Vacuum chuck for irregular shaped plastic part

I need to flycut the top face of an irregularly shaped cast epoxy part.
The part is cast using epoxy into an open top aluminium mold.
When the part is removed from the mold there is a meniscus type sharp flashing along the edge.
What I want to do is flycut about 1mm of material off this face to remove the sharp casting flashing and to create a smooth surface.
The only way I can think of holding it is in a vacuum chuck of some sort as the four sides are sloped (like an inverted flat top pyramid).
Part side view is as below;
Need to flycut this top face ---------------------------------------------------- \ / \ / \ / \ / -------------------------------------------
<-----------------about 7"/175mm long------------->
The part is about 2"/50mm wide and just over 1"/25mm tall. The bottom face is say 6" x 2" = 12" squared.
If the vacuum pump sucks down to say 1/2 of atmospheric then I should see 12 Sq Inch x 7.5 Pounds/Sq Inch = 90 lb down force.
The angled sides of the mold should prevent the part being thrust sideways when flycut.
As all four sides are sloped I had thought of using a second aluminium mold as a vacuum chuck. I would have to machine 5mm machined off the top to expose the top face of the casting that needs to be machined back. chuck.
I could plumb a vacuum line into the base and use a refrigeration style vacuum pump to hold the part down and then flycut the top.
Will it work?
Should I use a die grinder to run a few slots for the vacuum to spread around in the bottom of the mold base used for clamping?
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On Saturday, March 17, 2018 at 11:36:55 PM UTC-7, Perry wrote:

So, mount a similar mold to a lathe faceplate, apply pressure on the workpiece center with the tailstock, and face off the outer rim of the gizmo. Then apply clamps, retract the tailstock, and complete the cut.
Fly cutting something that I can't hold in a vise makes me nervous.
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On 18-Mar-18 5:30 PM, whit3rd wrote:

Yeah I'm not so keen on the fly cutting myself. Actually I may have misled you. As you've probably guessed I am not a machinist.
Facing would be done using a tool like this but with only 4 cutting inserts.
http://www.machiningnews.com/wp-content/uploads/2013/11/MILL4-15-1.-560x534.png
It cannot be machined in a lath chuck as the cast part has a long cable attached to it.
Cheers.
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On 3/18/2018 9:06 PM, Perry wrote:

A face mill.
Is there any reason you can't put the face mill in the lathe chuck and fasten the piece to the cross slide? Perhaps held tightly in stiff foam rubber?
Paul
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On Monday, March 19, 2018 at 2:28:56 PM UTC-4, Paul Drahn wrote:

What would be the advantage to that over doing the same in a mill?
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On 20-Mar-18 2:28 AM, Paul Drahn wrote:

I'm worried about holding the work piece. The sloped sides make holding it difficult.
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On Sunday, March 18, 2018 at 2:36:55 AM UTC-4, Perry wrote:

ck.

I've been doing a lot with vacuum work holding lately. It sounds like your plan is pretty good. If you have a reasonably close fit around the sides, y ou are safe from the part flying out of the fixture.
What I would do is machine slots for the vacuum to spread around and also m achine a groove around the periphery of the bottom to place a seal. I use 1 /8" neoprene round cord from McMaster. Make the groove about 3/32 deep and 1/8 wide and the neoprene will just sit in there and make a very good seal. If you have irregularities larger than that will absorb, go to a thicker s eal - I also have 1/4" material, but haven't had the need to try it yet.
If you are using a refrigeration pump and you have a good seal, you are goi ng to get a LOT better than 1/2 atm, much closer to 1 atm.
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On 19-Mar-18 8:32 PM, rangerssuck wrote:

Thanks. I was thinking of buying an eBay refrigeration vacuum pump. I can get a 6 or 12 CFM unit for a couple of hundred dollars. I've got a fridge compressor pump but its small and pumps a low volume.
Thanks for the info on the sealing bead.
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https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw sco+vacuum+pump&_sacat=0
Mine pulls 26".
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Perry - do all as you describe but skip the vacuum and use two hold down clamps on the surface you're cutting. Use the clamp on the left while you surface the right side and the opposite. If too costly timewise, you'll at least get a look at the finished appeareance.
Hul

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Thats not a bad idea Hul. Maybe some of those toggle type clamps would make it easy to do. Thanks.
On 20-Mar-18 5:54 AM, Hul Tytus wrote:

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