Super-simple vacuum clamping setup [Pictures] - created 01-02-2012
Rodgers, Jason - 01/02/2012.14:52:36
I'd like to put together a very simple vacuum clamping setup for gluing bridges and other small applications. I've read up on a lot of the info in the MIMF Library, as well as the great info on the Joe Woodworker site. What I'm wondering is: how simple can one of these setups be? Without going all high-tech with the auto-cycling regulators and such, can one get by with a fridge pump with a vacuum reservoir, a gauge, and a bleeder valve?
Yes, it's do-able, but don't try it for the first time on the instrument of interest.
I think that a constant (fridge) vacuum pump that is combined with ordinary kitchen PE film (e.g. Saran-Wrap or silicone rubber) can work. The key issue with vacuum-clamping is about establishing an airtight seal around surface, the part, and the air-evacuation port. A judicious application of masking tape can help with sealing the film to the surface.
The fridge pump works great, and is so quiet that you can leave it running if you wish. Also no need for a reservoir for small work; I use a huge "bag" for most everything, and I still get-by without a reservoir. For longer clamping times than you care to leave the pump running, you can use a timer of some sort to cycle the pump.
Vinyl sheet(the thicker stuff from the hardware store) is best for making jigs or bags. Glue it using vinyl cement, and if you can't find any(I couldn't) just buy a small swimming pool or waterbed repair kit!
I'm lazy. I bought a continuous duty Dayton vacuum pump off of Ebay for $55 and the vacuum bridge clamp from LMI. I got the hose from Lowe's for a couple of dollars. For several guitars, I used it with no valves (and never with a gauge after I checked it the first time with my automotive vacuum gauge). I later added a bleed valve (from Lowe's) for my convenience. It's simple. It works reliably. You don't need much. I made another, larger vacuum clamp myself with rubber sheeting, a plywood rim, and a weatherstrip gasket. It was easy to make. It works fine but I usually just use the gobar deck for stuff that large. I use hide glue and generally glue something with the vacuum clamp just before I'm done in the shop for a while so I don't have to listen to the pump.
This is all good information, and I'm glad to hear that it can be relatively low-tech and with minimal fittings. The only reason I want to build a reservoir (and they can be any sort of shape, right?) is that I get fairly limited windows of time in the shop; for glue-ups, I'll often leave clamps (or go-bars) on for a couple days at a time. If I was running a vacuum clamp, I'd want to charge the reservoir and be able to turn the pump off, walk away, and have enough suck left to clamp and cure (with fish glue), even if the inevitable leaks eventually draw down the mercury. Not sure if that's realistic, though.
The setup I was considering is: fridge pump > bleeder valve > PVC reservoir > bleeder valve > clamp. Think that would do what I described above?
Gonna have to keep looking for a pump. Tried calling 5 appliance repair places today and nobody had anything.
Jason,
Forget about your reservoir alone pulling suck long enough for your glue to dry, unless you have a gas station next door and they will loan you one of their underground fuel tanks.
I would put in my two cents for coughing up the few bucks it costs for the controller kit from Joe Woodworker. I also wanted to avoid a bunch of hassle and complication, partly because I know nothing about electricity, motors etc. but at the last minute gave in and got it and it is the best purchase I've made in this decade. It was easy to rig it up with the directions they give, even for me. It will let you set the suction you want and hold it,kicking in your pump when the vacuum has dropped 4 inches of mercury from where you set it.
Remember, the refrigerant pumps aren't designed for continuous duty, and you will burn them out if on continuously, plus they will get so hot that they boil of the coolant lubricant inside, and while it isn't toxic, it can leave a film of oil on your shop. Pop for the controller. Really. And buy a gallon of the lubricant oil, you fill the pump through one of the suction lines going into the pump and want to fill it until some starts coming out of the exhaust line. You just have a bare winding inside the pump that is constantly bathed with this oil, and if you don't have enough in there it will burn out the motor fairly quickly. You will need to periodically add a little replacement as some of it vaporizes and passes out the exhaust line.
Everything else you can go cheaper than cheap and still be fine. Any kind of tank will make a good reservoir. Look for old broken compressors at garage sales, the old obsolete propane tanks are what I used but HAVE THEM FLUSH IT WITH NITROGEN! before using. Regular compressor valves will work fine for vacuum too (20% won't hold a vacuum but 80% do).
You can also use a small hand vacuum pump from Harbor freight or buy a skateboard kit that includes the bag and a very effective hand pump. As long as you can eliminate the leaks you don't actually have to remove much volume.
I have a 1/4 HP Gast rotary vane vacuum pump that will run all day but it's quite noisy and only gets to 26"Hg.
I've printed out the Joe Woodworker plans for the smaller pump system, am making a shopping list for the basic plumbing, and will head to the Depot soon. The other fancy parts will be added in time.
Question about the reservoirs: the plans have two 15" long 4" PVC tubes that are plumbed as separate chambers. I'm planning to build this system into my benches, and I want to fit the reservoir in the top frame of a bench (a 2x4 cavity). Is the shape and size of the reservoir particularly important? What I'm thinking is 2" or 3" PVC shaped into a snake or fork shape with elbows and t's, like this...
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It would have the same volume, or more, as shown in the plans. Is there any benefit (or requirement) to having separate chambers, or can it be just one with the sum of the two?
Shape makes no difference, just volume. I use a four foot long piece of 4" PVC.
The fork shape with elbows would probably work better than the snake due to friction losses, which would admitedly be minor.
Jason,
Be sure to pick up some rubber corks the right size as the I.D. of your hose. You will want to check the integrity of each connection as you work back from the pump building your system. Start at the pump and build upstream from there As you connect everything together, if you check that it doesn't leak after each connection is done, it's a lot easier to find your leaks than if you put it all together and then try to find them. (The corks can be used to plug the line for checking leaks downstream)
Does anyone have any experience with a before and after when adding reservoirs?
I have a small vacuum setup I use for fixtures on my CNC machine and am finding that the more volume I'm trying to evacuate, the lower the suck I'm getting. I spoke with a friend who's been doing vacuum for a while and he's also heard that for clamping and fixturing a reservoir actually makes things worse.
My pump is only 1.1cfm (the small kit from veneer supplies/joe woodworker) which I'm sure has something to do with it as well but it would be interesting to find out if things get better or worse for people that add a reservoir to a setup like that.
In this first picture, I'm getting around 25" of vac. Notice the two pockets on the front of the neck (to drop weight). Those will be face down in the next setup.
With the neck flipped, I only get about 6" of vac. I can get it up to 9" if I coat the surfaces with packing tape to create a less porous surface but porosity doesn't explain all of the vacuum loss.
I have the same vacuum whether I use poplar or maple. The vacuum loss doesn't increase as the neck get's carved away on the back side leaving a thinner part.
p.s. you're not going to get much more than 26" of vac no matter what pump you're using.
A reservior does not create a lower vacuum pressure; it only creates more volume that requires more time to evacuate. Your problem is created by leaks. Tracking vacuum leaks down can be relatively frustrating because they harder to find than positive pressure leaks that can be heard or soap bubble tested.
When I built my venturi vacuum pump system I had a leak somewhere in the pipe fittings that I could not find. I ended up smearing silicone caulk on every connection and turned the unit on for just a few seconds and then left it alone to cure. When I came back the next day, presto changeo, it worked perfectly.
The one thing you want to be careful with when sizing your reservoir is not to oversize it for your pump. If you have a small capacity vacuum pump, make a smaller reservior so your pump doesn't have to work for a long time to build up the vaccum. If your pump works over 5 or 10 seconds or more to evacuate it then you are definitely too big. My little venturi pump can pull a vacuum on a 4' x 4' bag and a 4" x 4' reservoir in around 5 seconds. One thing nice about a PVC pipe reservoir is that you can always cut it down if you find out that it is too big.
I'm learnin' a bit about these systems and feeling more confident about putting something together. Thanks for more tips, Randy: I'll be picking your brain as I get more pieces together.
When it comes to reservoirs, if you do the math, the 30" of 4 inch PVC that JoeWoodworker suggests in his plans only amounts to about 1/5 of a cubic foot. If your vacuum can only pull around 1 CFM or less (like the refrigerator compressors), you're looking at about 10-20 seconds to evacuate. As Barry says, a bigger reservoir shouldn't create less vacuum, just longer reservior evacuation (bringing the system up to max Hg). The benefit of the reservoir is quicker clamp/bag draw-down and (if your seals are tight) a little more time between the motor kicking on to keep your Hg up.
Does all that sound right?
The question I still have is the relationship between a compressor's CFMs and maximum achievable Hg. Or is there a relationship?
Sorry Jason, it is not that easy. Gases being compressible make the whole thing a mess to calculate. Think of it as for any given unit of time you only remove half as much as the previous unit of time. So pressure drop would have a asymptotic curve.
As for relation ship between CFM rating of the pump and maximum negative pressure relationship, there is none. The CFM rating is a matter of swept volume/unit of time. Achievable pressure is a matter of how close the pump tolerances are and the design of the pump. The pump you use to achieve the relatively low vacuums for your clamping needs are very different from the pumps I use to evacuate a study chamber to 10^-23 torr.
The main reason for the reservoir is to prevent rapid cycling of the vacuum pump. Before I built a reservoir, my pump would come on every 5-10 seconds; after the reservoir was added the pump would only come on every 5-10 minutes. Huge difference.
Hi,
Good luck...
Remember that the maximum pressure differential is going to be under 15psi With PVC plumbing pipes being rated for over 100psi, you are not going to be able to do much that will render it failure prone.
Since I have said that PVC is rated to a rather high pressure, let me say for safeties sake, do not use it for compressible gases (air) if the pressure is going to be over a few 10's of pounds. PVC fails catastrophically, and the result is shrapnel. CPVC can be used for compressible gases at its working rating, because the cross linking means that it just ruptures with out flying shards