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Vacuum gauge/vacuum issue

Nick Valenza

Nick Valenza

Joined
Sep 29, 2021
Messages
105
Likes
198
Location
Hatboro, PA
Hey need some help Here. Yes I know my vacuum pump isnt the highest quality. 4.5cfm off amazon. Found here...4.5cfm unit off amazon. Oil level looks good. But thats not the direct issue. The gauge is showing -4hg without power to the pump. And I was able to pull off the piece with my hand without much struggle. There are no cracks or checks. It's a solid piece of hollowed out livedge dogwood urn. See video link. In my degassing/vacuum chamber there are two vacuum gauges. One was alittle more expensive from BWW and the other was from amazon. So my question... why do the vacuum gauges not go back to 0 when there is no vacuum, why was the expensive gauge no working at all, and why was I able to pull off the piece off the vacuum chuck when I pump was on (chuck has good seals).

Vacuum setup at lathe

Vacuum chamber 1

Vacuum chamber 2

thanks everyone!!

Nick
 
The gage not going to zero with the pump off means the gage is damaged. The net reading from the bad zero point to the peak is still a reasonable value, just low. This suggests a weak pump. I didn't see any filters protecting the gage or the pump so any sort of dust or debris will potentially cause harm.
The vacuum you are seeing with the pump running and nothing on the lathe is from restriction caused by the line itself. This vacuum restriction would read even higher if the gage was closer to the pump since your lines are very long and small diameter. With nothing on the lathe, there is still resistance to air flow through the tubing. This is the vacuum you are reading on the gage (ignoring the fact the gage no longer zeros out). With the piece on the lathe, the vacuum goes as high as possible since there is soon no air flow. Tube diameter no longer matters for all practical purposes when a piece is mounted on the chuck.
The fact you can pull the small piece off the vac chuck is not surprising to me. All it take is a minor leak (caused by you hitting with your hand) and the piece will always pop off. The chuck's net diameter (minor diameter of the inner seal) appears to be relatively small. Couple this with a less than strong vacuum pressure as indicated by your gage and the force on the piece could easily be 30-40 lbs or less. My pump pulls a strong vacuum and it is still relatively easy to pop a bowl off the chuck.
 
I agree with Dwayne and Mike. The small very long hose causes restriction to air flow, combined with the collapsed hose as it approaches the headstock (nice catch Mike) would mean the vacuum pump is likely pulling vacuum but the restrictions mean the pressure drop at the fixture is much much smaller than it should be. I would use larger diameter stiffer tubing (nylon reinforced tubing works great for me).

I’ve had the same problem with some gauges not zeroing and just pay attention to the differential.
 
How big is your chuck? Assuming an i.d. of 4" (a generous estimate) and pulling a vacuum of 21" hg (again, a generous estimate given a long, possibly restricted hose), the maximum hold available is about 65#. The force used in your video to pull the vessel loose doesn't seem that low considering its size and possible leakage through the work plus the hose restrictions . The real question is, does it hold well enough to do the work you are attempting?
 
Did it work OK when you first used the system? If so then you need to look for what's changed/wore down or malfunctioned.
 
Most of my thoughts are covered in the above responses and I agree with most of them.

1) What's the ID of your tubing? I use a quality 3/8"ID air hose ... and length doesn't matter - it only takes a second or two to evacuate the air from a long hose and then you're good to go.
2) Even if your gauge is off you can use a relative reading for your chucking needs - as long as its consistent.
3) Your gauge might read zero if you put in an upright position. Rotate your T fitting and turn your gauge 180º.

Nick, you are probably less than an hour from me and you're welcome to come over to see my set up and chucks (20+ ?). Look me up in the Bucks directory or PM here for location, etc.
 
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I could be wrong, but I believe that a vacuum gauge that does not read zero needs to be normalized to your elevation/atmosphere. The small label on the gauge or instructions will mention this. A better quality gauge will have a brass piece mounted in a rubber seal on the periphery of the gauge, typically at the 12:00 position that you can press to balance the pressure inside the gauge from where it was manufactured to equal where you are using it. Cheaper gauges will have a little rubber stopper at the same location with a rubber tit sticking out. You need to either cut the tit or puncture it with a needle. Two things to note. The brass piece can fall out or be be ejected over time. Not the end of the world but you will lose some gauge oil from inside. On the rubber one, once opened up it will weep oil for the rest of its life. A bit of a mess, but they will read true afterwards.

On the best gauges I am sure this leaking doesn’t occur, but I don’t spend that kind of money for my hobby.
 
Dave Landers said:
The cheap gauge I use with my vacuum chucks doesn't go to 0. And I totally don't trust the numbers it points to elsewhere on the dial. But I know where the needle points for "none" and "most the pump will do" - and that's good enough for me to know if I got a reasonable seal or not.
Click to expand...
Valid point. We are using vacuum for a basic process, so accuracy doesn’t matter. If you are 3 above zero when you start and 3 above max possible at full vacuum, or vice versa, it doesn’t matter if it is repeatable.
 
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