Vacuum Table Advice

Hi,

Apologies for so many questions for a first post.

I run a Heiz S-400 which spends a lot of its time making one-off aluminium front panels for specialist electronics equipment.

Today, however, I needed to machine holes into the top of a quantity (40+) of plastic 'project' boxes approximately 3" x 5". My plan was to do 6 at a time by cutting an alignment jig out of 3mm PVC sheet and using my VT4432SEAL vacuum table (powered from a workshop vacuum cleaner) to hold the boxes in place. However, even with a rubber mat, I just wasn't getting enough 'suck' which meant a couple of boxes lifted on the tool with all the problems that causes. I tried double-sided tape as a last resort but the nature of the boxes' surface, a textured finish, meant that getting the tape to stick was a problem. What should have been an easy job turned into a bit of a marathon.

This may become a regular job and so getting a reliable method would be nice.

My first question is 'Do I have the right sort of vacuum table?' Mine has a 1cm x 1cm grid of small holes and a holey rubber mat but I see other designs which use slots and rubber cord to make custom patterns. Looking around Stritzelberger Steuerungstechnik GmbH shows many different designs.

Or is my vacuum cleaner not powerful enough? I see other vacuum generators ranging from affordable side-channel blowers to quite expensive pump models.

Or would I be better off by only doing 3 at a time? Instinct says that I ought to get twice the 'suck' if I do that.

Any help and suggestions will be gratefully received.

Brian.

Sounds as if the seal on each box is variable.
Why not make a dedicated fixture so that each unit has the same cross section of vacuum. Remember that the force from the vacuum is dependant on the area under each unit. Is the vacuum from the cleaner sufficient! Most vacuum pomps are posituve displacement devices!
Just a few thoughts!

peter

Thanks Peter,

I've attached a photo to show how I was doing it. Obviously all 6 locations had a box in them. Each aperture in the jig is 140mm x 80mm. The unused holes around the edge are blanked with masking tape.

Playing around this morning I'm almost convinced that it's a lack of suck from the vacuum cleaner. Sticking a box onto the end of the hose, via a holey rubber mat, it felt like I had enough retention. However, I guess that via the vacuum table that amount of suction is being divided 6 ways.

When you say that most vacuum pumps are 'positive displacement' what does that mean?

hello mucker. when i used the sabre router, the vacum bed had less holes in it, probably spaced at 40mm centres. it wasnt just holes, if i can explain. you had 4mm holes through into the sacrifial bed into the vacum, with a 20mmmm hole in the sacrificial bed. looking at it from the side it was "t" shaped. the small hole gives suction, then the bigger recess gives more, resulting in a better hold. Thats as clear as mud i know but difficult to explain. I think you have to many holes there, less is best i feel.

regards alex.

By positive displacement I mean that the volume changes as in a piston engine.
To get a good vacuum you have to compress the remaining gas to extract it whilst a vacuum cleaner is just a fan.
You also have to remember that the force from the vacuum is proportional to the area at that vacuum and that the maximum force is about 14 lbs per square inch.
You may be better served with some gasket material at the edges of the boxes so as to get a good seal.
Hope I am explaining it ok.

Peter

Stick two vacuum cleaners in series, like a super-charged vacuum ;)

I wouldn't have used foam PVC to make the template, I doubt you will be getting a good seal all the way round & I wouldn't be surprised if it were porous enough for air to be sucked through it as well so the hoover will be fighting a losing battle trying to maintain any sort of vacuum. Try making the template out of a bit of rubber sheet rather than the foam.

Hi Martin,

it's actually 3mm rigid PVC which I machined on the CNC so that each cutout was a snug ( <1mm clearance) fit against the boxes. That sits on 1 mm holey rubber mat.

Hi Peter,

thanks, I thought you meant a piston based pump but best to check. I just need to find one which isn't going to break the bank.

it's actually 3mm rigid PVC which I machined on the CNC so that each cutout was a snug

Looking at the picture it looks very much like foamex which they may call a rigid PVC but because of the way is is made tends not to be great for sealing. If your using a vacuum cleaner to try & maintain a vacuum then an accumulation of very small leaks can cause you problems. One option would be more suck by using a bigger hoover, more hoovers or something else to pull a vacuum & the other option would be less leaks by sealing it better or reducing the area your sucking from. What sort of condition is the holey rubber mat in ?

There are a couple of threads on the forum about making vacuum tables so they might be worth a read.

I mentioned this in another thread recently - try putting a neoprene rectangle with a hole (or some other thin rubber sheet) under each box. That should provide a good seal and a high coefficient of friction, which you need to get them to stick without a substantial pressure difference.

I'm sure you can get small vacuum pumps, as in vacuum formers, I think they can be diaphragm type its not as if you need ultra high vacuum after all just a vacuum!

peter

Quote:

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Originally Posted by ptjw7uk

I'm sure you can get small vacuum pumps, as in vacuum formers, I think they can be diaphragm type its not as if you need ultra high vacuum after all just a vacuum!

peter

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Your right Peter but the problem is maintaining the vacuum because of all the leaks, with a good seal you would probably be able to use something like this
Composites Vacuum Pumps and Vacuum Equipment - Easy Composites

or if you didn't want to spend that sort of money you could build your own using an old fridge compressor.

I use a degassing chamber as a vacuum reservoir, but if you dont want to use one of those I've seen people coil air pipe and use that its alot cheaper

Sorry for not getting back to you all sooner - I've been bouncing around the country doing professional firework displays.

@martin54 - It's definitely rigid PVC and not Foamex. According to the web Foamex has a density of between 0.5 and 0.7 (and so will float). I've just measured the density of some offcuts of my material and it comes in at 1.4 which fits nicely with it being Rigid PVC (1.3 - 1.45).

@martin54 - the holey mat was new for this sets of cuts.

@Jonathan - thanks for the suggestion although I'd hope that the holey mat would have the same effect although I can see that a seal just around the edge might be better.

@martin54 - thanks for the link to the vacuum pumps. Although quite expensive it's a price I can stand if it'll do the job.