WTB Presto 75 Dashpot/Damper Assembly

[lotusone]

The title says it all - recently picked up a Presto 75A which appears to be without its damper/glycerin dashpot.

If anyone has a spare, I would be interested.

*Alternatively if anyone has come up with a DIY replacement I would welcome suggestions.

[EpicenterBryan]

Actually, what you want is a 171-A dampener like shown. That one doesn't use fluid.

I need one of those too. Let me know if you find more than one.
Bryan

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[jesusfwrl]

A few pictures of the Presto 171-A "inertia" damper I currently have in my lab:

IMG_8553.JPG

IMG_8554.JPG

IMG_8555.JPG

It works reasonably well on the 75A with a Presto 1C or 1D. I could try to make one, but it would be much cheaper to adapt an oil dashpot to fit the 75A. I can machine something similar to the 170-B for anyone interested.

[symatic]

that thing looks crazy!

I might be interested in a replica dashpot if I can't make my Airpot work.

Anyone got any advice on how to measure the improvements/benefits? what should I be looking for in the sound when I cut with/without dashpot?

[lotusone]

Thanks - I am aware the 171-A was the Presto 75-A specific unit.

jesusfwrl

Thanks for those fantastic pictures - Two rather specific questions for you:

- Any chance you could weigh your 171-A?
- What is the material suspended on the inner portion composed of? I was under the impression that inner piece is a rubber-like material (presto-flex).

I thought about having one machined, drilled, and tapped. I also considered making one myself from a relatively dense wood/neoprene

[jesusfwrl]

Symatic, you cannot really adjust damping properly just by ear, unless you have super accurate full range monitors and a high-end repro setup. I do, but even then it is tricky. A very rough/crude adjustment can be done by eye, just look for strange radial spokes (moire pattern) on the disk. For proper accurate adjustment, you need a microscope on the lathe to visually inspect groove depth variations while rotating the disk, at recording pitch, lead-in, spiral, lead-out, locked groove and point of entry.

It should also be pointed out that the suspension system is a mechanical high pass filter. Purely resistive damping will not affect the frequency but will affect the Q. An oil dashpot is as close to purely resistive damping as you can get. An airpot, on the other hand, is reactive, since the air is compressible and acts as a spring. This will affect various parameters and is much more difficult to get right. Same with the 171-A.

Lotusone, Some pointers:
The outer shell is cast brass, possibly SAE No.41, as it was called at the time. Looking up the data of a similar modern material, is has a density of 8470 Kg/m3. Compare to American Coast Douglas Fir, with a density of 450 Kg/m3, to get a rough idea in you much bigger a wooden part would need to be...
The density figures are from the reference material available to me through my ASME membership.
In other words:



The total mass of the entire assembly is approximately 435 grams.
But this is not what you need to know. The material you see could be Presto-Flex, wrapped around a flat reed (steel spring). To make something functional, you need to know the compliance of the spring and the effective mass contribution of the suspended brass casting to the stylus tip. You need to replicate the mass/compliance resonant system as accurately as possible.

Even then, this contraption simply mechanically damps the resonance of the suspension mass/compliance system, calculated not in the air, but with the mechanical impedance (and damping) of a lacquer disk... Which means, it won't work the same for other blank types!

While I do have the ability to design a part that does the same thing, it would take a lot more time, effort and exotic materials than simply turning an oil dashpot out of aluminum and milling a bracket to fit the same mounting point on the 75A. Not only would it be cheaper, but it would also be easily adjustable to work with any type of blank. But if you prefer the "dry" version, I could make one, either to identical parameters as the original 171-A, or modified to work with the blank of your preference.

[boryo]

Guys, my advice is if you don't know what exactly you are doing and don't have all the tools to do it just let jesusfwrl do it for you, it will make your life much more easier

PS: this is not paid comment

[markrob]

It should also be pointed out that the suspension system is a mechanical high pass filter.

Hi,

I think you meant low pass filter not high pass. As you move past natural frequency, the suspension responds less to disturbances. It is free to move slowly.

Mark

[jesusfwrl]

Mark, well, yes, you are right, the suspension unit itself is a mechanical low pass filter, but the effect it has upon the recorded modulation on disk is that of a high pass filter, since by "responding to disturbances" near and below the natural frequency, it essentially absorbs part of the energy that would otherwise end up on disk as modulation.

Boryo, thank you!

[symatic]

I'm thinking the same as Boryo - I might have to get one of Jesus' dashpots!

unless there is an easy way for me to mod the airpot i have already? I only got it becasue I saw on here that they would send a free sample if you wanted to try it out so i went for it, and I can see how it works, but the spring effect mentioned above is obviously going to be a problem.

If it's as simple as filling the cavity up with some sort of oil i could try that , if not, how much for a dashpot Jesus?

thnaks for the discussion so far guys - I'm trying to keep up!

when you talk about "natural resonance" do you mean the frequency at which the mechanism resonates?

[markrob]

Mark, well, yes, you are right, the suspension unit itself is a mechanical low pass filter, but the effect it has upon the recorded modulation on disk is that of a high pass filter, since by "responding to disturbances" near and below the natural frequency, it essentially absorbs part of the energy that would otherwise end up on disk as modulation.

I get it. Thanks for the clarification.

Mark

[jesusfwrl]

Symatic, to make an airpot work with oil, you would need to somehow "extend" the cylinder upwards of the valve (screw at the top) so there is oil both below and above it, moving between the two chambers. As it is, it is missing a chamber...

This is because for air, the "second chamber" is the "big one", the world around the first chamber and valve, is conveniently filled with the same medium (air).

Another point to consider is that leakage from the piston seal in an airpot is not a big deal, since the room is full of air anyway. Fill the airpot with oil and you will soon find out how fast it can make the room full of oil...! : - O

I will calculate some prices for a dashpot and mounting bracket for the 75A and get back to you about that in a couple of days.

Yes, the "natural resonance" would be the frequency at which the suspension unit tends to wobble up and down.

[jesusfwrl]

I finally found some time to measure up the 75A and calculate what will be needed. The dashpot cylinder itself will cost GBP 85 and the mounting bracket will cost GBP 30. The cheapest option for a plunger is a non-adjustable type, with exchangeable pistons to achieve different amounts of damping. They can be removed and replaced with a simple screw, no specialized tools needed. This assembly, along with mounting bracket for the 75A arm, will cost GBP 55, including three pistons, so you can achieve the required damping for different types of blanks and styli.

Total cost, GBP 170. This dashpot will work with standard SAE10 monograde motor oil, as should also be used for lubricating the lathe. No need for glycerin or silicone oil.

I will happily also supply any of the individual parts for anyone wishing to use them on other lathes. This is the cheapest version I can think of. You can of course choose to mix and match parts, such as using this simple cylinder with the Type 6022 Adjustable Oil Flow Regulator Plunger Assembly (http://agnewanalog.com/products/6022.shtml), which is the "luxury version".

Please place your order via email, not via the forum.
Payment via PayPal preferred.

[Jccc]

so this Damper you made will fit on a Presto K8 lathe?

i am thinking of needing it for my 5C Cutting head that is mounted on the K8

[jesusfwrl]

It will fit pretty much any lathe with the appropriate mounting plate. I can either supply the dashpot assembly without the mounting plate and you can make this at your end or I can also supply the mounting plate as well.

[lotusone]

Have you finished any units? Any pics to share? Interested