Queries :: Getting Summed Data From 2 Different Tables?

Here is my dilemma: I have a table where I am entering data for quantity of a product received. This data is further segregated by the mill it comes from, the grade it is, and the size it is. I have a second table where I track the output from when the product is processed through our system.

I need to be able to query / report the difference between these two sums (i.e. sum of total qty received from that mill/size/grade versus the total processed from that mill/size/grade). Basically, I am creating an inventory report showing how much "raw" product I have, grouped by the mill then the grade and then the size. I have to sort the data out this way because I purchase the exact same grade and size product from more than one mill.

My problem is that any query I have created so far wants to only show data from the two tables where the mill/grade/size match. In other words, if I have product that has been received, but nothing has been processed from that mill/grade/size combination, then I don't get data showing up at all. I need to be able to show all sums of product from the "receipt" side, and then only the "processing" data where I actually have processed something from that mill/size/grade combination, then show the difference. So the result would show something like "50 received, 0 processed, resulting in 50 on-hand" for that mill/grade/size.