Vaccum pods design

Hi, I'm new to the forum and I'm posting here cause I haven't seen a channel on fixturing.

I am designing a vacuum fixture for 5 axis machining that is going to be about 8" tall because I need to machine top, side, and bottom of part. I usually have a really tall fixed material and I wanted to give it a go with vacuum. Material is going to be wood just for information, non porous, hardwood.

In order to know if the the tangent force of the tools I'm using are greater than the vacuum holding force I need to calculate the vacuum pressure. However I'm not entirely understanding something.

The amount of holding force depends on the surface area shared by the two objects and the vacuum level. In the picture I attached, I created a simple vacuum cup and I was wondering if anyone knows if the surface area shared refers to the blue or the red area. The understanding of this aspect I think is important in the design of the pod.

Thank you in advance.

Vacuum is usually used for fixturing flat things, like plywood. It sounds like you're expecting it to hold onto the bottom of a fairly tall thing while you machine on the side of it. I really doubt that will work very well; the force of the tool plunging into the side near the top is likely to break it loose, or at least shift it a little. What you need for that is something like a "tombstone" fixture, which needs solid fastening to the workpiece - that or an awesomely powerful vacuum pump.

When using a vacuum pod, it's normal to mask off the area not covered by the workpiece; this conserves vacuum pressure that would otherwise go to waste, and concentrates it where it's needed. The "shared area" would be the surface of the pod that's covered by your part; the rest would be masked off.

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Originally Posted by awerby

Vacuum is usually used for fixturing flat things, like plywood. It sounds like you're expecting it to hold onto the bottom of a fairly tall thing while you machine on the side of it. I really doubt that will work very well; the force of the tool plunging into the side near the top is likely to break it loose, or at least shift it a little. What you need for that is something like a "tombstone" fixture, which needs solid fastening to the workpiece - that or an awesomely powerful vacuum pump.

When using a vacuum pod, it's normal to mask off the area not covered by the workpiece; this conserves vacuum pressure that would otherwise go to waste, and concentrates it where it's needed. The "shared area" would be the surface of the pod that's covered by your part; the rest would be masked off.

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Thanks for the answer. Tombstone is what I've bee using, but I wanted to try a 5 axis vacuum cup just for the sake of trying.
If I understood you right the red area from the pic I attached would be the shared area right? It seems counterintuitive though, cause you wouldn't need the channels to propagate the vacuum throughout the part on the pod. Just brainstorming

Hi Fribe - in your case it will be the area enclosed by the perimeter gasket including the passageways. If you use a vacuum pump, say one used for evacuating air conditioners this will produce 10 tonnes per sq metre clamp force at sea level. The "sliding" force is a function of friction so if your friction coefficient is say 0.1 which is for plastic you would get 1 tonne per sqm sideways. If 0.2 it will be 2T/sqm. I presume you will use a perimeter gasket. The timber will be porous so the pressure difference will be less. But if you seal the bottom of the timber with resin (epoxy, pva, paint) then you will get close to full vacuum. You can also use a vacuum venturi or vacuum generator via your compressor if you don't want to go with a pump. If you are making the pod then you could put some small spikes on it and it won't move sideways. Ot route a small bump or feature that registers to your pod. Cheers Peter

The channels are used for the initial evacuation and for scavenging air from micro leaks you will have in the pod and plumbing. The clamping pressure can inhibit scavenging if the surfaces are very smooth as they get crushed together very tightly. Sometimes its useful to put a thin cloth or paper on the surface to improve scavenging. I use the non slip table top stuff under my jobs.

This is called communication. The voids have to communicate the vacuum (or pressure) to the pump at all times otherwise the clamp pressure will decrease over time. The pump produces the pressure difference. If you have a perfect seal then you can pull down the job and turn the pump off. But this is rare, you always have micro leaks that will let you down!

+1 to what petting said. Also, if you are milling hard maple, the amount of leakage as a result of porosity is minimal. Red oak, very porous/leaky. Regarding your concerns about lateral movement, try applying sticky back 120 grit abrasive paper to the red squares within the gasket boundary. Depending on the degree of leakage, what your vacuum pressure is, what kind of lateral cutting forces are present and how many square inches are within the gasketed area, you may have enough vacuum to do the job.

Peteeng, sorry. My rogue spell check!

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Originally Posted by peteeng

Hi Fribe - in your case it will be the area enclosed by the perimeter gasket including the passageways. If you use a vacuum pump, say one used for evacuating air conditioners this will produce 10 tonnes per sq metre clamp force at sea level. The "sliding" force is a function of friction so if your friction coefficient is say 0.1 which is for plastic you would get 1 tonne per sqm sideways. If 0.2 it will be 2T/sqm. I presume you will use a perimeter gasket. The timber will be porous so the pressure difference will be less. But if you seal the bottom of the timber with resin (epoxy, pva, paint) then you will get close to full vacuum. You can also use a vacuum venturi or vacuum generator via your compressor if you don't want to go with a pump. If you are making the pod then you could put some small spikes on it and it won't move sideways. Ot route a small bump or feature that registers to your pod. Cheers Peter

The channels are used for the initial evacuation and for scavenging air from micro leaks you will have in the pod and plumbing. The clamping pressure can inhibit scavenging if the surfaces are very smooth as they get crushed together very tightly. Sometimes its useful to put a thin cloth or paper on the surface to improve scavenging. I use the non slip table top stuff under my jobs.

This is called communication. The voids have to communicate the vacuum (or pressure) to the pump at all times otherwise the clamp pressure will decrease over time. The pump produces the pressure difference. If you have a perfect seal then you can pull down the job and turn the pump off. But this is rare, you always have micro leaks that will let you down!

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Thanks man I appreciate the answer!
I'm thinking that it might not work then. The reason being, machining of the part will be done in 2 operations. The first would be to pur a radius on top of the part, then flip it holding it on the vacuum pod and using a shaper on both sides of the part. The shaper being 2-3/4" tall where it cuts might put too much tangential force for the pod to hold.
I might try and post the results.

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Originally Posted by MARV

+1 to what petting said. Also, if you are milling hard maple, the amount of leakage as a result of porosity is minimal. Red oak, very porous/leaky. Regarding your concerns about lateral movement, try applying sticky back 120 grit abrasive paper to the red squares within the gasket boundary. Depending on the degree of leakage, what your vacuum pressure is, what kind of lateral cutting forces are present and how many square inches are within the gasketed area, you may have enough vacuum to do the job.

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I'm using a Becker vtlf with 22.1 HgV so pump I dont think is a problem. My concern is the lateral force of the tool I'm going to use.

Thanks for the answer :)