It is metal. 

yes, the filter size is 55mm and your Nikon D3400 AFP-DX 18-55mm lens size is 55mm, so the filte can fit your camere 

hello Nikon 18-55mm f/3.5-5.6G lens size is 55mm, so the filter can fit it 

FD-EOS fit for Canon FD mount lens and canon EF mount camera body. canon 60d is EF mount, so the adapter ring can work with your camera, but you need to check your lens mount. could you pls let us know your lens specific model? service(AT)kentfaith(DO NOT)com, Please note: It is not same between Canon FD mount and EF mount, this adapter ring is only for Canon FD mount lens, please contact us if you want to purchase for Canon EF mount lens. 

Hi, we checked the Sony a6600 18-135mm lens seems 55mm, if you like this filter, please reconfirm the lens size, the 55mm lens need to order 55mm size filter,thanks.