Vacuum Hold down?

[stevespo]

I completely understand the appeal and strategy of merging the old methods and new. That's something I'm always attempting to do with my own work. I was a dedicated hand tool user for many years, until the CS/ME/EE side of my brain exerted control and moved me towards robotics.

I've really come to appreciate the "general purpose" nature of a CNC router. It has the ability to replace (nearly) every template and jig in the shop. The speed, accuracy and repeatability can't be beat.

If noise is such a concern, an inexpensive (and still very accurate) VSD and spindle can run at high speeds and maintain very low noise levels. For the type of work you're doing, the dust collection is going to be louder than the spindle. Even a good quality router running at 12K RPM is really not so loud. Just another thing to consider.

Also, you can get V-bits with many different angles (or have them ground to spec). Swapping out a single bit is going to be much faster and accurate than adjusting 2 saw blades. I know that this type of efficiency is not really what you're after, but I'll mention it anyway.

Personally, I'm looking forward to seeing some video of your machine in action. Very cool.

Steve

[CNCRouterm]

Hello Steve,
No dissagreement on reducing the number of tools for a job! That said, my router is a lot quieter than my cabinet makers saw! Dust collector doesn't really count as it should be used for either, but when I include the vacuum pump, then it just depends on where you stand to decide which is more annoying. Now, using a HAND router, even when new it is noisy.
The noise generated by the cut is comparable, though I would say most materials cut with more noise generated on the table saw than the CNC router. I suppose it depends a lot on what saw is compared to what CNC router though.

[wendtmk]

Surely you couldn't beat a carbon fibre rod with a bamboo veneer?
Ok guys, just joking..... By the way, nice projects and interesting reading!

Good Lord! I'd never be caught dead fly fishing with a rod birthed from a test tube, designed by Monsanto, and marketed and sold by Tupperware!

Mark

[wendtmk]

Steve,

Nothing against routers, except for the noise. I really don't need another multi-purpose machine in the shop, rather need a single task oriented machine for what I want it to do. I have the lathe and milling machines for tackling the other projects... ;-)

Mark

[wendtmk]

Silly arsed question about vacuum pumps. Lets say I have two identical vacuum pumps ganged. Will that theoretically double my CFM? I realize vacuum pull in inches theoretically won't change, but will this effectively double my CFM?

Thanks,
Mark

[CNCRouterm]

Mark,
If ganged in parallel, and for a given pressure, then I would say yes, you would have XX cfm per pump. If you were to run them in series, then (depends a lot on what kind of pump) you would not see much if any difference in cfm, but the max Hg vacuum likely would draw down further than with just one. Up until the point where the pump closest to the atmosphere inlet over heated and siezes up, shredds vanes, or whatever it's failure mode exhibits.

I have a twin table, each is 63" x 63" and one 25hp Travaine liquid ring vacuum pump (around 350 cfm, max 27-28 Hg vac). For most jobs that I run, this works pretty well, though on occasion I wish I had gotten the 40hp version or twin 25s, one per table. I have heard of some operators who will run two 40s, one per table, piano makers I think.

There is actually a lot more to vacuum clamping than just horsepower. What needs prompt your question?

[wendtmk]

Eric,

I'm thinking of making a setup either like this one or something similar:
http://www.joewoodworker.com/veneering/EVS/concept.htm and I just happen to have two Gast vacuum pumps laying around that I think are 3 CFM units. I figured if I could gang them together, the evac process would be that much quicker, and since I have them already laying around, cheaper than buying a new pump with the higher CFM rating.

Mark

[CNCRouterm]

Ok Mark,
You may not need both, but the premise is sound. Have you conducted any simple experiments yet with the pumps and the application? Something like a seeing how long it takes one pump to draw down a vac bag of the size you anticipate using? The reservoir tanks will help with initial snap down. The first CNC I ever ran used a pair of 5 hp ganged rotary vane pumps. It was set up such that under high demand both would run, under low demand only 1, and they would alternate based on duty cycle.

For your project:

First off, you will want to determine if the pumps will pull enough vacuum to do the job, the volume will be secondary. If they don't pull enough vac, it is a moot point.

Secondly you may want to determine if you need one or two. Conduct an experiment or two to confirm both absolute pressure performance and practical volume capabilities (assuming that there is no data-plate, or you don't know how to interpret whatever data is available).

Opinion.

Unless the pumps are very low performance (in terms of absolute pressure capacity), I think that one pump will be enough for most projects smaller than 4' x 4', as long as you can tolerate a minute or two of marginal clamping force while it fully draws down.

Disclaimer.

The only vacuum bagging I have done is with my CNC router table and the attached 25 hp Travaini. Worked swell, but it of course meant the pump ran continuously during the process, and the router was not making any "router" revenue during the process.

Resources.

Lots of places, but Graingers or McMaster Carr come to mind quickly for things like tubing, fittings, vacuum gauges etc.

[wendtmk]

Eric,

I haven't conducted any experiments as of yet. I was just asking in case I decided to try this idea out.

Again, I'm trying to work out a hold down solution that would secure a 3/8" wide, by 1/4" high, by a variable length, say 3' up to about 5' long strip of bamboo. By design of the rest of the machine to this point, the vacuum hold-down table won't be much wider than that strip itself, so I won't have a huge table to evacuate air from.

Here's what I'm thinking of doing as a test: Getting a couple of pieces of 1/2" square x 6' long pieces of plastic (some kind that could be glued together relatively easily). On one piece, mill out a 3/8" wide by 1/4" deep channel (the dimensions can vary depending on what works the best). The other piece would have holes drilled 1" apart (and the distance between can vary here too, depending on what works best). Not sure what diameter holes are needed, but I guess I can start small, and make them bigger as necessary. This top piece would have a gasket like was mentioned in an earlier post from AllStarAdhesives as a seal. Then, glue the two strips together so that the seams are sealed. Drill and tap one end for a hose fitting, and see if there is enough vacuum to hold the strip in place while the saws are doing the cutting.

Mark

[wendtmk]

Anybody have any decent links to web sites that help size a vacuum pump for this and similar kinds of applications? The ones I've found so far cover sizing a pump for evacuating a fixed sized chamber, and are more dependent on how quickly you want to be able to evacuate the chamber.

What I'm looking for is something that would help me figure out the minimum CFM and inches mercury necessary to make this vacuum "table" work.

Thanks,
Mark

PS - never mind on the ganging of the pumps. Turns out the CFM of each was a lot less than I originally thought. I'd used them for vacuum bagging RC pattern aircraft wings, and not for vacuum clamping.