What would be a preferred method to secure thin plywood (3mm) to the bed for cutting out parts ?
I’ve tried a vacuum system and it works ok until there is enough cults to decrease the vacuum holding the ply to the bed, then the vacuum is lost and the ply will not stay secured in place.
I then added small screws to hold the ply to the bed, this appears to be the best method so far.
The size of the wood being routeted out is 12x36 with multiple parts (model aircraft ribs) being cut out.
Thanks
Double sided tape was my first thought but your stock size is a bit big unless you can put the tape where the finished parts are going to be.
You have mentioned you have vacuum, if you search the internet for pictures of Wadkin CNC Router, these had a 2" thick nylon bed that a matrix was cut into, there were tapped holes that were used to allow the vac to areas of the bed where the finished part was going to be then sponge rubber was placed in the matrix within the perimeter of the finished part (to be) when the part was cut the vac still held the part, hard to explain fully but might give you some ideas.
Bill.
Put painters tape on the back of the plywood and on the bed. Put super glue on the painters tape and place the plywood on the bed.
This should hold it about as well as double sided tape, but will be easier to remove and won’t leave any glue residue.
And you don’t have to use tabs if you glue the entire surface of the plywood to the bed 
What I do is secure the 4 corners with screws or clamps, then have a v bit mark some spots where I know the bit won’t cut, then put screws down there. In my case, I actually just drill holes where my spoilboard holes are so I can secure with a washer.
Super glue and painters tape works well too.
just in theory it can be possible with electrostatic holder 
Thanks for the ideas everyone, they are greatly appreciated and will be useful
Metallic scrapboard and https://www.banggood.com/20pcs-N52-Super-Strong-Round-Magnets-10mm-x-4mm-Rare-Earth-Neodymium-Magnets-p-998141.html?rmmds=search&cur_warehouse=CN
I do the screw method on thin plywood. I use the self tapping metal screws that hav the washer type head.
I put screws in the 4 corners. Then I use my end mill to punch holes all over the plywood where I won’t be cutting. I then add more screws to all the little holes.
I like the idea of the masking tape and CA glue. I might have to give that a try in the near future.
I switched to the painters tape and glue deal, It works great.
The other item I like about it is that it raises the wood off the spoilboard just enough where the end mill doesn’t dig into the spoilboard.
You just have to be careful when removing the work from the spoilboard
by the way you can try to use milling bit with downward out of chips. such bit doesn’t move the material to up
ok, I’ll look into that
Thanks