O/T Vacuum fixture problem, long

[SLJ]

Hoping the brain trust here would have suggestions. Tried a vacuum fixture for the 1st time this morning. 1st part held fine, after 1st article insp. went back and it wouldn't even hold under finger pressure. Using a Gast vacuum generator with regulated air at recommended 68 lbs., getting a constant 25 inches vacuum.

The part is .125" alum. plate 5.25 x 9 inches. All we're doing is facing a step to .063with a 1.25 inserted mill, and then profiling with a .25 E/M. Profile is somewhat horse shaped(block legs, no small extentions). The fixture has a channel cut following the profile with a 3/16 Ball Mill x .14 deep that leaves about .05 offset from the part profile. I cut "vacuum tracks" on a checkerboard pattern .75 spacing and about .03 deep with the same BNEM. The tracks ended up about .13 wide. All tracks intersect each other and the outside profile channel. The profile channel is filled with 3/16 sponge cord.

Is there anything obvious to those of you that use Vacuum regularly that I may be missing?

[Rekd™]

Do you have gasket mat'l in there? Surgical tubing or something similar?

[SLJ]

Rekd

The customer supplied 3/16 spongecord(what they called it anyway) for the offset profile track. It's 3/16 dia. rubber like cord, very soft. They said they used it all the time. Forgot to mention, the vacuum comes into the fixture through a 1/4 dia. hole, placed at the intersection of two of the shallow tracks.

[Rekd™]

Everything sounds like it should work. Got pict's of the setup or cad files?

 

Also, how far from the top of the fixture is the top of gasket? I use about .005 to .015.

[Ripper3785]

we use vacuum 98% of the time to hold parts and it sounds like you've done everything right... but unfortunately my application is wood.. when this happens for us usually either some crap got in the vac hole in the vac fixture, or the part is not true or has a crack where vac leaks. but this is probably of no help as our application is quite different.

[SLJ]

Uncompressed, the cord is about .04 above the fixture. How do you butt the ends of the gasket cord? Would a guy dare use some RTV Silicone?

[Rekd™]

I use surgical tubing, and just cut it a little longer than the required length, and push them together. Seals real nice.

 

You should try to get it (uncompressed) to withing less than .01 of the fixture. You wouldn't think so, but just the preasure from the gasket alone can sometimes break the seal. How much vacuum do you have?

[Guest CNC Apps Guy 1]

He said 25 inches...

[SLJ]

Thanks guys, I'll try cutting the track deeper. The .14 was at the recommendation of the people we got the cord from.

[Chris Rizzo]

Keep us posted on how you get it working..just curious here.

[George_Gerhard]

If you are using an upspiral cutter it is probably lifting the part. This is common on thin gage material. Try using an o-flute cutter

 

rgds

George

[STKSHFTR]

SLJ

You mentioned a vacuum generator (compressed air powered?). Generally, these are filtered between the fixture and the generator and gaged at the generator. You may be seeing 25" at the generator, but getting a great deal less at the fixture because of a clogged filter. You may want to put a gage on a test plate to verify your vacuum at the fixture.

[Bruce Caulley]

Try some "belt grip" on the fixture. It will help the part seal without gluing it down for good. Sometimes vac pressure is not all that is needed. If the fixture is leaking anywhere the pump must be able to cope. I.e if a leak is absorbing 10L/min and your pump is only rated at 8l/min then you are in trouble.

 

Bruce

[SLJ]

Just got back to it this morning. Going to try cutting the track deeper to get the top of the spongecord down to .015-.02 above the fixture. We have 1/4 downspiral E/M to try also, however the 1 1/4 inserted mill has very high cup and helix angles. What would you guys think would be a general minimum for part size. We are cutting this out of 5.25x9 1/8in. plate, but the profile only leaves about 2/3 of the total sq. in. for the finished part.

[Jim at Gentex]

SLJ, I didn't notice if you mentioned the material your fixture is made of. The reason I mention it is I had a similar experience with a vac fixture that just wouldn't hold no matter what I did.

 

Finally, I discovered that the fixture was made of cast aluminum and that it had a small almost invisible crack that ran the length of it. Apparently the air was seeping through the crack and not allowing a seal. Our model shop guys patched it up with some kind of epoxy compound and it worked just fine after that. It may be worth a careful inspection of the fixture if you try everything else and it still won't hold.

 

quote:

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Would a guy dare use some RTV Silicone?

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IMHO it probably won't hurt anything. Anything that will prevent compromise of the seal will work, and silicone is particularly good for sealing hairline cracks where air can leak out.

[Rekd™]

I discovered that the fixture was made of cast aluminum

 

[SLJ]

Fixture is aluminum plate milled to fit the material size. Figured the guys could just lay the plate on flush all around and plug in the vacuum. We're going to try adding a couple of clamps and tabbing the profile since it's apparently a small part for vacuum application and the profile necks down fairly small in one location(still has "vacuum tracks through the area though).

[richardtraver]

WHERE IVE SEEN THIS HAPPEN THE MOST IS WHERE COOLANT OF OTHER CONTAMINATES GET INTO THE VACUME PUMP. LOOK AT THE GAUGE AND IT SHOULD BE REAL CLOSE TO 29" AND REALLY SHOULD BE ABOVE 28" IF TI'S NOT CHANGE THE OIL IN THE VACUME PUMP AND IT SHOULD THEN BE ABLE TO REACH THE RIGHT VACUME

[Rekd™]

Richard, take the pinky on your left hand, move it over about an inch, and press ONCE. Mkthx.

[MetalMarvels]

+1 to STKSHFTR

 

The last time I had a problem with my vacuum fixture - the port through the plate had filled with coolant and plugged the filter. I re-did my lines and I have the exhaust to air port below the vacuum port that goes to the pump - most of the coolant then is blown out through the exhaust port (with a little squirt of compressed air at changeover time).

 

I am also using the very soft, closed-cell sponge rubber cording in 1/8 and 3/16 sizes with very good results - I typically leave it 0.05 to 0.1 above the surface. This material is extremely soft and seals very nicely. I just cut it about 1/4 inch long and butt the ends together. I also found that I needed to "compress" the cording length-wise as I am packing the groove - if you stretch it, the diameter decreases too much and it won't seal properly (it will often pull out of the corners if you did stretch it during the packing process).

[MetalMarvels]

I re-checked my numbers - for the 1/8 cord, I am using a 0.115 width and a 0.100 depth groove. For the 3/16 cording, I am using a 0.160 width and 0.160 depth groove. In both cases the cording squishes down enough to almost completely fill the groove, but has just enough "push" to lift the plate up a little when the vacuum is released. My earlier trials used a deeper groove and the plates just would not pop back up because the cording is so soft.

[SLJ]

Guys, sorry for the delay, but this one has been set aside temporarily. Got a couple of hot jobs for that machine. When we get back on it I'll let you know our progress. Thanks Steve

[SLJ]

OK, got it going, but had to add a couple of small clamps and tab the profile. the part itself has two main sections 2 x 5 and 3 x 5 approx. with a one inch wide by 2.5 section connecting them. Got the idea the second section wasn't being serviced well by the single groove going through the "neck", so added a second vacuum port in that end. Still didn't hold. That's with the seal groove deepened as well. Total part area is 25.8 sq. in. according to analyse.