What dashpot oil to use is something we get asked about a lot, so we thought we'd give a little info based on what we've learnt over the years.
Firstly it is generally accepted that silicone oil is likely the best option as it maintains a very stable viscosity across a wide temperature range, unlike mineral oils which thin out significantly when warm. This means your damping will stay consistent whether your studio is cold in winter or warm in summer. Also it doesn't oxidise or degrade over time like petroleum based oils.
We have found that dashpot oil needs to match the type of suspension your lathe has. We have tested this on entry level diamond cutting lathes, like the Vinyl Recorder T560 and the MC-02. Below is what we have discovered over the years. As a point of reference, we have found that Neumann systems will often use 1000 cSt silicone oil. We will talk more about this later.
Vinyl Recorder T560:
The suspension on a VR is what we call 'front loaded', this means the dashpot and spring suspension all pull forwards with the head, we find this type of system needs thicker oil for optimum damping. We found 5000 cSt silicone oil worked best, and by our testing we found that 5000 is about the same viscosity as the oil the VR comes with.
MC-02:
This has what we call 'seesaw' suspension where there is a central pivot point, with the spring inline with the pivot and the dashpot behind this, so the cutterhead one end, spring and dashpot behind. From testing we have found 2000 cSt or 3000 cSt work the best.
As we mentioned previously, from what we've been told from mastering studios, Neumann use 1000 cSt, this is thinner and likely due to the fact that they are professional systems with sophisticated suspension, along with being designed for lacquers, which are 'usually' dead flat so less damping is required.
Other lathes like Presto's get upgraded for cutting all the time and Jed in Poland makes a great dashpot system for them, with them being front loaded systems we would assume closer to 5000 cSt works best, but we can't confirm this ourselves. However if you are unsure where to start, 3000 cSt is a reasonable middle ground and can be fine tuned from there, keeping in mind the suspension type will ultimately determine what works best for your specific lathe.
We should add, each of these systems has a rear counterweight, when we use terms like front loaded and seesaw, we are talking about the rest of the suspension, the spring and dashpot.
Alongside dashpots, many people are using Airpots, however we have no experience with these so it would be irresponsible to try and give info on these, but maybe someone with experience with them could make a guest blog post!