Air gap of a ferrite core. How?

[mAze]

Hi,
I want to ask how to do the air gap for a E type ferrite core, by myself?
I know the stock ferrites can be order with the air gap already made, but it is not an option for me, as my supplier has only non-gapped cores.
The stock gaps are in the middle leg, but I have seen DIY project using spacer on the all three legs, but this will increase the EMI, right?
Also, if a need a 4mm air gap in the middle leg, how much should it be, when I divide it among all of the legs... may be in half ( 2mm) ??

Thanks

 

[szhighstar]

Hi mAze, you can cut air gap by hand, we ofter mill core by hand when we make several samples.
When you make many experiments , you will get much experience.