Thread: Vacuum chuck system build WIP

Originally Posted by Hermit

G'day Rod. Thanks for your interest. Hopefully it will all work well when done, without much fine-tuning, (leak sealing). I'm sort of feeling my way along, learning as I go, so there's bound to be a bit of trial and error before it's completed.


I gave the vacuum pump a test connected only to the vacuum gauge in a closed system. It pulled a vacuum of 27 inHg, (27 inches mercury). Not too bad at all - a perfect vacuum at sea level would be 29.92 inHg. (1 atmosphere)
Incidentally, 27 inHg means that the vacuum can pull a circular column of mercury 27 inches upwards, against gravity.
The formula for calculating the total holding pressure, (P):
P = pi x r² x 0.491 x inHg (1 inHg = 0.4912 psi at sea level)

From the formula, with a 10 inch diameter bowl, 27 inHg would result in 1041 pounds of total pressure holding the bowl in place. Hence the vacuum adjust bleeder valve. I don't want to crush any thin-walled bowls. Generally, according to the 'Bruised Brothers' video, only about 75 pounds to 250 pounds of total pressure is needed.
So a 10" bowl would only need 5 inHg to get a pretty decent 193 pounds of holding force.

A handy chart:

Vacuum Pressure Guide.JPG


No parts in the mail today unfortunately, so I'm continuing with a couple more coats of poly inside and out to ensure a good seal. Although the vacuum pump can pull a good strong 27 inHg vacuum, it can't move much air, only 3.3cfm, so the whole thing has to be virtually leak-free to work well.

Thats a good result. By the look of it I thought might be a vane pump but the figures are a bit high for such a pump design. A very good WIP thoroughly enjoyed it and will keen to hear how it goes in practice.

Originally Posted by hughie

Thats a good result. By the look of it I thought might be a vane pump but the figures are a bit high for such a pump design. A very good WIP thoroughly enjoyed it and will keen to hear how it goes in practice.

Thanks hughie. And you're spot-on, it is an oil-bath rotary vane pump. I was surprised to see it could pull 27 inHg too, but I'm not complaining.
I guess it needs to go that high (low) to be able to properly evacuate an AC system.
Edit: I just did another test, set up better, and it actually pulls 28 inHg. It really sucks! Better still.

ax. Vacuum pump test.jpg


The parts are slowly trickling in. I've finished the handwheel assembly and started putting together the parts I have, while I wait for the rest. So far, I have no leaks, not even from the handwheel assy, apart from where the two grub screws go through the handwheel base. (Overlooked that one.)

ay. Handwheel assembly completed.JPG az. Handwheel grub screws.JPG

A bit tricky to seal, so I've settled on using a small blob of Blu-Tak on each after I fit it on the lathe, then a few wraps of tape to make sure they stay in place. For the test, I also used used an extra, big blob of Blu-Tak to block the spindle hole in the handwheel as well.
In the long run, I'll try to find an extra-small diameter bicycle tube and cut off an inch to form a 'collar' around the base of the handwheel, over the tops of the grub screws. I'm open to better suggestions for this.
With that done, according to my *high-tech testing, it doesn't leak at all.

* High-tech vacuum testing - block all except one hole, suck on it by mouth to create vacuum then place tongue over hole and wait to see if the vacuum fades off. I can get a 10 inHg vacuum that way.

I'm quite enjoying this build, but I'll be happier still when it's finished and I can actually use it. I've been dragging my feet on a plate I'm making, hoping to use the vac system to finish the bottom for a debut run.

I had a look on Ebay and the price is very good. Oil bath that explains the extra ommph! mine is with out so it pulls around 55%, probably be ok over a large area. The vacuum chuck has on and off my to do list for a few years, cant make up my mind.

Originally Posted by hughie

I had a look on Ebay and the price is very good. Oil bath that explains the extra ommph! mine is with out so it pulls around 55%, probably be ok over a large area. The vacuum chuck has on and off my to do list for a few years, cant make up my mind.

Yeah, mine was only $100, here. (The listing says 3.6cfm, 60Hz operation, but it's really for 50Hz and 3.3cfm.) Quiet running and smooth, high vacuum but, as mentioned, only 3.3cfm / 1.5 litres per second. Not bad, but a higher cfm rating wouldn't go astray, to compensate for any leaks. The bigger pumps of this type can move up to about 10cfm, but you pay for it. They're not so cheap.

I decided last night to use cable ties in pairs to clamp the hoses, instead of waiting for the spring clips to arrive. I think the cable ties do a better job.
So I went ahead and got most of the plumbing done. So far so good - I can block the end and still pull 28 inHg, and both the vacuum adjust and grip/release valves work well.

ba. Main plumbing done.jpg

I only need the 'o' rings and a couple of other incidentals and I can finally test it properly.

N.B. The circular hole in the RHS of the base is for oil-changing, to avoid having to remove the pump. It's directly below the oil drain plug.

Edit: I forgot to mention - I found that the neoprene seal that I made for inside the handwheel assembly crushed too much when firmly clamped up by the bolts, so I've ordered some 1.5mm 'natural insertion' rubber sheet, to make a better one. It won't compress as much.

Very nice and neat setup.

Originally Posted by Christos

Very nice and neat setup.

Thank you mate.
I've got a bit of an update....

I'm way too impatient to wait any longer for parts to arrive, so I searched around and found a rubber washer with the right thickness and ID for the handwheel spindle seal and trimmed the outside to size. (The washer came from a garden hose fitting that screws onto a tap - 3/4" ID.)
It sealed well, so I bodgied up a temporary short straight length to go to the rotary adaptor and set everything up. (I've got a 4" male to male fitting ordered to replace the two m-m bits and the f-f bit that I'm substituting with. Two less joints to fail.)

To ensure that bearing drag doesn't pull on the hose, I made a small wire bracket to keep things in place:

a. Handwheel adaptor & hose support.jpg


I also made a small, neoprene padded disc for the tail centre to hold the piece against the chuck disc when the vacuum is turned off. (That bowl is the first bowl I ever made some time back - 6" diameter, QLD Maple, finished in a few coats of WOP so I knew it would seal well for testing.):

b. Held by tail centre.jpg


First test, vacuum on and adjusted to 10inHg, but lathe turned off still. I found that the 'grip/release' lever is very useful for centring a piece that doesn't have a marked centre - I can back it right off until the piece is just gripped, allowing me to slide it around while sighting against the end of the toolrest:

c. Vacuum on & lathe off.jpg e. 10inHg setting.jpg


Second test - lathe running at 1000 rpm, vacuum at 10 inHg still. I didn't want to go higher because 10 inHg on this 6" bowl is already 139 pounds total holding force. I definitely can't budge it.

d. Lathe at 1000 rpm.jpg


It works great. I'm really pleased with it so far. The only thing I can't do just yet is any turning using it. I don't have a bowl at the right stage. I have to wait a day or two until I put another finish coat on the top of a plate I'm making, then I can turn it over and test the vac system fully.
After today's test, I already know that will go well.
I'm a happy camper - I didn't waste all this effort.

Congratulations. A first class job.
Bob

Originally Posted by Oldgreybeard

Congratulations. A first class job.
Bob

Thanks Bob, for your interest and your kind comment.
Also thanks again for your help the other day.


The only thing I'm not too sure of now is just how much pressure to use for a given bowl/plate size. The 'Bruised Brothers' vid says to stay between 75lb and 250lb, from memory, but that doesn't help a real lot. I'll do a 'Google' and see what I can come up with.

I've had the chance now to give this system a really good workout. It works beautifully, except for one little drawback - the 2mm neoprene I bought actually varies from 2.5mm to 4mm thick across it's width. Not the best for finishing the rear of a plate rim.(Wish I'd noticed before I glued it on. I blame my glasses. Time for a new pair. )
I contacted the seller, and he's sending me some that he's checked for consistency first. Also, this time I'm getting it with an adhesive backing, to save messing around with spray adhesive.

The second fix isn't so much a bug, as a precautionary measure. In my travels, I've read of a couple of cases of bearing seals popping inwards under maximum vacuum, resulting in a sudden loss of vacuum and a UFO. It sucks much of the grease out of the bearing and into the vacuum system too, apparently. Not good.

To help prevent this, I machined up a disc of 5mm acrylic, to act as a sort of seal retainer. I routed a groove for the bearing rim and a step, so that the acrylic sits firmly against the surface of the rubber, pressing it into place. It's held in place by four countersunk screws.
If the seal can't deform inwards, hopefully it'll never pop. I don't mind a gradual loss of vacuum over time as the seal wears, but don't want that sudden 'let go'.

bb. Bearing seal retainer mod pic1.JPG bc. Bearing seal retainer mod pic2.JPG

Hermit
I've been following this built from behind a sheet
Not having any thing use full to say.
But now it's all most done it looks fantastic and seems to be working a treat.
Well done
Something I hope to do one day my self

Originally Posted by Simplicity

Hermit
I've been following this built from behind a sheet
Not having any thing use full to say.
But now it's all most done it looks fantastic and seems to be working a treat.
Well done
Something I hope to do one day my self

Thanks for that mate.

It's well worth the effort. No more plywood discs with tabs for reversing & finishing bowls, plates etc.

One thing I really like is that unlike with a Longworth chuck or Cole jaws, you can work right to the edge of a plate without the rubber stoppers getting in the way. And it's sooo easy to use.

Next up, when I get some more 3/4 plywood, is a few cup chucks, for natural-edge bowls and smaller items. 2", 3", 4" and 5".
I'm not sure how the 2" will go - I'll need as much vacuum as possible, but we'll see. Even with the maximum 28 inHg of vacuum, the total holding pressure on a 2" item is only 43lbs. I'm not sure if that's enough. On other larger items, 4" to 6", I've found that about 120lbs to 150lbs is pretty good.

I've really enjoyed building this. A bit of a change from all the decorative stuff, and my first real precision turning on the handwheel adaptor.

Edit: The new adhesive-backed 2mm neoprene just arrived. It looks much better - a nice consistent thickness. In case anyone else is looking for some, it's here. (To get adhesive-backed, you need to specify it in the purchase note. Same price.)
It's thin enough to not be too spongy, and grips like s@#t to a blanket. The neoprene is sort of 'sticky'. You need to back the vacuum off to almost zero before you can move the workpiece.

How's the vacuum chuck going now hermit
Now that you have at some time to play with it
Matt

Originally Posted by Simplicity

How's the vacuum chuck going now hermit
Now that you have at some time to play with it
Matt

Really good, Matt. I've used it a few times now and it performs beautifully. Still pulls 28 inches of mercury when both bleeders are closed.

The only fault, if you can call it that, is that the little pump gets hot pretty quickly. Much more than 30 min in one go and it's almost too hot to touch. I give it 15 minute bursts, then pull up the tailstock with a padded disc and give the pump a break for a bit. Cuppa time.

I have a small 6" desktop fan on it now, which helps a lot, but in the longer run I'll either buy a bigger fan or a bigger pump. Probably the former.

I still haven't made any vacuum cups to go with it, either. Only the flat disc so far. I'll make them as I need them.