Vacuum workholding

[stevieboy]

Hi Guys,

I want to put together a Vacuum plate for workholding large thin parts. I don't have a lot to spend on the project so trying to make savings where I can. I can make the subplate no problem and do all the fittings etc. I've been looking at the prices of vacuum pumps and they all seem a bit steep as they have a tank like a compressor. I've found where I can buy just a pump so my question is this.

Can I use a receiver tank off a compressor and just mount the pump on top where the compressor motor used to be. Also how would I need to control it.

I don't know a lot about this subject so any help would be very much appreciated.

[Geof]

I used a compressor tank as a vacuum tank; actually I converted a compressor into a vacuum pump and it works very well.

http://www.cnczone.com/forums/work_f...ompressor.html

I have seen some people claim that it is possible to implode a compressor tank by putting it under vacuum but obviously mine is okay to 26"Hg and a full vacuum is only about 30".

The tank has two advantages: One is that your pump can pull the tank down to a good vacuum and you can have a large diameter ball valve between the tank and your vacuum plate. This allows you to apply a large vacuum quickly and that sometimes helps snap the part down tightly and compress all the sealing strips. Sometimes if you apply the vacuum slowly just by turning the pump on the part never pulls down enough to form a seal so the pump can never develop a vacuum. Another advantage is that the tank can act as a trap so that if you get coolant into the system at the vacuum plate it does not get into the pump. Acually on my setup I had a coolant trap ahead of the tank as well.

[lgalla]

For full vacuum a standard compressor tank should be rated 300PSI.The tank would have an inner structure.Propane tanks would meet this rating and be cheap.
Larry

[Hoooooter]

You should have a look at this - It just runs off shop air. No vacuum pump required. Vac-Clamp Vacuum Clamping System : CARBA-TEC

Cheers


Ken

[stevieboy]

Thanks for the responses guys, good info.

I can get my hands on an old propane bottle no problem. Might even be able to source a vacuum pump. Where I think I would stumble is a water trap and some sort of control. Would a water trap off a normal airline be ok or would the pressure be too much and cause the bowl to implode.

Also I would want some kind of control so that I can switch it on and when the negative pressure has reached a set level the pump cuts out until the pressure drops, then switches back on again. That way the pump's not running all day. Just like a compressor but the opposite way round.

[underthetire]

You can use a water trap, as long as it's not a auto drain type, and you purge it with the vac off. Obviously you want the inlet side to be on the work holding side, not the pump head. Vac switches are available to cycle the pump, i get them from Mcmaster Carr, but i'm guessing you don't have them on your side of the pond.

[lgalla]

cannot see the need for a water trap as water boils under vacuum and is exhausted as vapour
Larry

[underthetire]

Depends on the set up. We get plenty of water in our traps at 22" from the vacuum pulling coolant/water through from the cutting process. Your assuming a completely sealed system at ultra vac numbers.

[Geof]

You need to pull a very, very good vacuum to pull water off as a vapor, and if you do it will condense in your pump and cause all manner of problems.

A compressed air water trap may work but they have small passages so you negate the advantage of a vacuum reservoir tank that I described; namely that you want to be able to apply vacuum quickly to snap your part down onto the seals. The small passages restrict flow and the trap itself has a small volume.

Look at my link and you will see my water trap which is just a water filter housing with larger ports and a decent volume.

Also your idea of being able to turn the pump off once it has pulled a vacuum will probably not work. It is almost certain there will be leaks in the system and if you turn the pump off you will lose vacuum very fast. On my system I just run the pump constantly.

[stevieboy]

Yeah I was thinking of a water trap really just to catch any coolant that gets into the system. I've got some good info now so I'll do a bit of looking around and see what I can come up with.

I feel a project coming on.

Thanks.

[stevieboy]

Just one more thing.

I could do with a rough idea of what size pump to use. Is it measured by CFM. I want to make a 600mm by 300mm grid type plattern table for a base. Any more help?

[Geof]

It is almost impossible to predict what size pump you need. If you are working with really good seals and have negligible leaks you can get by with a very small pump, less than 1 cfm but with leaks you may need tens of cfm. Get, or build, the largest you can afford.