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View Full Version : Vacuum Clamp - Serious Metalworking or Toy



Bob La Londe
02-16-2012, 12:41 AM
Ok, how well does a serious vacuum clamp hold against cutting force?

Does it have to be gasketed?

I have played with some simple vacuum holding with a venturri style pump and a DIY vacuum plate (no gasket), but it wasn't satisfactory. I tried a basic drag engraving operation on a flat brass plate, and it moved. It held, but not hard enough.

I am curious if its possible/practical to use vacuum clamping for metal working against cutting force. If it has to be gasketed would that lead to slight imperfection of level of the work piece relative to the base clamp surface?

I haven't done any refrigeration work in ages, and I never owned my own vacuum pump so I hate to go out and buy one if its not going to be all that useful.

CaptainVee
02-16-2012, 01:41 AM
There are two main things here. The amount of pressure exerted on the piece by the vacuum and the friction between the vacuum table and the piece.

With a pump with a low flow the seal needs to be totally closed. if it is and the vacuum is high enough than it all depends on the friction between the piece and the table.

Bubba
02-16-2012, 09:15 AM
Bob,
I did some engraving (using a 1/8" EM) on some 1/4" AL plate about 6"x9" on a gasketed vacuum chuck and it worked perfectly.

Bob La Londe
02-16-2012, 10:22 AM
Thanks guys. Its looking like my lever clamps and step jaw screwless vises are still the way to go for most things.

datac
02-16-2012, 11:55 PM
"Closed" Vacuum jigs make for an incredible clamp if you get set up correctly. For starters, get a GAST rotary vane style pump that will pull 28 or so inches of vacuum, set it up on a tank and put a switch on to cycle it on and off as necessary. A vacuum leak or momentarily tipped part that causes a leak will deplete any clamping power immediately if you have no backup vacuum.... the tank will help with that.

The KEY to vacuum clamping though sits squarely in the amount of surface area you can expose to vacuum. The larger the area, the stronger the clamping power. You can easily see an example of this if you stick down a 1" vacuum cup to glass and pull it off, or try one of those double 4" cups they use for handling glass. One pulls off... the other does not. Its all about AREA.

Small parts, especially flat tags being diamond dragged became a specialty of mine some years ago when I was involved with a massive contract engraving operation. In order to have numerous parts located in know locations, I would end up making multi layered tables for the tags that incorporated a very shallow pocket the shape of the part. In the bottom of the pocket, I dropped in Volara closed cell foam that I had laser cut to provide a seal, but also off the maximum area possible to a vacuum port also located in the bottom of that pocket.

The pockets depth was calculated such that once vacuum was applied, the parts would drop just below the surface of the acrylic, and the pocket perimeters would restrict any movement while engraving. Those that had engraving near the edges just had the pockets edges chamfered as needed, and very close edge engraving sections had some "relief" made via Dremel. Some of these tables would have multiple "chambers" so that sections could be loaded separately from others.

To SEAT the parts securely, you would throw the vacuum on, and then rather than press down on each part and put finger prints on them, we'd throw a sheet of silicone rubber over the whole deal and they would all suck down instantly.

After about 100 uses, the foam would give up, so "leaker" gaskets were just replaced with new gaskets as needed. Some of the larger tables could hold 60-70 parts. You'd turn the engraver on and come back in a hour and swap out the table with a different loaded table and unload the finished parts with a vacuum cup on a wand. Made thousands upon thousands of tags !

I probably could take some photos of such a table if someone wanted to see it. Dont give up on vacuum.... its an awesome clamp method. On my router table here at home, I have a few 1/4" holes thru the table, tapped with a hose barbed fitting in them. Anytime I need to cut the perimeter of something generic, I have some various sized acrylic tubes.... 2" - 8" diameter, 1/4" thick wall, and various heights. Each has closed cell foam on the ends. Set one over the hole, put a blank on top and flip the vacuum on. When you get to 4" and up with 28" of vacuum, you couldn't pull it off with your truck !

rowbare
02-17-2012, 09:15 AM
Chris,

Thanks for the description of your vacuum tables. I would love to see pictures.

bob

Bob La Londe
02-17-2012, 10:52 AM
Yes I would like to see this as well.

datac
02-17-2012, 10:08 PM
I'll see if I can snap one or two tomorrow...

ger21
02-17-2012, 10:49 PM
You might want to check out All Star Adhesives - gasketing products for CNC routers and v-groove tape for v-grooving and coving applications (http://www.allstaradhesives.com) for foam gaskets for vacuum hold down applications.

datac
02-17-2012, 10:59 PM
Ah heck... I happened to think that I have a few old retired tables here where I am at that will show the method well enough.

Ok, this table fit on a 16" x 24" work area, two head machine. It held 60 parts for diamond drag work. The photos are from the rear of the jig. Those ports are just nylon quick connects glued into the plastic. When you fill this table, those point away from you as you set it on the machine. The ports go into properly spaced female ports on the back edge of the table, and simple ball valves allow you to turn the vacuum on to each section independently. On the front edge of the table, two holes are located to sit over pins located on the machine table.

The table construction is essentially a 1/2" piece of acrylic on the top, with a 1/4" piece under it.

To make it, you lay out the array of parts in a cad drawing, Define pockets for the parts, define a a main vacuum trough for each area under the parts, and finally define "ports" for each pocket.

I grabbed this table to show because it does show that I decided to make the gasket provide support in the middle so the part did not just bow under vacuum. The gasket as such has two separate cavities, so this part required two ports under each part, and the trough must feed each accordingly.

The machining involves cutting the part pockets from the top and tagging the port locations with a spot drill. The pockets depth gets to be pretty calculable once you have done it a few times. The worse case scenario is you go too deep, which then you just fly some surface off OR apply more chamfer to the edge of each pocket. If too shallow, you just drop it back on the machine and cut the pockets deeper. BTW, Too deep in a pinch, you can double gasket ! Regarding those ports, you could drill those ports right away, but there is some haphazard "science" behind not doing so. It's best explained by:

If you had a 5/8" hole under each part for vacuum, and if you developed a small gasket leak with only one of 60 parts, an undersized vacuum pump/tank system would QUICKLY be depleted and all the parts would fall loose. Now, I really did not have a problem with this because I had a 5 hp vacuum pump on a 60 gallon tank, with a 3 hp backup pump and a 1.5hp "maintenance" pump. All this Fed the engraving area by a 2" diameter tube. Others in the building also used the vacuum, so none of this was over capacity. Even then, I had low vacuum alarms in place to avoid having 500 to 1000 parts giving me a surprise by all popping off at once.

Back to the table ports... You can drill these small to start with and grow them as necessary, IF necessary. The rule of thumbl is, you only need to do make sure you can supply a smidget more volume thru the hole than what leaks past the gasket, and the vacuum will remain "high". Obviously no leaks are best, but doing thousands of parts a day, you do have an ornery part or two to deal with. The outcome of the smallest hole necessary is that you theoretically could have a few parts leak badly, but the small holes keep the vacuum supply from depleting and releasing all the other parts.

Once the pockets are machined, the parts test fitted and the ports are tagged, you flip the 1/2" sheet and machine in the troughs. These were just done with 1/4" tooling, making the depth to about the same 1/4". The troughs can be cut very near the edge where your barbed connections will be installed.

With the 1/2" panel done, we would use a thin 3M adhesive sheet to bond a piece of 1/4" acrylic on the trough side to seal it all up. We did glue the two together at first, but applying a wide adhesive turned out to be easier, cleaner, more reliable and more true to parallel. The 1/4" sheet was routed to match the 1/2" upper sheet with a following bit.

The ports to feed the troughs where then drilled in from teh edge so the hole being drilled hit the ports. Usually, I would go into those holes with a dremel and "pull a dentist", creating some "female barbage" in the hole before I would glue them in with silicone caulk. Silicone allowed me to easily replace a bad connector... people dropped these things, slid them into stuff... once in a while they would break one off. Easy fix.

I will try to remember to take a photo or two of the generic acrylic tubing vacuum clamp trick. There is no magic here... just simple stuff that works excellent.

HelicopterJohn
02-17-2012, 11:02 PM
Hi Chris,

Thanks for sharing your knowledge on the vacuum fixtures. I like the idea of a slight recess in the shape of the O.D. of the part to prevent lateral movement.

I have a Gast vacuum pump and use it to hold down 1/4" Plexiglas while engraving Edge Lite Signs. Works like a charm. I intend to update my vacuum pump with a auxiliary vacuum storage tank and cycling switch like you described in your post. I am able to obtain 26 to 28 inches of vacuum on my plexiglas vacuum plate fixtures according to the gauge I have installed.

Look forward to seeing your pictures.

Attached is a picture of a plexiglas vacuum fixture plate that I use on my HAAS TM-1P. I attached 2 each 3/8" X 3/4" aluminum support bars under the plexiglas fixture. The vise sqeezes on the aluminum support bars so that it does not distort the plexiglas vacuum fixture plate. I also made a similar plexiglas vacuum fixture plate that I use on my Shopbot CNC Router. The vacuum port on that one is on the side instead of on the bottom.

John

datac
02-17-2012, 11:11 PM
Well John,

Your obviously living the high life already ! Indeed, once you do have a decent vacuum source and make a few simple jigs, the sky is the limit.

Newcomers should know that 28" of vacuum laid on a jig like yours with just a gasket around the outside would probably visibly bow 3/8" 6061 aluminum laid over it.

On the other side of the vacuum clamp coin is the "open" variety. This is where you don't use gasketing, but rather high volumes of "air flow" around the parts to hold them. There are a few large format CNC routers that will put down 1.5" - 2" granular MDF and pull air THRU it. Of course, this take about 30 hp of centrifugal blower, and that makes some serious NOISE !

For milling, you must use a closed system to be effective, and you do have other quirks like keeping coolant out of your vacuum pump. LOL ! I love it though.

I've used vacuum for all sorts of stuff over the years. Its a MUST HAVE in my shop.

HelicopterJohn
02-18-2012, 12:32 AM
Hi Chris,

Yep, the ole retired life is the way to go. :D I am 65 and retired about 10 years ago.

The other update I intend to make has been discussed here on the Zone is to put a water filtration filter cylinder in the vacuum circuit to catch any coolant that enters the system and keep it out of the vacuum pump. If you use the clear polycarbonite style you can monitor and accumulation of coolant and drain as necessary.

The gasketing material I use is the more flexible type and easily allows the material vacuumed to lay flat on the machined plexiglas surface. Of course, as you know un-machined plexiglas varies in thickness so it would be hard to gauge any bowing of the clamped item. I am able t successfully clamp 1/4" plexiglas sheet and engrave at a depth of .010.

Here is a sample of a couple of my signs.

Again, I am just a hobbyist enjoying the high life and not a real machinist like you guys that do this for a living.

I love your fixtures. They really look nice. Thanks again for sharing your fixturing solutions as I really enjoy looking at others successful ideas.

John

datac
02-18-2012, 12:55 AM
I really like your signs ! I have seen the concept before, and even once searched for a reasonable supplier for the LED bases without luck.

Signage has always been one of my favorites..... I just don't get around to doing much of it ! A program like V-Carve really opened up the door for fast tool path generation for sign work... probably excellent for doing acrylic signs like yours.

I'm only bumping into 53 going on 80 year old bones (hard miles). I hope you have many many more. I already know I will be working until I keel over. Heck, I might even have to light the burners at my own cremation ! Har !