Z axis backlash

Q: What amount of mechanical backlash is acceptable?

Almost ready to start making chips with "this old Haas" my 1992 VF0 restoration project. While indicating the vice I decided to check the backlash on all axis. Comparing display to dial indicator the X and Y are better then 0.0005", I can get the needle on either side of the line, 0.0002" I'm guessing.

With the mag base on the casting / indicator to table, Z axis has a 0.0015" "pause" when changing direction. Using a lever to push up the spindle I get 0.0035" movement with force of 133lbs. Pressing harder makes no difference, releasing pressure returns to zero. Guess it is obvious that this 0.0035" is too much, just wanted an affirmation before I start taking back off the covers - at least they are clean

I also push-pulled with all my 200lb of self in Z and X and got not quite 0.001" of total +/- movement, not thinking this is excessive, right?

I have read all the posts about the thrust bearing, coupling etc. and will check all that just want some input so I can tell the wife I did not buy a complete POS and she's still getting the beach vacation this summer...
Thanks,

Yep, .0035 is too much BUT if that is your only issue, go tell the wife you bought a gem... listen, the heads are either supported with Nitrogen cylinders or counterweights and chains. Some people do not check the cylinder pressure and that will kill that screw and or thrust bearings in short order.

I personally have one that I think has thrusts out because I reballed the ball screw nut even WITH preload so guaranteed no backlash, yet I still have over .001 of lost motion and a growl in the Z.

Personally, I would slide down the Z screw covers, put an indicator on the end of the screw, and use a bottle jack to carefully apply some pressure and see if you can find screw movement. If that screw moves AT ALL, do the thrusts. I think they go out commonly on the Haas but I just do not replace anything on a guess when I can help it.

Those bearings are pretty easy to replace. Much better situation than the Y, that is for sure.. I would have to buy 2 cases of beer and take up weed smoking to get at that one. Who the F designed that???

I forgot to mention the servo coupler. They can really cause issues I guess but Haas does not use break away couplers so you should see damage if one is hurting. If bolts are tight, I would try to turn the screw by hand a bit with the servos lit and see if it wants to rotate at all. DO NOT put a big wrench on the screw with the servos hot. Good way to blow a fuse because that motor is trying to hold position.

Z axis backlash, the fix!

Well I got back to it today and pulled the covers to access the coupling. Under power I put a test indicator on the ballscrew side flange face and levered the casting, got a bit more then 0.002”. Decided to pull the thrust bearing assembly apart. Coupling looks perfect… a bit too clean and shiny compared to everything else, closer look reviled prying marks on the shaft thrust split collar nut - somebody’s been in here before! I killed the power and pulled the servo motor, when I loosened the split collar bolt I could easily tighten the nut about 30° I re-tightened and checked the ballscrew rotation - hard to move… that can’t be good. Pulled it apart, book actually has instructions that are fair. Thrust bearings look dry and dirty but cleaned up well, odd thing was the shaft at the lower thrust side had grooves worn in the surface almost like snap ring grooves, could not get the lower thrust collar off from the bur. I dressed it up with a file and cleaned it well greased the thrusts and reassembled and adjusted it for a very slight drag rotating but no axial play. Put the whole shebang together enough to test. Actually sounded smoother, checked backlash as before with lever and saw just 0.002” with considerably more force, sorry too lazy/tired to calculate, trust me it was more. Using the jog mode was interesting, when switching direction I would see 0.002” over compensation, as in down 0.001” back 0.003”. Checked the backlash parameter for Z and found it to be a value of 32. We then checked the X and Y and they were set at 3. Well all three axis are same pitch and steps per revolution, lets try that - perfect, go down 0.001” back up 0.001” same for 0.010”; sweet! I’m happy and wife is happy, even a bit proud of me - and as we all know, if mamas happy - everybody’s happy!

Thanks for the input Viper, gave me the incentive to attack! I got to say I’m not impressed with that thrust bearing, kind of wimpy and lots of room for a bigger ball type bearing ; let’s see what’s in the surplus box…

As I too have learned that initiative is half the repair. people love to waste money, even my own dad and brother.

Regarding wimpy bearings, you now know why some people insist in better machines. All those wimpy bearings add up to big savings on the machine build which means a lower price and more sales but no less margin than any quality build. I could make a DAMN long list of improvements with Haas. Actually, if they were local, I would probably be in Gene's office gaining employment and improving the line. I do like Haas believe it our not but some things about them just piss me off. We NEED a good USA machine builder you know. I really wish they could say it was made here but that ain't the case...