Half Nut Adjustment VMS-70

[concretecowboy71]

Can anybody tell me how to adjust the half nut on this?

I did it last year and Albert sent me a copy of the Neumann instructions for it and darn if I can only find half of the sheets...of course not the half I need.

I am getting a small irregularity in my grooves.

Even un-modulated, the head will drop, cut a few grooves and then I will see a groove that dives deeper and then shallow. Sometimes this results in a lift out, sometimes not.

I have realigned the suspension box, cleaned the pot for depth adjustment, cleaned the lead screw, reseated all of my pitch and depth cards, tried different setting for the dash pot and cannot seem to make it go away.

Any ideas out there what might cause this?

[jjgolden]

So when cutting a Un-modulated groove, do you see any erratic movement on the depth meter?

Check the audio connections and cabling at the preview Mod left in for loose/cold solder joints/shorts.

Is the spring/depth adjustment screw in the suspension box working correctly and free from any binding or null spots? I believe there's a small set screw at the back/top of the suspension box that holds the depth screw assembly in place. You could check that it's secure.


let us know what you find!

JJG

[opcode66]

Here is something I noticed on my lathe. The vaccuum hose I had was fairly heavy (thick rubber). And it was rather springy. As the cariage moved towards the inner diameters there would actually be some resistance to the movement that was caused by the hose. This actually caused cut grooves to jump a little. If you looked at them on scope there appeared to be a sort of S shape in the groove at the time when halfnut engage enough to push the carriage forward. Of course you could hear these points in the cut as small pops.

Once I replaced the tube with something light my carriage moved freely back and forth and with no noticeable resistance. Never had that problem again.

[concretecowboy71]

I have redone my tubing so nothing drags on the back, including my helium tube.

The lathe does this even when the amp rack is turned off, I have reseated all connections into the suspension box.

I have watched the depth gauge closely and do not see any movement when cutting with no signal.

I have tried to lubricate the bearings in the suspension box, but the one farthest inside is hard to get to.

Thanks for the suggestions, I can still cut, but this causes blown lacquers more than I want, so I will keep working on it.

[Aussie0zborn]

Where's Mossy when you need him? This sounds similar to an issue he solved on one of his lathes not too long ago.

[dietrich10]

The internal pitch computer of the VMS70 is notorious for capacitors etc going. is there a chance it is something in there?

[concretecowboy71]

I have reseated all of the cards in the pitch and depth section of the lathe.

If a cap went bad, it seems you would see it on the depth gauge as some kind of blip or movement.

I could be wrong about that however.

[gold]

Does this happen in only one spot or every time you drop the head no matter the diameter?

[concretecowboy71]

Over time it seems like it happens mostly on the outer edge of a 12" master plate, but after doing a lot of test cuts, it happens at all diameters and seems to happen 2-5 revs after the head is dropped, either manually or auto.

Gotta Groove started picking up the flaw a while back, skips near the lead-in and I have been seeing a pattern start to emerge. The pattern being that it happens just a few grooves after the head hits the lacquer.

[opcode66]

Some sort of build up on your lead screw? Dirt, debris, oil??? Or, worn in Half Nut???

[concretecowboy71]

I cleaned the leadscrew.

I am looking for the sheet that describes tightening the half nut. Al sent me one and I can only find half of the sheets and the one I need is not the one I can find.

I am bummed because I never throw away equipment info. I have manuals for stuff I don't even own anymore!

[gold]

I have sort of the same thing but to a much lesser degree. It's lateral instability a few revolutions after the head drop.

When you play around with the head suspension do you notice trends? Something like, if you loosen it makes it worse but if you go tighter it makes it better.

The general thing with the half nut is to make it as tight as it needs to be but not any tighter.

[concretecowboy71]

It is weird. If I change the dash pot setting, sometimes it seems like it helps and then it comes back. I have only spent a couple days so far trying to troubleshoot this. I will keep a better record of events in my head, now that I know I am looking for this. I inspect every drop now to see if it is there. Added work, but better than doing recuts down the road due to a skip in the lead in.

One question I do have in regards to the dashpot is:

When the head drops, should it bounce at all? I have noticed that at looser settings, when I look at a cut lacquer, I see the drop, a space and then the groove picks up again. Should I tighten that up until the bounce goes away? It seems really tight when I do that.

[gold]

When the head drops, should it bounce at all?

Mine does. I like the suspension a little loose. I find it rides over variations in the lacquer better. You also get less depth variation between the outside and inside diameter.

[fraggle]

bouncing? how does your stylus look like?

[gold]

It looks like sapphire with a facet ground into it. What does yours look like? If you are implying that it damages the stylus, it doesn't. The only downside is that that it can go deep and weaken the edge of the stamper.

[fraggle]

"It looks like sapphire with a facet ground into it"
Are you sure you cant break the tip if it bounces? That happened to me as well and I actually thought it was the reason for a broken tip i had.cheers

[opcode66]

I'm sure that Mr. Gold has a stylus scope. You can pretty easily see damage to the stylus using such a scope. So, if he says no, I would trust his assessment. I can say assuredly that Mr. Gold know what he is talking about and then some.

I tend to find too little damping will allow the head to move too much and it translates into a wooshing noise when cutting. You know when you've adjusted you damping correctly because things are working as expected and there is no noise in an unmodulated groove.

[concretecowboy71]

It seems that after a lot of cleaning and lubrication I have got this under control.

I cleaned the spring area that applies tension to the head suspension and lightly oiled a small bearing right up front where the spring attached to the head suspension.

I am running my dash pot pretty open. Not all the way, maybe about 3/4 of a turn from full open.

Yes, the head does bounce once when it drops, but I have never seen any damage to the stylus.

[fraggle]

ok that's interesting. cheers