 mm total movement max. Of course if trhe box ways are worn you will get more where it is worn. Like any machine you can't over tighten the gib to compensate for a worn spot if it makes the non worn points too tight!
As to wear that makes the table lean down at the front edge, this is a natural progression of the Deckel design.
There are two issues. If the out of square is significant the ways in the sliding casting have almost surly become worn. They tend to wear in a barrel shape staying relatively unworn in the center and most worn at trhe bottom. The cure here is to scrape the worn surface flat again. If large amounts are required to accomplish this then a fitting or Moglice or Turcite to bring everyuthing back to standars height will be required.
The clearance of the vertical slide in the front/rear direction is controlled by the plate gibs bolted to the rear of the slide. These (on the FP-NC's) are fitted with Turcite where they touch the box ways, and steel where they fit up to the slide face. The only way to adjust these is by grinding or scraping.
The drill goes like this....
Setup the plate on a surface grinder with the outside up. Check to be sure trhe plate is flat and the grinder is not goint to pull the plate flat by the magnetic chuck Any "pull down" here will translate into a curved and non flat way surface when you are done.
Grind the back side of both plate gibs, just to clean up.
Turn over and grind the inside surface to male the Turcite flat and remove any wear ridges. ...grind only the turcite. Zero the depth dial on the grinder.
Carefully measure the height between the back surface of the box way and the mounting face of the vertical slide. Check in several places and be sure you have the slide pulled up to the box way with some clamps and wood to protect the sliding surfaces. You are looking for the lowest number. (as the slide wears the value here will get larger)
The measured value between the box way and gib mounting face of the vertical slide is the value you need between the Turcite on the plate and its mounting face. Finish the plate gib by grinding the steel mounting face to get the measured value plus clearance. Somewhere around .001-.0015" of clearance is fine if the box ways are not too worn...allow more if thinfgs are not so good.
Be aware that if the position of the vertical slide is changed any amount (scrapingf it to correct wear) the position of the "Z" axis ball screw will change relative to the base mounting and cause it to run at an angle...this is not good.
The screw position can be adjusted by "floating" it with the vertical slide as far down as possible , blocking the slide , removing the servo belt, loosing the retaining bolts to allow the screw to move where it wants, then re tighten the bolts and finally, ream the dowel holes and fit OS dowels......
Cheers Ross