Vacuum Chucking a Bowl

Vaughn McMillan

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Brent Dowell asked about vacuum chucks, and as it just so happened, I was finishing off the bottom of a bowl tonight, so I thought I’d take a few pics of the process. In the true FW spirit, I got carried away on the pics. This is gonna take a few posts.

As Stu mentioned in Brent’s thread, Steve Schlumpf has a great pictorial of his vacuum pump setup. I figured I’d show the rest of the process.

My setup is a little different from Steve’s, but you’ll see some similarities in the rolling pump cart. (I built mine first. Steve improved on the idea and documented it very completely.)

The heart of the system (aside from the vacuum pump) it this little contraption. It’s the vacuum adaptor that allows me to run the vacuum line through the headstock. It’s just a threaded hollow tube, with holes on one end (where the chuck goes) and a swivel connection on the other end where the vacuum hose connects to the lathe. (Otherwise, the hose would be twisted into a knot before you could even start working.) This one is the Holdfast adaptor from Packard’s.

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Here’s the end that sticks out of the spindle into the base of the vacuum chuck. You can see the little holes where the air goes.

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And here’s the end that goes into the hand wheel side of the spindle. It has a quick-connect hose fitting on the swivel fitting.

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The cone-shaped part unscrews from the threaded tube, so you can feed the tube through the spindle from the chuck side of the headstock.

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In this shot I’ve pushed the threaded tube through the spindle, but not yet connected the cone to the other end.

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This is how much threaded tube sticks out past the end of the spindle. (It comes from the factory longer than needed, with instructions to cut it to size.) It’s the same kind of threaded tube used in lamps, so replacement should be easy if it’s ever necessary.

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In this shot I’ve just started screwing the cone/swivel onto the threaded tube.

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And here it’s cinched up tight. It only needs to be hand tight.

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Here’s the suck-u-tron. It’s the ubiquitous 220v Gast that Surplus Center was selling for a long time. As I said earlier, Steve has a good description of how the plumbing goes together in his thread. We’ll hook it up in a minute.

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Here’s the actual vacuum chuck. When I bought the vacuum setup, I decided to start with a factory-made chuck, just so I’d know how a good one is supposed to work. Now when I make others in the future, I’ll have something to compare them to. This one is a 6” Holdfast (again from Packard’s). I have the materials to make a few more, but so far this is the only one I’ve needed.

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Continued in the next post...
 
Part II

Here’s the business end. The bowl is some type of hard nylon, with aluminum threads that fit the spindle. It screws on over the tip of the vacuum adaptor that's been attached to the spindle. It has a hole through the middle to allow the air to get sucked out. On the rim, there are two rubber rings, but in between them is a strip of hard nylon. I’ve found that this can bruise the finished inside of a bowl, especially with softer woods, so I often add a large “washer” made of wetsuit material, cut a bit wider than the rim of the chuck. (You’ll see the edges of it in a minute.)

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Now for a little tangent. This gadget is not vital, but it sure is handy. The threaded end holds the scroll chuck, and the Morse taper part goes into the tailstock. It makes centering the bowl on the vacuum chuck a breeze.

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The only downside of the adaptor in the last pic is that is does not spin. You cannot run the lathe when it’s inserted into the tailstock. So…a while back I got the adaptor on the left side of this pic. It adapts the threaded end of the live center (Powermatic’s copy of the Oneway) to the 1 1/4” x 8 tpi thread in the scroll chuck.

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Assembled, it looks like this.

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And here’s how it all goes together. I remove the bowl from the lathe, but keep it attached to the scroll chuck. Turn it around, stick it into the tailstock, and it’s perfectly centered and ready to go on the vacuum chuck.

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Being able to spin the bowl while it’s mounted to the tailstock this way is handy but not critical. It just allows me to be sure everything runs right before detaching the chuck from the bowl.

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Back to the other side of the headstock, it’s time to hook up the vacuum hose. I use a good quality quick-connect fitting. (I had a cheaper one, but it started leaking.)

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Snaps on just like an air hose.

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In this shot, the lathe is running at about 500 RPM. You can see the bearing retainer on the right side of the hand wheel is blurry (since it’s spinning) but the quick-connect is just sitting there. The swivel connection in the vacuum adaptor is the trick.

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Turn on the vacuum pump, move the tailstock up into position, and the vacuum gauge shows there’s a decent seal. This is up into the “suck-start a Harley” range. You have to pull REALLY hard to get the bowl off when there’s this much vacuum. This bowl is maple…softer, more porous woods won’t seal quite as well.

Vacuum Chucking - 20 800.jpg

Continued in the next post...
 
Once I have the vacuum holding the bowl, I take the scroll chuck off and replace it with a live center. It’s just a bit of extra insurance. This bowl is about 14 1/2" across and 5/8” thick, so it’s pretty heavy.

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Here’s the inside of the bowl. You can see the edge of the wetsuit washer I mentioned earlier.

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A little spinnin’ and trimmin’…down to a small nub.

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Instead of turning the nub away, I prefer to cut most of it off with a little flexible flush-cut saw. It’s just less likely for me to mess up.

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A little more cleanup work with the lathe running, and it’s ready for sanding.

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Then a few more minutes with the little Grex random orbital sander, and the bottom's finished off. All the while, the vacuum pump is running and the chuck has a tight hold on the bowl. In fact, when I was sanding the finer grits, the dust was building up and clinging to the bottom like it was wet. It was the vacuum pulling air through 5/8” of hard maple, sucking the dust onto the surface of the wood. I could barely blow the dust off with the air nozzle. It truly does suck.

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And the result, after a quick swipe with mineral spirits.

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It should make a nice hefty salad bowl for someone.

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There ya have it.

Questions and comments are welcome. :wave:
 
Very well done, that is just great :thumb:

Really nice looking bowl as well, I imagine someone will get years of service out of that.

It is funny that you mention the wet feeling of the wood, I got that too when I first used the vacuum chuck, and I've learned to put "ZERO" finish on the bowl while on the vacuum chuck, as the vacuum WILL pull that finish all the way through the bowl and you will end up having to sand the inside all over again.........DAMHIKT :doh:

Cheers!
 
Very Nicely done Vauhgn, and a nice looking bowl to boot.

One question, do you leave the vacuum pump running all the time while turning?

I'm guessing so.

Thanks for the great Tutorial!
 
One question, do you leave the vacuum pump running all the time while turning?

ABSOLUTELY!!!

Think of the vacuum pump switch as the bowl launch switch. Turn off the vacuum pump while turning and launch the bowl. Instantly. :eek:
 
I wasn't sure if you were able to rig up a vacuum resevoir and use some sort of 'vacuum' switch, kind of like a compressor.

I like that. The Bowl Launch switch! I guess if there's a power 'blip' while you are turning, things can get interesting fast....
 
Although I do not have a lathe (well in fact I do but it is a hand drill powered one) I find all this post really formative and full of fantastic information, that I will probably use in the future.

However, and excuse my ignorance, I understand that you're using a separate vacuum pump but what would a the minimum requirement in bar or psi that a pump would need to supply to have a decent hold? Obviously the more the better. But what would that minimum?
I had a look at Steve's post and I cant find the vacuum that that pump delivers on his description of the pump.

Apart from that it is funny to find out that the vacuum created by the chuck was sucking the wooddust from the sander. It makes a lot of sense to me as it was sucking following the direction of the grain /sap channels as if a bunch of straws it was. If it had been perpendicular to them I believe it wouldn't have happened.
My only concern is that with that it was loosing suction power, has it happenned to you having to increase the suction pressure of the pump due to this fact or having to wax or seal the surface to provide a more airtight surface? Not on this one of course but in other more porous woods?

Thanks for such a thorough post Vaughn, and congratulations for the bowl:thumb:
 
Hi Toni :wave:

My pump pulls a lot of volume it was designed to empty rooms of air, that is why the darn thing weighs 26Kgs.

Mine will pull to 10-2 and moves a 160 liters of air a minute.

I think, in inches, most recommend at least 25" of mercury.

I've found that any finish put on, while on the lathe, just gets suck off or out by the vacuum, I've given up trying to finish things on the lathe with the vacuum rig, I just sand it really well, and then finish it off the lathe.

Cheers!
 
Thanks for the nice comments guys. :wave:

I wasn't sure if you were able to rig up a vacuum resevoir and use some sort of 'vacuum' switch, kind of like a compressor.

I like that. The Bowl Launch switch! I guess if there's a power 'blip' while you are turning, things can get interesting fast....
You can make a vacuum reservoir that has a sensing switch, much like a reverse compressor. A lot of people set up their pumps that way for vacuum veneering, so the pump doesn't have to run full-time.

The power blip bowl launch is a very real possibility. That's part of the reason I use the tailstock as much as I can.

...However, and excuse my ignorance, I understand that you're using a separate vacuum pump but what would a the minimum requirement in bar or psi that a pump would need to supply to have a decent hold? Obviously the more the better. But what would that minimum? I had a look at Steve's post and I cant find the vacuum that that pump delivers on his description of the pump.

Apart from that it is funny to find out that the vacuum created by the chuck was sucking the wooddust from the sander. It makes a lot of sense to me as it was sucking following the direction of the grain /sap channels as if a bunch of straws it was. If it had been perpendicular to them I believe it wouldn't have happened.
My only concern is that with that it was loosing suction power, has it happenned to you having to increase the suction pressure of the pump due to this fact or having to wax or seal the surface to provide a more airtight surface? Not on this one of course but in other more porous woods?...

No need to apologize, Toni. Like you, I'm not sure of the minimum amount of vacuum that would be necessary. I do know if you have too much, you can literally suck the bottom off the bowl. (Ask Stu all about that one.) ;) I believe this pump (which a lot of people use for vacuum chucking) pulls something like 26 inches of mercury (whatever the heck that means...I just know the higher the number, the more vacuum you have). The picture I posted above show gauge at about it's highest number (about 21 inches or so). There are some vacuum losses from small leaks, I'm sure. I do have a valve that lets me reduce the amount of vacuum, but unless a piece is real thin, I just keep it at the highest setting.

You're very correct about the wood fibers acting like straws. This bowl was turned in a side-grain orientation, so the end grain was exposed in two directions. The sanding dust was only sticking to those two spots. On the very porous woods (or wood with cracks or bug holes), I don't use the vacuum, but instead use something like a donut chuck.
 
...I think, in inches, most recommend at least 25" of mercury...

I just found this on the Joe Woodworker site...they recommend 19" of mercury or more:

http://www.joewoodworker.com/veneering/vacuumchucking.htm

And here's a link that explains "inches of mercury". There's also a link to convert it to other measurement units:

http://www.sensorsone.co.uk/pressur...hg-inch-of-mercury-0-deg-c-pressure-unit.html

[Edit to add...]

John, you beat me to it. :p OK, mine's another link. :)
 
How to think about using vacuum "pressure" for clamping ('chucking' to us turners)...

The "pressure" does not come from the vacuum pump. It comes from atmospheric pressure trying to fill the lack of air in a confined space.

The difficulty in understanding the characteristics of vacuum clamping is that many of the terms are the same as used with air pressure and psi. The physics are quite different however. Think of it this way. Atmospheric pressure is not something we can contain, and, atmospheric pressure is always seeking equilibrium. What it is on one side of a membrane is what it wants to be on the other side of the membrane. This is the natural property of atmospheric pressure. We don't control it. We simply find a way to utilize it.

There are pluses and minuses in trying to utilize vacuum 'pressure.'. The biggest minus for turners is that the weakest part of vacuum clamping is in sheer strength. That is the sliding sideways contact of two surfaces. And that is exactly the strength that is needed in holding a turning on the chuck. It isn't that the bowl will 'pull' off. It's that it can 'slide' off. It will only 'pull' off when there is no vacuum to hold it on.

So how much "pressure" is enough, by whatever means you measure it? That depends on several things. Concentricity of the piece is helpful. If you were turning an off-center piece, centrifugal force may be problematic. If the piece were of a larger diameter, the leverage of the gouge engaging with the out edge could well slide the piece of the vacuum chuck.

Then there is the species of wood. Some are 'leakier' than others. Here is a case of what your pump is capable of is not nearly as important as the seal between the piece and the chuck. And the piece IS part of the seal!

In the practical world, for vacuum to work for you, your pump needs to be capable of removing air from the vacuum chuck faster than atmospheric pressure can replace it. "Perfect" vacuum has to do with outer space. To duplicate that on earth requires a whole lot more money that a couple of hundred bucks worth of plumping we conjure up in the shop.

19 inHg is a reasonably good measurement. A real small piece of really dense wood where the gouge is only going to engage maybe 1" diameter of the very center of the bottom, may well allow for less inches of mercury. Additionally, we use inHg for measurement for the simple reason that those gauges are more easily found and more affordable!

I offer this explanation based on developing and teaching the course material for a class on vacuum clamping for woodworkers back in the '90's in a college woodworking setting. At that time there was no Joe Woodworker and the very good knowledge base he has brought forward for our use.

Vacuum technology for manufacturing, which includes shaping and clamping, came into its own in the production of aircraft for WW2. That is where I started with regard to gathering information. Over the next few years my students designed and built some pretty wondrous jigs and fixtures, not to mention vacuum pump systems. It was a fun class of experimentation.

I hope I've been helpful.
 
BTW Vaughn, I appreciated the pictorial tutorial. Based on my preference to make my own, I really appreciated the close-ups of the adapter. I have drawn up some vacuum chucks and a few adapters. When I get back home, I will do some experimentation and 'roll my own.'

I may even take some pictures....:)
 
Vaughn,

Thanks for putting together the good tutorial for the vacuum chuck. I've read about them but that's about it, so far. Great to have the step-by-step. I'm sure I'll have to put one together in the future as the vortex has a firm hold! Maybe I can just use the forces exhibited by the vortex to hold my bowls. :rofl::rofl::rofl:

Carol,

I appreciate you filling in some technical info in supplement to the tutorial. Good stuff.

John,

Thanks for the useful Wikipedia reference. Interesting how things have changed over the years (the comment about the reference temperature of the mercury being changed from 60 F degrees before and 0 F degrees now for what sounds like the same calculations. I'd guess there might be less stability issues at 0 F, but I could be out in left field).

Regards,
 
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