Stylus Rake Angle

From what I understand, the cutter head angle may be a particular value, but the resulting profile on the record is different due to the springiness of the lacquer material which varies with temperature, such that the resulting optimum SRA is not that at which the record was cut. Variations in modulation mean that the cutter has to dig more or less deep, thus varying temperature and hence the profile, so that an optimum SRA at one point on the side may not be so at another. Would that be correct?

No- but that is a common myth to which I subscribed before I started working with the cutting lathe. There really isn't 'springiness' in the lacquer surface. When modulation varies, the groove depth stays the same and the two can be set independently. The heated stylus cuts through the lacquer like butter if its set correctly - if not you get surface noise.

Csontos, I have been flying kites for some time :) seriously! But you are correct- if you can set SRA/VTA on the fly. LPs thicknesses vary as do the cutter angles. There is no other way to do it IMO. I don't think the thing you mentioned about standard cutting angles is correct though. What I have seen is that the cutter stylus needs a slight amount of rake (varies with each one as I said) and its really in the 1-4 degree range off of exactly perpendicular to the LP surface, so about 91-94 degrees. What we really don't know because no-one has really studied it is whether or not the best SRA position in playback is actually the same as the cutterhead was set to... I suspect that that varies with the individual cartridge.

I agree.

Atmasphere,
Surely the groove depth changes with signal? A right channel signal for example must cut into and out of the lacquer at 45 degrees. I can see that a heated cutting stylus will help the process, but then does that affect the final profile as the material cools?

Lewm,
The VTA is the angle from the cantilever pivot to the stylus tip relative to the record, which ideally should be the same angle as the equivalent angle a line from cutter pivot to the cutter stylus makes with the lacquer if there was no "give" in the material. Both can vary. Apparently there could be 10 degrees of a difference depending on the lathe.

But the cartridge is designed such that for an optimum downforce the generator is centred, and the resulting cantilever angle is the VTA. So the angle of the stylus relative to the cantilever is fixed and the two must vary together.

Now while the stylus might be set at 92 degrees, the cantilever may not be at the same angle as the the cutter. In which case there will be a distortion in the playback, similar to horizontal tracking error. Any adjustment to set VTA - the cantilever - to a particular angle will change the stylus rake as well, perhaps away from the average 92 degrees.

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You are mistaken. VTA has nothing to do with the angle of the cantilever. The angle of the stylus relative to the cantilever can vary wildly from one cart to another and therefore result in huge differences in cantilever/surface angles. VTA is set by the cutting stylus itself. Therefore there doesn't need to be a reference to VTA adjustment at all. It's done with as soon as the laquer is cut. The pertinent function to play back is SRA, the thing that follows. The fact that the cantilever angle changes with SRA adjustment is incidental.

Surely the groove depth changes with signal? A right channel signal for example must cut into and out of the lacquer at 45 degrees. I can see that a heated cutting stylus will help the process, but then does that affect the final profile as the material cools?

I used to hear about this thing called rebound associated with lacquers but yet to experience it. Yes, the groove wall is cut at a 45 degree angle, so if you were to cut an out-of-phase bass signal, the groove would indeed modulate up and down. Of course, the needle would not be able to track it, and such a signal fortunately does not exist in nature. If we encounter studio productions where out-of-phase information is a problem, we have a simple passive processor that takes care of it. So groove depth can be considered a constant.

Another way to look at this is if the cartridge body rises higher, that is a change in groove depth, if the needle simply traces a signal, the groove depth has not changed.