Fadal VMC40 Y-Axis Screw Coupling Question

[nineandtwothirds]

Hey All,

I've been having some trouble with the Y-Axis on my 1992 VMC40. When rapids are set to 100%, it makes this strange shuddering noise when moving on Y. I'm also getting terrible position repeatability. When moving the table by hand, I noticed a "thunk" whenever I changed direction. I could turn the screw by hand and feel that it was "landing" on one side, and then landing on the other when I changed direction. It's not backlash.

So I pulled the Y-axis motor off and found that the screw part of the spider insert coupling has a woodruff key that orients it to the shaft, but is able to turn ~1/16 of a turn on the shaft, even when the set screw is tight. So after pulling the coupling off of the shaft, I found that the keyway on the coupling spreads out in the middle. I've tried to attach photos to explain what the heck I'm talking about.

I sincerely cannot tell if the "spread", aka the widening of the keyway, is machined or from some mishap or another.

Does anyone have experience with this? I'm tempted to put a new set screw hole in and re-broach a new keyway so the coupling can't spin on the shaft but if it's meant to have some slop, I obviously don't want to do that.

Any help is always appreciated.

Thanks,
Mat

 

[Milland]

I'm not too familiar with Fadals, but if this is a box-way machine I'd bet you're missing Turcite on one side (maybe both) on the guideways. If it's a linear rail machine, one or more trucks on one side may be bad and binding.

Either of these would put excess load into the ballscrew and coupling, so could strain the key with back and forth loads. I doubt the keyway wallowed out on its own...

 

[nineandtwothirds]

I'm not too familiar with Fadals, but if this is a box-way machine I'd bet you're missing Turcite on one side (maybe both) on the guideways. If it's a linear rail machine, one or more trucks on one side may be bad and binding.

Either of these would put excess load into the ballscrew and coupling, so could strain the key with back and forth loads. I doubt the keyway wallowed out on its own...

Thanks. It's a box-way machine. I would agree with the Turcite point except that it's so easy to turn the screw by hand. There's a 1" nut on the motor end of the shaft and I can turn it and move the table with very little effort. It's buttery.

 

[Milland]

Well, you get a "thunk" and terrible repeatability, but you say it's not backlash. Thrust bearings do go out on these, but that would lead to backlash and perhaps even "grindyness" on the ballscrew.

I would set up a coupe test indicators on magnetic stands mounted to the head, with the needles contacting a keyway in the table, perhaps 8-10" to either side of center. Then rock the table and see what sort of movement you get on the needles, I'm expecting some variation, not identical readings.

That would indicate wear/slop on the Y box ways from whatever reason. You can also drive the ballscrew in and out, look for identical or variations of reading.

You can try similar movement diagnosis along whatever axis and load, see if you get a pattern of unusual motion. Get a friend to help with inputting loads if you can't get sufficient force on your own.

 

[Vancbiker]

......I sincerely cannot tell if the "spread", aka the widening of the keyway, is machined or from some mishap or another.

Does anyone have experience with this? I'm tempted to put a new set screw hole in and re-broach a new keyway so the coupling can't spin on the shaft but if it's meant to have some slop, I obviously don't want to do that......

Uhhhh, you mean you can't tell that that thing is beyond worn out????

Toss it and buy a new coupler and insert. It's the same thing as used on lots of farm equipment so won't be too expensive.

 

[Bobw]

Itty bitty little keyway, and one tiny little setscrew. They wear and go to shit.

What I did when I had the same problem. Grind some flats 90 degrees to the keyway
on both the motor shaft and then the end of the ballscrew. Drilled and tapped the
couplings for 2 3/8" set screws and put it all back together. I put a little loctite
in there just to make sure it all stayed in one spot.

You could buy a new coupler, but I'd bet my left nut that the keyway in the end
of the ballscrew, or the motor shaft is also messed up, so a new coupler isn't
really going to help.. Just use what you have and beef it up.

 

[nineandtwothirds]

Itty bitty little keyway, and one tiny little setscrew. They wear and go to shit.

What I did when I had the same problem. Grind some flats 90 degrees to the keyway
on both the motor shaft and then the end of the ballscrew. Drilled and tapped the
couplings for 2 3/8" set screws and put it all back together. I put a little loctite
in there just to make sure it all stayed in one spot.

You could buy a new coupler, but I'd bet my left nut that the keyway in the end
of the ballscrew, or the motor shaft is also messed up, so a new coupler isn't
really going to help.. Just use what you have and beef it up.

Thanks Bob. I'll do that! I figured that was the issue but I didn't want to introduce a ton of rigidity.

 

[Vancbiker]

...... but I didn't want to introduce a ton of rigidity.

It's a Fadal. Anything you do to add rigidity will be an improvement.

Measure the bore in the coupler halves. Would not be surprised if they were worn oversize. If there is more than a thou or maybe 2 you ought to think about replacing them. If you are on a budget, shop this place for the sizes you need.

Shaft Couplers | Power Transmission | www.surpluscenter.com

 

[D Nelson]

I think the new style clamp on the shaft no key used they have less slop then that style you show
Don


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