Good day looking to build a electromagnet that is very powerful and curious as to best power supply(110 or 12 volt) unit will be made from largecopper  coil I will source and will have a 24 x 24 inch metal plate on top unit is to attract metal filings from our small milling  mill (prototype) that I will attempt to build on our farm  with my son unit must be very strong able to pull metal  from at least 4 inches away. Any help with materials list greatly apreciated regards steve

The relevant spec for your supply is how much power it can provide, since that will ultimately limit the strength of your field. 

15 hours ago, Thewashguy said:

Good day looking to build a electromagnet that is very powerful and curious as to best power supply(110 or 12 volt) unit will be made from largecopper  coil I will source and will have a 24 x 24 inch metal plate on top unit is to attract metal filings from our small milling  mill (prototype) that I will attempt to build on our farm  with my son unit must be very strong able to pull metal  from at least 4 inches away. Any help with materials list greatly apreciated regards steve

Here is a free program that will help you design your magnet.  Not knowing much about the design, I'm concerned that if you get such a powerful magnet working it will interfere with equipment in the shop or even cause harm.  Also the magnetic field won't act uniformly under the square metal plate.  This program will help you get a feel for what is possible.

http://www.femm.info/wiki/HomePage

 

Thank you both for your response a little clarity here we are producing on a small scale a grown farmed  product and then milled in our 3 tests carried out its was noted that there was metal particles in our product minute microscopic particles(probably from machinery0 after looking online at magnetic plate magnets made for food industry I thought we could make one as ones sold for this purpose are super expensive and totally out of our budget thanks again for responses

This type of thing?

http://www.puritanmagnetics.com/1/magnet/plate_magnets.asp

Basically a strong magnet in a stainless steel box.   Since price is the issue, maybe source it from China - like aliexpress or alibaba.  If you really want to design your own, price out ceramic magnets and figure out some sort of plastic cover maybe?  Arrange magnets in a Halbach array (could be difficult to align the magnets).  Redesigning tends to be more expensive for a one-off.

 

Step 1) find out why there are metal particles in your product

Step 2) eliminate source

It seems easier to prevent contamination than attempt to remove it afterwards.

Also note that a magnet will not remove aluminum nor most common stainless steels. Have you checked that your metal particles are magnetic?