Vacuum System Limiter

[John Giem]

From my reading in the Forum and personal experience, most Turners adjust the vacuum level in their vacuum chucking system by manually adjusting a blead valve. This is an 'open loop' method in that the Turner must monitor it and adjust if it changes. In my own work, I have observed that the vacuum level can change over time. I have developed a Vacuum Limiter that when once set it will limit the maximum vacuum level. It is simple to use, just turn the knob to the level desired. Constructed using easily obtained hardware from the local big box store. Question: Is anyone interested in this kind of a tool? Would anyone like to see me write it up and have it published?
John Giem

 

[Greg Ketell]

From my reading in the Forum and personal experience, most Turners adjust the vacuum level in their vacuum chucking system by manually adjusting a blead valve. This is an 'open loop' method in that the Turner must monitor it and adjust if it changes. In my own work, I have observed that the vacuum level can change over time. I have developed a Vacuum Limiter that when once set it will limit the maximum vacuum level. It is simple to use, just turn the knob to the level desired. Constructed using easily obtained hardware from the local big box store. Question: Is anyone interested in this kind of a tool? Would anyone like to see me write it up and have it published?
John Giem

Absolutely!

 

[Bill Boehme]

From my reading in the Forum and personal experience, most Turners adjust the vacuum level in their vacuum chucking system by manually adjusting a blead valve. This is an 'open loop' method in that the Turner must monitor it and adjust if it changes. In my own work, I have observed that the vacuum level can change over time. I have developed a Vacuum Limiter that when once set it will limit the maximum vacuum level. It is simple to use, just turn the knob to the level desired. Constructed using easily obtained hardware from the local big box store. Question: Is anyone interested in this kind of a tool? Would anyone like to see me write it up and have it published?
John Giem

I will agree that strictly speaking a constant orifice is not an active feedback element to maintain a fixed vacuum level, but it is good enough for practical purposes. For very small orifices, the behavior approaches that of a perfect flow control valve that maintains a constant flow rate regardless of the pressure differential. For quasi steady state systems such as a vacuum chuck system, a flow control valve operates to perform essentially the same function as a pressure control valve which is our intention -- maintaining a reasonable degree of pressure regulation. For vacuum chucking systems in particular, precise pressure regulation is unimportant since we only use ballpark guesstimates anyway for the vacuum level based on our experience and that of others.

You mentioned that over time, the vacuum level can change. I presume you mean that as the bottom of a turning is being finished, the amount of leakage through the wood and around the seals will increase slightly and result in a slightly lower holding vacuum level. I will agree, but I have never observed much of a change -- maybe two or three inches of mercury at the most. However, it is a certainty that the change will be to a lower vacuum level and not a higher vacuum level. You state that your device will limit the maximum vacuum level, but that is not a problem since a simple bleed valve already accomplishes that function. I would be more impressed if your regulator could control the minimum vacuum level, but that could only be done within a very restricted performance envelope since the pump being used has fixed vacuum and flow rate performance capabilities that would restrict the performance improvement to a small region.

 

[John Giem]

Bill,
I appreciate your insight.
I don't know if the correct name should be 'limiter' or 'regualtor'. When in use, the amout of 'leakage' it provides will increase or decrease to keep the set level relatively constant. In one case, if the leakage through or around the bowl increases, the device will reduce its leakage to compensate. I have also observed mounted bowls with imperfect seals which resulted in levels around 15 in Hg. By the time I finished sanding, the level increased to around 19 to 20 in Hg. This device will help with that type of change also.
I do agree though that most of us are just guessing at what reduced vacuum level we should use. First, our gauges are not closely calibrated and second, we don't know the actual vacuum level that would cause our bowl to fail.
John

 

[Bill Boehme]

... I don't know if the correct name should be 'limiter' or 'regualtor'. When in use, the amout of 'leakage' it provides will increase or decrease to keep the set level relatively constant....

The function that you describe would be a pressure regulator.

 

[john lucas]

I would be interested. I've almost imploded a vessel once when the walls got thin on me and a dip developed. I felt the unusual vibration and stopped the lathe. there was a dime size divot where the wall was collapsing. I reduced the pressure and it popped right back out and let me finish the piece.

 

[Bill Boehme]

I would be interested. I've almost imploded a vessel once when the walls got thin on me and a dip developed. I felt the unusual vibration and stopped the lathe. there was a dime size divot where the wall was collapsing. I reduced the pressure and it popped right back out and let me finish the piece.

John, I think that you might need a "thin wall detection device" to control the applied vacuum. I get the impression that the depression (forgive me for the alliteration and punning) is the result of turning the bottom very thin (thus mandating the use of a much lower vacuum level) and not that the applied vacuum level had changed from where it was originally set.

I don't think that you could convince me that a vessel that is thin enough to noticeably flex without breaking will have less leakage through the wood than it did when it was much thicker. On the other hand, it could conceivably conform closer to the seal if it originally was not perfectly round.