My 3/8th clamping sets T-nuts I just got are just slightly too big for my 12mm slots for my X3 table. But the spindle of my head can reach all the slots of the table. Shall I put in an endmill and make the slots slightly wider? Or shall I put my T-nuts in a vice and mill them slightly narrower? Is the table heat treated and therefore would be hard to mill?

T Nuts, for sure!

not agood idea to mill the table or slots. 12 mm t-slots can be purchased cheaply on ebay. thats what I did although I had to drill and tap bolt holes.

Walden's has really nice pre threaded 12mm t nuts for less than $2.00 each.

http://stores.homestead.com/WaldenSpecialties/-strse-81/Set-of--6/Detail.bok

Little Machine Shop has the A2Z Tuff-Nuts

http://www.littlemachineshop.com/products/product_view.php?ProductID=3107&category=

I agree with everyone else. Don't mess up your table, either modify your T-nuts or get some new ones.I suspect you'll want the 'Chinese Mini Mill' size: http://soigeneris.com/A2Z_Workholding.aspx . (I don't stock those but can get them for you, other folks sell them as well.)

Why buy new nuts when you already have some, use the mill and cut them down.
Make them the same size as the stock T-nuts that came with the mill.
Hoss

Alright, thanks everyone! I'll mill the T-nuts.