cause of Z axis vibration

[dahui]

Hi folks,

Well my shoptask is "almost" there! Only problem is I made a new problem for myself when I replaced the mill spindle bearings. There has always been quite a lot of vibration when running the mill, causing a jittering Z on the DRO. I thought it probably had to do with all the pulleys and belts.

However, now that I have my VFD hooked up and driving the mill spindle with a single belt there is still a major vibration. As far as I can tell, it's caused by a sloppy fit between the splines on the spindle and the pulley. Basically it rattles. I've taken to jamming some shim stock in the gaps and it takes a lot of vibration out. I'd say the gaps range from .006-.010 and the more shims I jam in there the quieter it gets. That would be okay, except that I can't keep the shims in place if I move the quill up and down. Before I only had a major problem if I used a fly cutter, but now it just vibrates like crazy all the time.

Does anyone else have a loose fit? What can I do to take out the slop?

You can just grab the spindle in one hand and the pulley in the other and fell the "thunk" if you rotate them against each other. I'm not sure why, but it seems worse after my dissasembly/reassembly but I can't track down the problem. I've tried rotating the base with the four screws that bolt it onto the top of the mill head and also indexing the pulley on the spindle to find some "magic" arrangement to minimize the vibration, but there is no change. I've tried adjusting the preload from loose to tight and the only change is how hot the bearings get.

help!

[HuFlungDung]

If the spline on the shaft is straight and unworn, then possibly machining a new pulley would be the best solution. However, cutting a close sliding fit internal spline is a job for an expert with the correct equipment and know-how.

So what I would suggest is to purchase a new pulley, and see if the fit is improved just by replacement from a different batch of pulleys.

If that does not help, then perhaps you could make an anti-backlash pulley. Without seeing what space you have available, this could amount to cutting each pulley in half (by facing off half of the length) and bolting them back together with small bolts that lie parallel with the axis of the spline. If you make the holes slightly oversize in one half, then you can twist the spline hub a degree or so, thus reducing the clearance, and then tighten up the bolts to lock. When you have found a good setting that gives you quiet but smooth sliding action, then perhaps drill lengthwise through the hubs and pin the two halves of the hub together permanently.

[gerryv]

Possibly, the new bearings resulted in the spine being better aligned (maybe not exactly the correct term here) meaning that possible earlier misalignment "hid" the slack which is now showing up more. Trouble with splines, like many other things, it's a lot harder to make them tighter than looser. If the pulley doesn't work, maybe finding a quality supplier for one/both of the male/female spline mating parts? Just a thought.

[dahui]

Thanks very much for the tips. I contected Shoptask and they felt that my pulley was out of spec after I sent them the measurements. So, it looks like I'll be sending the part back for warranty. However, I'm a bit dubious that this will work since the splines could be a little off as well.

I may have to think about splitting the pulleys but I think I'll need two of them anyway. The splines only cover about an inch of the pulley stack, so getting that to work would be pretty drastic and tricky. Would be great to have it work properly though!

Thanks again and I'll let everyone know what works or what doesn't. I'm still curious to see if anyone else has this problem.

[salukikev]

Hi-
Just getting around to posting here, but yes- I know just what you mean about the Z vibration. I have the same issue, but I would say mine oscillates a total of .015" on the DRO, but of course its hard to tell as the #'s flash so quickly.

I've also had the same theory of something being up with the pulleys. I haven't had much opportunity to use my shoptask yet- but I already have my parts to CNC retrofit. Still working some kinks out of the benchtest, but it would be nice to rebuild to dial in the #'s before I get to CNC'ing it.

Anyway- I wish I had a solution to offer, but I don't. The problems I've had so far besides that include the following:
1. Leaking oil- I thought it was from the clear oil sight plug, but I keep on tightening it and it keeps leaking very slowly from somewhere.
2. The Y power lever- As the post is pressed in- it slipped loose. I took it to work and tig welded it in place and reassembled. I think it's cool now but not quite sure- seems like I can over-advance the driving gears if I rotate the handle too far (sliding them over too far one way or the other).
3. The Y axis seems really tight. I took apart the guides and various parts, thought I'd smoothed it out, but especially toward the limits of its motion, it continues to have a lot more resistance than the X.

First things first though- I'll be interested to see how you solve you Z issue. Maybe I need to contact John about this. Thanks for the info!
-kevin

[gerryv]

Is there too much play for a dose of Loctite (non-permanent). You couldn't slide for the test but it would maybe tell you if it's spline backlash or not as long as the startup torque didn't crack the bond right away. - Gerry