Venturi Vacuum for Work Holding and beyond... #3: Bits and bobs...

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This entry is going to focus on the bits and bobs that turn the venturi pump from a pump into a workholding device. This one rambles a bit, so get comfy...

A pump is a pump of course of course (doesn't rhyme, sorry to all you Mr Ed fans...). A vacuum pump is a pretty cool device on its own and (in my mind) the science of venturi action make it that much more of a 'cool' tool. But sitting and staring at a pump while is tries to suck the universe into its maw gets us nowhere...unless you're into mushrooms, and that's a different forum... So, in the quest to turn this pump into a work holding clamp, we needed to add a couple more pieces.

The first piece (albeit there are several of them) is some sort of a focusing and limiting device. That is, a fixture that will allow the vacuum device to apply pressure to a confined area and, thus, hold it fast. There are, in theory, a couple ways to go about this in the broad strokes: Either you've got a permanently installed fixture that provides vacuum force on one 'face' (and onto that face you would apply the workpiece to be held in place), or you've got a non-permanently installed fixture that will need to be (first) affixed to a non-moving surface (like a bench top), then (second) have a vacuum 'face' that allows the workpiece to be held in place.

In theory, each has advantages. A permanently fixed work holding site means there's nothing to be set up, and the installation can be made more bomb proof. But, this also means that it's (at least semi) permanent. And that means it's constantly taking up space, even when not in use. This is, in my mind, what you might see on a large production CNC machine.

A non-permanent installation has some additional challenges. Firstly, and IMO most importantly, is that you've got to have a way to fix the vacuum device/fixture to an immovable surface (a la the workbench). After all, what good is it to use a clamp if the clamp and the workpiece can still move about? So, we can use a number of different fasteners to get your fixture to hold in place...that could be screws, clamps, whatever. But now we're building additional steps into the setup of this system, and I didn't want that. However, we can also use the vacuum pressure itself to hold the fixtures in place on the bench if you allow the vacuum to occur on both sides of a fixture (think a hockey puck that can develop suction on both faces). The vacuum holds the puck in place on the bench, and the vacuum holds the workpiece in place on the puck. Simple and elegant! But not without challenges...

First, each time we ask the vacuum to hold something in place, we run the risk of leaks. A powerful enough vacuum may overcome those leaks, but enough leaks will overwhelm the system. As an example, a permanent, fixed system with 4 'pucks' need only supply vacuum to those 4 pucks. However, a non-permanent system that relies on vacuum to hold those 4 pucks in place to the workbench must now supply vacuum to both sides of those same 4 pucks, resulting in effectively 8 pucks worth of vacuum. And each of those pucks is subject to leaks.

'Vacuum' is an interesting concept...or rather a vacuum pump is. It doesn't care how much air it has to remove, it just keeps going. It may take longer to get there but, in a sealed system, it will eventually give you what you're looking for because, in this case, we're looking for a static pressure. Of course, again, this assumes no leaks. Compressed gases (shop air) isn't the same. A compressed air system, because it's action is dynamic in a shop setting (you want it to blow something around...meaning the pressure is being released to do work, not held to do work as in a vacuum), is subject to other physics and fluid dynamics 'flaws'....like friction loss. Trying to push air, at pressure, through a hose out into the open to do work at a specific pressure may be impossible with a given size compressor and size of hose because it cannot overcome the friction loss in the hose, no matter how long the compressor runs. If the system has a large enough compressor and properly sized hose, small leaks may not be a big deal since the system is dynamic. At little loss probably isn't a big deal...it just means the compressor runs more often. As an aside, a compressor may be able to develop a static pressure which, in some instances, may be beneficial, but that's not helping us (most likely) in the shop setting since we want the dynamic action in the case of shop air. But, since a vacuum clamp, vise or chuck relies on static pressure, we have some more latitude in pump size and hose diameter, and in the case of a venturi pump, compressed air volume. It'll mostly always work, it might just take a little longer to pull vacuum. Thus, where we really run into issue with vacuum is in having an airtight system. Back to leaks...always with leaks...

To summarize: the enemy of static vacuum pressure is leaks, period.

Ok, lets get back to the bits and bobs...this all ties together, I promise....

Anyone who knows me knows my constant lament is the size of my shop. It makes the most sense for me to use a portable vacuum clamp system since it can be put away and not unduly take up room that I don't have. But, I'm also lazy, which means if a system takes too long to set up I'm less likely to use it. So, all I want is a perfect system that stores away completely and set up instantly. Should be easy...

Owing to my space concerns and laziness, a non-permanent system that uses vacuum to both hold the clamping 'pucks' in place to the bench and vacuum to hold to the work to the pucks seems the right solution to me. Enter PODZ (not to be confused with POTTZ).



PODZ are a commercial product that are, essentially, a chunk of dense plastic (HDPE) with connected holes on both the top and bottom and on two sides. In this way, the vacuum can be pulled through the PODZ allowing them to be daisy chained together, but also to provide vacuum to hold the PODZ to the bench and to the workpiece. Since vacuum pressure needs to be confined to be useful as a clamp, a non-memory forming gasket material is applied to both sides which acts as a barrier to hold the vacuum. Since the PODZ material is a high density plastic, threading brass inserts into it to connect tubing creates a good* seal. You'll note the stainless looking disc in the middle of the pic above. That's a press in bushing with a spring loaded ball bearing that works as a seal so that the PODZ will stick to the bench even without a workpiece applied on top of it. This creates a good* seal as well.

*'Good' is not created equal. The brass threading into the HDPE is, as far as I can tell, vacuum tight. The stainless looking valve, called a Mach Ball valve, is in no way vacuum tight. It just limits the amount of vacuum loss to allow the PODZ to be placed on the bench, while a vacuum is being drawn, and not have them slide all over. The pump continues to run and will not reach 'full' vacuum until something seals off the top of the PODZ. It should also be noted that a leak affects the whole system...so one PODZ hanging out in space effectively nullifies any vacuum being produced since there's free flowing air running into the system. If all PODZ are in their place no the bench, the system will hold itself in place until the system can be sealed with a workpiece on top.

The PODZ seem to be a good option for the setup I wanted to build. Small, portable, modular and expandable. BUT, having them all tied together creates more potential leaks. But I soldier on...

Assembly of the PODZ is straight forward: Screw in the brass inserts...beit barb adapter or terminal plug...,push in the Mach valve, apply the sticky backed gasket, and push the 1/4" flex tubing onto the barb. Boom done. The kit comes with 4 PODZ. I knew I wanted modularity with it, so I assembled mine in two sets of two, with a quick connect to allow the two pairs to be joined and create a string of 4 PODZ. This allows me to use 2 PODZ for small projects, and 4 PODZ for larger ones. The quick connects work for my laziness as well. Incidentally, I think the quick connects are vacuum tight, or very nearly, but I can't be sure. So much easier to check for, and isolate, leaks in a positive pressure system than a negative pressure system. 

The grey pedal looking thing is a foot actuated valve...more on that below. 


The pedal: In order for this sort of vacuum clamp system to be useful, in my mind, it's got to be easy to turn on and off. That is, to apply and release vacuum. The foot pedal is a latching valve that leave the valve open to the system when vacuum is being applied...and releases the vacuum when unlatched. The V4 pump is designed to be self-regulating. It uses a pressure sensing valve that, when the vacuum falls off a couple inches, opens the venturi valve to renew the vacuum. How often that occurs depends on how much vacuum is lost (again with the leaks). Whats unique about the latching foot pedal is that, when it releases the vacuum pressure to the PODZ, it doesn't release the vacuum pressure upstream of the pedal (back to the pump). In this way, the pump isn't forced to rebuild the entire vacuum every time the workload in released. It just has to rebuild the vacuum in the small space between the PODZ and the bench/workpiece. This results in a very quick reapplication of work holding when actuated. So the pedal becomes an integral part to fast, efficient, vacuum clamping action.



Between the PODZ and pedal being assembled and built into a modular system, then added to the pump which is plumbed for air and power, we get this:



The red hose is compressed air into the pump (I added a valve since my main compressed air lines are not pressure regulated and the V4 pump likes between about 95-110psi to work). The clear/white braided hose is the vacuum line coming form the pump to the latching pedal (there's about 10' of this that stored thoughtfully behind a bungie built into the V4 pump, out of view here). The blue 1/4" tubing is the vacuum line from the pedal to the PODZ. This has a quick connect nipple at about 8'. The two sets of PODZ have a 'whip' of about 3' that gives plenty (almost too much) extra hose for positioning. All in, I could move the PODZ probably about 20' from the pump and still use them as clamps, but the area I intend to use them at is right above the pump on the auxiliary table.

But does it suck????

Tune in next time for the clamping action...same bat time, same bat channel...

Ryan/// ~sigh~ I blew up another bowl. Moke told me "I made the inside bigger than the outside".