Vacuum Platter on VR issue

[PTV]

Hi! 
Wondering if anyone could throw some ideas at me for troubleshooting an issue.


I’m getting bizarre rapid vertical movement on the same spot of the record for about 1 cm of the cut every time. You can see the movement on the VU meters and it shows up as visible kind of blotches on the disc. With any audio coming through it’s causing occasional skips too.



I’m using a vacuum platter from Agnew Analogue on a SP-10 on a VR. There’s less platter runout than any of my previous cutting setups and with the vacuum platter engaged the blanks are very flat. My initial thought was that maybe there was some gunk built up on my lead screw but the weird thing is that when I don’t use the vacuum suction and just clamp the blank down, the problem goes away. (of course then I don't have the benefits of the vacuum platter)



I’m using a Souri cutterhead and it is parallel to the blank. I’ve tried repositioning the head and recalibrating/using more and less weight - still the problem persists. Also, I’ve been using this vacuum platter several months now and this was not always happening.

I’ve changed the dashpot oil and have even tried a few different viscosities/types.


Any suggestions would be greatly appreciated!

[tragwag]

it sounds like a physical problem to me, maybe the extra platter height is causing the chip tube or positioner for the chip tube to rub against the turntable at a certain diameter.
Either that or tension from the chip tube, those are my thoughts because of the intermittent issue, and the issue not showing up when the vacuum isn't used.

let us know if you sort it out!
best of luck

[Thelatheofus]

I've had similar issues. They seem to be weight related. It makes regular patterns on the record, and the groove width changes (and the cutting head vibrantes). They pitch of the playback audio changes aswell. They happen more often at the end of the record. I thought they were alignement issues but I'm pretty sure it's not the case. At some point I thought they were related to suction issues but I guess also not. I have an oil dashpot but I doubt that it's causing issues

When changing the rake angle (by changing the angle of the cutting head) the issue is reduced a lot, but not always fixed. I guess it's because a bigger rake angle applies more resistance on the record.

I'll try changing the way my suction tube is connected and its material, because that's the biggest suspect I have for weight changes on the cutting head after eliminating other possible causes.

If you share similar symptoms let me know, if I find a solution I'll post it here !

[PTV]

I've been fiddling with this issue and it is truly puzzling.
The only thing I can think of is that perhaps my oil dashpot is not working properly and when the suction is engaged, the blank is rigidly held on the platter and there’s nothing to give, absorbing the oscillation. Without suction the blank edges (outer 1 inch or so) are floppier, so they absorb oscillating tendencies.?

Do y'all have a specific dashpot oil you recommend? Maybe mine isn't thick enough..

I've been cutting full time for 8 years and feel like I have a pretty good grasp on how the VR works so this is really quite frustrating as I've tried all the usual troubleshooting methods several times - different rake angles, different weight/depth settings, different heat settings, viscosities of dashpot oil and even different amounts of suction being applied to the platter.

The platter is calibrated such that it is flatter than any platter I've cut on before - very little runout. Also, the plunger in my dashpot is not hitting the bottom or coming too close to the top..

Any suggestions are greatly appreciated

[PTV]

Resolved!
It seems that one of the bolts that holds the carrier to the lead screw had been slightly stripped last time I took the whole thing apart so there was a tiny amount of play in the whole thing which meant that any tiny oscillations in the platter were causing the the head to wiggle ever so slightly.

I fixed the stripped bolt hole, fastened everything in nicely and the issue was fixed

[spinnertownblanks]

If it isn't too late, post images of the sections that you fixed, that along with an explanation might be helpful for other users