4020 Z axis bearing carrier.

[little bubba]

About 3 months ago, I notice that the tool holders were not pulled up tight against the taper, so while having the sheet metal off and looking at that a part timer, jogged the table and cought a clamp on the Z ways and blew a driver board, so now the machine is sitting for four days to get the driver board. Oh yeah, the tool holders aren't tight, clean everything, and finally figure out that its the belleville (sp?) springs and drawbar. (the previous owner seemed to think that spacers should just be dropped in the hole when a spring broke, dumbass). Anyway after 8 days of sitting we finally get everything all set and the Z axis is making a whole bunch of noise. From sitting or whatever there is a whole bunch of rust all around the bearing carrier of the Z screw. A welfare stethoscope(screwdriver) check confirms this. So now check the backlash, its fine.

Fastforward to yesterday. Running a part that uses four tools to make up a flat surface, small radiuses and whatnot. It was the worst looking thing I've ever seen, remeasured tools several times and still the surface was all over the place. Some of the finish tools weren't even cleaning up the roughing. This sucks, so I found something else to do, bring in the swamp cooler, check out E-bay for some inserts, etc.. So this morning, I ran it again,(who knows maybe the underpants gnomes fixed it). Same thing. So I set the backlash back to zero and check it. .00186" fairly consistant through the whole Z. I did find out that the controller doesn't like the number 186 for backlash compensation, it seemed to think I meant zero.

Now for the actual question, I can get a new bearing carrier for $480, but can I just change out the bearings for less. I know its going to be a matched set of thrust bearings, I've done that before on a lathe. Or is it a sealed unit. Anybody done this before, I'm not too worried about the actual mechanical part, shove a rod through the counter weights, block up the head, rip the motor off and the rest should be cake. Anybody have any tips or tricks or anything that I'm missing?

[alloyspec]

Hi Little Bubba,
You are right, two ball thrust bearings and a seal and you are ready to go. if I remember right I believe it uses the same bearings, seals and plates for all the axis. The only difference is if you have liquid cooled ball screws as on a 6030.
While you have it apart check the depth of the bearings in the housing and compare it to the lip on the retaing cap. It is supposed to crush the bearings against each other. Be sure and follow the directions in your manual about blocking the head. If you don't have manuals I believe they are available as pdf files from the fadal website. If that does not fix the problem check the brushes in the servo motor. I hope that fixes your problem, if not it is could be the ball screw itself.

Alloyspec

[carbidecraters]

Another thing to consider is the condition of the taper...it may need to be reground. Heavy cuts and steel chips on the #40 taper holders can cause scoring when they are sucked up tight in the spindle. usually around $200 to regrind a spindle....cheap insurance and may give you a better finny. I am getting a crap finish even with round inserts so I may do this as well soon.

[little bubba]

All done, got the bearings out of Mcmaster Carr. $52 a piece. Fixed it right up, back to about a thou of backlash, I can live with that. I didn't do the seal, I should have. The bearings in there were rusted beyond belief, they literally fell apart. I'm guessing that water got in there before we bought it, they probably pressure washed it to make it "shiny". Now I found out that the chiller pump is only putting out about 3psi and sounding really crappy, I'll leave that one for another day. As far as the spindle taper goes, I'm sure it needs a tune up, but with the new belleville(sp?) washers and bearings, its cutting to size within a half and leaving a good finish, now tomorrow I'm sure something else will break.

[DareBee]

My chiller pump sounds horrible as well. I was concerned it was bad and replaced it with a reman' and it still is horribly loud and grindy sounding.
All you need to do to fix the "3 PSI" issue is remove the filter screen from the bottom rear of the pump and give it a good washing, system should run about 40 PSI. I have to wash mine frequently.

[Edster]

I need to replace the z axis thrust bearings in vmc15. There making a horrible intermittent squeal thats getting worse.

little bubba,

Do you happen to have the part number for these bearings from mcmaster carr?

When you removed the servo motor do you have to mark it's position and the position of the ballscrew to make sure there in the same spot?


Thanks
Ed

[DareBee]

The Z servo sits on a tri lobe shaft coupling it doesnt matter where your height is at when you reinstall the servo but you DO have to get the coupling back on in the same teeth slots.
If when you do a toolchange (after the job is done) the tool and changer don't line up height wise you have got the coupling back on in the wrong lobe. You will also see this deviation on the Z axis indicater after you do a CS. I know this first hand :-O

[little bubba]

Part# 6680K26

There are a couple of little shims in there that help preload the bearings, don't lose them. Also check your backlash before you button everything up, just did the Yaxis bearings and it took 3 shots to get the bearings loaded right.

On the orientation, you can be dead on or 180 off, which will cause fits with the toolchanger. Not a big deal to fix.

[Edster]

how did you adjust the bearing preload?

What's the procedure to check the backlash on z axis? I watched a tech do it once on my fadal a few years ago, but I don't remember how to do it. It had about a thou of backlash in z and about 2-3 tenths in x and y. I'm sure the z has more backlash now. I'm hoping a new set of thrust bearings will cure the squeal and maybe tighten up the backlash.

[little bubba]

Ed, pretty simple, but first let me say that the tapered bearings are usually bought in a matched set. Those bearings from McMaster are not matched sets, though they aren't too bad for the price. When you rip it apart you will notice that the factory bearings most likely weren't matched sets, since they (or at least ours did in the 2axis I've done so far) were shimmed.

Basically a matched set when placed back to back will have no backlash. These bearing need up to a three thou shim in the center to eliminate the backlash. I think this is why Fadal trys to sell you a whole carrier, they buy cheap bearings then pay some guy $6 dollars an hour to shim them, then stiff you with a high bill for crap.

I know for sure there are more surefire ways than what I'm going to explain, but it seems to work for me pretty well. First check what your backlash settings currently are. You can set it back to zero or just add what the settings are to what your getting. Slap a test indicator in the spindle and come down on some surface, 1-2-3 block, gage block, ground angle plate, something nice and flat. Come down on it slow in .0001 increments, until you hit zero on the indicator(or pick a random number if there is "zero" fanatic standing over your shoulder), then crank it back up and keep track of how many .0001 bumps you give it until the needle starts moving again. I know its not the Fadal way of doing it, but it will get you in the ballpark. Try it at a couple of different heights just to see how your ball screw is doing.

I would say if your over .0008, then you need to add a shim to tighten up the bearings a bit, or you have a crappy ballscrew, your current backlash settings should let you know were you are at.

Just be thankful your working on the Z, thats the easy one, the X doesn't look too bad, but the Y is a *****.

[MidfieldJake]

One other thing to watch for when replacing Z-axis bearings: If this has a "Dow Frost' spindle chiller, make sure dow isn't leaking on screw. We replaced bearings on our 4020 about 5 or 6 years ago, and discovered a hole in dow line was the problem... it was leaking coolant onto the ball screw which quickly wasted the bearings. We haven't had a problem since - and yes, we ordered bearings from a bearing company (just took old ones in and cross referenced them).

[little bubba]

Leaking on the screw, do you have the wonderful option of cooled ball screws, what a fricken waste. That is how the thrust bearings went bad to begin with. I haven't done it yet(no time), but I plan on unplumbing the screws and just chilling the spindle. I can understand cooling the thrust bearings, and cooling the ball nut(if the lightning fast rapids required it), but running that coolant through a spinning screw, now that is just begging for a problem. Even worse, they are all in series, starting with the screws, so one seal goes bad, no cooling on the spindle, you know, where it actually might help, as opposed to cooling a ballscrew that was sloppy to begin with and spins at about 1% of the speed.