Vacuum chuck application..Calculator for axial pull out forces based on flute helix, material and cutting conditions ?. Is there such a thing ?.

[Wsurfer]

Like the title says, looking for a calculator if it exists, as I'm math challenged. The setup I have is currently working (Mic-6 .05 to .25" thick part in a custom vacuum fixture). What I have is working, but would like to decrease cycle time without scraping part and broken tooling. I realize there's a lot of trial and error / variables, but a gross reality check would be nice.

 

[crossthread82]

If you calculate the tangential force from a hp/torque calculator you can multiply that by the helix angle of the tool divided by 90. That should give you a rough approximation as to how much axial lift force your tool is generating in the cut.

 

[DavidScott]

It's way more complicated than that. Corners lift far easier than the middle of straight sections and those "fingers" on the right are prime examples of easy to lift areas. Corncob mills produce much less lift than any other end mills and my Mitsubishi ASX445 face mill doesn't lift parts, even if I forget to turn the vacuum on so these are good tools to rough with.

 

[crossthread82]

It's way more complicated than that. Corners lift far easier than the middle of straight sections and those "fingers" on the right are prime examples of easy to lift areas. Corncob mills produce much less lift than any other end mills and my Mitsubishi ASX445 face mill doesn't lift parts, even if I forget to turn the vacuum on so these are good tools to rough with.

That is true, however my point still stands. The force generated by the cutter is fairly constant through out the cut based on engagement. It's up to your own engineering insight whether or not your part/fixture combo has enough material being supported by vacuum around where the cutter will be at all times to counteract that force. The reason why corners lift in vacuum fixtures is b/c there is less square inches being affected by vacuum in that area (effectively 75% less).

 

[Scruffy887]

Is that a part shape cut from a rectangle? The outside shape is where you will have the most issues. You can get down spirals for outer perimeter and they will also work really well on other features as long as flood clears the chips. Rule of thumb is about 13 psi max holding power. Down spiral and straight bits can make a huge difference in cycle time. Up spiral did not work for my application even though the sheet had thousands of pounds of hold down. The smallish pieces were "onion skinned" but the up cut broke the vacuum.

 

[DavidScott]

It looks like the profile is cut when it's a plate and nrsted in the fixture to locate.

 

[crossthread82]

I might suggest you redo your gasket profile so you get more surface area under vacuum. I usually only leave .050" from the outside edge of my gasket to the edge of the part. Also you're giving up a lot of area by just lopping off sections that have holes in stead of O-ringing off the holes individually and having the gasket going around the full outside profile.

How many inHg is that fixture holding right now?

 

[Wsurfer]

I might suggest you redo your gasket profile so you get more surface area under vacuum. I usually only leave .050" from the outside edge of my gasket to the edge of the part. Also you're giving up a lot of area by just lopping off sections that have holes in stead of O-ringing off the holes individually and having the gasket going around the full outside profile.

How many inHg is that fixture holding right now?

Not sure I believe my gauge, but 30"Hg, which I don't think is possible using a venturi vacuum. I calculate holding force around 100lbs. Orings are a good point. The gasket slot is .0625 located at the wall of the fixture/part interface. The forked fingers was the challenge as there is no room for bend radius. Gasket was laser cut at .07" wide from a sheet of neoprene closed cell foam.

 

[Wsurfer]

It looks like the profile is cut when it's a plate and nrsted in the fixture to locate.

Yes, op-1 makes the profile for the fixture. The fingers are roughed out first to get the benefit of the existing carrier stock rigidity.

 

[crossthread82]

Not sure I believe my gauge, but 30"Hg, which I don't think is possible using a venturi vacuum. I calculate holding force around 100lbs. Orings are a good point. The gasket slot is .0625 located at the wall of the fixture/part interface. The forked fingers was the challenge as there is no room for bend radius. Gasket was laser cut at .07" wide from a sheet of neoprene closed cell foam.

I have good luck using the .0625" foam core gasket cord you can get from mcmaster. Usually cut the groove .0625" wide .050" deep. That way you can make the groove whatever profile you want. 30" is hard to believe, a venturi setup is usually only good to around 26-27". My full blown vacuum pump struggles to get 29.5" If you can get away with not drilling all the way through the part and leave a .005-.008 floor that you can avoid having all the perforations. Then take a counter sink and poke it through the thin floor and debur it away.

 

[Wsurfer]

If you calculate the tangential force from a hp/torque calculator you can multiply that by the helix angle of the tool divided by 90. That should give you a rough approximation as to how much axial lift force your tool is generating in the cut.

Thank you

 

[Wsurfer]

Is that a part shape cut from a rectangle? The outside shape is where you will have the most issues. You can get down spirals for outer perimeter and they will also work really well on other features as long as flood clears the chips. Rule of thumb is about 13 psi max holding power. Down spiral and straight bits can make a huge difference in cycle time. Up spiral did not work for my application even though the sheet had thousands of pounds of hold down. The smallish pieces were "onion skinned" but the up cut broke the vacuum.

I'll look into down cut tooling, thanks

 

[Scruffy887]

I do a lot with Onsrud down spirals. Like .35 deep in a single pass with a single flute 5mm O flute down. Plunging too, this is in .375 phenolic/linen.
30 hg may be a gauge tolerance issue. I get an easy 27" with the venturis I make and they use less than .4 cfm each. A few may pull a bit more.