Vacuum Chucking

[J D Thorson]

First question - Feb. 2011 American Woodworker article on vacuum chucking systems shows a vacuum reservoir in use. To adjust amount of vacuum you bleed off some of the pressure by passing air so (to me) you are always running the system at the capacity of the vacuum pump. What good can the reservoir do in this case? Do you all use one?

 

[Mark Mandell]

First question - Feb. 2011 American Woodworker article on vacuum chucking systems shows a vacuum reservoir in use. To adjust amount of vacuum you bleed off some of the pressure by passing air so (to me) you are always running the system at the capacity of the vacuum pump. What good can the reservoir do in this case? Do you all use one?

Morning JD,

Reservoirs were carried over from the vacuum veneering folks where the pumps are most ofter set to run intermittently over several hours or overnight with the rez maintaining the vacuum in between pump runs. This allows small CFM pumps to be used.

Some turners have posited that the reservoir works for turners to maintain vacuum in case of a power failure. Their quest for safety is laudable but unnecessary since, in the event of a loss of power, the vacuum in the chuck's "bell" will be enough to hold the piece while the lathe stops. You then just hold the piece 'till it gently comes free.

 

[J D Thorson]

Pressure loss in cold weather

In the cold weather my rotary vacuum pump system cannot get nearly the same vacuum as in the warmer months. In my barely heated outdoor shop in Minnesota it could be gasket leakage or other issue but in general is one type of pump more affected than the others?

 

[Mark Mandell]

It's more likely your chuck's rim seal is the problem. When it's stiff from the cold, small pieces of dirt imbedded can increase the leakage. Try stretching a piece of Saran wrap over your check (poke a hole in the center of the film) before positioning the workpiece. The fresh plastic surface will give you a better seal.

You can also recheck your hose fittings for tightness. If you're not using teflon tape on them, you'll find the tape can make a big difference.

 

[john lucas]

Mark I hate to tell you but when you lose power the bowl flies off. I lost power in my basement one night while reverse turning a big bowl using the vacuum chuck. The basement went dark and the bowl flew off. I could hear it hitting all kinds of things but didn't even know where to duck.
I haven't put a reservoir on yet but a longer hose holds the vacuum just long enough for my lathe to slow down.
Most of the time if that happens you have your tool rest pretty close and the bowl doesn't actually come off just makes lots of noise and tears up the bottom.
I just got my Journal last night and saw the vacuum article but haven't had tome to read it yet.

 

[Mark Mandell]

Mark I hate to tell you but when you lose power the bowl flies off. I lost power in my basement one night while reverse turning a big bowl using the vacuum chuck. The basement went dark and the bowl flew off. I could hear it hitting all kinds of things but didn't even know where to duck.
I haven't put a reservoir on yet but a longer hose holds the vacuum just long enough for my lathe to slow down.
Most of the time if that happens you have your tool rest pretty close and the bowl doesn't actually come off just makes lots of noise and tears up the bottom.
I just got my Journal last night and saw the vacuum article but haven't had tome to read it yet.

Hey John,

Follow The Bouncing Bowl?

My first vacuum "pump" was a recycled dehumidifier compressor. Actually did well but it had a feature I didn't know about. Those sealed compressors have a thermal overload built in. I was using the thing just sitting on the floor, had done three pieces with it running continuously, when right in the middle of the fourth, it just stopped. I hit the switch on the lathe it had about stopped before the bowl let go. Of course, I was pulling 24" of vacuum on an 6" chuck, so it took longer to to bleed off. I can understand that if you're pulling less than 18-20" and/or using a smaller chuck or have a leaky piece of wood, a power failure could cause such a quick release.

BTW I solved the thermal overload problem with a small muffin fan blowing on the compressor shell. Thing would run all day without a whimper.

PS Still waiting for my Journal. Oh wait, I think I hear the latter carrier's sled dogs now . . . .

 

[waltben]

I've had a Gast vacuum pump on my Stubby for at least five years without any reservoir tank and not had any issues at all - but then I haven't lost power for more than a few seconds while turning either. I've more interest in figuring out a better filter on the intake side of the pump that I can replace easier than my home made one.

 

[Mark Mandell]

I've had a Gast vacuum pump on my Stubby for at least five years without any reservoir tank and not had any issues at all - but then I haven't lost power for more than a few seconds while turning either. I've more interest in figuring out a better filter on the intake side of the pump that I can replace easier than my home made one.

Hi Walt,

These will work in-line, just have to check on element replacement availability.

http://www.surpluscenter.com/item.asp?item=4-1565&catname=air

m

 

[bonsaipeter]

Since I use 3/8" I.D. vacuum hose on my vacuum chuck system I went to an automotive parts store and purchased a fuel line filter with 3/8" inlet and outlet hose barbs on it. It cost $3-$4 and is readily replaceable if needed. I've had mine on my system for several years now and have not had to replace it yet. The best place to insert it in the system is just before the vacuum pump inlet.

Peter

 

[Bill Boehme]

.... in the event of a loss of power, the vacuum in the chuck's "bell" will be enough to hold the piece while the lathe stops ....

That is not the case at all if the system is set to bleed off any of the vacuum which would be likely if you have a thin walled vessel mounted on the chuck. Since the internal volume of components such as the filter, chuck, and hose are essentially nothing, the holding vacuum would drop to practically nil in a fraction of a second. In addition to that, there is always significant leakage at the seal, and some types of open grain wood leak like a sieve. I have sanded the outside of mesquite bowls while running on the vacuum chuck and discovered that an amazing amount of the dust was sucked right through the wood.

 

[Bill Boehme]

Since I use 3/8" I.D. vacuum hose on my vacuum chuck system I went to an automotive parts store and purchased a fuel line filter with 3/8" inlet and outlet hose barbs on it. It cost $3-$4 and is readily replaceable if needed. I've had mine on my system for several years now and have not had to replace it yet. The best place to insert it in the system is just before the vacuum pump inlet.

Peter

The best place to put the filter is as close to the rotary coupling as possible. By placing the filter just before the pump, it allows wood dust to accumulate everywhere in the lines and vacuum gauge. Make certain that you have the right type of filter. If it is a fuel filter, it will allow fine dust to pass right through it which might explain why it is not clogged up. What you want is a filter that WILL get clogged up with dust.

 

[Mark Mandell]

That is not the case at all if the system is set to bleed off any of the vacuum which would be likely if you have a thin walled vessel mounted on the chuck. Since the internal volume of components such as the filter, chuck, and hose are essentially nothing. The holding vacuum would drop to practically nil in a fraction of a second. In addition to that, there is always significant leakage at the seal, and some types of open grain wood leak like a sieve. I have sanded the outside of mesquite bowls while running on the vacuum chuck and discovered that an amazing amount of the dust was sucked right through the wood.

Morning Bill,

My view on the issue has been formed from personal experience a) with the vacuum source stopping and b) with a power failure. I have and use a bleed valve in my rig, and rarely use full vacuum. Perhaps the difference is that, since I only use the vacuum to finish turn the foot of the piece, I will have completed the inside and applied at least one coat of sealer which naturally makes for a better seal. I also make my own chucks out of schedule 80 pipe couplings, so my "bell" volume is more substantial than you may realize. Lastly, the only time that my tailstock is not engaged, even when using vacuum, is when I'm finishing off the very center of the foot. Up to that point I use the vacuum chuck as a hybrid jam chuck; a practice I highly recommend to all.

peace

 

[abrown]

I have not received my issue yet, nor read the article, so I apologize if this is covered in the article.

I don't believe that the reservoir is an absolute necessity, but it is a nice feature. I built my pump off of plans from www.joewoodworker.com, and yes, it is a veneering press. However, I can set a vacuum level through a vacuum sensor switch which lets the pump cycle very much like a compressor does.

Most of the work I do has relatively thick bottoms, so I can set it for about 20", and the pump cycles for maybe 15 seconds every four or five minutes for non-open grained woods. Granted that most of the wear and tear on a pump is probably during the startup phase, but I have to believe that reducing the on time also increases pump life. This particular design has a 3-way switch and check valve, so when the pump starts up from a cycle, the pump is beginning at atmosphere, not vacuum, which does ease the pump startup force considerably. From a noise perspective, the ratio of on/off in the cycling is worth having the reservoir.

For open grained woods where I don't have a great vacuum seal, I did install a bypass switch on the vac sensor to allow me to run the pump constantly.

For thin bottom bowls, where I am worried about collapsing the material, I then have the option of bypassing the vac sensor switch and opening the bleeder valve, or simply setting the vac sensor switch to around 10" or less and let the pump cycle.

Using the plans from Joe, the cycling pump with reservoir didn't cost that much more than just a pump from one of the major suppliers while meeting or exceeding performance. Yes, the vac reservoir does hold the bowl for some time should a power failure occur. For a closed grain wood like maple, the reservoirs would hold the bowl for probably hours (overkill and the reservoir is not necessary). I just turned an elm bowl that was moderately open grained, and it would probably hold the vac to better than 10" for about two minutes. I'm guessing oak would hold at least long enough until the lathe would finish spinning down. Without the reservoir, I'm not sure the oak bowl would stay on. Then again, I do have a Nova lathe which takes a while to slow down.... The bottom line is that it didn't cost much more, adds some safety and reduced noise, and I'm now dreaming of ways to use the pump now like stabilizing blanks and infusing dyes, and veneering.

Andy

 

[AlanZ]

Of course, one could simply add a small uninteruptable power supply (UPS) between the pump and the wall outlet.

Add a small light to the setup so when the power goes out you would have some time and visibility to do a reasonable shutdown.

A UPS would be an inexpensive and easy to implement (no plumbing) workaround to a power failure affecting the pump, and has the benefit of being backup for failed lighting.

 

[Steve Worcester]

I use a reservoir on my vacuum form "machine" but when you shut it down there is a leakdown to zero. With a 5 gallon reservoir, it takes about 20 seconds so it would probably hold the wood until the lathe stops. What I don't understand in his design is why there is an isolation valve. It would cut off or vary the vacuum, but the vacuum control valve does the same thing, sort of.

 

[Bill Boehme]

Morning Bill,

My view on the issue has been formed from personal experience a) with the vacuum source stopping and b) with a power failure. I have and use a bleed valve in my rig, and rarely use full vacuum. Perhaps the difference is that, since I only use the vacuum to finish turn the foot of the piece, I will have completed the inside and applied at least one coat of sealer which naturally makes for a better seal. I also make my own chucks out of schedule 80 pipe couplings, so my "bell" volume is more substantial than you may realize. Lastly, the only time that my tailstock is not engaged, even when using vacuum, is when I'm finishing off the very center of the foot. Up to that point I use the vacuum chuck as a hybrid jam chuck; a practice I highly recommend to all.

peace

Mark, Your situation is much like mine. I also make mine out of 4" schedule 80 couplings and bring the tailstock up with a cone center touching the bottom of the bowl. I also run the lathe at low speed. All of these things plus having a finish applied to the wood go a very long way towards holding things together. The only time that I remove the tailstock is when touching up the center of the foot. The only loss of vacuum that I have experienced was when my rotary coupling broke. The bowl didn't fly off, but it did rattle around a bit between the chuck and live center since the tailstock pressure was rather light. Heck, I have even used the vacuum chuck purely as a jam chuck without vacuum applied when turning on my mini lathe.

However, I have also on occasion done some non-traditional turnings that were more easily done with the tailstock backed off. I now favor doing the finishing AFTER the turning is completely done because I have ruined the finish on the interior of a couple bowls by vacuum chucking. The damage was not due to seal scuffing, but caused by the vacuum pulling air and dust through the wood which caused the finish to have a "sanded" appearance (sanded as in it looked like sand had been mixed with the finish -- great if you need a traction surface). One of the bowls had been finished with tung oil and presumed to be dry after a week or so. The other one had a film finish (don't remember what type) and dried for a couple days.

 

[Mark Mandell]

I use a reservoir on my vacuum form "machine" but when you shut it down there is a leakdown to zero. With a 5 gallon reservoir, it takes about 20 seconds so it would probably hold the wood until the lathe stops. What I don't understand in his design is why there is an isolation valve. It would cut off or vary the vacuum, but the vacuum control valve does the same thing, sort of.

Hi Steve,

You clearly were not in the Navy, having to conserve fresh water. His isolation valve appears to allow the pump and reservoir to remain "pressurized", but still allow removal of the bowl by cutting off the vacuum from the chuck; same as turning the shower on and off for quick wet and quick rinse.

Yo Ho, Yo Ho 😀

 

[Bill Boehme]

.... What I don't understand in his design is why there is an isolation valve. It would cut off or vary the vacuum, but the vacuum control valve does the same thing, sort of.

What he refers to as an isolation valve appears to me to actually be a check valve (i.e., a one-way valve). The reason for the check valve is that rotary vacuum pumps will "dump" their vacuum almost instantly once the motor stops. The reason that this happens is that centrifugal force is required to keep the vanes in contact with the rotary cylinder wall. When rotation stops, the vanes are free to float within their slots.

See you at WNT tonight.

 

[Bill Boehme]

..... Yo Ho, Yo Ho 😀

So how much does the anchor weigh, anyway?

You swabbies were always "weighing" the anchor so you should have the weight memorized by now ... right? 😀

 

[Bill Boehme]

BTW, the author's system apparently does not have two essential valves -- a shutoff valve and a dump valve. Both of those are needed if one bothers to read the owner's manual concerning the shutdown procedure for the vacuum pump. Proper shutdown involves running the vacuum pump with a blocked inlet port and also with it open to the atmosphere. The run times are apparently different for newer and older model pumps. Ball valves are bet for this application.

 

[Mark Mandell]

So how much does the anchor weigh, anyway?

Don't know. It was always "away." Concerned me, however, because what if we needed it and it wasn't there? Chain was pretty heavy, though.

BTW, you mentioned centrifugal pump (as in the article). That may account for our differing experience as I've always used piston pumps that hold their seal when stopped. My current pump is a 6 cfm oil-lubed Robinair to which I fit a coalescing filter; gets above 28" right quick.

 

[J D Thorson]

Lessons learned

Thanks all - I've repeated the 'leak search' done when constructing my system and found that for whatever reason air leaks around the spindle threads more when cold then when warm. Teflon tape on the threads plus the suggested help on the gasket to bowl seal brings things back up to par.

I will also be adding a reservoir to the system and moving my filter as suggested in the article and these comments.

I have always used the tailstock when vacuum chucking and second all comments on this. I have the OneWay livecenter so I can use a non-marring 'cone' as well as the point.

Sailor talk - When the anchor is 'aweigh' it is up and safely stowed away which means the ship is free to set whatever coarse is desired.

Thanks again everyone, very educational.

 

[Mark Mandell]

air leaks around the spindle threads more when cold then when warm.

The reason, J D, is because you've got 2 different pieces of metal that react differently. The male part of the equation (the spindle) shrinks slightly in the cold so its outside diameter is a hair smaller. The chuck's threads, the female part, also shrink in the cold, but being the inside diameter of a defacto pipe, when the metal shrinks the inside diameter increases by a hair. Thus the thread seal is diminished and you get your leaks. I know some who use a thin plastic washer between the spindle shoulder and the chuck. Care should be taken here because any slight variation in the thickness quickly translates as runout on the business end of the chuck.

Sailor talk - When the anchor is 'aweigh' it is up and safely stowed away

[I knew that] 😀

 

[John Giem]

Hi all, in case you don't recognize my name, I wrote the vacuum chuckiing artricle in the Feb 2011 issue of AW. Due to space limitations, I did not explain the functions of all of the components in my system. Most questions are regarding the isolation valve and the vacuum reservoir. These are not needed in most normal small systems but as an engineer, I'm doing a lot of not normal things. The isolation valve is just a standard ball valve and is used to isolate the manifold from the pump, filter and reservoir. In the article I use the isolation valve to be able to isolate the output side of the system for leakage testing.
Also, by having a vacuum established, I can get faster mounting and unmounting by using the valve and not cycling the pump. I have had bowls that could not be mounted using the relatively slow pump only but with the isolation valve and reservoir, I got a rapid suck down and mount. The reservoir was essential for this.
Some turners have a remotely located vacuum pump with the associated vacuum drop from long lines. Using the reservoir next to the lathe will yield better performance.
I have submitted another vacuum chucking related article to the AW and have several more under development. I hope this posting helps clear up the issues. Let me know if you have more questions.
John Giem (pronounced as geem)_

 

[davidwalser]

John,

Thank you for the additional information. It helped clarify things for me. I look forward to reading additional articles from you in the future.