new here sorry if wrong froum trying to found out about demagnitzers
would like to build one for large parts

How large part do you intend to work with?

I'm not sure, but maybe you'll get better response in the Electronics forum?

thanks will try different forum the parts are large gear blanks
400.mm dia. x 350.mm long this is the largest part i would need to demag

I imagine they consist of high energy AC 50/60hz coils, if so they possibly could be constructed using a welding transformer supply?
This is essentially what is used for crack & defect detection.
Al.

Must admit don't know how to build one,but loudspeaker repair shops have mag-demag service.Sometimes it is cheaper to send out than attempt to do it yourself.
http://www.speakerrepair.com/services.html

I have a primary winding of a scraped transformer, the wire is 1mm thick and about 1-2 kilo in weight. The opening is about 30x50 mm . I simply connect it to 220VAC and demagnet tools and parts. It only takes seconds. In order to make for a 400mm dia. you can wind a coil to your requirement. The only issue will be the time to demagnetize. If you need for example to demagnetize pipe ends for welding, then you need to elaborate more. There are lots of sources for its calculation, just google demagnetizing.
Hope that helps
Nader